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

Pairwise enthalpic interaction coeffs. of the virial expansion of the excess enthalpies were detd. at 298 K by measuring the enthalpies of diln. in aq. soln. of binary aq. solns. contg. 4-hydroxyphenylacetic acid, 2-phenylethanol, 3-phenylpropanol, 3-phenylpropionic acid, L-tyrosine, and L-phenylalanine. Coeffs. obtained are compared with those already reported in the literature for other arom. substances in aq. solns. Not withstanding the similarity of the substances employed, the values of the enthalpic coeffs. range from highly neg. to highly pos., 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 solns. of the amino acids depend on the pH of the medium and on the presence of hydroxyl group on the arom. ring. The data are discussed according to an interaction model which assumes the presence of a preferential configuration between two hydrated mols.