Three-phase behavior in a mixed nonionic surfactant system

The effect of monodisperse solubilities of each surfactant in an excess oil phase on the three-phase behavior was investigated in a water/octaethyleneglycol dodecyl ether (R₁₂EO₈)/tetraethyleneglycol dodecyl ether (R₁₂EO₄)/heptane system. The mid temperature of the three-phase region is defined as the HLB temperature. The HLB temperature is largely skewed to higher temperature in a dilute region due to the difference in the distribution of each surfactant between excess oil and microemulsion (surfactant) phases forming the three-phase body. Taking account of the monodisperse solubilities, the equation for the HLB temperature was obtained on the basis of geometrical calculation of a particular three-phase triangle. The equation well describes the three-phase behavior for a mixed surfactant system in a space of compositions and temperature.In the mixed surfactant system, the monodisperse solubility of R₁₂EO₈ in oil phase forming a three-phase body is monotonously increased with the rise in temperature, whereas that of R₁₂EO₄ is first increased and then is decreased. Consequently, the sum of both solubilities does not change greatly in a wide range of temperature.     

Use of the ternary phase diagram of a mixed cationic/glucopyranoside surfactant system to predict mesostructured silica synthesis

Mixed surfactant systems have the potential to impart controlled combinations of functionality and pore structure to mesoporous metal oxides. Here, we combine a functional glucopyranoside surfactant with a cationic surfactant that readily forms liquid crystalline mesophases. The phase diagram for the ternary system CTAB/H(2)O/n-octyl-beta-D-glucopyranoside (C(8)G(1)) at 50 degrees C is measured using polarized optical microscopy. At this temperature, the binary C(8)G(1)/H(2)O system forms disordered micellar solutions up to 72 wt% C(8)G(1), and there is no hexagonal phase. With the addition of CTAB, we identify a large area of hexagonal phase, as well as cubic, lamellar and solid surfactant phases. The ternary phase diagram is used to predict the synthesis of thick mesoporous silica films via a direct liquid crystal templating technique. By changing the relative concentration of mixed surfactants as well as inorganic precursor species, surfactant/silica mesostructured thick films can be synthesized with variable glucopyranoside content, and with 2D hexagonal, cubic and lamellar structures. The domains over which different mesophases are prepared correspond well with those of the ternary phase diagram if the hydrophilic inorganic species is assumed to act as an equivalent volume of water.     

Thermodynamic and structural characterization of a mixed perfluorocarbon-phospholipid ternary monolayer surfactant system

Pulmonary lung surfactant is a mixture of surfactants that reduces surface tension during respiration. Perfluorinated surfactants have potential applications for artificial lung surfactant formulations, but the interactions that exist between these compounds and phospholipids in surfactant monolayer mixtures are poorly understood. We report here, for the first time, a detailed thermodynamic and structural characterization of a minimal pulmonary lung surfactant model system that is based on a ternary phospholipid-perfluorocarbon mixture. Langmuir and Langmuir-Blodgett monolayers of binary and ternary mixtures of the surfactants 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1,2-dipalmitoyl-sn-glycero-3-phosphoglycerol (DPPG) and perfluorooctadecanoic acid (C18F) have been studied in terms of miscibility, elasticity and film structure. The extent of surfactant miscibility and elasticity has been evaluated via Gibbs excess free energies of mixing and isothermal compressibilities. Film structure has been studied by a combination of atomic force microscopy and fluorescence microscopy. Combined thermodynamic and microscopy data indicate that the ternary monolayer films were fully miscible, with the mixed films being more stable than their pure individual components alone, and that film compressibility is minimally improved by the addition of perfluorocarbons to the phospholipids. The importance of these results is discussed in context of these mixtures' potential applications in pulmonary lung surfactant formulations.     

Titanium-rich highly ordered mesoporous silica synthesized by using a mixed surfactant system

A new titanium-rich highly ordered 2-D hexagonal mesoporous titanium silicate has been synthesized using a mixture of cationic (cetyltrimethylammonium bromide, CTAB) and non-ionic (Brij-35, C₁₂H₂₅-(OC₂H₄)₂₃-OH, a polyether and aliphatic hydrocarbon chain surfactant) mixed surfactant system as the supramolecular structure directing agent (SDA) in the presence of tartaric acid (TA) as a mineralizer of Ti(IV). XRD, N₂ adsorption and TEM data suggested the presence of mesophase with hexagonal pore arrangements and the UV-visible, FT IR and XPS studies suggested the incorporation of mostly tetrahedral titanium (IV) species in the highly ordered silica network. This mesoporous titanium silicate material showed excellent catalytic activity and selectivity in the epoxidation of styrene using dilute aqueous H₂O₂ as oxidant.     

Effect of temperature on the rheology of wormlike micelles in a mixed surfactant system

The formation and rheological behavior of a viscoelastic wormlike micellar solution in an aqueous solution of a mixed surfactant system of alkyl ethoxylate sulfate (AES), C(12)H(25)(OCH(2)CH(2))(3)OSO(-)(3)Na(+), and polyoxyethylene dodecyl ether, C(12)EO(3), and the unusual effect of temperature on the rheological behavior have been studied. Upon successive addition of C(12)EO(3) to the dilute micellar solution of AES, viscosity increases swiftly and reaches its peak where a viscoelastic solution with nearly Maxwellian behavior is formed. With the further addition of C(12)EO(3), viscosity decreases sharply, which is attributed to the formation of micellar joints. With increasing temperature, the extent of micellar growth increases and the viscosity maximum is achieved at a lower mixing fraction of C(12)EO(3), but the maximum viscosity attained by the system decreases. The evolution of relaxation time and network density of the viscoelastic network also suggests that with increasing temperature, enhanced micellar growth takes place, but an additional, faster relaxation mechanism becomes increasingly favorable at high concentrations of C(12)EO(3). These results can be explained in terms of the increase in free energy of hemispherical end-caps (end-cap energy) of the micelles with increasing temperature.     

Microfluidic exploration of the phase diagram of a surfactant/water binary system

We investigate the behavior of a binary surfactant solution (AOT/water) as it is progressively concentrated in microfluidic evaporators. We observe in time a succession of phase transitions from a dilute solution up to a dense state, which eventually grows and invades the microchannels. Analyzing these observations, we show that, with a few experiments and a limited amount of material, our microdevices permit a semiquantitative screening of the equilibrium phase diagram as well as a few kinetic observations.     

Investigation of the blue phase in a compensable cholesteric mixed system

Pitch dependence of cholesteric blue phase stability has been investigated in a compensable mixed system of cholesteryl nonanoate/chloride. Two critical pitches above which no blue phase exists were found depending on the structure of the liquid crystal. The clearing enthalpy does not exhibit discontinuities over the whole mixing range.     

Aggregation behavior in mixed system of double-chained anionic surfactant with single-chained nonionic surfactant in aqueous solution

The aggregation behavior of mixed systems of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) or sodium bis(4-phenylbutyl) sulfosuccinate (SBPBS) with nonionic surfactant pentaethylene glycol mono-n-dodecyl ether (C12E5) have been studied by means of steady-state fluorescence, electrical conductivity, dynamic light scattering, transmission electron microscopy, electrophoretic light scattering and pyrene solubilization measurements. The critical concentrations for aggregation, micropolarity, mobility, solubilization capacity and morphology of aggregates are characterized. Two critical concentrations for aggregation are observed in the mixed surfactants, which may correspond to the formation of different kinds of aggregates. Moreover, it is more favorable for AOT-C12E5 to form mixed vesicles compared to SBPBS-C12E5 at higher mole fraction of C12E5. In addition, it is revealed that SBPBS-C12E5 mixture has larger solubilization capacity for pyrene than AOT-C12E5 system.     

Effect of a neutral water-soluble polymer on the lamellar phase of a zwitterionic surfactant system

We have studied the effect of adding a water-soluble polymers (PEG) to the lamellar phases of the ternary system tetradecyldimethylaminoxide (C14DMAO)-hexanol-water. The results of Freeze-Fracture Electron Microscopy (FFEM) and Small Angle X-ray Scattering (SAXS) experiments show that the addition of the polymer induces the spontaneous formation of highly monodisperse multilayered vesicles above a threshold polymer concentration.     

Phase behavior of mixed solution of a glycerin-modified cationic surfactant and an anionic surfactant

The phase behavior of mixed solution of newly synthesized monoglycerylcetyldimethylammonium chloride (MGCA) and sodium octyl sulfate (SOS) in water was investigated by cryo-transmission electron microscopy (cryo-TEM), dynamic light scattering (DLS), differential scanning calorimetry (DSC), and fluorescence polarizing for evaluation of the microviscosity of bilayers. No precipitate was observed in the mixed solution except at concentrations below 20 mM over all mixing ratios, and stable vesicles were formed in a considerably wide range of mixing ratio, even at the equimolar ratio. Vesicles formed in aqueous 1/1 MGCA/SOS mixture were found to exhibit no phase transition, and fluorescence polarizing measurements showed that the vesicle bilayers have a high fluidity. This flexibility allows the bilayers to have a spontaneous curvature, and thus vesicles rather than flat lamellae can be stabilized in the mixture even at the equimolar ratio. In addition, because the glycerin group of MGCA interacts strongly with water, the hydration repulsion contributes to prevent the bilayers consisting of MGCA and SOS from adhering and flocculating even though the charge neutralization between MGCA and SOS occurs at the equimolar ratio.     

Clouding: origin of phase separation in oppositely charged polyelectrolyte/surfactant mixed solutions

The phase behavior in the system of cationic modified poly(vinyl alcohol) (CPVA)-sodium dodecylsulfate (SDS)-water has been investigated. Samples were found phase separated near electroneutral mixing at CPVA concentrations < or =6%, while in a medium CPVA concentration of 7-12%, the phase separation disappeared and the system transformed into bluish homogeneous solution. At > or =13% CPVA concentrations, the mixed systems became colorless homogeneous. Preclouding phenomenon was observed in 5-8% CPVA-SDS mixed systems at an electroneutral mixing ratio. The addition of inorganic salts, such as Na2SO4, NaCl, NaBr, and NaSCN, could exclude the bluish and phase separation phenomenon that was found to be caused by the increase of clouding point in these systems. The clouding phenomenon was proven to be the origin of the phase separation in the CPVA-SDS mixed system. The ability for the inorganic salts to increase the clouding point follows the order of the Hofmeister series.     

Impact of model perfumes on surfactant and mixed surfactant self-assembly

The impact of some model perfumes on surfactant self-assembly has been investigated, using small-angle neutron scattering. A range of different model perfumes, with differing degrees of hydrophilicity/hydrophobicity, have been explored, and in order of increasing hydrophobicity include phenyl ethanol (PE), rose oxide (RO), limonene (LM), linalool (LL), and dihydrogen mercenol (DHM). The effect of their solubilization on the nonionic surfactant micelles of dodecaethylene monododecyl ether (C12EO12) and on the mixed surfactant aggregates of C12EO12 and the cationic dialkyl chain surfactant dihexadecyl dimethyl ammonium bromide (DHDAB) has been quantified. For PE and LL the effect of their solubilization on the micelle, mixed micelle/lamellar and lamellar regimes of the C12EO12/DHDAB mixtures, has also been determined. For the C12EO12 and mixed DHDAB/C12EO12 micelles PE is solubilized predominantly at the hydrophilic/hydrophobic interface, whereas the more hydrophobic perfumes, from RO to DHM, are solubilized predominantly in the hydrophobic core of the micelles. For the C12EO12 micelles, with increasing perfume concentration, the more hydrophobic perfumes (RO to DHM) promote micellar growth. Relatively modest growth is observed for RO and LM, whereas substantial growth is observed for LL and DHM. In contrast, for the addition of PE the C12EO12 micelles remain as relatively small globular micelles, with no significant growth. For the C12EO12/DHDAB mixed micelles, the pattern of behavior with the addition of perfume is broadly similar, except that the micellar growth with increasing perfume concentration for the more hydrophobic perfumes is less pronounced. In the Lbeta (Lv) region of the DHDAB-rich C12EO12/DHDAB phase diagram, the addition of PE results in a less structured (less rigid) lamellar phase, and ultimately a shift toward a structure more consistent with a sponge or bicontinuous phase. In the mixed L1/Lbeta region of the phase diagram PE induces a slight shift in the coexistence from Lbeta toward L1. The addition of LL to the Lbeta (Lv) region of the DHDAB-rich C12EO12/DHDAB phase diagram also results in a reduction in the lamellar structure (less rigid lamellae), and a shift toward a structure more consistent with a sponge or bicontinuous phase, or a coexisting phase of small vesicles. For the mixed L1/Lbeta region of the phase diagram LL induces a shift toward a greater L beta component.     

Infrared imaging of a solid phase surfactant monolayer

A new method for visualizing solid phase surfactant monolayers is presented. This method utilizes infrared (IR) imaging of the surface of a warm subphase covered by the monolayer. When the subphase is deep, natural convection occurs, resulting in a complex surface temperature field that is easily visualized using an IR camera. The presence of a surfactant monolayer changes the hydrodynamic boundary condition at the interface, dramatically altering the surface temperature field, and permitting the differentiation of surfactant-covered and surfactant-free regions. In this work, solid phase monolayers are imaged using this IR method. Fractures in the monolayer are dramatically visualized because of the sudden elimination of surfactant in the region opened up by the crack. The method is demonstrated in a wind/water tunnel, where a stearic acid monolayer is deposited and a crack is created through shear on the surfactant surface, created by suddenly increasing the velocity of the air over the water.     

Brownian dynamics of mixed surfactant micelles

We investigate micelle formation in a system containing two or more different amphiphiles with different geometries using a stochastic molecular-dynamics (MD) simulation method. For a binary system containing two amphiphiles, we calculate the critical micelle concentration (CMC) and cluster distribution for the mixture at several mole fractions and compare the simulation results with those predicted by analytic theories in the dilute limit and with experiments. We find that the CMC obtained from molecular mean-field theory agrees well with our simulation results. Motivated by the industrial use of mixed surfactant systems, we then extend our studies to a system containing six different chain lengths drawn from a Poisson distribution. We find that unlike a binary mixture of amphiphiles, the different species cancel the effects of each other so that the cluster distribution for the mixture has a shape of a system consisted entirely of amphiphiles of length equal to the mean chain length of the Poisson distribution.     

The pseudo-binary pulmonary surfactant system

Since the existence of amphiphilic molecules in the alveolar liquid has been unraveled, research aimed to clarify the process of breathing continuously grows and narrows the gap between in vivo and in vitro. Within the last 5 years progress has been made in understanding the structure–function relationship of pulmonary surfactant questioning classical models. Phase behavior, dynamical and thermodynamical aspects will be the focus of this review.     

Bilayer swelling of nonionic surfactant and sodium dodecylsulfate mixed system by refractive-index matching

Bilayer swelling behavior of nonionic and anionic surfactant mixed aqueous solution induced by adding glycerin was studied. The phenomenon were performed on a system, polyethylene glycol ether of tridecyl alcohol with the average number of ethylene oxide of 5 (CH₃(CH₂)₁₂(OCH₂CH₂)₅OH; abbreviation IT5) and SDS mixed aqueous solution, with white cream of the upper phase and micelles (L₁) of the lower phase. White cream containing densely packed multilamellar vesicles was revealed by freeze-fracture transmission electron microscopy and polarized microscope observations. Phase transition from white cream/L₁, two-phase, to clear unique vesicle phase can be induced by adding glycerin to replace water. The addition of glycerin lowers the turbidity of the dispersion and swells the interlamellar distance between bilayers, which could be explained by refractive-index matching between solvent and bilayers.     

The stability of L3 sponge phase in acidic solutions

In the synthesis of the disordered lyotropic liquid crystalline L3 sponge phase prepared with the cosurfactants cetylpyridinium chloride and hexanol, aqueous NaCl solution is used as the solvent. When this sponge phase is used as the template for L3 silica-phase processing, we replace NaCl with HCl to facilitate the acid catalysis of tetramethoxysilane in forming a templated silica gel, assuming that changing the solvent from NaCl(aq) to HCl(aq) of equivalent ionic strength does not affect the stability range of the L3 phase. In this work, we confirm that changing the pH of the solvent from neutral to acidic (with HCl) has negligible effect on the L3 phase region. Equivalent ionic strength is provided by either NaCl(aq) or HCl(aq) solvent; therefore, a similar phase behavior is observed regardless of which aqueous solvent is used.     

Flow-induced effects in mixed surfactant mesophases

Spatially resolved small-angle neutron scattering, SANS, has been used to investigate the response of the mixed microstructure of the dialkyl chain cationic and nonionic surfactant mixtures of (2,3-diheptadecyl ester ethoxy-n-propyl-1), 1,1,1-trimethyl ammonium chloride/octadecyl monododecyl ether, DHTAC/C18EO10, and DHTAC/dodecyl monododecadecyl ether, Coco20, over the velocity flow pattern of a crossed-slot elongational flow cell. The two different surfactant mixtures have different relative amounts of lamellar and micellar components, and this results in some differences in the flow-induced response. For the DHTAC/C18EO10, which is predominantly in the form of lamellar fragments, a complex pattern of orientational ordering is observed which reflects the competition between or demixing of the two principal flow directions in the cell.     

Pressure-induced hexagonal phase in a ternary microemulsion system composed of a nonionic surfactant, water, and oil

The pressure-induced phase transition in a microemulsion, consisting of pentaethylene glycol mono-n-dodecyl ether, water, and n-octane, was investigated by means of small-angle neutron scattering. A pressure-induced phase transition from a lamellar structure to a hexagonal structure was observed. The temperature-pressure phase boundary shows a positive slope with dTdP approximately 0.09 KMPa. The structure unit of the high-pressure hexagonal phase was an oil-in-water cylinder with the membrane thickness of 15.5 A, identical to the low-temperature hexagonal phase. Pressurizing was found to have the same effect by decreasing temperature. This behavior was satisfactorily explained with the pressure dependence of the spontaneous curvature of surfactant membranes. That is, the volume change of surfactant tails plays a dominant role in the structure change of the microemulsion with applying pressure.     

Markov chain model of mixed surfactant systems

A new model of mixed surfactant systems have been developed in the work presented. It includes two parameters only connected directly with the Gibbs free energy of the surfactant aggregation. They can be determined using both the aggregation equilibrium constant values or the phase composition data. It has been shown also the relation between these new parameters and the same of the regular solution approximation and the alternative models. The possibility to describe the available experimental information about the micelle composition as a function of the singly dispersed surfactant mixture composition better than by the other models has been shown also.