Phase behaviour of lecithinwateralcoholtriacylglycerol mixtures

Abstract

Although phospholipids, e.g. lecithin, are widely used to stabilize water-edible oil mixtures, so far the thermodynamic phase behaviour properties of such lecithin-containing blends have not been investigated systematically. The phase behaviour of a model system containing pure lecithin, i.e. phosphatidylcholine (PC), medium-chain triacylglycerol (MCT), water, and different types of C 3C 5 alcohols is studied in this paper. In the absence of alcohol, PC is dispersed in water-MCT mixtures as liquid crystals. Addition of alcohol decreases the rigidity of the PC bilayer structures and leads to the formation of isotropic phases. Moreover, depending on the type of alcohol, the spontaneous curvature of the surfactant monolayer is changed from zero, in the absence of alcohol, towards either negative values when forming microemulsion water droplets dispersed in an oil continuum, or to positive values by forming microemulsion oil droplets dispersed in an aqueous continuous phase. The results suggest that it is difficult to induce the formation of flexible monolayers having a zero net spontaneous curvature, a prerequisite for the formation of a bicontinuous microemulsion phase. In all likelihood, this is because addition of alcohol changes not only the rigidity but generally also the spontaneous curvature of the PC monolayers. An exception is observed on the addition of tert-butanol which induces the formation of a bicontinuous-type microemulsion middle phase. In this case, the bilayer rigidity seems to be decreased while maintaining a low spontaneous curvature. This is one of the few cases reported up to now which show microemulsion formation as encountered when using mineral oils, containing comparable amounts of oil and water.