The thermal transitions and structural properties of 5α-cholestan-3β-ol esters of aliphatic acids

Abstract

5α-Cholestan-3β-ol esters of aliphatic acids undergo both enantiotropic and monotropic changes of state. Ten saturated and three unsaturated esters have been examined by differential scanning calorimetry and polarizing microscopy to determine transition temperatures, enthalpies, and entropies. The results are compared with an analogous series of cholesterol esters. All esters of evennumbered n-alkanoic acids from C 22 to C 20 melt from a crystalline state to an isotropic liquid. The crystalline state has been studied by X-ray powder diffraction. The C 8 to C 20 esters have progressively increasing crystalline melting transition temperatures from 76 to 99°C and possess similar X-ray powder diffraction patterns, suggesting that these compounds form an isostructural series. Esters of C 2, C 4, and C 6 acids exhibit polymorphism. Crystalline cholestanol oleate melts to an isotropic liquid, whereas cholestanol linoleate and linolenate fail to crystallize, even after several months at −20°C. Esters of the even-numbered saturated acids from C 4 to C 14 form monotropic cholesteric liquid crystalline phases. Esters C 10, C 12, and C 14 form smectic liquid crystalline phases. Cholestanol oleate, linoleate, and linolenate form both cholesteric and smectic mesophases. The lower smectic to cholesteric and cholesteric to isotropic transition temperatures of the cholestanol esters compared to the corresponding transition temperatures of the analogous cholesterol esters suggest that the Δ 5 double bond in cholesterol increases the thermal stability of the mesophases of cholesterol esters.