Phase Transitions in Binary Mixtures of Cholesteryl Esters

Abstract

Phase transitions in mixtures of mesomorphic (liquid crystalline) cholesteryl esters were studied by visual thermal microscopy. Binary mixtures of saturated cholesteryl esters (cholesteryl nonanoate, myristate, and palmitate) and binary mixtures of these saturated cholesteryl esters and an aromatic cholesteryl ester (cholesteryl benzoate) show transitions from the solid phase to the mesomorphic phase at temperatures that are depressed well below the phase transition temperature of either pure compound. However, the mesomorphic to isotropic phase transition temperatures are virtually linear functions of the composition of the mixtures. The result is that the temperature range over which the binary mixtures are mesomorphic is greatly expanded when compared to the narrow range observed for the individual components. The solid to mesomorphic phase transitions of these mixtures occur above body temperature. However, binary mixtures of the saturated cholesteryl esters with unsaturated cholesteryl esters (cholesteryl oleate and linoleate) are mesomorphic at body temperature except at low concentrations of the unsaturated ester. The unsaturated cholesteryl esters along with small amounts of other cholesteryl esters are major components of lipid droplets found in atheromatous lesions. The mesomorphic behavior of mixtures of the cholesteryl esters may be related to their interassociations in these lesions.