Solvation of benzene and its simple mono derivatives in water-tertiary butanol mixtures

This paper focuses on the energetics of solvation of slightly soluble aromatic solutes - benzene (C6H6), nitrobenzene (C6H5NO2) and aniline (C6H5NH2) in the mixtures of water with hydrophobic tertiary butanol (t-BuOH). Standard enthalpies of solvation for all of the solutes studied are found to reveal the abnormal dependence from the alcohol content in the mixed solvent. It passes through the pronounced maximum at Xt-BuOH ≈ 0.05 mol fractions, the position of which is nearly equal to the mixed solvent composition, where alcohol molecules begin to aggregate forming small hydrophobic clusters. The enthalpic cost for accommodation of the hydrophobic benzene ring into these aggregates is likely to cause this abnormal solute behavior. The height of the maxima observed is found to depend on nature of a functional group in the solute molecule. Hydrophilic amino group forming strong H-bonds with surrounding water molecules decreases the extremum, whereas aromatic nitro group induces its significant increase.