A new model for predicting solubilities and enthalpies of solution for a given solute in a series of solvents

The unified solvation model Delta chi=Ps'+E-A*E-B+C-A*C-B+W, used to predict solvent dependence of dissolved solutes, is extended to enthalpies of solution and solubilities, by predicting the energetic component for forming a cavity in the solvent. The energy to create the cavity is given without adding a new solvent parameter using the term? QS'(2), where the S'(2) values are the square of the previously reported USM solvent polarity parameters, S'. Q is the solute-dependent cavity formation term. The resultant unified solvation cavity model (USCM) is applied successfully to 350 enthalpies and free energies (solubilities) of solution in effect unifying the separate areas of solvation polarity probes and regular solution theory. The resulting empirical parameters for all 35 solutes studied are consistent with the intuitive concepts that cavity formation, QS'(2) and separation of solid or liquid solute molecules upon dissolution, W, are endothermic processes, while nonspecific solvation of the solute, PS', is exothermic, This consistency, combined with the spectroscopic basis for S', supports the USCM interpretation as opposed to a meaningless multiparameter fit of a data set. Early approaches to predicting solubilities and correlating measurements to single parameter solvent polarity scales fail when solute-solvent specific interactions exist. As in earlier USM studies, the magnitude of specific donor-acceptor contributions is predicted with the electrostatic-covalent model, fur heats of solution and solubilities. Thus, in addition to an empirical correlation, USCM provides a quantitative separation of specific and nonspecific effects. Furthermore, the work provides a model that can be used to predict solubilities with confidence. Finally, the correlation produces solute parameters that provide new insights concerning solubility. For example, the magnitudes of Q for the cavity term of different solutes lends to the novel suggestion that solute polarity is more important than solute size in determining the energy of cavity formation.