Evidence of Sticky Contacts between Wormlike Micelles in Viscoelastic Surfactant Solutions.

Many cationic surfactants form highly viscoelastic solutions at concentrations of only a few percent. These solutions contain wormlike micelles (WLM) which can be several micrometers long. The structural relaxation time, τs in these solutions can be as long as many seconds, and the zero-shear viscosity can be in the range of 106 Pas. Electric birefringence measurements on such solutions showed four different relaxation times with increasing concentration. The two shortest ones, τ1 and τ2 in the µs region were due to the alignment of small rodlike or ringlike micelles. The third one, τ3, was observed in the viscoelastic region in the ms region and finally, a fourth one, τ4, which was the same as the structural relaxation time, τs. In this article, it is shown that the τ3 process is due to the formation and opening of contacts between the WLM. The reason for the contacts is the attraction between the WLM which is due to the hydrophobic surfaces of the micelles. The contacts crosslink the WLM and thus form a three-dimensional network. This network is the reason for the high viscosity and viscoelastic properties of the samples. If the attraction between the WLM is reduced by adding glycerol to the solution, the viscosity of the solution breaks down. The same happens if the surface of the WLM is made hydrophilic by the addition of small amphiphilic molecules. The WLM are not destroyed by these procedures.

Formation of Unique Unilamellar Vesicles from Multilamellar Vesicles under High-Pressure Shear Flow.

Vesicles in surfactant systems are influenced by a shear field. The high shear flow generated by a homogenizer is expected to affect the size of vesicles. Hence, it should be possible to control the size and dispersion of vesicles by tuning the shear. In this study, the influence of shear on the vesicle phase was studied by measuring the rheology and conductivity of a solution made of the nonionic surfactant trideceth-5, a polyethylene glycol ether of tridecyl alcohol with an average number of ethylene oxide of 5, and the anionic surfactant sodium dodecylsulfate. It was found that when shear was applied by a homogenizer, the bilayers of the multilamellar vesicles were stripped off and became unilamellar vesicles, which decreased the viscoelasticity of the system. However, because of the pressure provided by the homogenizer, the newly formed unilamellar vesicles were small and the relative distance between them was large. As a result, the vesicles were no longer crowded and could easily pass each other under shear. This is why the unilamellar vesicles generated by the homogenizer had low viscoelasticity and flow birefringence. Additionally, it took a long time for the unilamellar vesicles to relax back to the original state.

Rheological and calorimetric study of alkyltrimethylammonium bromide-sodium salicylate wormlike micelles in aqueous binary systems.

HYPOTHESIS: It is known that additives like glycerol and sucrose lead to the swelling of aqueous bilayer Lα phases. The swelling of the Lα phases can be explained by the increase of the refractive index of the mixed solvent, which lowers the van der Waals attraction between the bilayers. Afterwards, the undulation forces between the bilayers can push them apart. This hypothesis was previously extended to wormlike micelles (WLM) of cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal). These types of self-assembly structures have viscoelastic properties, and the zero shear viscosity of these solutions is dependent on the molar ratio NaSal/CTAB, R. At R = 0.6, R = 1.0 and R ≈ 2.6 the viscosity goes, respectively, through a maximum, a minimum and another maximum. These viscosities can be explained by differences in relaxation mechanisms predominant in each region. Similarly to what is observed to bilayer Lα phases, the additives would change the interaction between the WLM, affecting the relaxation processes of each region, altering the profile from two maxima and one minimum to a single maximum in viscosity. In the present manuscript, it is investigated whether it is only the refractive index, other solvent properties, or a combination of several factors that induce these changes in WLM. For this, several additives, forming binary mixtures with water, were studied, through rheology of CTAB/NaSal and calorimetry of tetradecyltrimethylammonium bromide (TTAB)/NaSal. EXPERIMENTS: Herein, we present the zero-shear viscosity diagrams of NaSal and CTAB with glycerol, sucrose, dimethyl sulfoxide, 1,3-butanediol and urea combined with water. Additionally, isothermal titration calorimetry was used to obtain the variations of enthalpy for formation of WLM of TTAB and NaSal in mixtures of water and such additives. FINDINGS: Based on our data, only the refractive index match is not enough to explain the rheological and calorimetric behaviors of the WLM. For instance, sucrose has little effect on the micelles, even at the same refractive index match conditions. Additional characteristics, such as dielectric constant, the cohesivity of the solvent (here symbolized by the Gordon parameter), and the interactions of the additive with the micelles, have to be considered to better describe the results.

Monitoring the different micelle species and the slow kinetics of tetraethylammonium perfluorooctane-sulfonate by 19F NMR spectroscopy.

Since we lack effective tools that can monitor the structures of surfactant micelles in situ, the different equilibrium species and the slow kinetics of micelles are still not well understood. Herein, by using 19F NMR, we simultaneously monitored that micelles of tetraethylammonium perfluorooctanesulfonate (TPFOS, C8F17SO3N(C2H5)4) in water grow more complex in virtue of hydrophobic counterions and the slow kinetic exchange process exists in the system. Apart from the monomeric signals, three sets of micelle signals which correspond to spherical micelles, wormlike/wormlike micelles with rings in end caps and toroidal micelles were successfully detected on the NMR time scale because of the slow exchange rate for surfactant molecules between the monomer and the micelle states. By comparison, other fluoro- and hydrocarbon surfactants with different tail lengths and counterions (+N(CH3)4, +N(C3H7)4, Li+ and Na+) have been studied, and the coexistence of different micelles could also been observed for the aqueous solution of C9F19COON(CH3)4. However, only one set of averaged NMR signals could be observed for these surfactants. The micellization of TPFOS in water is demonstrated to be a predominantly entropy-driven process. Molecular dynamics (MD) simulation revealed an unusual distribution of counterions, providing further understanding of the mechanism of the micelle formation process.

Surfactant-Modified Ultrafine Gold Nanoparticles with Magnetic Responsiveness for Reversible Convergence and Release of Biomacromolecules.

It is difficult to synthesize magnetic gold nanoparticles (AuNPs) with ultrafine sizes (<2 nm) based on a conventional method via coating AuNPs using magnetic particles, compounds, or ions. Here, magnetic cationic surfactants C16H33N+(CH3)3[CeCl3Br]- (CTACe) and C16H33N+(CH3)3[GdCl3Br]- (CTAGd) are prepared by a one-step coordination reaction, i.e., C16H33N+(CH3)3Br- (CTABr) + CeCl3 or GdCl3 → CTACe or CTAGd. A simple strategy for fabricate ultrafine (<2 nm) magnetic gold nanoparticles (AuNPs) via surface modification with weak oxidizing paramagnetic cationic surfactants, CTACe or CTAGd, is developed. The resulting AuNPs can highly concentrate the charges of cationic surfactants on their surfaces, thereby presenting strong electrostatic interaction with negatively charged biomacromolecules, DNA, and proteins. As a consequence, they can converge DNA and proteins over 90% at a lower dosage than magnetic surfactants or existing magnetic AuNPs. The surface modification with these cationic surfactants endows AuNPs with strong magnetism, which allows them to magnetize and migrate the attached biomacromolecules with a much higher efficiency. The native conformation of DNA and proteins can be protected during the migration. Besides, the captured DNA and proteins could be released after adding sufficient inorganic salts such as at cNaBr = 50 mmol·L-1. Our results could offer new guidance for a diverse range of systems including gene delivery, DNA transfection, and protein delivery and separation.

Pulsed-field gradient NMR measurements on hydrogels from phosphocholine.

Gels from diacylphosphatidylcholine in glycerol/butylene glycol mixtures were investigated by pulsed-field gradient NMR measurements. Previous measurements had shown that the gels are formed by networks from crystalline multilamellar vesicles (MLV). The obtained self-diffusion coefficients for water and butylene glycol molecules indicate that both molecules occur in two different environments, even at temperatures above the phase transition T(m) where the system is still in a liquid crystalline state. While the larger fraction of the molecules shows a free self-diffusion process like in a homogeneous phase, the smaller fraction seems to be encapsulated in closed domains and undergoes only hindered self-diffusion. It is concluded that the hindered diffusions are due to the solvent molecules trapped between the bilayers of the multilamellar vesicles, while the free diffusion is assigned to the solvent molecules outside of the MLV. Since the fraction of the entrapped molecules does not change during phase transition, we assume that the structure of the network in the samples remains the same when gelation occurs. The gelation process is simply due to the transformation of the vesicle bilayers from the liquid crystalline to the crystalline state. The permeability of the bilayer for the solvent molecules is drastically changed by this transition. The exchange of water molecules through the bilayers slows down significantly below T(m): while the average residence time of water molecules inside the vesicles is smaller than 50 ms in the liquid crystalline state, this value increases to more than 1 s for the gel state. In the case of pure butylene glycol, no vesicles are present, and it is likely that these gels are formed from crystalline fibers.

Hydrophobin coated boehmite nanoparticles stabilizing oil in water emulsions.

Hydrophobin coated boehmite nanoparticles have been used to establish tooth-paste like, homogenous emulsions. The surface-modified nanoparticles were simply obtained by mixing aqueous solutions of cationic boehmite particles with the anionic hydrophobin H Star Protein B® (HPB). Surface tension measurements clearly show that 1 wt.% boehmite binds up to 1 wt.% HPB. The strong interaction and aggregation of hydrophobin coated boehmite nanoparticles was proven by Cryo-TEM measurements, too. Interestingly, the combined use of 0.5 wt.% HPB and 0.5 wt.% boehmite as emulsifying agents resulted in very stable, homogenous, high internal phase emulsions (65 wt.% oil) that are stable over months. The established emulsions have also been characterized by rheological measurements. Storage moduli of more than 1000 Pa are characteristic for their high gel-like properties. Furthermore, light microscopy showed an average droplet size close to 1 µm with low polydispersity. Cryo-SEM confirmed that the hydrophobin coated nanoparticles are located at the interface of the oil droplets and therefore stabilize the emulsion systems.

Dynamic properties of microemulsions in the single-phase channels.

We have studied the dynamic and rheological properties in the single-phase channels of a microemulsion system with a mixed anionic/nonionic surfactant system and decane from the aqueous to the oil phase. One isotropic channel, called the "upper" channel, begins at the L(3) phase (sponge-like phase) of the binary surfactant mixture on the water side and passes with a shallow minimum for the surfactant composition to the oil side. The other "lower" single-phase channel begins at the micellar L(1) phase and ends in the middle of the phase diagram. Both isotropic channels are separated by a huge anisotropic single phase L(α) channel that reaches from the water side to 90% of oil in the solvent mixture. The structural relaxation time of the viscous fluids could be measured with electric birefringence (EB) measurements, where a signal is caused by the deformation of the internal nanostructure of the fluids by an electric field. For the L(3) phase, the EB signal can be fitted with a single time constant. With increasing oil in the upper channel, the main structural relaxation time passes over a maximum and correlates with the viscosity. Obviously, this time constant controls the viscosity of the fluid (η(o) = G'·τ). It is remarkable that the longest structural relaxation time increases three decades, and the viscosity increases two decades when 10% of oil is solubilized into the L(3) phase. Conductivity data imply that the fluid in the upper channel has a bicontinuous structure from the L(3) phase to the microemulsion with only 10% oil. In this oil range, the conductivity decreases three decades, and the electric birefringence signals are complicated because of a superposition of up to three processes. For higher oil ratios, the structure obviously changes to a HIPE (high internal phase emulsion) structure with water droplets in the oil matrix.

Microemulsions from silicone oil with an anionic/nonionic surfactant mixture.

Microemulsion phases have been prepared for the first time from the silicone oil "M(2)" (hexamethyldisiloxane) and a surfactant mixture of a nonionic surfactant "IT 3" (isotridecyltriethyleneglycolether) and an ionic surfactant Ca(DS)(2) (calciumdodecylsulfate). For such a surfactant mixture the hydrophilicity of the system can be tuned by the mixing ratio of the two components. With increasing IT 3 content, the surfactant mixtures show a L(1)-phase, a wide L(α)-region and a narrow L(3) sponge phase. For constant temperature, two single phase channels exist in the microemulsion system. The lower channel (low IT 3 content) ends in the middle of the phase diagram with equal amounts of water and oil, the upper channel begins with the L(3)-phase and passes all the way to the oil phase. Conductivity data show that the upper channel has a bi-continuous morphology up to 40% oil while the lower channel consists of oil droplets in water. In contrast to previous studies on nonionic systems, the two single phase channels are not connected and microemulsions with equal amount of oil and water do not have a bicontinuous structure.

Cyanosis by methemoglobinemia in tadpoles of Cochranella granulosa (Anura: Centrolenidae)

Tadpoles inhabit generally well oxygenated rivers and streams, nevertheless they were found in areas with limited oxygen availability inside the rivers. To assess this feature, I examined factors that influence centrolenid tadpole behaviour using Cochranella granulosa. The tadpoles were reared in well-oxygenated and hypoxic environments and their development, survivorship and growth were compared. The tadpoles in oxygenated water acquired a pale color, while tadpoles in hypoxic water grew faster and were bright red and more active. In the oxygenated water, the ammonium, which had its origin in the tadpoles’ urine and feces, was oxidized to nitrate. In contrast, in the hypoxic treatment, the nitrogen compounds remained mainly as ammonium. Presumably, the nitrate in oxygenated water was secondarily reduced to nitrite inside the long intestine coils, because all symptoms in the tadpoles point to methemoglobinemia, which can occur when the nitrite passes through the intestine wall into the bloodstream, transforming the hemoglobin into methemoglobin. This could be checked by a blood test where the percentage of methemoglobin was 2.3% in the blood of tadpoles reared in hypoxic condition, while there was a 19.3% level of methemoglobin in the blood of tadpoles reared in oxygenated water. Together with the elevated content of methemoglobin, the growth of the tadpoles was delayed in oxygenated water, which had high nitrate content. The study about quantitative food-uptake showed that the tadpoles benefit more from the food in hypoxic water, although they spent there more energy moving around than the tadpoles living in oxygenated but nitrate-charged water. Rev. Biol. Trop. 58 (4): 1467-1478. Epub 2010 December 01.

Los renacuajos por lo general viven en ríos y arroyos bien oxigenados, sin embargo, como han sido encontrados en áreas con disponibilidad de oxígeno limitada en los ríos, se estudió como influye este factor en su comportamiento. Renacuajos de Cochranella granulosa fueron criados en ambientes bien oxigenados y de hipoxia para comparar su desarrollo, supervivencia y crecimiento. En el agua que no fue cambiada durante al menos un mes, los renacuajos mostraron diferencias en su desarrollo cuando vivían en agua hipóxica u oxigenada. Los renacuajos en el agua aireada tenían un color pálido, mientras que en la hipóxica fueron más activos y de un color rojo brillante. En el agua hipóxica, el nitrógeno que se originó de la orina y las heces de los renacuajos se mantuvo principalmente en forma de amonio; en cambio, el amonio fue oxidado a nitrato en el agua aireada. Presumiblemente, el nitrato en el agua oxigenada se redujo secundariamente a nitrito dentro del intestino, ya que todos los síntomas en los renacuajos que vivían en esta agua apuntaron a una metahemoglobinemia, que se produce cuando el nitrito pasa a través de la pared del intestino a la corriente sanguínea transformando la hemoglobina en metahemoglobina. Esto pudo comprobarse mediante un análisis sanguíneo en donde el porcentaje de metahemoglobina fue del 2.3% en la sangre de los renacuajos criados en condición hipóxica y de un 19.3% de metahemoglobina en aquellos criados en agua aireada. En la misma forma en que la metahemoglobina aumenta en la sangre de los renacuajos que viven en agua oxigenada, su crecimiento disminuye en agua con alto contenido de nitrato. El estudio cuantitativo de la ingestión de nutrientes mostró que el crecimiento de los renacuajos se beneficia más de los alimentos en agua hipóxica, a pesar de que los renacuajos son más activos en sus movimientos que los que viven en agua oxigenada pero cargada de nitratos.

Cyanosis by methemoglobinemia in tadpoles of Cochranella granulosa (Anura: Centrolenidae).

Tadpoles inhabit generally well oxygenated rivers and streams, nevertheless they were found in areas with limited oxygen availability inside the rivers. To assess this feature, I examined factors that influence centrolenid tadpole behaviour using Cochranella granulosa. The tadpoles were reared in well-oxygenated and hypoxic environments and their development, survivorship and growth were compared. The tadpoles in oxygenated water acquired a pale color, while tadpoles in hypoxic water grew faster and were bright red and more active. In the oxygenated water, the ammonium, which had its origin in the tadpoles' urine and feces, was oxidized to nitrate. In contrast, in the hypoxic treatment, the nitrogen compounds remained mainly as ammonium. Presumably, the nitrate in oxygenated water was secondarily reduced to nitrite inside the long intestine coils, because all symptoms in the tadpoles point to methemoglobinemia, which can occur when the nitrite passes through the intestine wall into the bloodstream, transforming the hemoglobin into methemoglobin. This could be checked by a blood test where the percentage of methemoglobin was 2.3% in the blood of tadpoles reared in hypoxic condition, while there was a 19.3% level of methemoglobin in the blood of tadpoles reared in oxygenated water. Together with the elevated content of methemoglobin, the growth of the tadpoles was delayed in oxygenated water, which had high nitrate content. The study about quantitative food-uptake showed that the tadpoles benefit more from the food in hypoxic water, although they spent there more energy moving around than the tadpoles living in oxygenated but nitrate-charged water.

Phase transition of precipitation cream with densely packed multilamellar vesicles by the replacement of solvent.

The swelling of lamellar phase can be induced by the replacement of solvent in a tetradecyltrimethylammonium bromide (TTABr) and sodium laurate (SL) aqueous mixed solution that contains cream floating precipitates on the upper phase and L1-phase (micelles) at the lower phase. The cream floating precipitates contain densely packed multilamellar vesicles, which were determined by freeze-fracture transmission electron microscopy (FF-TEM) images. Phase transition, from cream floating precipitates to swelling birefringent vesicle phase, to two-phase Lalpha/L1, and finally to micelle phase, can be induced by adding glycerin as solvent in the aqueous solution. At first, densely packed multilamellar vesicles of cream floating precipitates on the upper phase swelled throughout the whole phase with increasing content of glycerin. The replacement of solvent lowers the turbidity of the dispersion and swells the interlamellar distance between the bilayers, which is explained by matching of refractive index of the solvent to the refractive index of the bilayers of the surfactant mixtures. With an increasing amount of glycerin, the swelling Lalpha phase turned to two-phase Lalpha/L1, and finally to L1 phase (micelles). This phase transition can also be explained because of the increasing critical micelle concentration of the cationic and anionic (catanionic) surfactant mixture (TTABr and SL) at high glycerin concentration. The phase transition induced by addition of sorbitol can also be studied and compared to the case of adding glycerin. These results may direct toward acquiring an understanding of the phase transition mechanism of catanionic surfactants induced by solvents.

Aggregation behavior of fluorocarbon and hydrocarbon cationic surfactant mixtures: a study of 1H NMR and 19F NMR.

The aggregation behavior and the interaction of four mixed systems for a cationic fluorocarbon surfactant, diethanolheptadecafluoro-2-undecanolmethylammonium chloride (DEFUMACl), mixing with cationic hydrocarbon surfactants, alkyltrimethylammonium chloride, CnTACl (n=12, 14, 16, and 18; where n=12 is DTACl, n=14 is TTACl, n=16 is CTACl, and n=18 is OTACl), were studied by 1H and 19F NMR in more detail. The results of 19F NMR measurements strongly indicate that in the three mixed systems of DEFUMACl/DTACl, DEFUMACl/TTACl, and DEFUMACl/CTACl at different molar fractions of fluorocarbon surfactant (alphaF=(cDEFUMACl/cDEFUMACl+cCnTACl)), with an increase of the total concentration of fluorocarbon and hydrocarbon surfactants (cT=cF+cH), the mixed micelles at the first break point and the individual DEFUMACl micelles at the second break point form. However, three different types of micelles were determined in DEFUMACl/OTACl mixtures by 19F NMR measurements, OTACl-rich and DEFUMACl-rich mixed micelles and individual DEFUMACl micelles, respectively. The chemical shifts of proton Deltadelta (1H) for -CH3 in the mixed systems of DEFUMACl/CnTACl (n=12, 14, 16, and 18) have different variation trends from the 19F NMR measurements. For the two systems of DEFUACl/DTACl and DEFUMACl/TTACl, the mixed micelles form at the first break point. At the second break point, for lower alpha F values the DTACl-rich and TTACl-rich mixed micelles form with a strong downfield shift and for higher alpha F values DEFUMACl-rich mixed micelles form with a strong upfield. For the other two systems of DEFUMACl/CTACl and DEFUMAC/OTACl, the chemical shifts of proton Deltadelta (1H) of -CH3 increase with an increase of the total concentration of DEFUMACl/CTACl or OTACl, and mixed CH- and CF-surfactant micelles form. At higher total concentration, the greater effect of fluorinated chains of DEFUMACl on CH-chains was obvious, resulting in the strong upfield chemical shifts. In cationic fluorocarbon and hydrocarbon surfactant mixtures, the different kinds of micelles observed by 19F and 1H NMR measurements could be caused by the increase in alkyl chain length of hydrocarbon surfactants with different critical micelle concentrations. Combining two theoretical models for mixing, for the four different chain-length hydrocarbon surfactants studied, one can conclude that the two components of mixtures interact with each other and form mixed micelles in two completely different ways according to their molecular properties and cmc values in a certain range of total concentrations. One is close to an ideal mixing case with the formation of one type of mixed micelles, such as the DEFUMACl/DTACl and DEFUMACl/TTACl systems. The other is a demixing case with the formation of two types of micelles, i.e., fluorocarbon-rich and hydrocarbon-rich mixed micelles, such as DEFUMACl/CTACl and DEFUMACl/OTACl systems. However, as the total concentrations of the mixed systems are high enough, the four systems tend to demix and to form individual micelles of corresponding components due to the initial respective interaction between fluorocarbon and hydrocarbon chains. That is to say, at high total concentration, the individual DEFUMACl micelles in all four systems could form. These results may be primarily directed toward acquiring an understanding of the mechanism of CF-CH mixtures in aqueous solution and secondarily directed toward providing more detailed information on nonideal mixing.

Reversible phase transition from vesicles to lamellar network structures triggered by chain melting.

Reversible phase structural transition from densely packed multilamellar vesicles of cationic and anionic (catanionic) tetradecyltrimethylammonium laurate (TTAL) with an amount of salt (NaBr) to network structures was triggered by chain melting. Phase behavior of catanionic TTAL multilamellar vesicles in aqueous solutions at different concentrations of NaBr with increasing temperature was studied. This phase structural transition is a progressive process and happens at the chain melting, which was monitored by means of Fourier transform infrared (FT-IR) spectroscopy, turbidity and viscosity measurements. Transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS) were used to demonstrate the phase structural conversion from vesicles to three-dimensional structures consisting of extended bilayer networks. We found that the phase transition temperature (Tm) was influenced by adding amount of salt but not by being diluted. This is the first time that the phase conversion from catanionic surfactant vesicles to bilayer networks triggered by chain melting has been observed. The phase structural transition should arise from the enhanced membrane elasticity accompanying the catanionic surfactant state fluctuations on chain melting and the solvent-associated interactions including cationic and anionic surfactant electrostatic interaction, which favors a change in membrane curvature. We hope this phase conversion observed in catanionic surfactants in aqueous solution will provide good insight into the nature of the fusion or fission processes and the fluctuation of catanionic vesicular systems.

Glycerol-induced swollen lamellar phases with siloxane copolymers.

Phase behavior is established for a block copolymer polyethyleneoxide-b-dimethylsiloxane-polyethylenoxide (EO)(15)-(PDMS)(15)-(EO)(15) (IM-22) a in glycerol/water mixed solvent. In water alone, the block copolymer forms biphasic micellar/lamellar (L(1)/L(alpha)) systems over the range 10-70 wt%, with single L(alpha)-phases between 70-90 wt%. Strong solvent effects on the phase behavior were noted. For example, using a mixed 60:40 vol% glycerol/water solvent, the single L(alpha)-phase region appears at much lower concentrations, only 20 wt% IM-22, as compared to the biphasic L(1)/L(alpha) system observed in water alone. This interesting observation of L(alpha)-phase swelling on addition of glycerol may be explained by a decrease in attraction between the bilayers, as it is also found that in this mixed glycerol/water solvent there is a close refractive index matching with IM-22. Rheological measurements show the L(alpha)-phases with added glycerol have low shear moduli. The influence of added ionic surfactant sodium dodecylsulfate (SDS) on these swollen IM-22 L(alpha)-phases was studied. Small-angle X-ray scattering (SAXS) indicated the interlamellar distance d remains essentially constant up to 3 mM SDS, and then decreases with increasing SDS content. This weak effect is consistent with the fact that the L(alpha)-phases are most swollen when the mixed solvent contains 60 vol% glycerol. The results suggest that glycerol/water solvent mixtures can be used to tune the refractive index of the background solvent, modifying DLVO-type interactions, and causing significant effects on the phase stability of simple block-copolymer systems.

A cationic fluorocarbon surfactant DEFUMACl and its mixed systems with cationic surfactants: 19F NMR and surface tension study.

A cationic fluorocarbon surfactant system of diethanolheptadecafluoro-2-undecanolmethylammonium chloride (DEFUMACl) and both mixed systems of DEFUMACl/cationic dodecyltrimethylammonium chloride (DTACl) and DEFUMACl/cationic Gemini copolymer was investigated by 19F NMR spectroscopy and surface tension measurements. The critical micelle concentration (cmc) of DEFUMACl by 19F NMR is about 3.40 mmol/L, which is completely consistent with that obtained by the surface tension method. The studies of salt and temperature on the cmc values of DEFUMACl suggest that both salt addition and temperature increase decrease the cmc values of DEFUMACl. 19F NMR measurements provide much richer information on both mixed systems. For the DEFUMACl-DTACl system, two break points were observed with increased total surfactant concentration. The first break point means the DEFUMACl and DTACl mixed micelles and the second one implies the individual DEFUMACl micelles. Results of 19F NMR and surface tension measurements for DEFUMACl/cationic Gemini copolymer mixtures show three peculiar break points, corresponding to the critical association concentration (cac) of DEFUMACl, the concentration where cationic Gemini copolymer molecules become saturated by DEFUMACl micelles, and the concentration where DEFUMACl micelles and cationic Gemini copolymer coexist. These peculiar points in the cationic-fluorocarbon and cationic-copolymer systems were first reported by 19F NMR and surface tension measurements. These results should broaden the useful information for a better understanding of the mechanism of interaction and the behavior of surfactant-polymer mixtures.

Swelling of Lalpha-phases by matching the refractive index of the water-glycerol mixed solvent and that of the bilayers in the block copolymer system of (EO)15-(PDMS)15-(EO)15.

The swelling of Lalpha-phases from the block copolymer polyethylenoxide-b-polydimethylsiloxane-polyethylenoxide (EO)15-(PDMS)15-(EO)15 in water/glycerol mixtures is reported. At low and medium polymer concentrations (<60%), the block copolymer forms a turbid vesicular dispersion in water. With time, the small unilamellar vesicles (SUV) and the large multilamellar vesicles (MLV) separate into a two phase L1/Lalpha-system. The turbid dispersions of the Lalpha-phase below 60% of the compound become more and more transparent with increasing glycerol and at 60% of glycerol become completely clear. Replacement of water by the solvent glycerol thus lowers the turbidity of the dispersion and swells the interlamellar distance between the bilayers. A 20% aqueous L1/Lalpha-dispersion can thus be transformed into a single birefringent transparent Lalpha-phase. The swelling of the Lalpha-phase in water and the decrease of the turbidity of the dispersion by the addition of glycerol is explained by the matching of the refractive index of the solvent to the refractive index of the bilayers of the block copolymer. The matching of a refractive index lowers the Hamaker constant in the DLVO theory between the bilayers and therefore decreases the attraction between the bilayers what allows them to swell to a larger separation. The microstructures in the phases were determined by cryo- and FFR-TEM. The interlamellar distance between the bilayers was determined by SAXS measurements. The viscous properties of the Lalpha-phases were determined by oscillatory rheological measurements. In comparison to other Lalpha-phases from normal surfactants, the Lalpha-phases from the block copolymer (EO)15-(PDMS)15-(EO)15 have low shear moduli. This is probably due to the high flexibility of the poly dimethylsiloxane block in the bilayers what can be recognized on the non-spherical shapes of the SUV's.

Molecular exchange through the vesicle membrane of siloxane surfactant in water/glycerol mixed solvents.

The effect of glycerol on the permeability of vesicle membranes of a siloxane surfactant, the block copolymer polyethyleneoxide-b-polydimethylsiloxane-polyethyleneoxide, (EO)15-(DMS)15-(EO)15, was studied with freeze-fracture transmission electron microscopy (FF-TEM) and pulsed-field gradient nuclear magnetic resonance (PFG-NMR) spectroscopy. The FF-TEM results show that, in pure water, the surfactant can form small vesicles with diameters of less than 25 nm, as well as a few multilamellar vesicles with diameters larger than 250 nm. Gradual substitution of water with glycerol to a glycerol content of 40% leads to significant structural transformations: small vesicles are gradually swollen, and large multilamellar vesicles disappear. A glycerol content of 60% results in the complete disintegration of the vesicles into membrane fragments. PFG-NMR measurements indicate that the vesicle membrane does not represent an effective barrier for water molecules on the NMR time scale; hence, the average residence time of water in the encapsulated state is below tau b = 2 ms. In contrast, the average residence time of glycerol molecules in the encapsulated state can be as large as tau b = 910 ms. The permeability of the vesicle membrane increases with increasing glycerol concentration in the solvent: At a concentration of 40%, the residence time tau b is lowered to approximately 290 ms. After vesicle destruction at higher glycerol concentrations, a small glycerol fraction is still bound by membrane fragments that are formed after the disintegration of the vesicles.

Influence of ionic charges on the bilayers of lamellar phases.

The influence of ionic charges on the mesophases in the ternary system of C(12-16)E(6) (LA 070), ethylhexylglycerid (EHG), and water was studied. The charge was introduced by adding the ionic surfactant SDS (sodium dodecyl sulfate). The single lamellar phase (5 wt % LA 070 and 240 mM EHG in water) yields a bluish homogeneous solution. With the addition of SDS, the samples become more and more clear. Rheology measurements indicate that increased charge density increases the storage modulus G', and the lamellar phases show typical behavior of a viscoelastic fluid with a yield stress at higher SDS concentration. SAXS measurements show that the interlamellar distance D decreases with SDS concentration. The addition of ionic surfactants suppresses the Helfrich undulations, flattens the bilayers, and decreases interbilayer spacing due to electrostatic repulsions of the ionic surfactant head groups. Furthermore, the L(alpha) phase transforms into vesicle phases as the SDS concentration is increased. Second, it is shown that with added NaCl electrolyte the phase with charged surfactant behaves again in the same way as the initial uncharged system. The addition of salt screens the electrostatic interaction, which leads to a higher flexibility of the bilayers and a decrease of the storage modulus G'. Theoretical calculations show that the shear moduli of the L(alpha) phases are much smaller than the osmotic pressure of the systems. Several models are proposed for the explanation of the shear moduli. The model due to Lekkerkerker for the electric contribution of the bending constant of the bilayer seems to yield good results for the transition to vesicles.