ABSTRACT
This chapter begins with a description of ideal solution theory because ideal behavior is the reference point by which all other solution behavior is judged. Regular solution theory best describes solution behavior of nonelectrolytes in nonpolar solvents where the enthalpy of mixing is not negligible. J. H. Hildebrand designated nonideal solutions as regular solutions if there was sufficient thermal agitation to overcome the segregating effect of unequal cohesive densities between the solute and the solvent. Regular solution theory is largely limited to describing the solution behavior of nonelectrolytes in nonpolar solvents. The solubility of the drug is determined in multiple solvents that possess different functional groups, both nonpolar and polar, to allow each of the three-dimensional solubility parameters to be estimated. The theoretical equations in the Hildebrand and C. M. Hansen approaches can be effectively applied to predicting the solubility of a new chemical by employing the experimental solubility data of a structurally related chemical.
