Catanionic Surfactant Mixtures and Binary Systems of Modified Fatty Acid Surfactants. Phase Equilibria and Structure

University dissertation from Malin Bydén Sjöbom, Dept. of Natural and Environmental Sciences, Mid Sweden University, SE-851 70 Sundsvall, Sweden

Abstract: Surfactant self-assembly is an important phenomenon that occurs in a number of processes in our everyday life. It is important to understand the forces and parameters that control and influence the self-assembly, as this knowledge can be used both in order to design surfactant systems with desired properties and for creating model systems, for example for biological processes. The papers of this thesis all concern the phase equilibria of ionic surfactant systems, which have been investigated by means of 2H NMR, optical methods and SAXS. The aqueous ternary phase diagrams of 1-dodecylpyridinium bromide, with dodecane and dodecanol as the respective third component, were studied. The sodium 2-methyldecanoate was proved to have a very low Krafft-temperature in the solution phase, about 1 °C. Both the pure (R)- and the racemic surfactant formed the same phase sequence with water as have been seen for sodium decanoate, although at lower temperatures. They also form a very narrow intermediate phase, with a so far unknown structure. The phase equilibria of the binary aqueous systems of a-methylated sodium octanoate, -nonanoate and -dodecanoate respectively, were studied. The two former show a phase sequence very similar to sodium octanoate. The latter has a phase sequence different from its unsubstituted analog, as we have not been able to isolate any hexagonal structure, and only one intermediate phase, most likely a rhombhohedrical structure. In the pseudo-ternary DoTAC-sodium carboxylate-water systems, the level of asymmetry was proven to play an important role for the extension of both micellar solutions and liquid crystalline phases. The combination DoTAC-sodium octanoate seemed to be the most suitable for the formation of a mixed hexagonal structure, whereas sodium decanoate gave the largest lamellar phase. All three systems form mixed micelles at carboxylate/DoTAC molar ratios <1, and coexisting aggregates or aggregates and monomers at higher ratios. Within the DoTAC-decanoate solution phase, there is a narrow coacervation area around equimolarity. The complete pseudo-ternary phase diagram of the DDAB-sodium taurodeoxycholate-water system was presented. The DDAB-rich side was studied by means of 2H NMR and SAXS, whereas NMR self-diffusion measurements were performed in the large solution phase. The latter results indicate that the solution phase is consistent of aggregates with positive curvature throughout its whole range of existence.

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