Phase Studies on Binary Systems of Cholesteryl Esters A. Two Aliphatic Ester Pairs

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Abstract
Binary blends for two pairs of esters of cholesterol, the myristate plus acetate and the myristate plus the nonanoate have been characterized to determine the equilibrium phase conditions for the isotropic liquid, the cholesteric and smectic mesophases, and the stable crystals. The phase diagrams were determined using a Differential Scanning Calorimeter (DSC-1B), which also provided the heats of transition. For the more complicated acetate system, morphology was also studied with a polarizing microscope puls hot stage and structure determinations were made using X-ray diffraction. Results indicate that both ester pairs form a single eutectic mixture. The crystal melting points of the pure components in both pairs follow the van't Hoff equation for melting point depression due to impurity over most of the composition range. A homogeneous cholesteric mesophase was formed in both pairs. The temperatures and heats of the smectic-cholesteric transition for the acetate blend decrease as the concentration of the myristate ester is decreased. Indeed the smectic mesophase was not observed below 78% myristate. Monotropie behavior seriously limits the study of high acetate content blends. In the nonanoate systems the cholesteric and smectic mesophases exist over the full composition range. This ester pair forms virtually ideal comesophases. Transition temperatures and heats agree well with the theoretical results based on a model which involves ester separation only at the lowest (crystalline solid) transition. The results suggest methods of predicting phase diagrams for other binary mesophase systems.

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