Nonlinear enthalpy-entropy compensation for the solubility of phenacetin in dioxane-water solvent mixtures.

The solubility of phenacetin was determined at five temperatures in solvent mixtures of aprotic-amphiprotic mixtures of dioxane and water. Enthalpy-entropy compensation analysis is used to study the effect of changing polarity of the medium on the solute. The apparent heats of solution and free energies of solution are nonlinear functions of the cosolvent (dioxane) ratio. The free energy curve goes through a minimum at 80-90% dioxane in water, whereas the apparent heat of solution displays a maximum at low cosolvent ratio (40% dioxane) and a minimum at high cosolvent ratio (90% dioxane). A plot of delta H against delta G shows a nonlinear compensation effect. Two different mechanisms (entropy and enthalpy) are suggested to be the driving forces to increase solubility. These two mechanisms can be related to the nonlinearity of the compensation effect. The slope changes from positive to negative at 40% dioxane. The overall nonlinear function can also be considered as two linear relationships that intersect at 40% dioxane. The results support the usefulness of enthalpy-entropy compensation analysis for a better understanding of the solubility of drugs in aqueous mixtures as related to the role of cosolvents.