Differences in the structures of highly polar and hydrogen-bonded liquids

Abstract

For apolar liquids the molar energy of vaporization per unit volume at 25°C appears to be proportional to the square of the polarizability per unit volume, and is found to be equal to 3.97 kJ cm −3 ( n 2 D−1) 2/( n 2 D+2) 2, where n D is the refractive index. The standard deviation is of the order of 3.5 kJ mol −1. With this expression n D can be used to estimate the contribution of the dispersion forces to the molar energy of vaporization of other liquids, either highly polar or hydrogen bonded. The remainder is the share of dipole—dipole interactions or hydrogen bonds. For the normal nitriles (μ⋍3.5 D) this contribution decreases when progressing through the homologous series. In contrast, for the normal alcohols upto hexanol, the part of the vaporization energy which is not due to dispersion forces remains practically constant (25–29 kJ mol −1). This demonstrates that for most of the time the OH groups in these pure alcohols are involved in hydrogen bonds and exhibit highly preferential contacts with the oxygen atoms of the neighbouring molecules at the expense of the other groups. Such a situation provokes a decrease in the entropy of the pure alcohols and this decrease becomes still larger when inert substances are dissolved in the liquid. This phenomenon is at the origin of the hydrophobic effect.