SOLVENT EFFECTS ON CHEMICAL PROCESSES .4. COMPLEX-FORMATION BETWEEN NAPHTHALENE AND THEOPHYLLINE IN BINARY AQUEOUS-ORGANIC SOLVENTS

The standard free energy change for complex formation is written as a sum of effects arising from solvent-solvent interactions (the general medium effect), solvent-solute interactions (the solvation effect), an d solute-solute interactions (the intersolute effect). The general med ium effect is given by gDELTAA(gamma - gamma0), where g is a curvature correction factor to the solvent surface tension gamma, DELTAA is the change in surface area as the two solvent cavities containing the sub strate (naphthalene) and ligand (theophylline) collapse into a single cavity containing the complex, and gamma0 is the value of surface tens ion at which there is no net solvophobic interaction; gamma is defined to be the value appropriate to the equilibrium mean solvation shell c omposition. The solvation effect is modeled by equilibrium stoichiomet ric formation of solvated species. All data are related to the fully a queous system to give delta(M)DELTAG-degrees, the solvent effect on th e free energy change, as an explicit function of solvent composition. Solvent effects on bimolecular association are related to solvent effe cts on the solubilities of the substrate, ligand, and complex. Approxi mation methods for interpreting such systems are described and are app lied to the naphthalene-theophylline complex. It is shown that complex destabilization in mixed aqueous-organic solvents (relative to the fu lly aqueous system) may receive contributions from both the general me dium and the solvation effects, and that these contributions can be qu antitatively estimated.