Abstract
The phase-transition temperatures, T*, molar enthalpies, ΔH2*, and molar heat capacities, ΔCp, of pure cholesteryl esters, i. e. myristate (ChM), palmitate (ChP) and oleate (ChO), were measured by using a differential scanning calorimeter. The observed kinds of transition were crystal (k)/smectic phase (s), s/cholesteric phase (c), and c/isotropic phase (i) for ChM, k/c and c/i for ChP, and k/i transition for ChO, respectively. From the values of T*, ΔH2*, and ΔCp, the values of T* and ΔH2* of k/i transition for ChM and ChP were theoretically estimated. The molar transition enthalpies of the k/i transition, which occurs at the co-existence of solute (cholesteryl ester) and its solution with solvent, were also calculated for ChM, ChP and ChO at 298.15 K. By use of the above quantities, the solubility vs. temperature relationship of cholesteryl ester in ethanol, 1-pentanol, acetone, toluene and carbon tetrachloride was analyzed thermodynamically as well as in terms of regular solution theory. The activity coefficient of cholesteryl ester, f2, the product of solution lattice coordination number and solute-solvent interaction energy, zw, and the solubility parameter of cholesteryl ester, δ2, were estimated at 298.15K. Toluene and carbon tetrachloride were almost ideal solvents. The values of δ2 at 298.15 K were 18.0, 17.8, and 14.8 J1/2. cm-3/2 for ChM, ChP, and ChO, respectively.
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