Studies on solute-solvent interactions in aqueous solution of isomeric mono-hydroxybenzoate salts

Abstract

Measurements of the relative viscosities of aqueous solutions of sodium salts of <em>o-, </em><em>m- </em>and <em>p</em>-hydroxybenzoic acids in the temperature range 298-308 K have revealed that among the three salts, <em>o- </em>hydroxybenzoate has the smallest B-coefficient and it behaves as a structure breaker while the <em>meta- </em>and <em>para-</em>isomers act as water structure makers, the structure forming capacity of the latter two being almost the same. The energy of activation data support the above conclusions. The smaller B-coefficient of <em>o- </em>hydroxybenzoate ion may be due to involvement of the <em>ortho-</em>hydroxyl group in intra- and inter-molecular hydrogen bonding with the carboxylate group and the solvent water respectively. Since the probability of intra-molecular hydrogen bonding is absent in the case of <em>meta- </em>and <em>para </em>hydroxybenzoates<em>, </em>these have B-values higher than that of the <em>o</em>-isomer, A comparison of B-coefficients of the three isomeric <em>mo no</em>- hydroxybenzoate salts with that of sodium benzoate at 298 and 308 K shows that introduction of an hydroxyl group destroys partly the hydrophobic character of the benzene ring in the benzoic acid molecule.