The stability of the molecular complex between methyl trans-cinnamate and theophylline can be correlated with the surface tension of the solvent for various mixed solvents containing water. Among the solvents used were aqueous binary mixtures with methanol, dioxane, acetonitrile, and ethylene glycol, as well as aqueous solutions of sodium chloride and lithium chloride. A linear relationship exists between the standard free-energy change for the complexation equilibrium and the surface tension of the solvent. This was extrapolated to zero surface tension, presumably eliminating the solvent contribution to the overall complex stability. This extrapolation procedure was applied to five complex systems: methyl inm-cinnamate-theophylline; methyl trans-cinnamatetheophylline anion; methyl fra«s-cinnamate-8-chlorotheophylline anion; naphthalene-theophylline; and methyl 2-naphthoate-8-nitrotheophylline anion. The extrapolated intercepts, interpreted as free energies of substrateligand interaction, were found to be linearly related to the vertical deviations for the same complexes from the average regression line in the correlation between complex stability and maximal overlap area as described earlier. This extrapolation procedure may provide an approach for separating overall complex stability into a solvent contribution (which is dependent upon surface tension) and a substrate-ligand interaction contribution. These experimental results are consistent with the functional relationships proposed by Sinanoglu and coworkers to account for solvent effects on association equilibria.

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