Yujie Wang

University of Porto, Oporto, Porto, Portugal

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Publications (16)31.12 Total impact

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    ABSTRACT: The aqueous phase behavior of gemini surfactants of the type alkanediyl-α,ω-bis(dodecyldimethylammonium) bromides has been widely studied, and it has been shown that the interfacial and self-assembly properties are largely influenced by the spacer length of these molecules. In contrast, a systematic investigation of the thermotropic behavior of the anhydrous compounds, as a function both of chain and spacer length, is still lacking, with only incomplete or conflicting reports available in the literature. These studies can nevertheless provide valuable information on the direct effect of gradual changes of molecular architecture on polymorphism, the structure of crystalline phases, and the formation and structure of liquid crystals. In this work, we present a differential scanning calorimetry and polarizing light microscopy study of the thermotropic phase behavior of a series of gemini amphiphiles, designated here as 12-s-12, with spacer lengths s=2, 3, 4, 5, 6, 8, 10 and 12. All the surfactants are shown to form thermotropic mesophases—with presence of oily streaks, focal conics, mosaic and fan-shaped birefringent textures—but the surfactants with s=3, 4, 5, and 6 decompose and do not form disordered (fluid) liquid crystalline phases. On the basis of the experimental data and a simple geometric model, some insight into the structure for the smectic mesophases and its relation with the observed mesogenic behavior is presented.
    Proceedings of the 4th Iberian Meeting on Colloids and Interfaces, Porto; 07/2011
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    ABSTRACT: Cationic liposomes have been extensively studied from the experimental and theoretical standpoints, motivated both by their fundamental interest and by potential applications in drug delivery and gene therapy. However, a detailed understanding of the nature of interactions within mixed bilayers containing cationic gemini surfactants is still lacking. This work focuses on the structural and dynamic properties of DODAB membranes in the presence of dicationic gemini surfactants. A thermodynamic characterization of the phase transitions in the mixed systems has been carried out by differential scanning calorimetry, while insight into the molecular interactions in the bilayer has been provided by molecular dynamics. For this purpose, variations in the gemini spacer and tail length, as well as in the respective molar fraction, have been included in both experimental and simulation studies. The results indicate that the influence of cationic gemini surfactants upon the thermotropic behavior and degree of order of DODAB structures is controlled by a complex interplay between charge density, conformation and hydrophobic effects, for which a detailed rationale is provided.
    Physical Chemistry Chemical Physics 06/2011; 13(30):13772-82. · 4.20 Impact Factor
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    ABSTRACT: While cationic/anionic surfactant mixtures have been reported to form strongly non-ideal mixed aggregates in aqueous solution, the micellization of true catanionic surfactants, i.e. the salt-free ion-paired compounds obtained from the mixtures, has been much less investigated. However, if there is a significant solubility mismatch between the paired chains, these surfactants may show less conventional aggregation features in water, such as a temperature-driven vesicle–micelle transition and the coexistence of two lamellar phases in equilibrium. In this work, we carry out micellization studies for a catanionic surfactant of the type C16+C8−, cetyltrimethylammonium octylsulfonate (TASo), by surface tension and conductivity measurements. For comparison purposes, the catanionic mixture cetyltrimethylammonium bromide (CTAB)/sodium octylsulfonate (SOSo) at different mixing ratios and the neat SOSo have also been studied. For the non-ideal CTAB/SOSo mixture, Rubingh's model has been applied to determine the β interaction parameter and the mixed micelle composition. The cmc of TASo has been investigated as a function of temperature, salt and excess ionic surfactant, displaying some differences with respect to conventional ionic and nonionic surfactants. The trends in the thermodynamic parameters of micellization of the three types of systems are comparatively rationalized.
    Journal of Molecular Liquids 01/2010; 157:113-118. · 2.08 Impact Factor
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    ABSTRACT: Monolayers of cationic gemini surfactants and their catanionic mixtures with sodium dodecyl sulfate (SDS), have been investigated with the Langmuir trough technique, at the air–water interface. The gemini surfactants are of the alkanediyl-α,ω-bis(alkyldimethylammonium) type, here designated as m-2-m, where m and 2 are the alkyl chain and spacer lengths, respectively. For the neat geminis, the stability of the monolayer increases as the chain length increases, starting from soluble films of 12-2-12 to stable films of 18-2-18. For the equicharged m-2-m/SDS mixtures (with m = 12, 14, 16 and 18), stable monolayers are obtained. The effect of the gemini chain length on the phase behavior and molecular organization of the films is discussed on the basis of pressure–area isotherms for compression or compression–expansion cycles. The pressure–temperature plots, at constant trough area, yield the desorption temperature and suggest the desorption mechanism of the film molecules. Furthermore, the effect of the mixing molar ratio between m-2-m and SDS on the isotherms, for m = 12 and 14, has also been investigated. It is concluded from the mean area per molecule that the gemini molecules when in excess with respect to equicharged composition desorb from the film, so that the electroneutral composition is maintained.
    Thin Solid Films 01/2008; · 1.87 Impact Factor
  • Yujie Wang, Eduardo F. Marques
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    ABSTRACT: Mixtures of cationic and anionic surfactants (catanionic mixtures) are often highly non-ideal, exhibiting strong synergism in their interfacial properties, manifested for instance in significant reduction of the mixture critical micelle concentration (cmc) and enhanced adsorption onto surfaces. The magnitude of such effects is of fundamental interest and has important application-related uses (e.g. in detergent formulation). In this work, the micellization process of mixtures of cationic gemini surfactants of the alkanediyl-α,ω-bis(alkyl dimethylammonium bromide) type, denoted by 12–n–12 (where n is the spacer length), with several common anionic surfactants has been investigated by electric conductivity. For the purpose of comparison, cationic–cationic mixtures, where dodecyltrimethylammonium bromide is the second cationic surfactant, have also been investigated. The cationic/anionic mixtures show relatively significant deviations from ideal behavior, depending on the structure of the gemini surfactant and the anionic surfactant. The interaction parameter β12, within Rubingh's non-ideal model for mixed micelles, has been calculated for each mixture, as well as the mixed micelle composition as a function of mixture composition. The observed synergism in the different mixtures is interpreted in terms of the molecular structure of the surfactants and corresponding head–head and chain–chain interactions.
    Journal of Molecular Liquids - J MOL LIQ. 01/2008; 142(1):136-142.
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    ABSTRACT: Understanding factors responsible for the fluorescence behavior of conjugated polyelectrolytes and modulation of their behavior are important for their application as functional materials. The interaction between the anionic poly{1,4-phenylene-[9,9-bis(4-phenoxy-butylsulfonate)]fluorene-2,7-diyl}copolymer (PBS-PFP) and cationic gemini surfactants alpha,omega-(CmH2m+1N+(CH3)2)2(CH2)s(Br-)2 (m-s-m; m=12, s=2, 3, 5, 6, 10, and 12) has been studied experimentally in aqueous solution. These surfactants are chosen to see whether molecular recognition and self-assembly occurs between the oppositely charged conjugated polyelectrolyte and gemini surfactant when the spacer length on the surfactant is similar to the intercharge separation on the polymer. Without surfactants, PBS-PFP exists as aggregates. These are broken up upon addition of gemini surfactants. However, as anticipated, the behavior strongly depends upon spacer length (s). Fluorescence measurements show three surfactant concentration regimes: At low concentrations (<2x10(-6) M) quenching occurs and is most marked with the small spacer 12-2-12; at intermediate concentrations (approximately 2x10(-6)-10(-3) M), fluorescence intensity is constant, with a 12-carbon spacer 12-12-12 showing the strongest fluorescence; above the critical micelle concentration (CMC; approximately 10(-3) M) increases in emission intensity are seen in all cases and are largest with the intermediate spacers 12-5-12 and 12-6-12, where the spacer length most closely matches the distance between monomer units on the polymer. With longer spacer length surfactants, surface tension measurements for concentrations below the CMC reveal the presence of polymer-surfactant aggregates at the air-water interface, possibly reflecting increased hydrophobicity. Above the CMC, small-angle neutron scattering experiments for the 12-6-12 system show the presence of spherical aggregates, both for the pure surfactant and for polyelectrolyte/gemini mixtures. Molecular dynamics simulations help rationalize these observations and show that there is a very fine balance between electrostatic and hydrophobic interactions. With the shortest spacer 12-2-12, Coulombic interactions are dominant, while for the longest spacer 12-12-12 the driving force involves hydrophobic interactions. Qualitatively, with the intermediate 12-5-12 and 12-6-12 systems, the optimum balance is observed between Coulombic and hydrophobic interactions, explaining their strong fluorescence enhancement.
    The Journal of Physical Chemistry B 05/2007; 111(17):4401-10. · 3.61 Impact Factor
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    ABSTRACT: We present the phase behavior and thermodynamics of the catanionic mixture of the gemini surfactant hexanediyl-alpha,omega-bis(dodecyldimethylammonium bromide), designated here as 12-6-12Br(2), and sodium dodecyl sulfate (SDS) over the full range of composition, at the water-rich corner. Visual and turbidity measurements of the mixtures provide some basic macroscopic information on phase behavior. The structure of the aggregates formed spontaneously in the mixtures has been observed with TEM. As the molar fraction of SDS, X(SDS), is increased, at constant total surfactant concentration, the aggregation morphologies change gradually from gemini-rich micelles, through multiphase regions containing a precipitate (catanionic surfactant) and a vesicle region, to SDS-rich micelles. From isothermal titration calorimetry measurements, the phase boundaries and corresponding enthalpy changes for phase transitions have been obtained. The formation of the different microstructures, in particular, the spontaneously formed vesicles in the SDS-rich side, is discussed on the basis of geometric and electrostatic effects occurring in the SDS-gemini mixture.
    The Journal of Physical Chemistry B 04/2006; 110(11):5294-300. · 3.61 Impact Factor
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    ABSTRACT: Catanionic surfactants result from the pairing of oppositely charged amphiphilic molecules, forming a new class of surfactant molecules with various interesting lyotropic and thermotropic properties. With the aim of probing the role of both headgroup chemical nature/structure and molecular shape, a series of catanionic surfactants were synthesized. The cationic portion of the molecule is kept constant, being the dioctadecyldimethylammonium double chain. Different single-chained surfactants with varying headgroups and chain lengths are used as the anionic pair. The thermotropic behavior has been studied by DSC and the mesophase structural investigated by polarized light microscopy. The results indicate that, for a given chain length, parameters such as headgroup polarity and charge density, as well as volume, influence the catanionic surfactant behavior. The thermodynamic parameters are qualitatively evaluated, considering the headgroup chemical nature and the overall molecular structure.
    Journal of Colloid and Interface Science 03/2006; 294(1):240-7. · 3.55 Impact Factor
  • Yujie Wang, Eduardo F Marques
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    ABSTRACT: The lyotropic phase behavior for the neat cationic gemini surfactants alkanediyl-alpha,omega-bis(alkyldimethylammonium bromide), designated here as m-s-m, has been investigated previously in several works, but the thermotropic behavior has not been well characterized. Only for 15-s-15 and 14-s-12 have thermotropic liquid crystals (Lc) been reported. In this work, for the first time and in contrast to previous reports, we observe thermotropic Lc formation for m-2-m geminis with m = 12, 14, 16, and 18, by means of polarizing microscopy and differential scanning calorimetry (DSC). Furthermore, we investigate mixtures of m-2-m and SDS, m-2-m Br2.2SDS, which exhibit crystal-to-crystal phase transitions at lower temperature and, at high temperature, smectic Lc phases. The transition temperatures and enthalpies for Lc phases, obtained by DSC, present clear trends upon increase of the chain lengths. Combining Langmuir film experiments, possible lamellar arrangements for the different phases are tentatively discussed.
    The Journal of Physical Chemistry B 02/2006; 110(3):1151-7. · 3.61 Impact Factor
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    ABSTRACT: A catanionic surfactant consists of paired oppositely charged surfactant ions, acting as counterions to each other. In this work, a series of catanionic surfactants with the cationic quaternary ammonium group and anionic sulfate group, with variation of chain length and number of chains, have been synthesized. The results of the Langmuir pressure–area isotherms, for compression and cycles of compression–decompression, have been used to discuss the effect of length and number of hydrophobic chains on the phase behavior and molecular organization of the spread catanionic film. The molecular desorption from the catanionic film, involving either dissolution of the catanionic surfactant or desorption of the organic counterion (short chain ionic moiety), is critically discussed.
    Thin Solid Films 01/2006; 515(4):2031-2037. · 1.87 Impact Factor
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    ABSTRACT: We have used microcalorimetry to measure the critical micelle concentrations (CMC) and enthalpies of micellization (ΔHmic) of the series of dissymmetric cationic gemini surfactants, [CmH2m+1(CH3)2N(CH2)6N(CH3)2CnH2n+1]Br2, designated as CmC6CnBr2, with constant n + m = 24 and n = 6, 8, 10, 11, 12. There is a small decrease in CMC as the ratio m/n increases but this decrease is not much larger than the error. On the other hand there is a very large decrease in the enthalpy of micellization with m/n. This is consistent with a large increase in the hydrophobic contribution to micellization with m/n. To explain the relative invariance of the CMC, there must be a corresponding large increase in the entropy of micellization with m/n. The change in the hydrophobic contribution is explained in terms of the relative strengths of internal and external hydrophobic contacts down the series.
    Journal of Physical Chemistry B - J PHYS CHEM B. 06/2002; 106(26).
  • Guangyue Bai, Yujie Wang, Haike Yan
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    ABSTRACT: Microcalorimetry has been used to study the interactions between two gemini surfactants, [C12H25(CH3)2N(CH2)SN(CH3)2C12H25]Br2, with S = 3 and 6, and several modified poly(acrylamides) (PAM) including hydrophobic groups and acrylic acid residues. Values of enthalpies, entropies, and free energies of interaction have been derived for the two gemini surfactants and two single-chain surfactants, sodium dodecyl sulfate and dodecyltrimethylammonium bromide. The interaction with the polymers was found to be much stronger for the geminis in comparison with the single-chain surfactants and was also found to be strongly dependent on the spacer length. Inclusion of acrylic acid residues in hydrophobically modified PAMs generates a strong specific interaction between polymer and gemini surfactant that acts to disrupt the aggregation of the surfactant.
    Journal of Physical Chemistry B - J PHYS CHEM B. 02/2002; 106(9).
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    ABSTRACT: Microcalorimetric measurements have been made on double chain cationic surfactants with the formula [C(N)H(2N+1)C(M)H(2M+1)N(CH(3))(2)]Br, referred to as C(N)C(M)DAB, and a cationic gemini surfactant with the formula [C(12)H(25)(CH(3))(2)N(CH(2))(12)N(CH(3))(2)C(12)H(25)]Br(2), referred to as C(12)C(12)C(12)Br(2). The CMCs and enthalpies of micellization for the compounds C(12)C(6)DAB, C(14)C(4)DAB, C(16)C(2)DAB, and C(12)C(12)C(12)Br(2) are obtained from experiments. The effect of the ratio (N/M) of the two alkyl chain lengths for the C(N)C(M)DABs on the micellization process has been investigated. The contribution of DeltaH(mic) to DeltaG(mic) is about 3.4% at N/M=2, about 11% at N/M=3.5 and about 39% at N/M=8, suggesting that when the total carbon atoms (N+M) remain constant, the micellization process will be driven by entropy or entropy and enthalpy jointly with increasing ratio (N/M). In addition, the comparison between C(12)C(12)C(12)Br(2) and its corresponding monomer (C(12)C(6)DAB) is interesting. The micellization process for C(12)C(12)C(12)Br(2) is driven jointly by enthalpy and entropy, whereas the micellization process for C(12)C(6)DAB is driven mainly by entropy. Copyright 2001 Academic Press.
    Journal of Colloid and Interface Science 09/2001; 240(1):375-377. · 3.55 Impact Factor
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    ABSTRACT: The interaction enthalpies between didodecyldimethylammonium bromide (DDAB) and sodium dodecyl sulfate (SDS) have been measured at 298.15 K using an isothermal titration microcalorimeter. From the variation of enthalpy with SDS concentration, the enthalpy of interaction between DDAB micelles and SDS, the enthalpy of formation of the DDA+SD-(cryst), the enthalpy of vesicle formation of DDAB−SDS, and the enthalpy of transition from vesicles to SDS-rich micelles have been determined to be −29.53, −125.8, 41.23, and 32.10 kJ/mol, respectively. The phase behavior of the DDAB−SDS aqueous mixture has also been investigated by turbidity measurements and transmission electron microscopy.
    Langmuir. 05/2001; 17(12).
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    ABSTRACT: A titration microcalorimeter with the sample cells of 1 mL and 3 mL volume was constructed by combining LKB-2107 ampule microcalorimeter with an improved Thermometric titration microcalorimeter. Its sensitivity and precision were tested with the baseline noise and stability, the measurement of energy equivalent, and the linear relation of electric energy and integral area as the function of voltage(V)-time (t). Its accuracy was demonstrated by measuring the dilution enthalpy of a concentrated sucrose solution and the micelle-forming enthalpy of sodium dodecyl sulfate (SDS) in aqueous solution respectively. At the same time, the enthalpy of interaction between SDS and didodecyldimethylammonium bromide (DDAB) was measured by using the titration microcalorimeter, and the phase behavior of SDS-DDAB aqueous mixture was discussed. The microcalorimetric results show that the enthalpy of interaction between SDS and DDAB micelles is -29.53 kJ/mol, the enthalpy of formation of 1:1 SDS-DDAB salt is -125.8 kJ/mol, the vesicle-forming enthalpy of SDS-DDAB is 41.23 kJ/mol, and the enthalpy of phase transition from vesicles to SDS rich micelle is 32.10 kJ/mol.
    Science in China Series B Chemistry 11/2000; 43(6):617-624. · 1.20 Impact Factor
  • Yujie Wang, Eduardo F. Marques
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    ABSTRACT: The thermal behavior for three homologous series of cationic geminis surfactants of the type n-2-n, alkanediyl-α,ω-bis(alkyldimethylammonium bromide), with n=12, 14, 16, and 18, and sodium alkyl sulfates, SC m S, with m=12, 14, and 16, is reported here. The cationic/anionic molar ratio is kept at 1:2 (equicharged mixtures), and salt is also present. Polarizing light microscopy and differential scanning calorimetry show a stepwise fusion for the mixtures with appearance of several mesophases between the crystalline structures and the isotropic liquid. A main endothermic transition is observed, associated with partial chain melting and consequent loss of crystalline order, followed by a transition to a smectic liquid crystal. The phase transition thermodynamics is interpreted in terms of an interplay between van der Waals chain–chain interactions and ionic head group interactions. KeywordsCatanionic mixture-Mesophase-Soft crystal-Smectic liquid crystal-Van der Waals interactions-Electrostatic interactions-Birefringence
    Journal of Thermal Analysis and Calorimetry 100(2):501-508. · 1.98 Impact Factor