Solubilisation of different medium chain esters in zwitterionic surfactant solutions – Effects on phase behaviour and structure

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 L1–Lα–L1 is observed with increasing ester concentration, where the Lα-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 Lα-phase is directly related to a corresponding increase in the bending constants of the surfactant/ester monolayers.

Phase and SANS study of surfactant/ester/water systems with differently long esters. Systematic correlations exist between structural and phase behaviour of the systems and the bending rigidity of their amphiphilic film.Figure optionsDownload high-quality image (52 K)Download as PowerPoint slideHighlights
► Phase in ternary surfactant systems shift systematically with length of ester.
► Quantitative analysis of the rod-to-lamellae-to-sphere transition.
► Bending constant 2κ+κ¯ increases with chain length of the ester.
► Stability of the lamellar phase directly related to 2κ+κ¯.