Studies on Molecular Interaction of Organic Molecules in Solution. I. Effects of Solvent on Molecular Interaction of Salicylic Acid with Caffeine

Abstract

A new interative method to study molecular interactions spectrophotometrically in case of two interacting species being present in comparable concentrations was developed and applied to salicylic acid-caffeine interaction to examine the effects of carbon tetrachloride, benzene, isoamyl acetate, and water on the apparent stability constant of salicylic acid-caffeine complex. The results of the present study have indicated (a) that salicylic acid interacts very strongly in the nonpolar solvents presumably by hydrogen bonding, (b) that their interaction is minimal in the moderately polar solvents, and (c) that they interact strongly in aqueous solution. These differences are discussed in terms of the difference in mechanisms of interaction in various solvents. Because the nature of complexes in organic phase is different from that in aqueous phase, complexes themselves are not likely to penetrate through the phase boundary when caffeine and salicylic acid are allowed to partition between the organic and aqueous phase.