Phase Behavior, Self-assembled Structures And Rheological Properties Of Salt-free Catanionic Surfactants Systems Containing Bile Acids

In colloid and interface science, the study on the self-assembly, structural transition and properties of surfactants in aqueous solutions is one of the important researches. Surfactants in solutions can form various aggregates, such as spherical micelles, wormlike micelles, vesicles, lamellar liquid crystalline, etc. In most cases, the mixtures of surfactants usually have better practical application than the single surfactant solutions. The salt-free cationic and anionic (catanionic) surfactants mixture systems have unique characteristics as no excess salt exists in these solutions, such as high osmotic pressure and unscreened electrostatic interactions, and the systems do not form precipitates even at the equimolar ratio of the cationic and anionic surfactants. For these reasons, such systems are more stable and exhibit a much richer phase behaviors and aggregate structures. Thus, such systems are valuable for theoretical study and can be taken as a model system. Bile acids and their salts are natural biosurfactants and display vital biological importance in many physiological processes such as the digestion of lipids. They have a rigid steroidal skeleton with several chiral carbon centers, and exhibit facial amphiphilicity and biocompatibility. Thus, they can form some unique aggregates in solutions, and the microstructures and structural transition of the aggregates are different from conventional surfactants. The research on the catanionic surfactant systems containing bile acids can provide fundamental knowledge for the nano materials, biotechnology and pharmaceutical technology.In this thesis we investigated the interactions between bile acids and catanionic surfactant systems and the aggregates structural transition. Various techniques, such as cryogenic transmission electron microscopy, freeze fracture transmission electron microscopy, dynamic light scattering, small angle X-ray scattering, etc. are employed to investigate the formation of aggregates, microstructures and structural transition in detail by modulating the molecular structures, molar fractions, concentrations, temperature, salty and shearing, and we give the theoretical explanation. In addition, the rheological properties are studied detailed, too. The specific contents of this doctoral thesis are shown in the follow:Chapter I is a brief introduction of the basic knowledge, the research background and recent improvement related to the research of the thesis. The object and scientific significance of this thesis are also discussed at the end of this chapter.In chapterⅡ, we studied the solubilization of bile acids in the vesicles formed by tetradecyltrimethyl ammonium (TTAOH) and lauric acid (LA). The influence of the solubilization on the microstructures and rheological properties of the vesicles are also investigated. The vesicles have hydrophilic and hydrophobic areas in the structures. The bile acids can solubilized into the hydrophobic bilayers and the palisade layers of the vesicles, due to the insolubility of bile acids in water. The ability of solubilization depends on the concentration of vesicles. Because of the rigidity and facial amphiphilicity of bile acids, the solubilization can decrease the curvature of the bilayer membrane, resulting in the transition from multilamellar vesicles to unilamellar, multi-room vesicles and tubular vesicles. The transition caused the decrease of rigidity of the vesicle membranes, and the viscosity and the viscoelastic properties are also decreased. When the concentration of bile acids was further increased, the multi-room vesicles and tubular vesicles are transited to smaller uni-and bi-lamellar vesicles. Then, the rigidity of the vesicle membranes is increased and the viscosity and the viscoelastic properties are increased again.In chaperⅢ, by modulating the molar fraction of the bile acid in the mixed acids, we investigated the structural transition of aggregates in the salt-free catanionic surfactant systems containing bile acids at equimolar ratio, and the salt-free, zero-charged vesicles, planar lamellar phase and wormlike micelles are prepared. With the increase of the molar fraction of bile acids, the transition from vesicles to planar lamellar phase and then to micelles occurred, and the viscosity and the viscoelastic properties are decreased. In the bilayer membranes, the packing parameter of TTAOH-LA is between 0.5 and 1, while the packing parameter of TTAOH-bile acids is larger than 1. With the increase of the molar fraction of the bile acid, the fraction of the TTAOH-bile acid ionic pairs is also increased, which induced the increase of the average packing parameter of the system. When the molar fraction of the bile acid achieved a certain value, the average packing parameter is around 1, which resulted in the gradual transition from vesicles to planar lamellar phase. With the further increase of the molar fraction of bile acids, the orderly alignment of the bilayer membranes is destroyed, and micelles are formed in the solution. Because the hydrophobicity of deoxycholic acid (DeCA) is larger than that of cholic acid (CA), in the bilayer membranes, the interactions between DeCA and the alkyl chains of TTAOH and LA are stronger than that in the CA systems, and the variation of the curvature of the membranes is smaller, which caused the transition from La phase to the L1 phase occur at a higher molar fraction of DeCA. In the wormlike micelles region, with the decrease of the molar fraction of bile acids, the zero-shear viscosity of the solutions increases first and then decreases;in the La phase, the solutions exhibit shear thinning behaviors, and shear thickening behavior is observed at intermediate shear rate range at some molar fractions of bile acids. In the longer chain cationic surfactant CTAOH systems, due to a little larger of the packing parameter of CTAOH than that of TTAOH, the hydrophobicity of CTAOH is larger and the viscosity and the viscoelastic properties of the aggregates are also larger than that in the TTAOH systems at the same conditions. The results above can provide important directions for the practical application of the salt-free catanionic surfactant systems containing bile acids.Shear flow has been well-known to have a strong influence on the aggregates in the solutions. In chapter IV, we investigated the aggregates structural transition induced by shear and the structure recovery after shear. In the salt-free catanionic surfactant systems containing DeCA, because of the larger volume, rigid steroidal skeleton and facial amphiphilicity, with the variation of the molar fraction of DeCA, the influence of shear on the solution was different. In the TTAOH/(DeCA+LA) mixture systems at equimolar ratio, at xDeCA=0.3, the shear can induce the transition from planar lamellar structures to vesicles, but the transition was not complete in the experimental period. After shearing, the solution can recover to the original state spontaneously when it was put in the 25℃environment without disturbance. At xDeCA=0.25, planar lamellar structures and vesicles coexisted in the solution. The shear can induce the transition from planar lamellar strctures to vesicles. Shear thickening behavior was observed, and the viscosity and viscoelastic properties were increased. But the sheared sample can not recover to the original state spontaneously after shear, indicating a permanent transition occurred. At xDeCA=0.2, vesicles are the dominant aggregates in the solution, and the shear can cause the transition from multilamellar vesicles and elongated vesicles to uni- and bi-lamellar vesicles, which can result in the increase of the number density of the vesicles and the viscoelastic properties. With the decrease of the molar fraction of DeCA, the packing parameter of the aggregates decreased, and the curvature of the membrane increased; thus, it was prone to form vesicles with a higher curvature.In chapter V, we investigated the helical twisted ribbons formed in the lithocholic acid (LCA) and CTAOH at equimolar ratio, and the structural transition to vesicles and gel. When the temperature was above 25℃, uni- and multi-lamellar vesicles are formed in the solution, with a large polydispersity. When the sample was put at room temperature (about 20℃) for several days, straight helical twisted ribbons with a large aspect ratio were observed. The diameter was about 3-4μm, and the length could be longer than 1 mm and can be seen with naked eyes. After a further longer time, the helical ribbons grew longer and intertwined to each other to form a network. The samples were opaque, and behaved as gels, which had strong gelation ability. By modulating the temperature and the aging time, the transition from vesicles to helical ribbons was achieved, and gel was formed in the end. The formation of helical structures was the cooperation of hydrophobic interactions between the hydrophobic moieties, and the hydrogen bonding and the electrostatic interactions between the polar moieties of the molecules, together with the chirality of LCA. The transition from vesicles to helical ribbons may have potential applications in the biological and materials fieldsIn the last chapter, the tubular aggregates formed in the LCA and TTAOH mixture solutions at equimolar ratio were investigated. After the complete dissolution of LCA in TTAOH solution, unilamellar vesicles are formed. After several hours at 25℃, the transition from vesicles to tubes was occurred, and gel was formed at a high concentration. Compared to the CTAOH/LCA system, due to the two methylene difference of TTAOH and CTAOH in the molecular structures, the structural transition can be completed within several hours, indicating a lower stability of the vesicles in the system; while in the CTAOH/LCA system, the structural transition was a much low process, which needed a higher concentration and a longer time. The results indicated the important influence of the molecular structures on the formation and stability of the aggregates. When equimolar NaBr was added into the solution, giant spherical vesicles were formed at higher temperature;in the process of cooling to the room temperature spontaneously, crystals appeared inside of the vesicles and spread to the whole vesicles quickly. The results of rheological properties measurements indicated that the strength of the gel formed at high concentration was weak.