Solvent-induced sponge (L ₃) phases in the solvent-monoolein-water system

Abstract

The phase behavior of the monoolein-water system in the presence of polar solvents such as dimethyl sulfoxide, propylene glycol, polyethylene glycol (M _w ≈ 400) and ethanol was investigated. These solvents share the common feature of being completely miscible with both monoolein and water. At water contents in the range 30–60% w/w, an isotropic liquid phase is found in all the four systems. The liquid phase shows many characteristic features of a sponge (L ₃) phase, i.e. a long but narrow phase region, surrounded by two-phase regions which include a lamellar phase on the water-poor side and another liquid phase on the water-rich side. At the monoolein-rich end of the narrow phase region, a cubic liquid crystalline phase is in equilibrium with the isotropic liquid. At high solvent content, the sponge phase shows shear birefringence and scatters light in the case of ethanol. The formation of the sponge phase in the present systems is interpreted as a consequence of a subtle partitioning of the solvent between the monoolein and aqueous domains in the system, which in turn leads to the desired, slightly negative, interfacial mean curvature of the sponge phase.

This paper is dedicated to Professor Kåre Larsson at the time of his retirement from academic duties