AB-INITIO QUANTUM-CHEMICAL STUDIES OF THE PK(A)S OF HYDROXYBENZOIC ACIDS IN AQUEOUS-SOLUTION WITH SPECIAL REFERENCE TO THE HYDROPHOBICITY OF HYDROXYBENZOATES AND THEIR BINDING TO SURFACTANTS

Using quantum chemical methods, the thermodynamic stabilities of benzo ic, o-, m-, and p-hydroxybenzoic, and acetic acids and their conjugate bases were studied in both gas phase and aqueous solution, enabling t he computation of the pK(a)'s of the above acids. The electronic struc ture calculations were carried out at the Hartree-Fock SCF and MP2 lev els of theory, using 6-31G(d,p) and 6-311+G(2d,p) basis sets. Solvatio n energies were calculated using two dielectric continuum methods: SCI PCM and PCM. o-Hydroxybenzoic acid and its anion were found to possess intramolecular hydrogen bonds, which in the case of the anion are so strong as to result in a ''bridged'' structure in the gas phase with t he proton effectively midway between the oxygens of the carboxylic and hydroxide groups. As a consequence of this strong hydrogen bond, the ortho anion appears to have the lowest solvation energy, lower (in mag nitude) by similar to 8-10 kcal/mol than that of benzoate itself, desp ite the presence of a potentially hydrophilic hydroxy group, The trend s in the calculated solvation energies of the anions provide an explan ation for the unusual binding strength of o-hydroxybenzoate to surfact ant films. The computed pk(a)'s of the acids were analyzed in terms of the contributing energetics, and while these calculations have not yi elded quantitatively accurate predictions of the pk(a)'s, they do exhi bit the correct qualitative trend among the acids studied.