Solubilisation of different medium chain esters in zwitterionic surfactant solutions--effects on phase behaviour and...

We studied the effect of solubilisation of methyl esters with different chains of medium length into the binary surfactant system tetradecyldimethylamine oxide/water at constant surfactant concentration of 200 mM. As esters we employed valeric, capronic, enanthic, and pelargonic methyl ester, thereby decreasing the polarity. Always a phase sequence L(1)-L(α)-L(1) is observed with increasing ester concentration, where the L(α)-phase increases in extent and goes to much lower temperatures with increasing chain length of the ester. Viscosity measurements show a maximum at intermediate concentrations of additive that is independent of the type of ester. From SANS measurements detailed information about the structural changes occurring during the rod-to-sphere transition in the system of the shortest additive is deduced, which proceeds first through a pronounced rod growth. Interestingly, for the different esters an almost constant value of the volumic solubilisation capacity is observed, in agreement with the relatively constant interfacial tension. For the different esters no effect on the radius and the area requirement at the amphiphilic interface is observed at the solubilisation boundary. The microemulsions present here are spherical aggregates where the ester is partitioned between core and shell. From the SANS and interfacial tension data the effective bending constants of the surfactant monolayers were deduced and they show that the extension of the L(α)-phase is directly related to a corresponding increase in the bending constants of the surfactant/ester monolayers.