Phase Behavior Of Catanionic Surfactant Vesicles Affected By External Conditions

It is well known that surfactants can self-assemble into various aggregates such as spherical micelles, rod-like micelles, worm-like micelles, vesicles, lamellar liquid crystallines and so on, when the concentration is above the critical micelle concentration (cmc). The wide-ranging applications of surfactant aggregates as drug delivery, selective biomembranes and microreactors depend on the structure and properties of aggregates. The synergistic effect of mixed surfactant systems makes them with fascinating properties, such as low critic aggregates concentration, low surface tension, colorful morphologies of colloid particles. Besides the properties of surfactants, total concentration, some external conditions such as ionic strength, temperature, solvent and shear force also can affect the phase behavior of surfactant aggregates.In this thesis we investigate the phase transition of surfactant aggregates influenced by external conditions in order to control the morphologies of colloid particles. The outline and contents of this doctoral thesis are as follows:ChapterⅠis a brief introduction of basic knowledge of colloid and interface science and the research background and recent improvement of cationic and anionic surfactant mixed system. The object and scientific significance of this thesis are also discussed at the end of this chapter.In chapterⅡ-Ⅳthe phase behavior of cat-anionic surfactant system influenced by ionic strength, temperature and solvent was investigated in more detail. In chapterⅡthe phase transition of salt-free catanionic (mixtures of cationic and anionic surfactants) tetradecyltrimethylammonium laurate (TTAL) system affected by salt (NaBr) was described. With increasing concentration of NaBr, the salt-free catanionic birefringent Lα-phase formed by cationic tetradecyltrimethylammonium hydroxide (TTAOH) and lauric acid at equlimolar mixtures was transferred into a two-phase precipitate/L1-phase and finally a birefringent La-phase again at much higher salt concentration. The interlamellar distance of the precipitated vesicles is much smaller than that of salt-free or high-salinity catanionic vesicles. It is therefore supposed that the phase transition is aroused from the reduction of the repulsive between the bilayers by the excess salts and the interlamellar forces between the bilayers become attractive.In ChapterⅢ, the phase behavior of precipitate/L1 two-phase system with different amount of NaBr was investigated at different temperature. These densely packed multilamellar vesicles of two-phase system can be transformed into three-dimemtional network lamellar at high temperature. This phase transition is progressive process and happens at the chain melting temperature of surfactants. We also found that the phase transition temperature (Tm) was influenced by adding different amounts of salt but not by being diluted. This is the first time to observe the phase conversion from catianionic surfactant vesicles to bilayer networks triggered by chain melting, which 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 that favor a change in membrane curvature. The energy of the fluctuation attained by heating surpasses the loss of the edge energy (the hydrocarbon tails of the surfactant bilayers are exposed to water). And then the multilamellar vesicles open up around the chain melting temperature and restructure to three dimensional networks.Besides ionic strength and temperature, the polarity of the solvent also plays an important role in the morphology of surfactant aggregates. In chapter IV we found that the swelling of lamellar phase can be induced by the replacement of solvent in tetradecyltrimethylammonium bromide (TTABr) and sodium laurate (SL) aqueous solution which contains cream floating precipitates on the upper phase and L1-phase (micelles) at the lower phase. Phase transition, from cream floating precipitates to swelling birefringent vesicle phase, La/L1 two phase, and finally to micelle phase, can be induced by adding glycerin as solvent in aqueous solution. At first, densely packed multilamellar vesicles of cream floating precipitates on the upper phase swelled to the whole phase with increasing the 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 more glycerin, the swelling La phase transfered La/L1 two-phase, and finally L1 phase (micelles). This phase transition can also be explained because of increasing cmc of 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.Because of the high electrostatic interaction of cat-anionic surfactant system, shear force almost can not influence the morphology of cat-anionic surfactant aggregates. So in chapter V we chose viscoelastic multilamellar vesicle phase formed by certain amount of nonionic surfactant, polyethylene glycol ether of tridecyl alcohol with the average number of ethylene oxide of 5 (CH3(CH2)(12)(OCH2CH2)5OH, abbreviated Trideceth-5 or IT5) and anionic surfactant SDS (sodium dodecyl sulfate) in aqueous solution and investigated in different shear field. The bilayers of multilamellar vesicles will be stripped off and become densely packed unilamellar vesicles by shear which increases the viscoelasticity of the system. However, through homogenizer the new-formed unilamellar vesicles become so small that they have relative larger distance between each other. The vesicles will not be crowded any more and can easily pass each other under shear. So the unilamellar vesicles through homogenizer only have very low viscoelasticity and flow birefringence. It takes very long time for the unilamellar vesicles to come back to the original state. So it is a good method to control the size of the vesicles.In the last chapter we studied the influence of surface charges and shear force simultaneously to the nonionic surfactant system. A sponge phase (L3 phase) was observed in the mixed system of nonionic surfactants, polyethylene glycol ether of tridecyl alcohol with the average number of ethylene oxide of 3 (CH3(CH2)(12)(OCH2CH2)3OH, abbreviated Trideceth-3) and tetradecyldimethylamino oxide , abbreviated C(14)DMAO), in aqueous solution. The L3 phase can be transferred to lamellar phase (L(αl) phase) after the bilayer was protonated by the formic acid formed through the hydrolysis of methylformate. The L(αl) phase can be transformed into multilamellar vesicles (L(αv) phase) under shear. The properties of L3 phase were investigated by conductivity and rheology measurements. The phase transition from L3 to vesicles was characterized by rheology measurements,2H NMR spectra, polarized microscope, scanning electron microscope (SEM), and transmission electron microscope (TEM) observations.We hope that the report of the controlled phase transition through protonation and shearing forces in nonionic surfactant mixtures may direct primarily us how to achieve the phase transition in surfactant solution by changing the conditions and secondarily to acquire the applications such as the controlled materials from the phase transition as templates.