Salt Effect On The Aggregation And Adsorption Of Single Surfactant And Catanionic Surfactant Mixed Systems

The salt effect on surfactant systems has always been one of the main focuses in the surfactant physical chemistry study. Electrolytes can not only influence the aggregation behaviors, rheological and tribological properties of surfactant and surfactant mixed systems, but also affect their adsorption at air-liquid and solid-liquid interfaces. It is of great value both for theoretical investigations and practical application to study the salt effect on surfactant systems. In this thesis, we studied the salt effect on the aggregation and adsorption behaviors of surfactants and surfactant mixtures in detail from the following points:1. In the cationic and anionic (catanionic) surfactant mixed system, tetradecyltrimethylammonium hydroxide (TTAOH)/decanoic acid (DA)/H2O, abundant phase behaviors were obtained in the presence of hydrophilic and hydrophobic salts. The microstructures of typical La phases with the different compositions were characterized by the Transmission Electron Microscope (TEM) images. Aqueous double-phase transition induced by addition of hydrophilic salts was observed when the cationic surfactant was in excess. Salt-induced reversible vesicle phases could be obtained when the anionic surfactant was excess, whereas the vesicle phase at lower salinity behaves highly viscoelastic but is much less viscoelastic with high salinity which was demonstrated by measuring their rheological properties. The La phase with the positive membrane charges can be finally transferred into an L1phase with added salts. The ion specificity of hydrophilic and hydrophobic salts is discussed and the order of cations is summarized, which is significant for the further study of the Hofmeister effects on catanionic surfactant mixed systems.2. For the first time we present that the high salinity can dramatically eliminate the minima in the surface tension-concentration curves. The minima vanishment occured at enough high ionic strength, not only for salt-free catanionic surfactant tetradecyltrimethyl-ammonium hydroxide/decanoic acid (TTAOH/DA) system, but also for the single surfactant system sodium dodecyl sulfate (SDS), sodium dodecyl benzene sulphonate (SDBS). Moreover, it occurs regardless of the types of salts. Through the present investigations and analysis, we deduced that the main reason why the minima exist is the electrostatic interactions between impurities and surfactants at the air-liquid interface layers, which can be screened by high ionic strength, resulting in the minima vanishment. We propose that our findings are significant for the further study of salt-containing surfactant mixtures.3. In the study of Chapter2, we found that the two Lα phases with different compositions and surface charge densities show completely different phase transitions when electrolytes were added. Based on this finding, we performed systematical investigations on the relationship between the surface charge densities and the rheological properties of the salt-free catanionic surfactant TTAOH/DA vesicle systems. We found out that the storage modulus and the yield values of the vesicle phases both first increased and then decreased with the increasing surface charge density.