Differential Molar Heat Capacities to Test Ideal Solubility Estimations

Purpose. Calculation of the ideal solubility of a crystalline solute in a liquid solvent requires knowledge of the difference in the molar heat capacity at constant pressure of the solid and the supercooled liquid forms of the solute, ΔCp. Since this parameter is not usually known, two assumptions have been used to simplify the expression. The first is that ΔCp can be considered equal to zero; the alternate assumption is that the molar entropy of fusion, ΔSf, is an estimate of ΔCp. Reports claiming the superiority of one assumption over the other, on the basis of calculations done using experimentally determined parameters, have appeared in the literature. The validity of the assumptions in predicting the ideal solubility of five structurally unrelated compounds of pharmaceutical interest, with melting points in the range 420 to 470K, was evaluated in this study.