Solubility of polar organic solutes in nonaqueous systems: role of specific interactions

The changes in solubility of several polar organic solutes when polar organic solvents are added to a relatively inert solvent such as isooctane were determined. The relative changes in solubility predicted from regular solution theory using solubility parameters often did not agree with the observed results. However, the solubilities could be rationalized mathematically by assuming the formation of specific solute-solvent complexes. Agreement of the thermodynamic data reported here with such models provides further evidence that specific interactions, when they occur, are more important than the bulk properties of the pure components in determining drug solubilities in nonaqueous systems. Specific examples demonstrate the relationship between the solubility and molecular structure of the solute and solvent. For example, solubility can be related to the hydrogen-donating and hydrogen-accepting abilities of the solute and solvent. Steric factors also appear to play a role in solubility, while structural modifications in a solute or solvent molecule far removed from the interactive functional group have little influence on molar solubility changes with the added polar cosolvent.