Aqueous phase behaviour of choline carboxylate surfactants—exceptional variety and extent of cubic phases

Abstract

Choline carboxylate surfactants are powerful alternatives to the well-known classical alkali soaps, since they exhibit substantially increased water solubility while maintaining biocompatibility, in contrast to simple quaternary ammonium ions. In the present study, we report the aqueous binary phase diagrams and a detailed investigation of the lyotropic liquid crystalline phases formed by choline carboxylate surfactants (ChCm) with chain lengths ranging from m = 12–18 and at surfactant concentrations of up to 95–98 wt%. The identification of the lyotropic mesophases and their sequence was achieved by the penetration scan technique. Structural details are elucidated by small-angle X-ray scattering (SAXS). The general sequence of mesophases with increasing soap concentration was found to be as follows: micellar (L₁), discontinuous cubic (I₁), hexagonal (H₁), bicontinuous cubic (V₁) and lamellar (L_α). The main difference to the phase behavior of alkali soaps or of other mono-anionic surfactants is the appearance and large extent of a discontinuous cubic phase with two or even more different symmetries. The obtained phase diagrams further highlight the extraordinarily high water solubility of ChCm soaps. Finally, structural parameters of ChCm salts such as the cross-sectional area at the polar–nonpolar interface are compared to those of alkali soaps and discussed in the terms of specific counterion binding and packing constraints.