Abstract
The phase transitions among various phases, i.e. crystalline (k), isotropic liquid (i), cholesteric (c) and smectic (s) phases, of cholesteryl oleate (ChO) were studied by using differential scanning calorimetry (DSC) and polarizing microscopy. We used two commercial samples of ChO and a gift with a known history of synthesis and purification. The transition temperatures, Ttr, and enthalpies, ΔtrH, of these samples were all in good agreement with each other. The DSC thermograms of k/i transition showed the coexistence of two kinds of crystals with a higher melting temperature (k1) and a lower one (k2) in k. By analysis of the thermograms, we determined the enthalpic and molar fractions of k1 in k, f1 and x1, respectively, which depend on the method and conditions of crystallization. Three types of crystallizing procedures, i.e. solvent evaporation, cooling of ChO solution in ethanol, acetone or 1-pentanol, and in situ crystallization in the DSC pan, were examined at various temperatures. At higher temperature, a higher value of f1 or x1 was obtained. Using the values of Ttr and ΔtrH at various transitions, we could draw the curves of relative chemical potential, μrel, of the k1, k2, i, c and s phases of ChO vs. temperature, The stability of various ChO phases and the thermodynamic nature of the transition paths are discussed on the basis of the above μrel diagram.
