Interaction of hydrophobic molecules with water influenced by the clustering conditions of acetonitrile–water mixtures

Abstract

In acetonitrile–water mixtures, dynamical processes of hydrophobic substrates, such as deprotonation of excited-state 2-naphthol and hydrolysis of tert-butyl chloride, hardly occur at water mole fractions (x_w) less than 0.8. In contrast, the reaction rates increased markedly with increasing x_w for x_w > 0.8. These results were correlated with properties which reflect the solvent clustering conditions, such as partial vapour pressure and IR absorption of acetonitrile–water mixtures. IR absorption of water molecules due to the antisymmetric O—H stretching vibration showed a step-wise change of the absorption peak position suggesting the presence of four different solvent structures in the regions 0 < x_w < 0.2, 0.2 < x_w < 0.5, 0.5 < x_w < 0.8 and 0.8 < x_w < 1.0. Mass spectrometric analyses of liquid fragments of acetonitrile–water mixtures and the solutions with phenol, benzyl alcohol and 2-n-butoxyethanol showed clustering conditions characteristic of the water mole fractions in the mixtures. In 0.8 < x_w < 1.0 the hydrate clusters of phenol and benzyl alcohol increased very quickly with increasing x_w. In x_w < 0.8, the signals of the hydrate clusters of the solutes were very weak, while the acetonitrile hydrates increased with decreasing x_w. The observed mass spectral change with varying x_w is discussed in relation to the structure of acetonitrile–water solutions and its effect on the dynamical processes of hydrophobic substrates.