[show abstract][hide abstract] ABSTRACT: It is time to review latest activities on the dilational surface rheology of the two nonionic surfactants n-dodecyl-β-d-maltoside (β-C12G2) and hexaoxyethylene dodecyl ether (C12E6) and their 1:1 mixture as a lot of different data generated with different techniques have been published in the last years. As the data are scattered throughout different papers and were generated with different techniques, we carried out an extensive study with one technique, which we will use as reference for the discussion of different data sets. We found that the results are in most of the cases in line with already published data as regards the general trends. However, a quantitative comparison reveals differences, which may result in different interpretations of the data. In the review at hand, we summarize, compare and discuss our latest and previously published data.
Advances in colloid and interface science 05/2013; · 5.68 Impact Factor
[show abstract][hide abstract] ABSTRACT: The synthesis of porous “sponge-like” TiO2 via a polymer gel coating technique is presented. The experimental procedure involves the preparation of a gelled polymerizable microemulsion. The polymerization of the latter leads to porous poly-N-isopropylacrylamide which forms a hydrogel in the presence of water. Via solvent exchange, a suitable TiO2 precursor is infiltrated into this structure after which its in situ hydrolysisis triggered to form porous amorphous TiO2. The subsequent calcination step allows the removal of the polymer template and the transformation of amorphous TiO2 in to porous, crystalline anatase with domain sizes ranging from 200 to 250nm. As a means of verification and proof of concept, this material is tested as light-scattering layer in dye-sensitized solar cells (DSSC), and it is found that the resulting solar cell performance is comparable to commercially available TiO2. However, an increased tendency to form rutile during DSSC fabrication was noticed when compared to commercial TiO2. As there is a large potential for optimizing the synthesis, the proposed procedure is a promising route towards porous TiO2 that performs significantly better as scattering layer in light-harvesting and optical devices.
Colloid and Polymer Science 09/2012; 291:805. · 2.16 Impact Factor
[show abstract][hide abstract] ABSTRACT: Although it is known that foaming a surfactant solution results in a depletion of the surfactant in the bulk phase, this effect is often overlooked and has never been quantified. Therefore, the influence of surfactant depletion on foam properties using solutions of the two nonionic surfactants, n-dodecyl-β-D-maltoside (β-C(12)G(2)) and hexaethyleneglycol monododecyl ether (C(12)E(6)), were investigated. These investigations were conducted in two steps. First, different foam volumes were generated with the same surfactant solution at a concentration of c = 2 cmc. It was found that the higher the foam volume, the larger the surfactant depletion. Second, two different bulk concentrations (c = 2 and 1.33 cmc) were used for the generation of 50 and 110 mL of foam, respectively. For a foam volume of 50 mL, no differences were observed, whereas generating 110 mL led to different results. The surfactant loss in the bulk solution was measured via surface tension measurements and then compared to the results of purely geometric considerations that take into account the amount of interface created in the foam. Both results were in very good agreement, which means that surfactant depletion can be calculated in the way suggested here. Under conditions where depletion plays a role, our approach can also be used to estimate the bubble size of a foam of known volume by measuring the surfactant concentration in the bulk solution after foaming.
[show abstract][hide abstract] ABSTRACT: We showed in a previous study that a water-nonionic surfactant system, where the surfactant is a 9:1 mixture of tetraethylene glycol monodecyl ether (C(10)E(4)) and pentaethylene glycol monodecyl ether (C(10)E(5)), forms a disconnected lamellar (L(α)) phase. Thus, the isotropic phase spans the whole concentration range from the water-rich L(1) region to the surfactant-rich L(2) region of the phase diagram. The L(1) and L(2) regions are connected via an isotropic channel that separates the two regions of the L(α) phase. In this letter, we monitored the structural evolution of the isotropic phase along a path through this isotropic channel via (1)H NMR self-diffusion measurements. We used this technique because it enables us to distinguish between discrete and bicontinuous structures by comparing the relative self-diffusion coefficients (obstruction factors) D/D(0) of the solvents (i.e. of water and surfactant in the present case). We found that the obstruction factor of water decreases whereas the obstruction factor of the surfactant increases with increasing surfactant concentration and increasing temperature. This trend is interpreted as the transition from a water-continuous L(1) region, which contains discrete micelles, to a bicontinuous structure, which may extend to very high surfactant concentrations. Although there is good evidence of bicontinuity over a broad concentration range, there is no evidence of inverse micelles or any other microstructure at the highest concentration studied in the surfactant-rich L(2) phase.
[show abstract][hide abstract] ABSTRACT: The adsorption isotherms of n-decyl-β-D-glucoside (β-C(10)G(1)) as well as various n-alkyl-β-D-maltosides (β-C(n)G(2)) with n=8, 10, 12 and 14 were determined from surface tension measurements. Based on the analysis of the adsorption isotherms, the total free energy change of adsorption was determined and a novel method was proposed to determine the maximum adsorbed amount of surfactant. It can be concluded that the driving force for adsorption first increases with increasing adsorbed amount of the sugar surfactants and then levels off in a plateau. This peculiar behaviour is interpreted as formation of a thin liquid-like alkane film of overlapping alkyl chains at the air/water interface once a certain adsorbed amount is exceeded. The driving force of adsorption depends on the alkyl chain length only and is not affected by the type of the head group. The hydrophobic contribution to the standard free energy change of adsorption was compared with the values of sodium alkylsulfate and alkyltrimethylammonium bromide surfactants. This comparison reveals that the hydrophobic driving force of adsorption is the largest for the sodium alkylsulfates, whereas it is the same for the sugar surfactants and the alkyltrimethylammonium bromides.
Journal of Colloid and Interface Science 04/2012; 379(1):78-83. · 3.17 Impact Factor
[show abstract][hide abstract] ABSTRACT: Carbohydrate- and oligoethylene oxide-based surfactants behave quite differently despite the fact that they are both classes of nonionic surfactants. Intensive studies of a mixture at fixed molar ratio (1:1) of two very common sugar- and oligoethylene oxide-based surfactants, namely n-dodecyl-β-D-maltoside (β-C(12)G(2)) and n-dodecyl hexaethylene oxide (C(12)E(6)), revealed that most properties of the mixture are similar to those of the oligoethylene oxide-based surfactant. In the present work, this mixture is compared to respective "hybrid surfactants". Such hybrid surfactants are surfactants whose head group contains chemically linked carbohydrate and oligoethylene oxide units. In order to study the behaviour of this sort of compounds, we synthesised a new class of surfactants whose head group consists of one carbohydrate-like unit (myo-inositol) and three ethylene oxide units. New regiochemically defined ethoxylated inositol derivatives (referred to as C(12)I(1)E(3) and C(12)E(3)I(1) in the following) were synthesised and studied for their thermotropic and lyotropic liquid crystalline properties as well as for their surface activities. The results are compared with those of the reference systems β-C(12)G(2) and C(12)E(6), and their 1:1 mixture, respectively, and are discussed in terms of structure-property relations.
Journal of Colloid and Interface Science 04/2012; 371(1):82-8. · 3.17 Impact Factor
[show abstract][hide abstract] ABSTRACT: An ideal template for the production of macroporous polystyrene can be prepared from foamed oil-in-water emulsions containing styrene, water, glycerol, and sodium dodecylsulfate. After addition of a photoinitiator the mixture is polymerized with UV light and the foam structure of the precursor is transferred to the polymer. The resulting materials display densely packed cells with windows between adjacent pores (see SEM image; scale bar: 250 μm).
Angewandte Chemie International Edition 02/2012; 51(9):2213-7. · 13.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: Carbohydrates are versatile materials of natural origin and thus they are interesting hydrophilic head groups for surfactants. Especially myo-inositol derivatives have advantages over pyranosidic or furanosidic sugar derivatives, namely a higher thermal and chemical stability. The main disadvantage turned out to be the poor water solubility of myo-inositol surfactants, which, however, can be overcome by introducing an oligoethylene oxide group between the inositol head group and the apolar chain. Our goal was to combine the favourable properties of surfactants based on myo-inositol with those of fluorinated surfactants, which are becoming increasingly important as CO2 solvation mediators or stabilizers for reverse water-in-fluorocarbon microemulsions. Thus we synthesized a new surfactant which combines the three units of interest, namely an inositol head group, an oligoethylene oxide linker to provide sufficient solubility and a fluorinated chain. We studied the thermotropic and lyotropic liquid crystalline behaviour as well as the surface tension and compared these results with those of a fully protonated surfactant which has a similar structure.
Colloids and Surfaces A Physicochemical and Engineering Aspects 01/2012; 414:320-326. · 2.11 Impact Factor
[show abstract][hide abstract] ABSTRACT: PtPb intermetallic nanoparticles (3-6 nm) with precise control (±0.8 nm) were prepared under mild conditions by using water-in-oil (w/o) microemulsions as reaction media. The amount of the aqueous phase has no significant effect on the composition of the NPs, while it indeed affects the size of the resulting NPs in a systematic way. A precise size control can be obtained by controlling the size of the w/o-microemulsions containing the metal salts and the reducing agent, respectively.
Physical Chemistry Chemical Physics 05/2011; 13(20):9134-6. · 3.83 Impact Factor
[show abstract][hide abstract] ABSTRACT: In water/oil systems, surfactants partition between the water phase and the oil phase according to their solubility in both phases. The ratio between the concentration of the surfactant in the oil phase and in the water phase at equilibrium is known as the partition or distribution coefficient (K(p)). The partition coefficient (K(p)) is an important fundamental parameter essential to understanding and controlling phenomena in water-oil-surfactant systems under both equilibrium and non-equilibrium conditions. In the present work we report on the partitioning of three different classes of nonionic surfactants in the pre-cmc regime, namely polyoxyethylene alkyl ethers (C(i)E(j)), alkyl dimethyl phosphine oxides (C(n)DMPO) and alkyl glycosides (β-C(n)G(m)) between water and different n-alkanes. We focus on the influence of the surfactant's molecular structure (alkyl chain length, head group size and type), and oil chain length on K(p) to derive systematic structure-property relationships. Moreover, we discuss the influence of the surfactant purity on partition coefficients of technical grade alkyl glycosides and polyoxyethylene alkyl ethers, respectively.
Journal of Colloid and Interface Science 03/2011; 355(1):150-6. · 3.17 Impact Factor
[show abstract][hide abstract] ABSTRACT: Not Available Bibtex entry for this abstract Preferred format for this abstract (see Preferences) Find Similar Abstracts: Use: Authors Title Return: Query Results Return items starting with number Query Form Database: Astronomy Physics arXiv e-prints
[show abstract][hide abstract] ABSTRACT: Structural features of bimetallic nanoparticles synthesized in w/o microemulsion have been examined by using a coarse model solved by Monte Carlo simulations. The microemulsion was modeled as spherical droplets containing either two metal salts or reducing agent, and the processes occurring during the collision of the droplets and leading to nucleation and growth of the bimetallic nanoparticles were determined by a set of variables. The bimetallic nanoparticle structure is mainly determined by the difference in the reduction rates of the two metal ions and the excess of reducing agent. An intermetallic structure is always obtained when both reduction reactions take place at about the same rate. When the metal ions have very different reduction potentials, a core−shell to intermetallic structure transition is found at increasing the excess of the reducing agent. An enhancement of the intermetallic structure at the expense of the core−shell, one can be obtained either by decreasing the concentration of both metal salts or by increasing the interdroplet exchange rates. Predictions of the Monte Carlo simulations at large differences in the reduction rates of both metal ions compare well with the experimental data on the size and structure of PtBi nanoparticles, leading to prediction of the experimental conditions for designing specific bimetallic structures.
[show abstract][hide abstract] ABSTRACT: This paper describes the synthesis of intermetallic Pt/Bi and Pt/Pb nanoparticles (NPs) using water-in-oil (w/o) microemulsions (μe) as template. For that purpose, w/o-microemulsions containing H2PtCl6 + Pb(NO3)2 and H2PtCl6 + Bi(NO)3, respectively, were mixed either with a w/o-microemulsion containing the reducing agent (NaBH4) or with solid NaBH4. A variation of the amount of reducing agent led to different particle compositions and sizes, while different ratios of the two metal salts only affected the composition but not the size of the resulting NPs. The size and structure of the microemulsion droplets were studied via small angle X-ray scattering (SAXS), and the intermetallic NPs were characterized by high resolution transmission electron microscopy (HRTEM) in combination with energy dispersive X-ray spectroscopy (EDX) and selected area electron diffraction (SAED). The results revealed that it is indeed possible to synthesis Pt/Pb and Pt/Bi intermetallic nanoparticles of 3−8 nm in diameter at low temperatures.
Chemistry of Materials, v.22, 6263-6271 (2010). 11/2010;
[show abstract][hide abstract] ABSTRACT: For the synthesis of Pt nanoparticles we used water-in-oil droplet microemulsions as templates. The focus was on the correlation between the size of the microemulsion droplets and that of the resulting Pt particles. To study this correlation in a systematic way, all particles were synthesized at the water emulsification failure boundaries where the microemulsion droplets are spherical and where their size can easily be tuned by the amount of added water. The metallic particles were synthesized by mixing two microemulsions one of which contains the metal salt H(2)PtCl(6) and the other the reducing agent NaBH(4). The size and structure of the microemulsion droplets was studied via small-angle X-ray scattering, while the Pt particles were characterized by high-resolution transmission electron microscopy in combination with energy-dispersive X-ray spectroscopy and selected area electron diffraction. The clear correlation between droplet and particle size was further supported by accompanying Monte Carlo simulations.
Physical Chemistry Chemical Physics 09/2010; 13(8):3048-58. · 3.83 Impact Factor
[show abstract][hide abstract] ABSTRACT: Mixtures of the two non-ionic surfactants hexaoxyethylene dodecyl ether (C(12)E(6)) and n-dodecyl-beta-D-maltoside (beta-C(12)G(2)) were studied with regard to surface properties, bulk properties, foam films, and foams. The reason for studying a mixture of an ethylene oxide (C(i)E(j)) and a sugar (C(n)G(m)) based surfactant is that despite being non-ionic, these two surfactants behave quite differently. Firstly, the physico-chemical properties of aqueous solutions of C(n)G(m) surfactants are less temperature-sensitive than those of C(i)E(j) solutions. Secondly, the surface charge density q(0) of foam films stabilized by C(n)G(m) surfactants is pH insensitive down to the so-called isoelectric point, while that of foam films stabilized by C(i)E(j) surfactants changes linearly with the pH. The third difference is related to interaction forces between solid surfaces. Under equilibrium conditions very high forces are needed to expel beta-C(12)G(2) from between thiolated gold surfaces, while for C(12)E(6) low loads are sufficient. Fourthly, the adsorption of C(12)E(6) and beta-C(12)G(2) on hydrophilic silica and titania, respectively, is inverted. While the surface excess of C(12)E(6) is large on silica and negligible on titania, beta-C(12)G(2) adsorbs very little on silica but has a large surface excess on titania. What is the reason for this different behaviour? Under similar conditions and for comparable head group sizes, it was found that the hydration of C(i)E(j) surfactants is one order of magnitude higher but on average much weaker than that of C(n)G(m) surfactants. Moreover, C(n)G(m) surfactants possess a rigid maltoside unit, while C(i)E(j) surfactants have a very flexible hydrophilic part. Indeed, most of the different properties mentioned above can be explained by the different hydration and the head group flexibilities. The intriguing question of how mixtures of C(i)E(j) and C(n)G(m) surfactants would behave arises organically. Thus various properties of C(12)E(6)+beta-C(12)G(2) mixtures in aqueous solution have been studied with a focus on the 1:1 mixture. The results are compared with those of the single surfactants and are discussed accordingly.
Advances in colloid and interface science 03/2010; 155(1-2):5-18. · 5.68 Impact Factor
[show abstract][hide abstract] ABSTRACT: A thin film pressure balance was used to investigate the disjoining pressure Pi as a function of the film thickness h of surfactant-stabilized formamide foam films. Nonionic (alkylpoly(ethylene glycol)s) and cationic surfactant (alkyltrimethylammonium bromides (C(n)TAB) with n = 14 and 16) solutions were studied in the absence and presence of electrolyte. The resulting Pi-h curves were fitted with the DLVO theory from which we extracted surface charge densities q(0) and surface potentials Psi(0). Investigating formamide foam films is of interest for studying the electrostatic component of the stabilizing forces in foam films. We know that the aqueous foam films are stabilized via electrostatic forces. In this case the self-dissociation of water contributes to the charges in the foam film. As formamide has a dissociation constant which is about 2 orders of magnitude lower than that of water, the number of charges in the solution due to self-dissociation is much smaller, which, in turn, should lead to lower electrostatic forces. Indeed, we found that formamide solutions of nonionic surfactants did not form stable foam films at concentrations below the critical micelle concentration. Regarding the cationic surfactants, the main difference between the formamide and the aqueous foam films is the fact that the concentration of ionic surfactants to form stable foam films is about 2 orders of magnitude higher compared to water. Consequently, the screening length for the electrostatic interaction and thus the film thickness are much smaller compared to films formed by the respective aqueous solutions.
[show abstract][hide abstract] ABSTRACT: An extensive study of the foaming properties of a surfactant mixture consisting of the non-ionic dodecyldimethyl phosphineoxide (C(12)DMPO) and the cationic dodecyltrimethyl ammonium bromide (C(12)TAB) with mixing ratios of C(12)DMPO:C(12)TAB = 1:0, 50:1, 1:1, 1:50, 0:1 is performed both above and below the critical micelle concentration. Foamability and foam stability were examined using the commercially available FoamScan (sparging), the standardised Ross-Miles (pouring) and a home-built winding (shaking) technique. The focus, however, was on FoamScan measurements as they allowed for the evaluation of the foam's liquid content. The foamability and foam stability of C(12)TAB was found to be larger than that of C(12)DMPO. The foamability continually increased with increasing C(12)TAB content in the surfactant mixture, which reflects the reduction of the diffusion relaxation time (i.e. faster adsorption). Where possible correlations are drawn between the foam properties on the one hand and adsorption and foam film properties on the other hand, which were studied previously. Interestingly, the 1:1 mixture shows weak/negligible surfactant interactions but--counterintuitively--an increased foam stability compared to the single surfactant systems. However, at this ratio charge neutralization occurs, which leads to the formation of a Newton Black Film thus suggesting that the foam film type plays an important role in the foam stability.
Journal of Colloid and Interface Science 03/2010; 346(2):414-23. · 3.17 Impact Factor