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Salt induced micellization and micelle structures of' PEO/PPO/PEO block copolymers in aqueous solution
Abstract
Aqueous micellar solutions of two moderately hydrophilic polyethylene oxide/polypropylene oxide/polyethylene oxide (PEO/PPO/PEO) triblock copolymers, pluronics P84 and P104 are examined by small angle neutron scattering (SANS), viscosity and cloud point measurements. Micellar structures of pluronic P84 in aqueous solution are determined as a function of its concentrations (5 and 10 wt%) and added KCl concentrations (0–2 M). 5 wt% solutions of both the block copolymers contain spherical micelles with hydrophobic core of PPO and corona of highly hydrated ethylene oxide subchains. The presence of added neutral salt favors micellization at markedly lower concentration/temperature unlike conventional surfactants. The addition of the salt causes dehydration of ethylene oxide units from hydrated PEO shell from the side of PPO core, leading to an increase in core radius (RC) of spherical micelles. Micellar volume fraction (φ) increases with pluronic concentrations. It however increases at lower salt concentrations (up to 0.5 M) but is found to be independent at higher salt concentrations. Aggregation number (Nag) of P84 in aqueous salt solutions increases from 72 in water to 141 in 2 M KCl. No growth of spherical micelles is observed in unimer-to-micelle transition region and at temperatures below 20–22°C to its cloud point (CP) as hard sphere remains almost constant (Rhs∼70–74 Å) with increasing salt concentration. Micelles remain spherical throughout the KCl concentration range studied.
... Literature reports have shown a great concern for the presence of additives, which influence significant changes in the physicochemical properties of BCPs [18][19][20][21][22][23][24][25][26][27][28]. This aspect has finely tuned the performance of Pluronic in formulating the desired dispersed system. ...
... In particular, the role of kosmotropic and chaotropic salts in promoting micellization is attributed to their ability to disrupt the structure of water molecules [17][18][19][20][21][22]. Specific anions, including I − and SCN − , contribute to the increased water solubility of organic solutes by acting as water structure disruptors, described as the salting-in effect [20][21][22][23][24][25][26][27][28]. On the contrary, water structure stabilizers, including ions such as Cl − , Br − , F − , and SO 4 2− , are recognized for their ability to induce a salting-out effect [23][24][25][26][27][28][29][30][31][32][33]. ...
... Specific anions, including I − and SCN − , contribute to the increased water solubility of organic solutes by acting as water structure disruptors, described as the salting-in effect [20][21][22][23][24][25][26][27][28]. On the contrary, water structure stabilizers, including ions such as Cl − , Br − , F − , and SO 4 2− , are recognized for their ability to induce a salting-out effect [23][24][25][26][27][28][29][30][31][32][33]. Sails et al. investigated F127-salt formulations, noting that their CMT and hard-gel formation temperature (T hg ) values are lower than 20% w/v F127, following an anionic Hofmeister series: SO 4 2− < Cl − < SCN − [18]. ...
Kosmotropic salts lower the cloud point (CP) in triblock copolymers (BCPs) and exhibit micellization at lower concentrations and temperatures. Our investigation utilizes various physiochemical techniques, including tensiometry, calorimetry, viscosity, spectroscopy, and scattering, to understand the self-assembly phenomenon. Furthermore, by employing photophysical methods, using Coumarin 151 (C-151) dye as a probe, the self-assembly mechanism in the highly hydrophilic F98 in water and NaCl solutions with temperature is reported. Adding NaCl decreases the CP of 10% w/v F98, inducing a significant influence on the micellization behavior. Such an effect is attributed to the dehydration of the PEO blocks of F98. The surface tension (γ) measurements provided an insight into the intermolecular hydrophobic interactions at the air–water interface, indicating the enhanced surface activity in the presence of NaCl as a function of temperature. As the temperature or NaCl concentration increases, the solution flow behavior in terms of relative viscosity notably rises due to the enhanced intermicellar interactions with the continued dehydration of the PEO shell of F98. The salting-out effect of the PEO and PPO blocks of F98 in the presence of NaCl was evaluated by observing changes in bond stretching through Fourier-transform infrared (FT-IR) spectroscopy. Two-dimensional nuclear Overhauser effect spectroscopy (2D-NOESY) revealed insights into the spatial arrangement and dynamic interactions of NaCl binding with the PEO corona (~ 3.0–4.0 ppm) of F98. This binding reduced hydration and significantly altered the micellar dynamics. Additionally, minor shifts observed in the PPO signals (~ 1.0–1.5 ppm) suggested indirect interactions, pointing to changes in the internal environment and segmental dynamics of the micellar core. The spectral behavior is further validated by evaluating optimum descriptors using computational simulations performed with the DFT/B3LYP method within the 3-21G basis set framework, utilizing the Gaussian 5.0.9 software. The absorption spectra under increased NaCl concentration and higher temperatures render the microenvironment around the probe C-151 more hydrophobic, suggesting the formation of H-type aggregation. Additionally, fluorescence excitation spectra indicated a blue shift with increasing NaCl concentration, further supporting H-type aggregation. Here, the average fluorescence lifetime remained constant at ~ 5.4 to ~ 5.7 ns, as observed from fluorescence emission decays. Such consistency in fluorescence lifetime, despite forming H-type and J-type aggregations, indicates that the structural changes around the probe do not significantly affect its excited-state dynamics. It has been observed that the addition of NaCl influences the spectral behavior of F98 due to the dehydration of thermosensitive regions as a function of temperature, likely due to the salting-out effect. Dynamic light scattering (DLS) analysis exhibited temperature-dependent variations in F98 micelle size in the presence of NaCl at varying concentrations, attributed to the screening and salting-out effects, which was further validated from small-angle neutron scattering (SANS) investigation, utilizing a core–shell spherical model. Furthermore, our investigation included a spectral analysis that depicted an improved solubilization assessment of the hydrophobic dye Orange OT expressed in dye loading efficiency (DL%) and encapsulation efficiency (EE%), thereby inferring NaCl to dehydrate the copolymeric micelles due to the salting-out effect.
Graphical abstract
Scattering and photophysical profile of 10% w/v F98 in water (blank) and with 2 M NaCl at 30 °C.
... This is because the ions in the solution shield the electrostatic repulsion between ionic surfactants and enable them to pack more tightly at the air-water interface, thereby reducing surface tension more effectively [33,34]. The lower electrostatic repulsion between surfactants also facilitates the formation of micelles in cement pore solution [33,35]. Lunagariya et al. [36] explored the effect of different active ions, including Cl − , Br − and I − , on copolymers and observed small changes of approximately 3-5 mN/m at CMC. ...
... Despite this, it was shown that some nonionic surfactants still exhibit a small reduction in surface tension and CMC in the presence of ions [33]. Some researchers attributed this to the dehydration of PEG blocks, which can make the copolymer molecules more hydrophobic and therefore lower surface tension [35]. ...
... Previous studies have also indicated that in higher concentrations, the surface tension of copolymers is slightly less affected [33]. This phenomenon is attributed to the formation of micelles, which are less susceptible to interference from ions [35,36]. Figure 6a,b depicts the foaming characteristics of the copolymers studied in cement pore solution. ...
This study investigates the relationship between the molecular structure and foaming of poly(ethylene glycol) and poly(propylene glycol) triblock copolymers in Portland cement pastes. Four copolymers with different molecular structures were studied at varying concentrations. All copolymers showed a reduction in surface tension of the cement pore solution; however, only some of them demonstrated foaming and air entraining in cement paste. The results indicated that the molecular structure parameter, hydrophilic-to-lipophilic balance (HLB), has a direct relationship with the foaming and air-entraining performance of the copolymers. The total organic carbon measurements showed very small adsorption of these non-ionic copolymers on hydrating cement particles due to the lack of surface charge needed to interact with the heterogeneously charged surface of hydrating cement. In addition, these copolymers did not seem to affect the flow of cement paste due to a lack of adsorption on cement particles. The cement paste modified with the copolymers showed increased water sorption compared to the control paste due to the increased capillary porosity and slight increase in pore surface hydrophilicity. However, the freeze-thaw resistance was shown to improve with an increase in the number of air voids in the modified cement pastes. The findings establish the relationship between molecular properties of copolymers and their air-entraining performance in cement paste to mitigate the damages caused by freeze-thaw action.
... The hollow crystals are formed through many factors. Firstly, the micelles aggregate with increasing salt concentration [28]. The inorganic salt particles originally generated inside the micelle agglomerates further fuse with each ...
... The hollow crystals are formed through many factors. Firstly, the micelles aggregate with increasing salt concentration [28]. The inorganic salt particles originally generated inside the micelle agglomerates further fuse with each other, but the concentration of inorganic salts inside the agglomerates after fusion is not enough to support its growth into a solid structure, leaving holes inside the inorganic salt particles. ...
... With the concentration of 200 g/L, the size of KCl nanocrystals is 57 ± 18 nm at pH 6.9 (Figure 5a,b) and 472 ± 176 nm at pH 7.3 (Figure 5c,d). When the P123 concentration approaches its gelation concentration, the micelle number density increases, leading to a denser micelle [28], slowing down the ion permeation rate and decreasing the ion concentration inside the micelle. So small-particlesize KCl nanocrystals were generated inside micelles at pH 6.9. ...
The current work introduces the synthesis of inorganic salt nano/micro-crystals during the reduction of hydrogen tetrachloroaurate(III) by Pluronic triblock copolymers (P123, PEO20–PPO70–PEO20). The morphologies and component were confirmed using an electron microscope with an electronic differential system (EDS), and the crystal structures were determined with X-ray diffraction (XRD). The morphologies highly depend on the concentrations of Pluronic and pH values. The mean size of the nanocrystal and hollow micro-crystal were controlled typically in the range of 32–150 nm (side length) and 1.4 μm, respectively. Different from the electrospray–ionization (EI) method, a model in which KCl forms a supersaturated solution in the micellar core of Pluronic is used to explain the formation process. This work provides the new insight that inorganic salt nanocrystals could be synthesized with the template of micelles in pure aqueous solutions.
... One class of compounds which combines all four features (i-iv) is triblock copolymers with an ABA architecture (A: PEO and B: PPO). ABA triblock copolymers are present as unimers (unaggregated form) at low temperature and as micellar aggregates at elevated temperatures (above their critical micellization temperature (CMT)) in aqueous solution [26][27][28][29]. A broad spectrum of nonionic systems was probed regarding their binding with superchaotropic NIs, such as small aromatic molecules [17], surfactant self-assemblies [11,15,18,19], oligo-and polymers [18,20,21], macrocyclic molecules [14,16,[22][23][24] and short-chain amphiphiles [25]. ...
... One class of compounds which combines all four features (i-iv) is triblock copolymers with an ABA architecture (A: PEO and B: PPO). ABA triblock copolymers are present as unimers (unaggregated form) at low temperature and as micellar aggregates at elevated temperatures (above their critical micellization temperature (CMT)) in aqueous solution [26][27][28][29]. Such well-known polymeric surfactants (Pluronic ® or Polaxamer ® (BASF, Ludwigshafen, Germany)) are used in a broad field of applications, as they solubilize oily compounds [26,30], are used as drug delivery systems [31,32] or templates in the synthesis of mesoporous materials [33,34]. ...
... For c(P84) > 1 wt%, the phase transitions appeared at rather constant temperatures: 60 ± 4 • C (transparent to blueish) and 74 ± 3 • C (blueish to turbid), respectively. Here, the blueish to turbid phase transition temperature was attributed to the CP [29,36]. For c(P84) < 1 wt%, the two phase transitions were shifted to much higher temperatures by decreasing P84 concentration. ...
Low charge density nanometric ions were recently shown to bind strongly to neutral hydrated matter in aqueous solution. This phenomenon, called the (super-)chaotropic effect, arises from the partial dehydration of both the nano-ion and the solute, leading to a significant gain in enthalpy. Here, we investigate the chaotropic effect of the polyoxometalate α-PW12O403− on the triblock copolymer P84: (EO)19(PO)43(EO)19 with (EO)19 the polyethoxylated and (PO)43 the polypropoxylated chains. The combination of phase diagrams, spectroscopic (nuclear magnetic resonance) and scattering (small angle neutron/X-ray scattering) techniques revealed that: (i) below the micellization temperature of P84, PW12O403− exclusively binds to the propylene oxide moiety of P84 unimers; and (ii) above the micellization temperature, PW12O403− mostly adsorbs on the ethylene oxide micellar corona. The preferential binding of the PW12O403− to the PPO chain over the PEO chains suggests that the binding is driven by the chaotropic effect and is reinforced by the hydrophobic effect. At higher temperatures, copolymer micellization leads to the displacement of PW12O403− from the PPO chain to the PEO chains. This study deepens our understanding of the subtle interplay between the chaotropic and hydrophobic effects in complex salt-organic matter solutions.
... The reducing power activity was performed using FRAP where different concentrations were prepared by diluting the mixture and incubating at 50°C for 20 min. About 1 mL 10% trichloroacetic acid was added to stop the reaction followed by 1 mL of 10% ferric chloride and then the absorbance was recorded at~700 nm [37][38][39]. ...
... Another standard antioxidant activity to reduce ferric (FRAP) was examined which measured the absorbance of the solutions depending on the increase in the parabens concentration (Fig. 7b). Reported studies have shown that higher the absorbance greater is the reducing power of the solution [37][38][39]. Here, the obtained correlation analysis revealed that parabens in micellar solution environment enclose more reducing power capacity than the parabens in water following the similar order as obtained in DPPH activity. ...
... This nepotism about the solubilization of BzP is found more acceptable in the both the polymeric micellar core giving an account on a linear dependency between the antioxidant capacity and the hydrophobicity resulting in to successive micellar transition in such a multiphase system. Furthermore, the IC 50 (half maximal inhibitory concentration) values which is a measure of the antioxidant potency between the examined systems were derived taking the reference of the positive control, AA which exhibit highest ability to scavenge the ·OH that are presented in supplementary information (Table S4) [37][38][39]. Looking its trend, it was observed that with increasing the hydrophobicity of the parabens irrespective of the media the IC 50 values decreases. Also, considering the media, the IC 50 values of parabens in non-micellar media were higher than in micellar media which depict that the parabens in micellar (5% w/v P84 and 5% w/v T1304) systems possess high ability redox potential to scavenge ·OH than aqueous media. ...
Micellar transition induced employing pharmaceutically-active antioxidants specifically p-hydroxy benzoic acid esters (parabens) viz., methyl paraben (MP), ethyl paraben (EP), propyl paraben (PP), butyl paraben (BP), and benzyl paraben (BzP) in ~5% w/v thermoresponsive triblock linear: Pluronic® (P84) and star-shaped: Tetronic® (T1304) copolymers was investigated in aqueous solution. The solution behavior of these block copolymers in presence of parabens is well scrutinized from cloud point (CP), relative viscosity (ηrel), spectral (FT-IR, UV–vis), computational simulation, and scattering (dynamic light scattering (DLS) and small angle neutron scattering (SANS)) study. Varied phase behavior and a dramatic change in ηrel with temperature ageing is attributed to the plausible induced micellar transition (spherical micelle to polymersomes) is accounted to the hydrophobic effect of parabens onto the block copolymers which is depicted from spectral study and further rationalized using computational simulation approach. The prominent shifts observed in the hydrodynamic diameter (Dh) of parabens loaded and unloaded copolymeric micelles was inferred from DLS while fitted SANS findings clearly depicted the micellar dimensions along with the aggregation number (Nagg). Furthermore, the performed antioxidant (free radical scavenging and reducing capacity) assay in non-micellar and micellar media trait the ability to shuttle between the reduced and oxidized forms that prevents the oxidative damage which makes this study biologically important.
... Generally, nonionic surfactants display optimal effectiveness when they are used at a temperature near below their cloud point. Pluronic surfactants are known to exhibit cloud point behavior [34][35][36][37]. ...
The interaction of antiprotozoan drug Ornidazole (ONZ) with single and mixed micellar surfactant systems were probed through FTIR, UV-Visible spectrophotometry, Cloud point, SEM and Cyclic Voltametric measurements. The surfactants employed were two polymeric surfactants, viz., Pluronic L-35 and F-127. The FTIR studies pointed at shorter and strong bonding between the drug and mixed micelle compared to the drug and single micellar systems. The cyclic voltametric study revealed active interaction between ONZ and the all the three micellar systems. This could be due to electron transfer operative around the electrode which is reflected in the oxidation and reduction peaks. The UV absorption spectrum of ONZ showed higher absorbance with the mixed micellar system L-35 + F-127 compared to that with single micellar systems of L-35 and F-127. This reflected greater solubilization of drug in the mixed micellar system. Based on the results we propose that ONZ can be solubilized in a mixed micellar medium of L-35 + F-127 for better efficacy compared to the single surfactant F-127 which is the most widely used surfactant in pharmaceutical industry.
... Since the shape and size can vary due to irradiation, the solubility also is affected. There can be dehydration of ethylene oxide units from hydrated PEO shells present at the sides of the PPO core, resulting in an increased core radius similar to salt addition [47]. Fusion of inter-micellar core can also take place resulting in large micelles or micellar clusters. ...
The present work has used pre-irradiation of surfactants in order to enhance their capacity to solubilize hydrophobic drugs and dyes. Aqueous solution of two pluronic surfactants viz., F-127 and P-123 were subjected to gamma radiation. These irradiated surfactant solutions were used to solubilize five drugs such as ornidazole, tinidazole, ciprofloxacin, aceclofenac, and methylparaben, and two dyes, methyl orange and eosin yellow. Their capacity for enhancing solubilization was compared with that of unirradiated surfactants. The irradiated surfactant solutions displayed 0.88%–31.9% higher solubilization compared to unirradiated surfactant solutions. This phenomenon of enhancement in solubility is explained through changes in the physical properties like decreased surface tension and increased hydrodynamic radius in irradiated block polymeric surfactants. Irradiation of surfactants for enhancing their capacity to solubilize hydrophobic chemical entities has been used for the first time.
... Bahadur co-workers [55,56] extensively studied the influence of added salts on the micellization and clouding phenomenon of block copolymers. Interpretation of CP is mainly based on the dehydration of PEO moiety [57] at a particular temperature above CMC and CMT when phase separation happens as the hydrogen bonding between ethereal oxygen of PEO and water molecule is disrupted [58]. The impact of different sodium salts on the CP of 5 wt.% ...
Aqueous electrolyte presents in batteries have high ionic conductivity. They are economic, non-flammable and offer high energy density. However, restricted thermodynamic electrochemical stable potential window of water and its spilling suppresses the use of these batteries. To address these problems, development of a polymer-based electrolyte system is an upcoming area of research. Even, the uses of sodium salts in battery electrolytes are gaining importance over lithium salts due to their abundance and economic feasibility. The present study examines the self-aggregation of star block copolymer Tetronic® 1107 in aqueous medium in the presence of sodium salts viz. sodium chloride, sodium fluoride, sodium nitrate, sodium hexafluoro phosphate and disodium hydrogen phosphate with different anions by cloud point (CP), gelation, dynamic light scattering (DLS) and small angle neutron scattering (SANS). Conductivity and DC polarization measurements are also performed for polymeric systems in the absence and also in the presence of these salts. The gelation behaviour of the block copolymer to reduce leakage problems and proper ionic conductivity of it in the presence of sodium salts make the system a credible choice as a polymer gel electrolyte (PGE).
... 10,14 The salt-induced decrease in solubility correlates with an increase in the size of the micelles, which has been revealed by light scattering and small-angle neutron scattering. [7][8][9]14,19,20,22 Systematic studies of the effect of the molecular weight of the polymer at a constant ratio of (EO) and (PO) do not show a strong correlation of the MW of the polymer with its response to salt. 18,20 Ionic liquids (ILs), which are commonly defined as salts that are liquids below 100°C, can be made with a wide variety of cations and anions. ...
There are many studies on the self-assembly of triblock poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) copolymers in aqueous solution. These polymers display a rich phase diagram as a function of block length, concentration, temperature, and additives. Here, we present a small-angle neutron scattering study of the impact of two water-miscible ionic liquids, 1-butyl-3-methylimidazolium chloride ([C4C1mim][Cl]) and 1-butyl-3-methylpyrrolidinium chloride ([C4C1pyrr][Cl]), on the temperature-dependent self-assembly of (EO)6-(PO)34-(EO)6, also known as L62 Pluronic, in aqueous solution. Both ionic liquids depress the temperatures of the various structural transitions that take place, but ([C4C1pyrr][Cl]) has a stronger effect. The structures that the triblock copolymer self-assembles into do not dramatically change nor do they significantly change the series of structures that the system transitions through as a function of temperature relative to the various transition temperatures.
... There- fore, the copolymer with a lower cloud point was easier to separate from the water phase. When the concentration of the copolymer was higher than 10% (v/v, %), the phase separation was restricted by the dissolving capacity of water instead of the collision of micelles [29]. ...
Recyclable aqueous two‐phase systems with thermo‐responsive phase‐forming materials have been employed to separate macromolecules; however, these systems have achieved very limited separation efficiency for small molecules, such as antibiotics. In this study, aqueous two‐phase systems composed of the ethylene oxide/propylene oxide copolymer and water were developed to extract alkaline antibiotics from the fermentation broth. In the aqueous two‐phase systems with an ethylene oxide ratio of 20 and propylene oxide ratio of 80, the partition coefficients of tylosin and spiramycin reached 16.87 and 20.39, respectively, while the extraction recoveries were 70.67 and 86.70%, respectively. Coupled with mechanism analysis, we demonstrated the feasibility of extracting alkaline antibiotics using this aqueous two‐phase system, especially for 16‐membered macrolide antibiotics. The molecular dynamic simulation was employed to visualize the process of dual‐phase formation and the partition behavior of antibiotics in an aqueous two‐phase system. The dynamic simulation revealed the binding energy between the antibiotic and ethylene oxide/propylene oxide copolymers, which provides a simple indicator for screening suitable antibiotics in aqueous two‐phase systems. Our recyclable aqueous two‐phase systems provide a robust approach for the extraction of 16‐membered macrolide antibiotics with ease of operation and high recovery rates, which is appropriate for large‐scale extraction in the fermentation industry.
... [27] However, owing to a remarkable influence on the aggregation behavior exerted by salt additives in block copolymers, limited groups are actively dealing with such trials which are resulting in less understanding of micellization phenomena at the molecular level. However, salt-induced micellization, clouding, and rheology behavior of such block copolymers in aqueous solution have been comprehensively discovered experimentally by several groups [28][29][30][31][32] and theoretically investigated by many groups. [33][34][35][36] It is well documented that the addition of salt leads to change in water structure especially salts as KCl and KF called structure makers because they increase the selfhydration of water through hydrogen bonding and resulting decrease in polymer solubility, while structure breakers salts such as KI and KSCN lead to increase polymer solubility due to reduced hydration. ...
... The low intrinsic viscosity with increasing concentration of the salt additives in turn leads to high viscosity due to the larger effective size of the copolymer micelles in solution. Addition of salts cause dehydration of ethylene oxide units from dehydrated PEO shell from the side of PPO core leading to an increase in the core radius of the spherical micelles [30]. The U-Shape displayed in presence of AlCl 3 is attributed to the complexation of polyoxyethylene chains of the copolymer. ...
... [27] However, owing to a remarkable influence on the aggregation behavior exerted by salt additives in block copolymers, limited groups are actively dealing with such trials which are resulting in less understanding of micellization phenomena at the molecular level. However, salt-induced micellization, clouding, and rheology behavior of such block copolymers in aqueous solution have been comprehensively discovered experimentally by several groups [28][29][30][31][32] and theoretically investigated by many groups. [33][34][35][36] It is well documented that the addition of salt leads to change in water structure especially salts as KCl and KF called structure makers because they increase the selfhydration of water through hydrogen bonding and resulting decrease in polymer solubility, while structure breakers salts such as KI and KSCN lead to increase polymer solubility due to reduced hydration. ...
... [27] However, owing to a remarkable influence on the aggregation behavior exerted by salt additives in block copolymers, limited groups are actively dealing with such trials which are resulting in less understanding of micellization phenomena at the molecular level. However, salt-induced micellization, clouding, and rheology behavior of such block copolymers in aqueous solution have been comprehensively discovered experimentally by several groups [28][29][30][31][32] and theoretically investigated by many groups. [33][34][35][36] It is well documented that the addition of salt leads to change in water structure especially salts as KCl and KF called structure makers because they increase the selfhydration of water through hydrogen bonding and resulting decrease in polymer solubility, while structure breakers salts such as KI and KSCN lead to increase polymer solubility due to reduced hydration. ...
Considerable attention is clutched by aqueous metal ion rechargeable batteries owing to their high energy density, low cost, nonflammability, and much environmental benignity with a safer alternative. However, effective manipulation in the electrolyte, such as the addition of polymers in an aqueous electrolyte system, is ongoing to suppress the leakage and water splitting behavior to expand its utility as improved energy storage devices. Here, the structural self-assembly of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO53-PPO34-PEO53, Mol. Wt.= 6600 gm mol⁻¹) block copolymer, Pluronic® F77 in aqueous solutions were investigated, and the effect of ionic additives especially cation influence onto micellar size by small-angle neutron scattering (SANS), dynamic light scattering (DLS) and proton nuclear magnetic resonance (¹H NMR) were systematically studied. The micellar structural parameters obtained from SANS data analysis with core-shell model noticed a decrease in core radius, aggregation number, and hard-sphere radius of micelles proved overall demicellization phenomena when salts were added. The results were supported by DLS study where a similar trend in radial decrement and NMR peak shifting supported the SANS results. Moreover, elementary electrochemical investigations such as ionic conductivity and transport number were also carried out for the applications into aqueous rechargeable batteries and supercapacitors.
... In parallel, the area per headgroup increases significantly. It is noteworthy that here the strong steric constraint is not a consequence of covalent bonds holding the chains together, like in Pluronics™ [64], or of covalent cross-linking, like in Gemini-type surfactants [65]. ...
Hypothesis:
The surfactant C8EO8CH2COOH (Akypo LF2) and its salts have a small hydrophobic and a significantly longer hydrophilic part. As a consequence, there must be a significant steric constraint, once these surfactant molecules form micelles. In addition, the partially charged headgroups should bring some additional fine-tuning via electrostatic interactions to this “essentially non-ionic” surfactant.
Experiments:
Phase diagrams of binary mixtures of water and C8EO8CH2COOH are established over large concentration and temperature ranges, also at different pHs and in the presence of sodium and calcium ions. Surface tensions and osmotic pressures are measured to understand the systems. To evaluate the microstructures, also Dynamic Light Scattering and Small-Angle X-ray Scattering are performed.
Findings:
Apart from the formation of coacervates at very low surfactant concentrations, spherical micelles persist over the whole concentration and temperature range and do not change in size and shape. At very high surfactant concentrations, above 60 % by weight, where the headgroups are no longer fully hydrated, the standard core-shell structure of micelles vanishes and highly stabilized aggregates of 8 to 26 octyl chains are suspended in interdigitated polyoxyethylene layers and form an “osmotic brush”. When the acid is partially transformed to a sodium salt, the repulsion between the micelles increases, whereas bridging between micelles prevails, when the counterions are calcium cations. Remarkably, the negative charges of the headgroups are randomly distributed in the hydrophilic ethylene oxide shell. Altogether, a phase diagram without lyotropic liquid crystalline phases and an extreme shift of the cloud-point in temperature and composition is found, similar to the phase diagram of C8EO8OH already known in literature. The phase properties can be explained by the curvature and packing constraints together with the Lindemann rule applied to short hydrocarbon chains.
... However, with 25 min sonication in the same saponin condition, the bigger vesicle size was observed (Fig. 3(b)). The sonication actually extended the core size, resulting in a bigger hydrophobic spherical diameter [47,48]. ...
The biosurfactant was used to remove diesel from contaminated montmorillonite, which was enhanced by humic acid sodium salt (HASS) addition or microbubbles resulted from sonication. It was found that montmorillonite exhibited strong adsorption for saponin, causing the effective critical micelle concentration shift of saponin from 0.2 g L-1 to 0.6 g L-1. The washing efficacy (diesel removal) with 0.6 g L-1 saponin and 2% HASS (HASS/saponin ratio=2%) increased 14.9% compared to the control condition (without HASS), whereas the diesel removal with microbubble enhancement (25 min sonication) increased 15.2% compared to the control condition. Bright-field microscopy, microscope, DLS, PSS, and TEM were used to measure the size of micelles and count micelle numbers. In the addition of 2% HASS, the mean diameter of saponin micelles decreased from 824.9 ± 12.3 nm to 91.3 ± 7.6 nm, and the number of micelles increased from 42000 to 110000 at saponin solution of 0.3 g L-1. The hydrophobic core volume increased accordingly. HASS molecule might cause the extension of hydrophobic interior space in micelles to solubilize the hydrocarbon contaminant. In addition to micelles solubilization, the microbubbles enhancement washing process increased the diesel removal by floatation, cavitation in sonication.
... The low intrinsic viscosity with increasing concentration of the salt additives in turn leads to high viscosity due to the larger effective size of the copolymer micelles in solution. Addition of salts cause dehydration of ethylene oxide units from dehydrated PEO shell from the side of PPO core leading to an increase in the core radius of the spherical micelles [30]. The U-Shape displayed in presence of AlCl 3 is attributed to the complexation of polyoxyethylene chains of the copolymer. ...
Physico-chemical properties of aqueous solutions of non-ionic tri-block copolymer (PEO)19(PPO)43(PEO)19(Pluronic P84) and anionic sodium dedocylsulphate (SDS) surfactants have been studied. Additives such as salts, co-solvents, polymers have marked effect on micellization, clouding and solubilisation behaviour of surfactant solutions. Studying effects of various additives on the critical micelle concentration (CMC) and other micellar properties of individual surfactant components and their mixtures can be useful for optimizing their properties for various applications. The CMCs for SDS in pure water and in aqueous mixed solutions were determined with electrical conductivity whereas those for amphiphilic block copolymer were obtained using UV-Visible spectroscopy. The micellar behaviour of anionic SDS surfactant in aqueous media has been investigated over the temperature range 298.15-313.15 K. Thermodynamic parameters of micellization evaluated from CMC data are also reported. Other thermodynamic and solution properties of aqueous mixtures of block copolymer P84 were investigated with cloud point and viscometric techniques. Analysis of conductometric data indicates that inorganic salts lower CMC values of aqueous SDS. The CMC also shows a decrease in presence of poly(ethylene glycol) (PEG) and no dependence on change in concentrations of P84. The intrinsic viscosity values of P84 (1 wt. %) in presence of inorganic salts were significantly lowered. The values of free energy of activation of flow indicates the existance of spherical micelles being dependent on the nature and concentration of the additive. The copolymer also exhibited cloud point depression in presence of inorganic salts, whereas PEG did not significantly affect both the intrinsic viscosity and cloud point.
... as illustrated in Fig. 2, is fitted with the Debye function [34,35] to find out that the P84 molecules in the aqueous solution exist in the form of unimers with a radius of gyration of 2.0 nm. When the system temperature increases, more P84 unimers are assembled to form micelles, consistent with that of Liu et al. [36] and Jain et al. [37]. ...
The binary P123 + F108, + F98, + F88, + F68, + F87 and + P84 systems were used to systematically explore the effect of molecular weight and hydrophobicity of Pluronic on the tendency of cooperative binding between parent copolymers and solubility of drug (ibuprofen) in these mixed Pluronic systems. Temperature-dependent co-micellization process in these systems was carefully investigated by using high sensitivity differential scanning calorimeter (HSDSC), dynamic light scattering (DLS) and small angle X-ray scattering (SAXS). All the HSDSC thermograms for these systems consistently exhibit two endothermic (micellization) peaks apart by at least 13.3 °C. It was evidenced that micelles are mainly formed by P123, the copolymer with a lower critical micelle temperature (CMT), at low temperatures. Raising temperature would dehydrate the other Pluronic with a higher CMT to be integrated into the neat P123 micelles developed at low temperatures. When the temperature is further increased beyond the second endothermic peak, the mixed micelles with a two-shell structure and characteristic corona lengths of their parent copolymers are observed to prove the existence of cooperative binding between parent copolymers. All the binary mixed Pluronic systems used in this study exhibit cooperative binding to form unimodal distribution of mixed micelles, except the P123 + F68 system. The SAXS results show that P123 + F68 system at 65 °C exhibits bimodal distribution of aggregates with coexisting of neat F68 micelles (65% in number) and P123 + F68 mixed micelles (35% in number). It is interesting to find out that P123 and F68 with distinct polypropylene oxide (PPO) moieties (i.e., a difference of 37 PO units) would exhibit very weak cooperative binding to partially form mixed micelles. Addition of ibuprofen in the P123 + F68 system would substantially enhance the cooperative binding between P123 and F68 to form bimodal distribution of aggregates with coexisting of neat F68 micelles (drops down to 30% in number) and P123 + F68 mixed micelles (increases up to 70% in number). For the systems with ibuprofen incorporated, SAXS results demonstrate that the drug is mainly encapsulated in the core of neat micelles developed at low temperatures. The solubility of ibuprofen in the 0.5 wt% P123 + 0.368 wt% P84 system is as high as 2.62 mg/ml, which is 114 times more than that in pure water at 37 °C.
... Sartori et al. [113] showed significant binding between PEO (concentrations from 5 to 200 mM expressed in monomer units), and KCl (concentrations of 2.4 and 7.5 mM). Dehydration of ethylene oxide units due to salt effect has been reported [124]. Bordi et al. [125] showed a decrease in conductivity of PEO in a 0.1 M KCl solution for various PEO molecular weights. ...
This review provides a better understanding of the underlying molecular mechanism of the association between poly(ethylene oxide) (PEO) and cofactor, a flocculation system used in papermaking. This association is necessary to induce fibre-fines flocculation. The binding of PEO or PEG based molecules with electrolytes is discussed. Salt is essential to induce the association between PEO and cofactor. Molecular Orbital PM3 calculations, supported by experimental evidence, prove the original idea, that the driving force for the association between PEO and the cofactor was hydrogen bonding, is invalid. The theoretical method provides structures and energies not obtainable from experiment, and predicts properties and interactions that can be tested experimentally.
... Generally, salt induces hydrophobicity in PO blocks and dehydrates EO shell, making copolymer chains more hydrophobic which brings into micellization at lower concentration. Further, salt being a strong chaotrope (water structure maker) lowers down the CP[70,71]. It can also be explained that, with the addition of salts, the change in H-bonding within EO chain and water molecules takes place and salts draws back some water molecules from the core regions of the micelles and therefore phase separation occurs at a lower temperature[26]. ...
Here we report, the effect of biologically important additives such as glucose, glycine, sodium chloride, urea and ethanol on the clouding and micellar behaviour of two commercially available ethylene oxide-propylene oxide (EO-PO) block copolymers (one linear triblock, Pluronic® P104 and other branched, starblock Tetronic® T1304) in aqueous environment. The cloud point (CP), viscosity, and dynamic light scattering (DLS) were measured. The solubilization of poorly water soluble drug, quercetin (QN) was also checked for the copolymer micelles in the presence of these additives using UV–visible spectroscopy. These additives depending upon their structure either decrease or increase CP and hydrodynamic diameter (Dh). Salt, glucose and glycine favoured micellization while urea and ethanol showed opposite behaviour. Markedly enhanced solubilization of QN was observed by the copolymer micelles which further improved in the presence of salt, glucose and glycine but decreased in the presence of ethanol and urea. The conclusions derived from this work will be highly useful in optimizing the performance of the copolymers in different pharmaceutical applications and in drug delivery systems as drug carriers.
... The low intrinsic viscosity with increasing concentration of the salt additives in turn leads to high viscosity due to the larger effective size of the copolymer micelles in solution. Addition of salts cause dehydration of ethylene oxide units from dehydrated PEO shell from the side of PPO core leading to an increase in the core radius of the spherical micelles [30]. The U-Shape displayed in presence of AlCl 3 is attributed to the complexation of polyoxyethylene chains of the copolymer. ...
The thermal degradation of complex plant materials such as tobacco gives rise to a variety of
toxic organic substances known to affect the clinical health of tobacco consumers. In this
paper we explore the theoretical characteristics of common toxic molecular products;
toluene and ethyl benzene from the pyrolysis of Sportsman cigarettes between 373 and 1223
K under conditions that simulate actual cigarette smoking. Accordingly, this work was
conducted at 1 atmosphere in increments of 50 K. Gaussian 03 computational suite of
programs was used to perform thermochemical calculations of toluene and ethyl benzene.
Toluenyl and ethyl benzyl radical stabilization energies were also calculated. HyperChem
computational package with Quantitative Structural Activity Relationship (QSAR) technique
was used to calculate the relative toxicities of ethyl benzene and toluene, and their
corresponding radicals. The toxicity values were 1.75 and 1.77 for toluene and ethyl benzene
respectively while those for the corresponding radicals were 2.15 (toluenyl) and 2.17 (ethyl
benzyl) based on octanol-water partition coefficient. To complement computational
calculations, the experimental Gas Chromatography-Mass Spectrometry (GC-MS) data on
the evolution of toluene and ethyl benzene between 200 and 700 ˚C has been presented to
determine the smoking temperatures at which toluene and ethyl benzene are produced in high
yields. Accordingly, the concentration of toluene and ethyl benzene was found to peak
between 400 and 600 ˚C. This is consistent with thermochemical data which predicted that in
the same temperature range, the formation of ethyl benzyl and toluenyl radicals were
generally favoured. The clinical impacts of molecular toluene and molecular ethyl benzene,
and their respective free radicals have also been discussed.
... The low intrinsic viscosity with increasing concentration of the salt additives in turn leads to high viscosity due to the larger effective size of the copolymer micelles in solution. Addition of salts cause dehydration of ethylene oxide units from dehydrated PEO shell from the side of PPO core leading to an increase in the core radius of the spherical micelles [30]. The U-Shape displayed in presence of AlCl 3 is attributed to the complexation of polyoxyethylene chains of the copolymer. ...
... The studies on PEO -PPO, PEO -PBO di-and triblock copolymers were completed recently by Bahadur et al. [162] who examined the role of various additives on the micellization behavior, by Guo et al. [141] who used FT-Raman spectroscopy to study the hydration and conformation as a function of temperature, by Chaibundit et al. [163] who were mainly interested in PEO/PBO block copolymers with long PEO sequences. ...
Glioblastoma multiforme (GBM) is the most common and lethal primary intracranial tumor in humans. Monotherapeutic interventions have not been successful. The objective of the current studies was to establish the effective combination therapy consisting of pifitrin as a sensitizer, and curcumin as therapeutic incorporated into miktoarm micelles. A2B type miktoarm stars were prepared using a combination of click chemistry with ring opening polymerization on a core with orthogonal functionalities. These self-assemble into spherical micelles with hydrophobic core and hydrophilic corona structure. Micellar delivery systems for curcumin based on these miktoarm star polymers were prepared, characterized and tested on cultures sensitized with pifitrin. The results show that: (1) pifitrin and temozolamide in combination with curcumin cause significant cell death compared with the individual therapeutics (incorporated or not in micelles), and (2) repeated exposure to the same treatments is necessary to fully prevent a re-growth of glioblastoma cells both in 2D and 3D cultures. Although the incorporation of curcumin into A2B star polymer micelles did not increase the extent of cell death compared with curcumin alone, the advantage of micelles is that they significantly increase the aqueous solubility of curcumin and sustain its release; this will likely reduce the frequency of its administration required to be effective in vivo. A2B miktoarm polymers could be a new viable delivery system for curcumin and other anticancer drugs with similar limitations.
Ordered mesoporous silica materials have attracted considerable attention due to their unique structural, textural, and morphological properties, alongside their versatile applications in catalysis, adsorption, and drug delivery systems. This study explores the influence of ionic liquids (ILs) on the synthesis of SBA-15 and SBA-15/CeO2 nanocomposites. DMIBr (1-dodecyl-3-methylimidazolium bromide) and DMIBF4(1-dodecyl-3-methylimidazolium tetrafluoroborate) were the analyzed ILs. The objective was to evaluate how these ILs modulate the structural, textural, and morphological properties of the resulting nanocomposites. SAXS analysis confirmed the formation of well-ordered mesoporous structures with hexagonal arrangements, corroborating with physisorption data and TEM images. XRD measurements confirmed the presence of CeO2 nanoparticles within the nanocomposites, exhibiting a fluorite-type cubic crystal structure, supported by Raman spectroscopy, which identified characteristic peaks corresponding to Ce–O vibrational modes. XPS analysis provided detailed insights into the surface chemistry, revealing the presence of Ce³⁺ and Ce⁴⁺ ions, oxygen species, and their interaction with the silica matrix. The Ce³⁺ ions, associated with oxygen vacancies in the CeO2 lattice, were identified as key active sites for adsorption and catalysis. SEM images displayed distinct morphologies of the nanocomposites, attributed to the specific used IL. These results underscore the pivotal role of ILs in tailoring the properties of SBA-15/CeO2 nanocomposites, offering valuable knowledge for their potential applications in diverse fields.
Cancer is a major pharmaceutical challenge that necessitates improved care. Modern anti-cancer drugs require well-designed carriers to deliver and release them at the intended location. Hydrogel delivery systems are efficient and therapeutically beneficial in situ anti-cancer drug delivery platforms. Hydrogels exhibit excellent biocompatibility, biodegradation pathways, and minimal immunogenicity. Their stimuli-responsive characteristics, such as thermo-response and reactive oxygen species (ROS)-response, make hydrogels ideal for sustained local drug delivery and controlled drug release. Recently, synthetically manufactured thermal and ROS-responsive smart hydrogels enhance treatment efficacy by minimizing toxicities associated with non-carrier-mediated anti-cancer drug delivery. In this review, we discuss synthetically manufactured thermal (poly(N-isopropyl acrylamide) (PNIPAM), Pluronic, and polyethylene glycol (PEG)) and ROS-responsive copolymer hydrogels and their application in local anti-cancer drug delivery. We also addressed an extensive bibliometric analysis of the keyword “hydrogels for cancer treatment” to generate the country, source, documents, and author-based rankings maps.
The paper presents the preparation and rheological response of thermosensitive gels of poloxamer 407 (P407) in the presence of a versatile polysaccharide, pullulan (PULL). Prior to material formulations, the phase diagram was established for PULL and P407 mixtures in aqueous solutions at 37 °C. Due to the competition between the hydrophobic and hydrophilic interactions, an extended island of immiscibility was observed. Homogeneous solutions of polymer mixtures near the phase boundary conditions were used to prepare gels and their rheological behavior was carefully investigated. Steady shear and oscillatory rheological investigations indicated a temperature induced gelation, shear induced yielding and soft gel characteristics at physiological temperature. Hyperimmune egg white was added to the samples to provide the target gel with appropriate antimicrobial effects coming from the valuable proteins from egg white. The gel samples were characterized and their biocompatibility and antimicrobial properties were compared. PULL/P407 homogeneous mixtures combine the unique characteristics of the two polymers and they are suitable to design polymeric materials with potential bioapplications, such as dressings for wound healing or injectable hydrogels.
We examined the structure–property relationship of a triblock copolymer comprising azobenzene sequences, namely, poly(6-[4-(4′-ethoxyphenylazo)phenoxy]hexyl methacrylate)-block-poly(ethylene glycol)-block-poly(6-[4-(4′-ethoxyphenylazo)phenoxy]hexyl methacrylate) (PMMAzo-b-PEG-b-PMMAzo), in mixed dimethylformamide (DMF)/H2O solvents, with the effects of solvency and salts being the main focus. When H2O (a PEG-selective solvent) was added to DMF (a neutral solvent), the poor compatibility between H2O and PMMAzo molecules led to the formation of micelles comprising PMMAzo and PEG in the core and corona, respectively. Increases in the H2O content in the mixed solvent decreased the solvency, which caused increases in the micelle association number and hydrophobic core radius. The azobenzene moieties confined in the micellar core inhibited their photoisomerization, which caused decreases in the photoisomerization kinetics and cis isomer content. The addition of KCl to the PMMAzo-b-PEG-b-PMMAzo solution increased the hydrophobicity of PEG chains and the core radius and decreased the corona thickness. Moreover, the KSCN addition to the aforementioned solution increased the hydrophilicity of PMMAzo segments, which caused the core radius and corona thickness of micelles to decrease and increase, respectively. Salt addition disturbed the compact packing of mesogens and reduced the mesogen association. Consequently, with an increase in the salt concentration, the content of H-type mesogen aggregates increased, whereas that of nonassociated mesogens decreased.
We investigated the effects of solvency and salt addition on the morphology, photoresponsivity, and fluorescence of the block copolymer poly(ethylene glycol)methyl ether-block-poly(6-[4-(4′-methoxyphenylazo)phenoxy]hexyl methacrylate) (PEG-b-PAzo). The morphology of PEG-b-PAzo was manipulated through dissolution in mixed solvents of dimethylformamide (DMF) and H2O. DMF served as a good solvent for both blocks, whereas H2O was a PEG-selective solvent. Increasing the concentration of H2O in the mixed solvent increased the repulsive interaction between H2O and PAzo chains, resulting in the assembly of PEG-b-PAzo into micelles composed of PEG in the corona and PAzo in the core. The confined geometry forced the mesogens to assemble into H-type aggregates. Adding KCl to the PEG-b-PAzo solution increased PEG segment hydrophobicity, causing an increase in the core radius and a reduction in the corona thickness. The opposite effect was observed when KSCN was added to the PEG-b-PAzo solution. The addition of salts led to slight reductions in H-type aggregate percentage and increases in nonassociated mesogen percentage. The fluorescence behavior of PEG-b-PAzo was closely correlated with a mesogen dispersion state. In the saltless PEG-b-PAzo solution, the increase in the percentage of the H-type aggregates, which was due to the addition of H2O, caused fluorescence loss. The small changes in mesogen aggregation upon the addition of salts led to only a small change in fluorescence emission.
Pluronics® and Tetronics® are used in industry for solubilizing various kinds of hydrophobic compounds since decades. In present study, moderately hydrophobic linear and star block copolymer viz. Pluronic® P104 and Tetronic® T1304 are employed for the solubilization of four polycyclic aromatic hydrocarbons (PAHs) viz. naphthalene (Np), anthracene (An), phenanthrene (Ph) and pyrene (Py). The aggregation behaviour of these block copolymers in an aqueous media has been investigated by cloud point (CP), viscometry, dynamic light scattering (DLS), and small angle neutron scattering (SANS) techniques. The aqueous solubilities of these PAHs are quite poor, hence their extractions from contaminated areas are difficult. Cloud point of block copolymers linearly decreases as a function of PAHs concentration and strongly depends on the structure of PAHs. The aqueous solutions of both block copolymers are Newtonian fluid. The solubilization of PAHs in these copolymeric micelles leads to increase in the viscosity of solution suggesting the growth of micelles. The effect of concentration and temperature on the size and shapes of micelles were evaluated from DLS and SANS studies. The study provides an alternative method for the remediation of PAHs using inexpensive commercially available block copolymers.
Ionic liquids (ILs) are seen as liquids with great and novel potential in various applications such as separation materials, reaction medium, and energy devices. They have garnered great interest in their potential as a liquid-based molecular self-assembly medium that can produce innovative materials with distinctive criteria that have never been observed in traditional soft materials such as water and solvents. Here, recent advances and practical application of block copolymer (BCP) self-assembly in ILs are discussed focusing on the formation of the microstructure depending on dilute or concentrated BCPs. On the other hand, ion gels prepared from self-assembled BCPs attracted significant interest due to the outstanding tenability features (physical, viscoelasticity, and solution processability) without hindering ILs properties (nonvolatility, nonflammability, and high ionic conductivity). Lastly, the applicability of such technology is already in progress with a great interest seen in electrochemistry, batteries, actuators, electrolyte-gated transistors, and nano-delivery vehicles.
The present study is based on the interaction between three polyethylene oxide-polypropylene oxide-polyethylene oxide (PEO-PPO-PEO) triblock copolymers namely Pluronics® L81, P84 and F88 (with same PPO Mol. Wt. but 10,40 and 80% PEO, respectively) and the ionic surfactants (with 12-C chain but different polar head group charge viz. sodium dodecyl sulfate (SDS, anionic), dodecyl trimethylammonium bromide (DTAB, cationic) and N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (C12PS, zwitterionic) using plethora of techniques. The most hydrophobic Pluronic® L81 (cloud point, CP = 16.5 °C (5%w/w)) remains as molecularly dissolved below the CP and forms unstable vesicles close to CP, moderately hydrophobic, P84 (CP = 73.6 °C), remains molecularly dissolved below 15 °C and forms core-shell micelle above critical micelle temperature (CMT) and the highly hydrophilic F88 forms micelles at elevated temperatures. Micellization and dye solubilizing ability improved in the presence of salt (sodium chloride, NaCl). The presence of ionic surfactants leads to small sized surfactant rich micelles with progressive addition as confirmed by dynamic light scattering (DLS) and small angle neutron scattering (SANS). Amongst the three copolymers, P84 (Nagg ≈ 72) showed maximum solubility of dye and lowering power of CMC (CAC < 2 mM), hence SANS studies were limited to P84 and it was observed that SDS (Nagg ≈ 45) showed stronger interactions with P84 as compared to DTAB (Nagg ≈ 53) and C12PS (Nagg ≈ 83).
The biosurfactant was used to remove diesel from contaminated montmorillonite, which was enhanced by humic acid sodium salt (HASS) addition or microbubbles resulted from sonication. It was found that montmorillonite exhibited strong adsorption for saponin, causing the effective critical micelle concentration shift of saponin from 0.2 g L⁻¹ to 0.6 g L⁻¹. The washing efficacy (diesel removal) with 0.6 g L⁻¹ saponin and 2% HASS (HASS/saponin ratio = 2%) increased 14.9% compared to the control condition (without HASS), whereas the diesel removal with microbubble enhancement (25 min sonication) increased 15.2% compared to the control condition. Bright-field microscopy, microscope, DLS, PSS, and TEM were used to measure the size of micelles and count micelle numbers. In the addition of 2% HASS, the mean diameter of saponin micelles decreased from 824.9 ± 12.3 nm to 91.3 ± 7.6 nm, and the number of micelles increased from 42000 to 110000 at saponin solution of 0.3 g L⁻¹. The hydrophobic core volume increased accordingly. HASS molecule might cause the extension of hydrophobic interior space in micelles to solubilize the hydrocarbon contaminant. In addition to micelles solubilization, the microbubbles enhancement washing process increased the diesel removal by floatation, cavitation in sonication.
Maintaining the integrity of cell membranes is indispensable for cellular viability. Poloxamer 188 (P188), a poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymer with a number-average molecular weight of 8700 g/mol and containing 80% by mass PEO, protects cell membranes from various external injuries and has the potential to be used as a therapeutic agent in diverse applications. The membrane protection mechanism associated with P188 is intimately connected with how this block copolymer interacts with the lipid bilayer, the main component of a cell membrane. Here, we report the distribution of P188 in a model lipid bilayer comprising 1-palmitoyl-2-oleoyl-glycero-3-phosphocholine (POPC) using neutron reflectivity (NR) and atomic force microscopy (AFM). We also investigated the association of a PEO homopolymer (PEO8.4K; Mn = 8400 g/mol) that does not protect living cell membranes. These experiments were conducted following incubation of a 4.5 mmol/L polymer solution in a buffer that mimics physiological conditions with supported POPC bilayer membranes followed by washing with the aqueous medium. In contrast to previous reports, which dealt with P188 and PEO in salt-free solutions, both P188 and PEO8.4K penetrate into the inner portion of the lipid bilayer as revealed by NR, with approximately 30% by volume occupancy across the membrane without loss of bilayer structural integrity. These results indicate that PEO is the chemical moiety that principally drives P188 binding to bilayer membranes. No defects or phase-separated domains were observed in either P188- or PEO8.4K-incubated lipid bilayers when examined by AFM, indicating that polymer chains mingle homogeneously with lipid molecules in the bilayer. Remarkably, the breakthrough force required for penetration of the AFM tip through the bilayer membrane is unaffected by the presence of the large amount of P188 and PEO8.4K.
As a major source of renewable resources, the valorization of lignocellulosic biomass is one pillar of the future economy. Lignin consists of aryl ether units connected by ether and alkyl bonds, whose cleavage yields monomeric phenols and methoxy phenols. Thus, the pyrolysis of lignin produces oxygenated bio-oils (20-45%) with poor fuel efficiency, such as phenols, aromatics, aldehydes, ketones, esters, acids and alcohols. Hence, those bio-oils have to be upgraded, mainly through hydrodeoxygenation (HDO), which is a thermal catalytic process under H2 pressure leading to the production of oxygen-free molecules and water as by-product. Our objective is to elaborate new catalytic formulations for the HDO process by combining Density Functional Theory (DFT) calculations with experimental studies (synthesis, characterization, and catalytic tests). HDO of phenolic compounds includes two deoxygenation routes: (i) the hydrogenation of aromatic ring before C-O bond cleavage (HYD) or (ii) the direct C-O bond cleavage - Direct DeOxygenation (DDO). The present work aims to promote the second route in order to increase the production of aromatics under lower H2 pressure. As described in the literature, Fe@silica catalysts show a good activity and selectivity toward aromatics, with being rather resistant towards deactivation. Thus, our work focused on designing silica-supported metallic Fe (or bimetallic Fe-Cu) catalysts with specific properties allowing higher performances.
The adsorption energies of phenol (as a model molecule of bio-oils) and inhibitors (CO and water) over several amorphous and crystalline silica surfaces, with various silanol densities and types, were computed by DFT. Three interaction modes were investigated for phenol: the “perpendicular O-interaction”, the “flat π-interaction”, and the “flat O-interaction”. Results show that crystalline surfaces promote the HYD route since the “flat π-interaction” dominates. For amorphous silica, the highest adsorption energies were found for the “flat O-interaction” mode, and a specific interaction of 120 kJ/mol (including a C-Si bond and phenol deformation) was observed for surfaces with a silanol density between 2 and 3.3 OH/nm2. The CO competitive adsorption is negligible for all silica surfaces, which make them more attractive than conventional sulfide catalysts. Water shows an inhibiting effect for all surfaces whose silanol density is higher than 5 OH/nm2. Hence, these results motivated the synthesis of silica-supported catalysts with a silanol density between 2 and 4 OH/nm2.
Single iron atom catalysts (SACs) supported on mesoporous silica were elaborated following the Sol-gel mechanism using non-ionic (P123) and metallic (CTAF) surfactants as porogens. Iron distribution within mixed P123/CTAF micelles, which is used as template for the silica framework polymerization, allows the fine dispersion of those atoms within silica mesopores. Tuning the P123/CTAF molar ratio enables to control the iron load, as well as the silica structural properties.
Résumé - Abstract
Therefore, different micellar solutions were prepared and analyzed (critical micellar concentration CMC, dynamic light scattering DLS, small angle neutron scattering SANS, magnetic studies) in order to understand their structuration depending on this ratio. The lack of iron clusters and nanoparticles was confirmed by synchrotrons - pair distribution function PDF, scanning transmission electron microscopy STEM mapping, magnetic and nuclear magnetic resonance NMR measurements. Both experimental results and DFT calculations confirmed that those atoms are predominately present as Fe(III) with high spin. Obtaining single metallic iron atoms increases the number of active sites, which improves the catalytic performance. However, the results of catalytic tests were unsatisfactory because the reduction of Fe(III) appeared to be difficult.
Silica-supported metallic (Fe & Cu) and bimetallic (Fe-Cu) catalysts were synthesized by simple impregnation, and Fe catalysts were elaborated following the co-precipitation method with urea decomposition under heating, enabling obtaining Fe(II) species. Catalysts were tested for guaiacol (as a model molecule of bio-oils) HDO conversion and results proved that bimetallic Fe-Cu have a better performance (90% conversion, 70% phenol selectivity) than iron-based catalysts since incorporation of Cu facilitates the reduction of Fe(III) species into Fe(0). Similarly, catalysts prepared by co-precipitation with urea decomposition proved a better performance than those prepared by impregnation since the reduction of iron species was easier.
Mesoporous aluminosilicates (MAs) with high hydrothermal stability by assembly Y precursors have been a promising materials for cracking heavy oil. A means to reduce the template consumption and water discharge is of vital importance in synthesis of MAs. Mother liquor recycling (MLR) is an eco-friendly and effective method to synthesize (MAs) with largely reduced template consumption and water discharge. However, inorganic salts accumulation especially for SO4²⁻ in the mother liquor with continuous recycling has a negative influence on the micellization of the surfactants. The P123 molecules tend to aggregate into much larger particles and the assembly mother solution becomes cloudy. Less ordered MAs with poor hydrothermal stability would be obtained. In this work, well-ordered MAs with high hydrothermal stability were synthesized via MLR method by introducing urea as additives. Urea acting as salting-in additives behaves opposite to the inorganic salts, which could clear the assembly mother solution by breaking down the P123 aggregates into micelles. After hydrothermal treatment in 100% water vapor at 800 °C for 16 h, the obtained sample maintains 37.7% of specific surface area and 62.1% of total pore volume. In contrast, the sample prepared without adding urea just maintains 15.0% of specific surface area and 20.0% of total pore volume. This strategy provides a high efficiency and low cost route to the synthesis of MAs, which sheds a light on the practical application of MAs.
Thermoresponsive polymers (TRPs) in different solvent media have been explored over a long time and are important from both scientific and technical points of view. In spite of numerous studies on TRPs behavior in various additives, the additives interactions with TRPs are still poorly understood. Despite the vast available literature regarding the biomolecular interactions between various TRPs and naturally occurring additives, it is not possible to provide a unifying declaration about the behavior of different additives, particularly on the phase transition temperature of the polymer. However, the absolute potential review is also absent to describe the behavior of additives as stimuli on the phase transition of TRPs. A lack of sufficient knowledge regarding the response of TRPs to stimuli of additives hinders the expansion of wide spectrum of applications of these polymers. Therefore, it is proposed to review the response of TRPs in the presence of various additives in aqueous media. The in-depth knowledge through literature survey has drawn our attention towards the filling of this gap by analyzing the interaction of the TRP with different additives. In this perspective, we have systematically examined the stability behaviour, aggregation behaviour, phase transition behaviour of various polymers in the presence of different additives. The perspective of the influence of additives as stimuli on TRPs behavior in the aqueous medium will provide new reliable information about intramolecular interactions between interior polymer segments as well as inter-molecular interactions between TRPs and molecules of additives, which will be helpful for industrialists in preparation of new polymer materials for drug delivery system.
The rheology of the aqueous solution of poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO−PPO−PEO) triblock copolymer, Pluronic F68 in the presence of KF was studied in the temperature range from 15 to 60°C. The variation of the shear stress according to the shear rate shows that independently from the temperature and concentration, the F68 solutions exhibit a Newtonian behavior. The results show that the Critical Micelle Temperature of Pluronic F68 in a KF aqueous solution decreases with the increase in the salt concentration.
For the last decades, amphiphilic block copolymers have been at the focus of extensive scientific interest, due to their unique properties and numerous potential applications. Their technological potential evolves from their ability to self-assemble into a plethora of morphologically diverse nanostructures such as micelles, polymersomes, cylinders and others. In this chapter, we discuss the basic principles governing amphiphilic block copolymer self-organisation in solutions and the variety of morphologies observed so far, as well as the experimental techniques available for characterising such nanostructures. Some representative examples of self-assembly from nonionic and ionic amphiphilic block copolymers of different macromolecular architectures are also presented.
Ionic liquids (ILs), in particular imidazolium-based ILs, have proved to be effective media for the synthesis and stabilization of Pt and Ni nanoparticles (NPs). Synthesis of Ni and Pt NPs includes reduction or decomposition of different metal precursors and reduction agents, using simple transfer of previously prepared water- or classical organic solvent-soluble colloids to the ILs. The techniques used for characterization include X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and small-angle X-ray scattering (SAXS). The obtained Ni and Pt NPs exhibit small sizes and narrow size distribution. The XPS analyses showed the interactions of the ILs with the metal surface displaying the formation of an IL protective layer, which is probably composed of imidazolium aggregates surrounding the surface of the nanoparticles. The SAXS analysis indicated the formation of a semi-organized IL layer surrounding the Ni and Pt NPs. The Ni and Pt NPs were tested as nanocatalysts in biphasic or heterogeneous conditions in hydrogenation reactions of alkenes and arene. The results exhibited high catalytic activity and selectivity of Ni and Pt NPs.
The adsorption of amphiphilic molecules of varying size on solid-liquid interfaces modulates the properties of colloidal systems. Nonionic, poly(ethylene oxide) (PEO)-based amphiphilic molecules are particularly useful because of their graded hydrophobic-hydrophilic nature, which allows for adsorption on a wide array of solid surfaces. Their adsorption also results in other useful properties, such as responsiveness to external stimuli and solubilization of hydrophobic compounds. This review focuses on the adsorption properties of PEO-based amphiphiles, beginning with a discussion of fundamental concepts pertaining to the adsorption of macromolecules on solid-liquid interfaces, and more specifically the adsorption of PEO homopolymers. The main portion of the review highlights studies on factors affecting the adsorption and surface self-assembly of PEO-PPO-PEO block copolymers, where PPO is poly(propylene oxide). Block copolymers of this type are commercially available and of interest in several fields, due to their low toxicity and compatibility in aqueous systems. Examples of applications relevant to the interfacial behavior of PEO-PPO-PEO block copolymers are paints and coatings, detergents, filtration, and drug delivery. The methods discussed herein for manipulating the adsorption properties of PEO-PPO-PEO are emphasized for their ability to shed light on molecular interactions at interfaces. Knowledge of these interactions guides the formulation of novel materials with useful mesoscale organization and micro- and macrophase properties.
Poly(ethylene oxide)- poly(propylene oxide) (PEO-PPO) block polyethers are typical nonionic polymeric surfactants, which allow for wide structural design, exhibit temperature-dependent micellization of the copolymers, and function in a variety of solvent systems. It greatly enriched the investigation of their aggregation behaviors in various solutions. In this paper, an overview based on our research work was provided about the basic properties of linear and branched PEO-PPO block polyethers in aqueous solutions. Furthermore, the effects of additives including acid/base, inorganic salts, alcohols, surfactants and polymers on the aggregation behaviors of PEO-PPO polyethers are examined. PEO-PPO block polyethers have good biocompatibility. They can form micelles in aqueous solutions, with a hydrophobic core and a hydrophilic corona around the micelle interior. This micelle structure provides local hydrophobic microenvironments for hydrophobic drugs. Thus, the application of PEO-PPO polyethers in the field of drug delivery is presented; they can be the theoretical dosage support structure in future drug discovery research.
This chapter summarizes progresses achieved to date in the formation of micellar structures prepared from anionically synthesized block copolymers. Generalities about the preparation, the characterization, and the dynamics of such block copolymer micelles are first outlined. Selected examples of micelle formation in aqueous and organic media are shown for anionically synthesized block copolymers with various architectures. The different types of micellar morphologies and the strategies devised to control them are discussed.
The effect of hydrophilic block length on the surface and micellar thermodynamic properties of aqueous solution of E20B8: E80B8 and E120B8 diblock-copolymers, were studied by surface tension measurements over a wide concentration and temperature range; where E stands for an oxyethylenc unit and B for an oxybutylene unit. Like conventional surfactants, two breaks (change in the slope) were observed in the surface tension vs logarithm of concentration curve for all the three copolymers. Surface tension measurements were used to estimate surface excess concentrations (Γm), area per molecule at air/water interface αtx and thermodynamic parameters for adsorption of the pre-micellar region in the temperature range 20 to 50 °C. Likewise the critical micelle concentration, CMC and thermodynamic parameters for micellization were also calculated for the post-micellar solutions at all temperatures. For comparison the thermodynamic parameters of adsorption and miccllization are discussed in detail. The impact of varying L-block length and temperature on all calculated parameters are also discussed. This study shows the importance of hydrophobic-hydrophilic-balance (HHB) of copolymers on various surface and micellar properties.
A viscoelastic system was developed by forming hybrid wormlike micelles with poly(styrene-co-methacrylic acid) (P(St-co-MAA)) and cetyltrimethylammonium bromide (CTAB) in aqueous solution. The molar monomer ratio of St to MAA in the copolymer and the mixing ratio (r) of P(St-co-MAA) to CTAB were the two key factors to obtain the viscoelastic system. Phase behaviors of P(St-co-MAA)/CTAB aqueous solutions as the functions of the monomer ratio in the copolymer, as well as the mixing ratio were investigated. It was found that there was no phase separation in any mixing ratios as monomer ratio of 6:4 and 7:3. On the other hand, the biggest viscosity was observed as the molar ratio of phenyl group in copolymer to CTAB was close to 1, rather than at charge stoichiometry between MAA and CTA+. It is found that as r between 1.0 and 1.24 and the monomer ratio of St to MAA at 7:3, the mixing system had the largest viscosity. The effects of the total concentration of the mixture system, the salt concentration, the temperature, and the shear rate on the viscosity were studied in detail. The experimental results showed that the complex system had excellent thickening property even the copolymer concentration as low as 0.90% (w/v), good electrolyte tolerance as inorganic salt concentration lower than 1.8 M, and acceptable heat-resistance at room temperature. The complex system had a potential application as the thickeners in various fields.
A new approach to improve the cloud point extraction of congo red (a hazardous anionic azo dye) from aqueous solution using Triton X-100 surfactant is reported. The interaction of congo red with Triton X-100 during micellar solubilization has also been investigated by visible absorption spectral studies. The optimum conditions for surfactant concentration, additives (salts, glycols and their ethers) concentration, temperature and pH have been determined to achieve higher extraction. Among the various glycol ethers studied, ethylene glycol monohexyl ether shows the most efficient extraction of the dye even at ambient temperature, with almost complete removal of the dye from aqueous solution.
Smart materials, which can change properties when an external stimulus is applied, can be used for the targeted drug delivery of an active molecule to a specific site in the correct dosage. Different materials such as liposomes, polymeric systems, nanomaterials and hydrogels can respond to different stimuli such as pH, temperature and light and these are all attractive for controlled release applications.
With so many papers available on smart and stimuli-responsive materials for drug delivery applications it's hard to know where to start reading about this exciting topic. This two volume set brings together the recent findings in the area and provides a critical analysis of the different materials available and how they can be applied to advanced drug delivery systems.
With contributions from leading experts in the field, including a foreword from distinguished scientist Nicholas Peppas, The University of Texas at Austin, USA, the book will provide both an introduction to the key areas for graduate students and new researchers in the stimuli-responsive field as well as serving as a reference for those already working on fundamental materials research or drug delivery applications.
Colloidal stabilization of nanoparticle dispersions is central to applications including coatings, mineral extraction, and dispersion of oil spills in oceanic environments, which often involves oil-mineral-aggregates (OMAs). We have an ongoing interest in the modulation of amphiphile micellization and adsorption behavior in aqueous colloidal dispersions in the presence of various additives. Here we evaluate the effect of added salts CaCl2, MgCl2, and NaCl on the micellization and adsorption behavior of the poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer Pluronic P105 (EO37PO56EO37). In 0.10 wt% silica nanoparticle (10.6 nm average diameter) dispersion, adsorbed block copolymer layer formation begins at a critical surface micelle concentration (csmc) of 0.02 wt%, well below the critical micellization concentration (cmc) of Pluronic P105 in water. Dye solubilization experiments demonstrate an increase in the csmc upon addition of each salt. Each added salt reaches a level of maximum effectiveness in its ability to disfavor Pluronic P105 adsorption at the silica surface. These peak levels occur at concentrations of 0.005, 0.03, and 0.05 M for CaCl2, MgCl2, and NaCl, respectively, in the presence of 0.10 wt% silica nanoparticles. We explain these results in the context of an electrostatic displacer mechanism and discuss possible connections to OMA-dispersant formation.
The aggregation and phase behavior in water of several triblock copolymers of poly(oxyethylene)-poly(oxypropylene)-poly(oxyethylene)[(EO)x(PO)y(EO)x] has been studied. The y-values of the compounds ranged from 16 to 70 and the x:y ratios from 0.1 to 2.5. All studied compounds form micelles and lyotropic liquid crystalline phases. For a constant temperature the critical micelle concentrations (cmc) of the compounds decrease exponentially with y. The energy increment for the transfer of a PO group from the aqueous to the micellar state is about (0.25 +/- 0.05) kT. The cmc values for all compounds decrease strongly with increasing temperature. As a consequence the solutions undergo a monomer-micelle transition for constant concentration and increasing temperature. This micellization process is associated with a large endothermic heat which is linearly dependent on the size of the PO block. It is concluded that this heat is due to the dehydration of the PO groups, and it is called the heat of micellization DELTAH(m). For most of the studied compounds DELTAH(m) = 3.0 +/- 0.5 kJ/mol for one PO group. The large change of the cmc values with temperature can quantitatively be explained by the large DELTAH(m) values. The sequence of the lyotropic mesophases is mainly determined by the x:y ratio. Systems with x:y > almost-equal-to 0.5 form spherical micelles for c > cmc. The size of the micelles is independent of the concentration and temperature, if the temperature is about 20-degrees-C above the micellization temperature T(m); in a transition region around Tm the micellar size increases strongly with temperature. Below T(m) or the cmc only monomeric block copolymer molecules are present in the solution. At higher concentrations and temperatures solutions with spherical micelles form in a first-order transition a transparent, optically isotropic, highly viscous, and elastic cubic phase. The formation of this cubic phase can be understood by hard-sphere interaction between the aggregates. With further increasing concentrations transitions to hexagonal and to lamellar phases are observed. Samples with a smaller hydrophilic EO block, i.e., with x:y almost-equal-to 0.25, usually form a hexagonal phase as the first liquid crystalline mesophase, while for systems with ratios xy almost-equal-to 0.15 a lamellar phase is found as the first mesophase; samples with x:y much less than 0.1 are no longer soluble in water. The lyotropic mesophases show also a thermotropic behavior; i.e., reversible transitions cubic --> hexagonal --> lamellar or from isotropic solutions to mesophases occur at constant block copolymer concentration with increasing temperatures. The mesophases usually melt at temperatures below 100-degrees-C to systems consisting of one or more isotropic liquid phase.
This chapter discusses that colloidal solutions are ideal systems to be studied by the small-angle neutron scattering (SANS) technique. This is especially true for the association colloids, such as micelles and microemulsions. The size range of these micellar systems ranges from 30 Å to about 100 Å that, in terms of the scattering experiment, translates into an intensity distribution in the Q range of 0.01 to 0.2 Å-1. This is the most easily accessible range of the SANS spectrometer for which the available neutron flux is also the highest. The resulting simple particle structure factor P(Q), in turn, allows an accurate determination of the interparticle structure factor S(Q) from the measurement. The internal structure of the micellar core, its water content, and its hydrocarbon conformations can also be addressed once the SANS data are properly analyzed. In microemulsions where one has an inverted micellar structure, the effective attractive interaction leads to the cloud point effect or the critical point phenomenon. SANS is a unique tool for simultaneously measuring the structure of the aggregates and their mutual correlations. An important aspect in recent developments of the application of the SANS technique in colloids is the ability to take into account the interaction realistically. An obvious direction to pursue in the future is to explore more of the phase transitions and the structure of the dense phases of these surfactant solutions.
The surface tension of aqueous solutions of a range of ABA polyoxyethylene-polyoxypropylene nonionic copolymers has been measured over a wide range of concentrations. The surface tension versus concentration plots show an inflection typical of that shown by conventional surfactants, but this occurs at very low concentrations (0.5–5 × 10−6M liter−1) and it is thought to be the result of monomolecular micelle formation. At higher concentrations (0.5 × 10−4 to 1.5 × 10−2M liter−1) polymolecular aggregation probably takes place. The extent of uptake of benzopurpurine and iodine in aqueous solutions increases at high concentrations but the inflection points do not agree with any observable in the surface tension plots. Areas per molecule calculated using the simple form of the Gibbs' equation indicate that there is considerable folding of the polymers at the air-water interface. For a given hydrophobic polyoxypropylene block, increasing the size of the hydrophilic polyoxyethylene chains causes an increase in interfacial molecular area. Increasing the length of the central hydrophobic moiety allows the molecule to fold more extensively and results in a decrease in the occupied area per molecule. Raising the temperature causes an increase in solution viscosity, especially near the lower consolute temperature (cloud point) of the system, a further indication of polymolecular association in these systems.
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The structural characteristics of aqueous solutions of the Pluronic triblock copolymers of poly(ethylene oxide) - poly(propylene oxide) - poly(ethylene oxide), PEO - PPO - PEO, and their self-associated assemblies are reviewed. It is shown by small-angle neutron scattering that at low temperatures and/or concentration the individual copolymers exist in solution as individual unimers. Thermodynamically stable micelles are formed with increasing copolymer concentration and/or temperature. The unimer-to-micelle transition is not sharp, however. Micelles of well defined spherical shape and size coexist with unimers over a relatively wide temperature/concentration range. The micellar volume fraction increases accordingly with increasing temperature, increasing copolymer concentration and decreasing hydrostatic pressure. The copolymer suspension undergoes as a result a transition from a Newtonian liquid to a soft solid material when the micellar volume fraction crosses the critical value for hard-sphere crystallization. Crystallographic investigations on shear-aligned monodomain samples prove that the micelles in the solid phase are organized on a body-centred cubic lattice. As a result of an increasing micellar size upon increasing the temperature, the micelles themselves undergo a sphere-to-rod transition at elevated temperatures. In a shear field these rod-like micelles form a macroscopic nematic phase for low copolymer concentration, and a hexagonal solid phase for higher concentrations. For even higher concentrations, lamellar phases are observed: one lamellar type which is still governed by the hydrophobic interactions, and one type which appears as a result of crystallization of the PEO blocks.
The mode of association of a purified sample of block-copoly(oxyethylene/oxypropylene/oxyethylene) L64 in aqueous solution has been examined as a function of temperature and concentration by surface tension and light-scattering techniques. The association at 34.5 and 40 °C is described by a cooperative association model which assumes aggregate growth by stepwise addition of unimers. Inflections in the curves of surface tension vs. log concentration are shown to be consistent with the concentrations at which enhanced light scattering is first detectable. Only limited association has been detected at 27 °C.
The cloud point of high-molecular-weight poly(ethylene oxide), PEO, in aqueous salt solution has been determined as a function of the salt concentration for all potassium halides, alkali-metal chlorides and alkali-metal hydroxides. Our theoretical model for the pure PEO+water system (J. Chem. Soc., Faraday Trans. 1, 1981, 77, 2053) has been extended to include the effects of salt on the phase separation. Basic features of the present model are a hydration shell with enhanced structuring of water as well as a zone with decreased salt concentration surround each chain. Overlaps of such regions are involved in polymer–polymer contacts and imply transfer of water molecules and ions from the proximity of the chains to the bulk solution, which gives important contributions to the free energy of interaction. The existence of the salt-deficient zone is explained as a consequence of asymmetric hydration of the ions near the polymer. The effects of the zone are large enough to account for the influence of salts on the clouding. The experimental differences found for the alkali-metal halides have been rationalized mainly in terms of varying degrees of salt penetration into the region around the chain.
The phase behaviour of aqueous solutions of poly(ethylene oxide), PEO, is analysed by means of a structural model of the system and a simple statistical-mechanical model based thereupon. The intention is to elucidate the structural questions involved in the water–PEO coupling and to gain some insight in possible consequences of this coupling. The experimental partial molar enthalpy and entropy of water can be reproduced, at least in fairly dilute solutions, if a zone with increased structuring of the water is assumed to exist around the PEO chain. The phase separation that takes place at high temperatures is traced back to the increase in total extension of the zones of enhanced water structure that occurs when the water content is increased. The chain-length dependence of the location of the solubility gap is mainly determined by the combinatorial entropy of the chains. The water solubility of PEO, which is unique in this respect among the polyethers, can be explained in terms of a good strutural fit between the water and the polymer.
The structure of a dispersion of charged colloidal particles interacting through a screened Coulomb potential is well described at moderate to high densities by calculating the correlations in the mean spherical approximation (MSA). In this paper we extend the validity of the MSA calculation to arbitrarily low densities, using a physical rescaling argument which preserves the analytic form of the MSA solution. Our results are in excellent agreement with other numerical calculations and with experimental low-density light scattering data. The method may be viewed as a generalization of Gillan's prescription for one component plasmas to systems interacting through a Yukawa potential. An advantage of the present prescription is that it allows a smooth transition from strong to weak coupling, and it implies no functional relation between the experimentally independent parameters.
We assume the time-averaged structure of a macroion solution to be determined only by the repulsive screened Coulomb pair potential between finite macroions. The counter ions and solvent are treated as a uniform neutralizing background which determines the screening of the potential. We slove the Ornstein-Zernike equation in the mean spherical approximation to obtain closed analytic forms for the direct correlation function c(r) and the structure factor S(Q). In the zero charge limit, the Percus-Yevick hard sphere solution is recovered. As charge is added to the macroions at given volume fraction, the isothermal compressibility decreases and S(Q) shows increasing structure, eventually exhibiting solid-like behaviour. The results provide a useful model basis for studying interacting colloidal systems, and finite ion screened one component plasmas in general.
Nonionic polymers and surfactants in aqueous systems show in a large number of properties unusual or ‘anomalous’ temperature dependences which are briefly reviewed. It is also reported that these solutes in the presence of ionic amphiphilic cosolutes show anomalous effects such as a strong synergistic ionic surfactant-electrolyte effect in the phase behaviour and the formation of stiff gels at higher temperature. With the aim of understanding the effect of ethylene oxidecontaining and related systems in general, including homogeneous solutions, microheterogeneous self-assembled solutions and macroscopic interfaces, a general discussion of possible interactions is presented. Different observations suggest that in all these systems, the solute or interface becomes less hydrophilic at higher temperature, which is in agreement with recent quantumchemical calculations. The effects of conformational equilibria on phase diagrams is considered and in particular it is shown that complex phase equilibria of nonionic polymer-ionic surfactant systems can be rationalized by a simple model.
We have performed static and dynamic neutron small-angle scattering experiments on sodium dodecyl sulphate solutions, in the concentration range 0.04–0.8 mol dm–3, with up to 0.2 mol dm–3 of added NaCl. We have analysed the data taking intermicellar correlations specifically into account by calculating an analytic structure factor for the fluid of interacting charged micelles. We have thus been able, for the first time, to derive micellar charge and aggregation numbers from the small-angle scattering patterns of concentrated micelle solutions. Our results largely reconcile the two opposed current views of ‘wet’ or ‘dry’ micelle structure. We effectively retain the basic micelle geometry proposed by Hartley, but take dynamic thermal fluctuations into account by allowing a rough outer layer from which monomers may penetrate into the solvent. We find the micelles are generally less than 25% ionised.
We investigate the microstructure and mutual correlations of micellar aggregates formed by two tri-block co-polymers of the type (PEO)n(PPO)m(PEO)n in aqueous solution. At room temperature, both PPO and PEO are hydrophilic but at elevated temperatures PPO becomes much less hydrophilic than PEO; thus the co-polymers aquire surfactant characteristics and self-assemble into spherical micelles in an aqueous environment. The aggregation number of the micelle is highly temperature dependent but practically concentration independent. We proposed a cap-and-gown model for the microstructure of the micelle, taking into account hydration in both the core and the corona regions. We treat the inter-micellar correlation by a mono-dispersed sticky-sphere model. The result of the analysis shows excellent agreement with SANS intensity distributions for the entire Q-range in an absolute scale. We thus obtain a consistent set of the structual and interaction parameters over the entire range of the disordered micellar phase.
The three-dimensional classical many-body system is approximated by the use of collective coordinates, through the assumed knowledge of two-body correlation functions. The resulting approximate statistical state is used to obtain the two-body correlation function. Thus, a self-consistent formulation is available for determining the correlation function. Then, the self-consistent integral equation is solved in virial expansion, and the thermodynamic quantities of the system thereby ascertained. The first three virial coefficients are exactly reproduced, while the fourth is nearly correct, as evidenced by numerical results for the case of hard spheres.
The phase behavior and aggregation properties of a triblock copolymer of ethylene oxide (EO) and propylene oxide (PO), with a measured composition (EO)29(PO)40(EO)29, in aqueous solutions containing salt, have been examined using dynamic light scattering, rheological techniques, and sedimentation and viscosity measurements. The copolymer is dissolved as a unimer at low temperatures and forms spherical micelles with increasing temperature. At higher temperatures, a sphere-to-rod transition is seen for the micelles. Two types of gel are formed at higher concentrations of the copolymer. With different inorganic salts, the micellization and gelation properties of the copolymer follow the same type of transitions as the salt-free system, but all transition temperatures are shifted. The spherical micelles thus transform into rod-like micelles at around 38 °C in 1 M KF, which is approximately 36 deg below the transition temperature in the salt-free system. Rod lengths in 1 M KF are between 1000 and 1800 Å, at 40 °C. The higher-temperature gel phase is seen at all concentrations down to 0.5 −1 wt %. The elasticity of this gel is due to hindered rotation of rods. Its relaxation time decreases with increasing concentration, indicating that the gel relaxes due to a partial breakdown or dissolution of the rods at the cross points. The strain dependence of this gel suggests that ordered structures of rods are formed at concentrations above 27 wt %.
Cryogenic temperature transmission electron microscopy (cryo-TEM) and small-angle neutron scattering (SANS) were combined to give complete microstructural characterization of aqueous solutions of Pluronic F127, an (EO)(99)(PO)(65)(EO)(99) triblock copolymer. The images of vitrified specimens observed by cryo-TEM provided direct information about the building blocks of these systems, i.e., spheroidal micelles. This confirmed the structural model obtained from analysis of SANS data, which, in addition, provide quantitative information about the system. Occasionally spheroidal micelles were pressed closer to each other during specimen preparation, thus giving rise locally to higher concentration. The cubic phase that was thus formed was directly imaged. The existence of this lyotropic liquid crystalline phase was confirmed by SANS of an originally higher concentration solution. When exposed to shear, the polycrystalline phase transforms into a monodomain crystal with cubic symmetry. The system was also used to demonstrate the potential of selective electron beam radiolysis to enhance contrast in radiation-sensitive, inherently low-contrast systems.
Triblock copolymers of poly(ethylene oxide) (PEG) and poly(propylene oxide) (PPO) form lamellar mesophases at low temperatures. The order-to-disorder transition is closely related to the melting transition of the PEO subunit. When water is incorporated into the copolymer melt, a single phase is formed at high temperatures, which is possibly a lamellar mesophase, formed as a consequence of the hydrophobic PPO blocks. At low temperatures two phases are present, one of which is similar to the high temperature phase while the other is similar to the lamellar melt, but swollen up to 10%.
Structural studies are presented on the Pluronic-R copolymer 25R8, which has a central poly(ethylene oxide) block (80 wt% of the copolymer) symmetrically surrounded by poly(propylene oxide) blocks. The studies include small-angle neutron scattering, static and dynamic light scattering, and rheology. Complex aggregation behavior is observed as a function of temperature and copolymer concentration. At low temperatures and low copolymer concentrations the (PO)n(EO)m(PO)n copolymers are dissolved as independent macromolecules. In a wide temperature range of dilute concentrations, the scattering patterns indicate formation of large domains of networks of copolymer strands interconnected randomly through the hydrophobic poly(propylene oxide) end blocks. At higher copolymer concentrations an interconnected network of micelles is formed in which micellar cores of hydrophobic poly(propylene oxide) are interconnected by poly(ethylene oxide) strands. For concentrations above 50 wt% this network constitutes the whole sample, resulting in a transparent homogeneous phase. Close to 60% copolymer concentration, the micellar network forms a low-temperature ordered solid-like mesophase. At higher polymer concentrations and low temperatures an elastic two-phase system is formed of a coexisting swollen lamellar and micellar network. In the pure melt, the 25R8 triblock copolymer forms a lamellar structure below T(m) = 55-degrees-C.
In dilute water solutions of a PEO-PPO-PEO triblock copolymer, polarized dynamic light scattering measurement data reveal a sphere-to-rod transition in the shape of the micelles at a temperature close to 70-degrees-C. This transition is anticipated from theory and was also inferred from earlier small-angle neutron scattering measurements. Regularized inverse Laplace transformation of the polarized dynamic light scattering data typically showed a fast rotational diffusion component of low amplitude in addition to a major slow component from translational diffusion. Good agreement was obtained between the rotational diffusion coefficient estimated from the polarized measurements and the value measured directly using depolarized dynamic light scattering. From the rotational diffusion coefficient a length of the P-85 rodlike micelle can be estimated, via Broersma's expressions for the rigid rod, as 1050 angstrom in the dilute limit. This corresponds to a length/diameter ratio of 7. Intensity autocorrelation functions were modeled on the measured dynamic light scattering data using Pecora's theoretical expressions for the time correlation function in terms of the dynamic form factors. Good agreement between theory and experiment was found at the lowest concentrations. Concentrations c > 0.05% correspond to the semidilute regime where interparticle friction effects preclude determination of the length.
The critical micellization temperature (cmt) and critical micellization concentration (cmc) values of 12 Pluronic poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymers, covering a wide range of molecular weights (2,900--14,600) and PPO/PEO ratios (0.19--1.79), were determined employing a dye solubilization method. A closed association model was found to describe adequately the copolymer micellization process for the majority of the Pluronics and used to obtain the standard free energies ([Delta]G[degree]), enthalpies ([Delta]H[degree]), and entropies ([Delta]S[degree]) of micellization. It was determined that the micellization process is entropy-driven and has an endothermic micellization enthalpy. The hydrophobic part of the Pluronics, PPO, was responsible for the micellization, apparently due to diminishing hydrogen bonding between water and PPO with increasing temperature. The cmc dependence on temperature and size of headgroup (PEO) of Pluronics follows a similar trend with lower molecular weight C[sub i]E[sub j] nonionic surfactants, the effect of temperature being more pronounced with the Pluronics. The PEO-PPO-PEO block copolymers were compared to PPO-PEO-PPO block and PEO-PPO random copolymers, in an attempt to probe the effect of molecular architecture in the formation of micelles. No micelles were observed in aqueous PPO-PEO-PPO block copolymer solutions with increasing temperature, up to the cloud point.
The phase behavior of a series of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide),EmPnEm, triblock copolymers dissolved in water has been studied using small-angle neutron and dynamic light scattering. The block copolymers were chosen with a common polymerization degree of the central PPO block, n=39, but different degrees of polymerization of the outer PEO blocks, m ranging from 6 to 97. All copolymers shows the characteristics of fully dissolved polymers at low temperatures, whereas aggregates are formed to elevated temperatures due to the hydrophilic-to-hydrophobic transition in PPO. The scattering function of the m = 27-96 aggregates has been analyzed in terms of hard-sphere interacting micelles, characterized by a dense core radius Rc, and a hard-sphere interaction radius Rha.
The gel formation in aqueous EO99-PO65-EO99 solutions has been studied (EO and PO being ethylene oxide and propylene oxide, respectively). In particular, the effects of the homopolymers (i.e., PEO and PPO) were investigated in detail. It was found that PEO of intermediate molecular weight causes the gel to ''melt'', at an amount of homopolymer which depends on the copolymer concentration. The efficiency of PEO in inducing the gel melting increases with its molecular weight, but at very high PEO molecular weights, phase separation, rather than gel melting, occurs. The gel melting behavior was found also for a cationic polyelectrolyte (PDADMAC). PPO, on the other hand, tends to increase the stability region of the gel, depending, however, on the PPO molecular weight. These findings are reported by means of rheological data and detailed phase diagrams, and discussed in terms of the temperature-dependent micellization occurring in this copolymer system.
The phase behavior of poly(ethylene oxide)-POLY(Propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers dissolved in water has been studied using small-angle neutron scattering. The structural properties have been studied as a function of polymer concentration and temperature. At low temperature (T less-than-or-equal-to 15-degrees-C) and low polymer concentrations the unimers are fully dissolved GaUssian chains with radius R(g) = 17 angstrom. Close to ambient temperature, the hydrophobic nature of ppo causes aggregation of the polymers into spherical micelles with core sizes of the order of 40-50 angstrom, somewhat temperature dependent. The concentration of micelles increases roughly linearly with temperature, until either a saturation is reached, where all polymers are part of a micelle, or the volume density of micelles is so high that they lock into a crystalline structure of hard spheres. In the 60-70-degrees-C temperature range, the micellar structure changes from spherical form to prolate ellipsoid, leading to a decreasing intermicelle interaction. At high concentration, this causes melting of the cubic lattice and leads successively to the formation of a rodlike structure with hexagonal symmetry. Close to 95-degrees-C, large aggregates of polymers ordered in lamellae structure are formed, leading to an opaque suspension.
The dynamics of poly(ethylene oxide)−poly(propylene oxide)−poly(ethylene oxide) micelle rearrangements were studied using iodine laser temperature jump experiments with light-scattering detection. Two relaxation processes were detected: the first, fast one was accompanied by an increase in scattered light intensity, while the second, slow process was accompanied by a reduction in light intensity. The Aniansson−Wall theory was used to interpret the experimental results. The equilibrium micelle structures at the start and end points of the temperature jump experiment were used as input for the Aniansson−Wall equations. The solution of these equations agrees qualitatively with experimental data and suggests the mechanisms associated with the two time constants. The first time scale, in the tens of microseconds to about 10 ms range, is attributed to unimer insertion into micelles. The other time scale, in the 1−100 ms range, is associated with the rearrangement of the micelle size distribution. It is shown that the second process is often not observed either because the unimer supply is insufficient or because the micelle number density is not temperature dependent.
Inorganic salts such as Na2SO4, MgSO4, and Na3PO4 have been reported to form aqueous two-phase systems with polyethylene glycols (PEGs). Recent studies show that the above phenomenon is very general in the sense that a number of inorganic salts, even certain uni-univalent salts, form aqueous two-phase systems with PEG. The relative concentration of various salts to form two-phase systems was found to depend upon the valency and hydration (size) of the ions as well as "specific" interactions of the ions with the polymer. Possible mechanisms leading to the formation of PEG-inorganic salt-water aqueous two-phase systems are discussed.
The change in aggregation behavior of L-64, a poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer, with the concentration of added potassium fluoride in aqueous solutions has been studied with fluorescence probes and light scattering and viscosity measurements. An important feature is that the L-64 preparation is polydisperse, with some 2% of the material in a more hydrophobic form, probably diblock polymers, which form micellar aggregates at lower temperature and lower salt addition than the bulk of the polymers. This explains nicely the peculiar excimer formation and fluroescence quenching behavior and also a number of other erratic observations previously made.
The aggregation behavior of an ethylene oxide-propylene oxide triblock copolymer, Pluronic P-94 (EO24-PO47-EO24, MW = 4600, 40 % poly(ethylene oxide)) in water was examined using several independent methods. Dynamic light scattering studies revealed the existence of micelles (R(h) approximately 90 angstrom) along with monomers and clusters (R(h) approximately 20 angstrom and >1000 angstrom, respectively). Only micelles with low poly dispersity were seen at high temperature/concentration. While surface tension measurements at 40-degrees-C gave a critical micelle concentration of approximately 0.002 wt % and molecular area at closest packing of 128 angstrom2, viscosity results showed that micelles were spherical. Fluorescence spectra of solubilized pyrene conformed the presence of clusters, which dissolve at high concentration/temperature. Oscillatory shear measurements and phase diagram showed reversible thermorheological behavior in concentrated solutions: An increase in temperature changes a Newtonian liquid to a solidlike gel which further dissolves at higher temperatures. This solution is again gellified with further increase in temperature ultimately leading to phase separation at cloud point.
The surface tension of aqueous solutions of seven poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO-PPO-PEO) Pluronic copolymers, covering a wide range of molecular weights (3400-14600) and PPO/PEO ratios (0.19-1.79), was determined over the 10[sup [minus]5]-10% w/v concentration range, at two temperatures (25 and 35[degree]C). Two breaks (changes in slope) were observed in the surface tension vs log concentration curve for most of the copolymers. The low-concentration break, occurring at bulk copolymer concentrations of approximately 10[sup [minus]3]%, is believed to originate from rearrangement of the copolymer molecules on the surface at complete coverage of the air/water interface. The breaks at the high-concentration part of the surface tension curve occurred at concentrations that correspond to the critical micellization concentration values as determined by a dye solubilization technique. The surface area per copolymer molecule, A, increased as a function of the number of EO segments, N[sub EO], obeying a scaling law (A [approx] N[sub EO][sup 1/2]) similar to that of lower molecular weight C[sub i]E[sub j] nonionic surfactants. 56 refs., 6 figs., 2 tabs.
Micellar solutions of a highly hydrophilic ethylene oxide−propylene oxide triblock copolymer, pluronic F88 (EO103PO39EO103), in aqueous solution are examined by small-angle neutron scattering (SANS) at different concentrations and temperatures and in the presence of different salts. At temperature less than 30 °C, F88 solution in water (5 wt %) showed unimers which are fully dissolved Gaussian chains. The unimer-to-micelle transition takes place when the temperature or concentration is increased. Added neutral salt favors micellization of the copolymer at lower concentration/temperature. At temperature close to ambient, block copolymer forms micelles that consist of a central core, presumably dominated by the propylene oxide blocks, surrounded by a corona of highly hydrated ethylene oxide subchains. SANS analysis shows that the size of hydrophobic core increases as a part of PEO adjacent to the PPO core loses water with increasing temperature or salt (KCl) concentration. The micellar volume fraction increases with increasing concentration of block copolymer. The salting out effect of different electrolytes on structure of micelles is in the order KCI > KBr > KI. The effect of temperature on micellar solutions in the presence of KCl is also examined. It has been observed that the effect of added salt on the structural phase behavior of block copolymer is analogous to that of temperature. In all the above study, micelles are found to be spherical.
The phase diagram for the poly(ethylene oxide)-water system has been calculated by using Flory-Huggins theory and assuming that each segment of the poly(ethylene oxide) chain may exist in two forms due to the rotations around C-C and C-O bonds. The agreement with experiment is semiquantitative.
This paper describes a comparison of low molecular weight, nonionic, triblock copolymers in aqueous solution. The samples were chosen to have a common block length (39 propylene oxide units) and with varying endblock lengths (6, 67, and 96 ethyleneoxide units, respectively) analogous to the previously studied copolymer with the same central block and 27 ethyleneoxide units. Relaxation time distributions obtained by Laplace inversion of the dynamic light scattering (DLS) correlation functions demonstrate complex states of aggregation in solution. Monomer, micelles, and larger aggregates coexist in proportions that depend sensitively on temperature and concentration. The monomers have hydrodynamic radii in the size range 15-30 angstrom, micelles 80-130 angstrom (in sequence of increasing ethylene oxide block length); the clusters are 800 angstrom and larger. At higher temperatures and concentrations the micelles close-pick to form a glass-clear gel and at even higher temperatures the gel ''dissolves'' again. The poly(ethylene oxide) chains in the micellar mantle interpenetrate extensively in the gel, yielding a dynamic correlation length of magnitude 8 angstrom at the highest concentration used (about 0.35 g/mL). Oscillatory shear measurements show that the gelation onset temperature and the thermal stability range of the gel increase with increasing poly(ethylene oxide) (PEO) length.
The properties of aqueous solutions of low molecular weight triblock copolymers of PEO/PPO/PEO have been investigated, mainly by dynamic and static light scattering. At low concentrations (C < 10%) and temperature (< 25-degrees-C) the relaxation time distributions from dynamic light scattering show the coexistence of the monomer (R(H) almost-equal-to 18 angstrom), micelles (R(H) almost-equal-to 80 angstrom), and micellar aggregates in relative proportions which depend critically on temperature and concentration. Micelles are formed at about C = 5% at 25-degrees-C. At 40-degrees-C and above micelles are present at all concentrations used (C > 0.3%). At infinite dilution the hydrodynamic radius of the micelles is approximately constant over the temperature range 15-50-degrees-C. At finite concentrations the apparent micellar radius increases with increasing temperature. The growth into asymmetric particles with increasing concentration is stronger as demonstrated by ultracentrifugation and combining static and dynamic light scattering data. At higher concentrations, a solidlike gel is formed at a well-defined temperature as shown by oscillatory shear measurements. It is characterized by a dynamic correlation length which decreases monotonically with increasing concentration to about 20 angstrom.
Ultrasonic velocity and light-scattering measurements on aqueous solutions of the ABA polyoxyethylene-polyoxypropylene block copolymer, Pluronic F127, have clearly indicated a micellar mode of association over the temperature range 10–40°C. The extent of association was limited below 25°C but increased markedly at higher temperatures giving micelles with a mean weight-average aggregation number of 44 at 40°C. A pronounced decrease in the critical micelle concentration accompanied the micellar enlargement. Aggregates were asymmetric at low temperatures but increased in symmetry with temperature increase becoming spheroidal above 25°C.A theoretical treatment of the concentration-dependent changes in ultrasonic velocity is proposed from which information concerning the volume change on micellization may be obtained. Micellization of F127 over the temperature range 20–30°C has been shown to be associated with a very small change in the molar volume of the monomers and this volume change decreased with increases in temperature to the extent of 2%/°C. The temperature-dependent changes in micellar properties have been related to the reversible thermal gelation of this block copolymer.
Aqueous solutions of the surface active poly(oxyethylene)-poly(oxypropylene)-poly(oxyethylene) block copolymers (poloxamers) were studied using photon correlation spectroscopy (quasi-elastic light scattering) and viscosity measurements. Poloxamers 184 and 237 showed detectable aggregates at 25° only at concentrations above about 6% with size increasing with concentration and with significant polydispersity, probably indicating a multiple association process. At 35°, however, essentially invariant values for the hydrodynamic radius were found over a wide concentration range and the systems were essentially monodisperse: these systems are more likely to be represented by a closed association model. The more hydrophilic poloxamer 188, however, retained its concentration dependence of aggregate size up to 55°. The variation with temperature of both the hydrodynamic radius of aggregates and the intrinsic viscosity of several poloxamers was rationalized by relating the temperature-dependency curves to the cloud point of the poloxamer. In some cases only certain sections of the curve are observable when the cloud point is high, e.g., >100°, or low, e.g., <40°.
The aggregation behaviour of two ethylene oxide-propylene oxide block copolymers (PEO-PPO-PEO) in aqueous solution has been investigated in the presence of added salts (KCNS, KI, KBr, KCl and KF) by viscosity, cloud point, light scattering, pulse gradient spin echo NMR, and solubilization measurements. The salts have a strong effect on the cloud points of the pluronics. Both P-85 and L-64 form micelles which increase in size and change into elongated shapes when the cloud point is approached. The changes of size and shape of the micelles, revealed by the intrinsic viscosity and rheological properties, seem to occur at the same temperature relative to the cloud point, independent of the nature of the salt. The onset of micelle formation is also shifted in the same direction as the cloud point by the salts, but by a much smaller amount.
The phase behavior and aggregation properties of block copolymers of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (Pluronics, poloxamers) in aqueous solution have recently attracted much attention. Both experimental and theoretical studies are reviewed, not comprehensively, but with the focus on studies, partly cooperative, partly independent, performed by groups in Uppsala (light scattering and fluorescence), Roskilde (rheology and calorimetry), Risø (SANS), Graz (x-ray and speed of sound), and Lund (theoretical model calculations).
The phase behavior of these copolymers is similar in many respects to that of conventional nonionic surfactants, with the appearance of hexagonal, cubic, and lamellar liquid crystalline phases at high concentrations. In the isotropic solution phase the critical concentration for micelle formation is strongly temperature dependent, and at a given concentration the monomer to micelle transition occurs gradually over a broad temperature range, partly due to the broad size polydispersity of both the PO- and EO-blocks. For some Pluronic copolymers a transition from globular to long rod-like micelles occurs above a transition temperature, resulting in a strong and sudden increase of viscosity and viscoelasticity of the solution.
Size and aggregation numbers have been determined for the globular micelles in some cases, and also the rod-like micelles have been characterized. NMR and fluorescence measurements have provided further information on the properties of the micellar core and mantle. In combination, results from different measurements on the same Pluronics material indicate that the aggregation number of the micelles increases with the temperature, whereas the hydrodynmic radius varies much less. The PEO-mantle of the micelles seems to contract with increasing temperature. The core appears to contain appreciable amounts of PEO in addition to PPO (and also some water). The segregation between core and mantle is not as distinct as in normal micelles, a conclusion which is in line with the predictions from the model calculations.
Aqueous solutions of blockcopolymers, consisting of a polyoxypropyleneblock (POP) with a polyoxyethylene-block (POE) at each side, were studied using surface and interfacial tension measurements, static and dynamic light scattering and smallangle neutron scattering techniques, electric birefringence, rheological and DSC-measurements. The compounds were commercial samples and had an approximate average composition EO20PO70EO20, EO18PO58EO18, and EO106PO69EO106. All three compounds formed micelles above a critical concentration. The size of the micellar core is determined by the length of the hydrophobic poly-propyleneoxide block. The transfer energy of a propyleneoxide unit from the aqueous to the micellar phase is about 0.3 kT at room temperature and hence a quarter of the corresponding value for a CH2-group.
The aggregation number of the micelles increases strongly with increasing temperature while the hydrodynamic radius remains constant in first approximation. The smallangle neutron scattering (SANS) data show at higher concentrations a strong correlation peak. Both the SANS- and the light-scattering data can be interpreted on the basis of the theory of hard sphere particles.
Solutions with a volume fraction beyond about 0.2 gellifie when the temperature is raised above a characteristic value that is at the lowest concentrations slightly above room temperature, shift to lower values with increasing concentrations. Below this gelation temperature DSC-measurements show a phase transition with enthalpies between 40J/g and 80J/g, which is probably due to the dehydration of the PO-groups; this transition can also be observed at low concentrations where no gelation takes place. The position of the correlation peak of the SANS-data is not affected by the gel formation. Some samples, however, show clear evidence of long-range order and seem therefore to consist of cubic liquid crystalline phases. The shear moduli of the gels can qualitatively be understood on the basis of hard sphere models.
We present a general method of determining the structure and charge of globular ionic micelles, using neutron small-angle scattering. The micellar solutions may have any concentration within the micellar phase. The method is based in part on an analytic calculation of the interparticle correlations between monodisperse spherical micelles, and we discuss the theory in some detail to justify its application to polydisperse globular particles. Experimental results are presented for several cationic and anionic micellar systems.
Aqueous solutions of a triblock copolymer of the type polyethyleneoxide-block-polypropyleneoxide-block-polyoxyleneoxide (Pluronic L-64, average composition 26 EO-units and 30 PO-units), were examined with static and dynamic light scattering (DLS), pulsed-gradient spin-echo (PGSE) NMR and fluorescence spectroscopy over a range of concentrations (0.2–25 wt%) and temperatures (15–60°C). Relaxation time distributions from DLS show L-64 to be molecularly dissolved at 21°C, and to form micelles at higher temperatures, which remain at high concentrations (25%) without formation of gel or liquid crystalline phases. The temperature where micelle formation starts is strongly concentration dependent, in contrast to the cloudpoint with remains fairly constant at 60°C. The hydrodynamic radii of the micelles, as obtained from DLS and PGSE NMR, are in reasonable agreement (60–80 Å), and also agree with an aggregation number reported from fluorescence-quenching studies, whereas static light scattering, evaluated according to normal practice, indicates much smaller aggregates. This is due to the presence in the solution of a mixture of monomers, micelles, and at low temperatures, also some strongly scattering larger aggregates, possibly emanating from a small percentage of a diblock impurity in the preparation. The diblock impurity aggregates are dissolved by the proper micelles at higher temperatures. Their presence is indicated also by the anomalous excimer formation at low temperatures, caused by pyrene becoming concentrated in the premicellar aggregates.
The association properties of poly(ethylene oxide)-block-poly(propyleneoxide)-block-poly(ethylene oxide) (PEOPPOPEO) copolymers (commercially available as Poloxamers and Pluronics) in aqueous solutions, and the adsorption of these copolymers at interfaces are reviewed. At low temperatures and/or concentrations the PEOPPOPEO copolymers exist in solution as individual coils (unimers). Thermodynamically stable micelles are formed with increasing copolymer concentration and/or solution temperature, as revealed by surface tension, light scattering, and dye solubilization experiments. The unimer-to-micelle transition is not sharp, but spans a concentration decade or 10 K. The critical micellization concentration (CMC) and temperature (CMT) decrease with an increase in the copolymer PPO content or molecular weight. The dependence of CMC on temperature, together with differential scanning calorimetry experiments, indicates that the micellization process of PEOPPOPEO copolymers in water is endothermic and driven by a decrease in the polarity of ethylene oxide (EO) and propylene oxide (PO) segments as the temperature increases, and by the entropy gain in water when unimers aggregate to form micelles (hydrophobic effect). The free energy and enthalpy of micellization can be correlated to the total number of EO and PO segments in the copolymer and its molecular weight. The micelles have hydrodynamic radii of approximately 10 nm and aggregation numbers in the order of 50. The aggregation number is thought to be independent of the copolymer concentration and to increase with temperature. Phenomenological and mean-field lattice models for the formation of micelles can describe qualitatively the trends observed experimentally. In addition, the lattice models can provide information on the distribution of the EO and PO segments in the micelle. The PEOPPOPEO copolymers adsorb on both airwater and solidwater interfaces; the PPO block is located at the interface while the PEO block extends into the solution, when copolymers are adsorbed at hydrophobic interfaces. Gels are formed by certain PEOPPOPEO block copolymers at high concentrations, with the micelles remaining apparently intact in the form of a “crystal”. The gelation onset temperature and the thermal stability range of the gel increase with increasing PEO block length. A comparison of PEOPPO copolymers with PEOPBO and PEO PS block copolymers and CiEj surfactants is made, and selected applications of PEOPPOPEO block copolymer solutions (such as solubilization of organics, protection of microorganisms, and biomedical uses of micelles and gels) are presented.
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