Co-solubility of saturated cholesteryl esters: a comparison of calculated and experimental binary phase diagrams

Based on ideal solution theory, phase diagrams are calculated for binary compositions of cholesteryl esters and compared to experimental data from pairwise combinations in a saturated acyl chain series from caprylate to arachidate, which encompasses three crystal packing motifs in the solid state. Within a crystal structure class, nearly ideal co-solubility is found for binary solids, where the acyl chain lengths of the pure components differ by one methylene group. Beyond this chain length difference, nonideal solutions occur until fractionation occurs at e.g., six methylene unit increments between the components. The observed liquidus lines of the eutectic are near the theoretical curves when the combinations of two compounds packing in the same crystal structure fractionate. Fractionation also is found when liquids composed of two esters which favor different crystal structures are solidified from the melt, no matter what the chain length difference is; the liquidus curves for re-heated solids, however, are not necessarily predicted by the Schröder equation. In general, co-miscibility can be found in mesophases formed from compounds with two different crystal structures.