Studies on the changes in interaction energies of salicylic acid and salicylate ion from solubility and dissociation constant measurements of salicylic acid in isopropanol+ water and t-butanol +water mixtures at 298 K

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

Manuscript received 13 August 2001

The solubilities and dissociation constants of salicylic acid in isopropanol (2-propanol) + water and t-butanol (2-methyl-2- propanol) + water mixtures have been deter·mined. The changes in Gibbs energies of transfer of neutral salicylic acid are coupled with the changes in Gibbs energies of transfer for the dissociation processes to determine the Gibbs energy changes of transfer for salicylate ion from water to aquo-organic solvents. Δ\(G_t^o\) (HSal^-) is positive and passes through minima at ~34 mass% of isopropanol and ~25 and 65 mass% of t-butanol. The results arc interpreted in terms of ion-solvent interactions and the characteristic changes in the solvent properties in these regions. In order to comprehend the ion-solvent interactions in a better way, attempts have been made to calculate the inter·action energies of benzoic acid, benzoate ion, salicylic acid and salicylate ion utilizing the scaled particle theory and the data obtained from the present work and the previous works.

The Gibbs energy changes due to interaction for neutral acid, Δ\(G_{t(int)}^o\) [water → org. solv.] are negative and favor·able but Δ\(G_{t(int)}^o\) due to OH group is increasingly positive indicating that OH group in the ortlro-position hinders dissolution process. The Δ\(G_{t(int)}^o\) (BZ^-) and Δ\(G_{t(int)}^o\) (HSal^-) are positive and Δ\(G_{t(int)}^o\) (BZ^-) > Δ\(G_{t(int)}^o\) (HSal^-). The Δ\(G_{t(int)}^o\) due to OH group in the anion is seen to be increasingly negative which can be correlated with the fact that the salicylate ion is stabilized due to hydrogen bonding leading to an increased dissociation of salicylic acid. The behavior of OH group in case of neutral acid and charged anions have been found to he exactly opposite.