Water-mediated interactions between benzene rings: Calorimetric studies of aromatic model compounds in aqueous solutions at 298 K

Pairwise enthalpic interaction coefficients of the virial expansion of the excess enthalpies were determined at 298 K by measuring the enthalpies of dilution in aqueous solution of binary aqueous solutions containing 4-hydroxyphenylacetic acid, 2-phenylethanol, 3-phenylpropanol, 3-phenylpropionic acid, l-tyrosine and l-phenylalanine. Coefficients obtained are compared with those already reported in the literature for other aromatic substances in aqueous solutions. Not withstanding the similarity of the substances employed, the values of the enthalpic coefficients range from highly negative to highly positive, an indication that the interactions between the benzene rings are largely dependent on the nature of the functional groups. For hydroxylated substances, enhanced hydrophobic interactions are operating, probably for the simultaneous interaction between the benzene rings and the alkyl chains, forced by the hydroxyl group. On the contrary, the strength of hydrophobic interactions in the solutions of the amino acids depend on the pH of the medium and on the presence of hydroxyl group on the aromatic ring. The data are discussed according to an interaction model which assumes the presence of a preferential configuration between two hydrated molecules.