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+ Water Mixtures

Department of Chemistry, University of Kalyani, Kalyani-741 235

Department of Chemistry. Santipur College, Nadia

Department of Chemistry, Vivekananda Mission Mahavidyalaya, Viveknagar, P O. Chaitanyapur (Haldia)-721 645

Manuscript received 27 March 1992. revised 21 July 1992, accepted 10 August 1992

Solubilities of salicylic acid in ethanol+ water (0 - 87.6 wt%) mixtures have been determined. The Gibbs energy changes of transfer of salicylic acid from aqueous to aquo ethanolic solvents have been coupled with the Gibbs energy changes of transfer for the dissociation process to determine the Gibbs energy changes of transfer for salicyl ate ion The results are interpreted in terms of ion-solvent interactions and structural variations of the solvent mixtures. Attempts have been made to calculate the interaction energies of benzoic acid and bezoate ion, salicylic 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. ∆G\(_t^0\)_(int) (water→organic solvents) are negative and favourable in case of salicylic acid and benzoic acid but ∆G\(_t^0\)_(int) due to OH group is increasingly positive indicating that OH group in the ortho-position of benzoic acid hinders dissolution process.

The ∆G\(_t^0\)_(int) (Bz ) and ∆G\(_t^0\)_{(int) (Hsal^-)} are positive and ∆G\(_t^0\)_(int) (Bz^-) > ∆G\(_t^0\)_{(int) (Hsal^-).} The ∆G\(_t^0\)_(int) due to OH group is seen to be increasingly 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 method have also been discussed.