STUDIES ON THE CHANGES IN INTERACTION ENERGIES OF SALICYLIC-ACID AND SALICYLATE ION FROM THE SOLUBILITY AND DISSOCIATION-CONSTANT MEASUREMENTS OF THE ACID IN ETHANOL PLUS WATER MIXTURES

Solubilities of salicylic acid in ethanol + water (0-87.6 wt%) mixture s have been determined. The Gibbs energy changes of transfer of salicy lic acid from aqueous to aquo ethanolic solvents have been coupled wit h the Gibbs energy changes of transfer for the dissociation process to determine the Gibbs energy changes of transfer for salicylate ion. Th e results are interpreted in terms of ion-solvent interactions and str uctural variations of the solvent mixtures. Attempts have been made to calculate the interaction energies of benzoic acid and bezoate ion, s alicylic acid and salicylate ion utilising the scaled particle theory and the data obtained in the present work and the previous works. The Gibbs energy changes of transfer due to interaction, i.e. DELTAG(t(int ))o (water-->organic solvents) are negative and favourable in case of salicylic acid and benzoic acid but DELTAG(l(int))o due to OH group is increasingly positive indicating that OH group in the ortho-position of benzoic acid hinders dissolution process. The DELTAG(t(int))o (Bz-) and DELTAG(t,int)o(HSal-) are positive and DELTAG(t,int)o (Bz-) > DEL TAG(t,int(HSal-))o. The DELTAG(t,int)o due to OH group is seen to be i ncreasingly favourable in qualitative agreement with the fact that the salicylate anion is stabilised due to hydrogen bonding leading to an increased dissociation of salicylic acid. The limitations of the metho d have also been discussed.