A cholesterol–isopropanol gel

Abstract

IN the course of measuring solubilities of cholesterol in pure and mixed solvents we have observed the formation of a transparent gel in the cholesterol+isopropanol system. Because of the historical importance of liquid crystals of cholesteryl esters¹ and the biological importance of cholesterol, we are reporting here some observations on the properties of this gel. Liquid crystals of cholesterol with fatty alkanols (C₁₂–C₁₈) are known^2,3, but have not been reported for smaller alcohols. The transparent gel of cholesterol and isopropanol seems to be stable at 4 °C for concentrations of 4.6–5.3% (w/w) cholesterol. At higher concentrations and temperatures (up to 10.2% at 22 °C), the transparent gel can be observed for a day or more, then white ‘snowflakes’ appear throughout the gel and grow slowly into a translucent or opaque mass. On warming, the transparent gels liquify, and the translucent gels seem to go directly to the liquid state. The transparent gel is unstable to vigorous shaking; samples with less than 5.5% cholesterol liquify then return to the transparent gel state on standing at 4 °C. Samples with larger concentrations form the translucent or opaque state on shaking and do not return to the transparent state over a period of several weeks. Observations of the transparent gel under a polarising microscope at room temperature showed the sample to be mostly isotropic with some small points of optical activity.