Polyoxyethylene cholesteryl ether-based aqueous wormlike micelles

Abstract

Formation of wormlike micelles (WLMs) in an aqueous mixture of polyoxyethylene cholesteryl ether (ChEO n; where n = 20 and 30) and polyoxyethylene dodecyl ether (C 12EO m; where m = 3 and 4) has been reported; rheological and small angle X-ray scattering (SAXS) measurements have been performed in the micellar solutions of ChEO n as a function of C 12EO m for the structural elucidation. When lipophilic cosurfactant, C 12EO m is added to the micellar solutions of ChEO n, it favors the sphere-to-cylinder transition due to the penetration of C 12EO m in the palisade layer of ChEO n micelle accompanying an increase in viscosity. When the concentration of C 12EO m is increased, entangled network of WLMs is formed. A strong shear thinning has been observed in highly viscous samples indicating the presence of transient networks. Such samples exhibited viscoelastic behavior and could be described by the Maxwell model with a single stress relaxation mode. A maximum is observed in zero-shear viscosity-C 12EO m plot. With further addition of C 12EO m, viscosity declines and ultimately a phase separation occurs with the formation of turbid solution of vesicular dispersion. This decline has been interpreted in terms of micellar branching induced by an increase in endcap energy, E c (which is compensated by the formation of branch points, having a mean curvature opposite to that of endcaps). The C 12EO m induced one-dimensional micellar growth has been confirmed by SAXS. © 2011 Springer-Verlag.