Partial molar volumes of molecules of arbitrary shape and the effect of hydrogen bonding with water

Abstract

The paper presents a critical review of approaches for the description of partial molar volumes of aqueous solutions and offers a new procedure for calculation of these volumes which allows one to analyze organic molecules of arbitrary shape. Quantitative analysis of the volume effect of hydrogen bonding of different polar groups with water is given. The model is based on the assumption that thermal (or empty) volume may be cosidered geometrically as a layer of empty space adjacent to the molecule surface with the thickness of the layer being independent of local differences in surface curvature. The assumption makes it possible to describe partial volumes of molecules of arbitrary shape within the framework of a unitary approach. The estimation of the volume effects resulting from the interaction of polar groups with water was made by comparing the experimental partial molar volume of a polar molecule with the calculated partial molar volume of a hypothetical reference nonpolar molecule of the same shape. The volume effect of formation of one hydrogen bond in solution was found to be −2.2 cm2-mol. Using this value the stoichiometry of hydration of polar groups was analyzed. As a result, the −O−, −CO− (in ketones) and ≥N groups were found to form one hydrogen bond with water (in ethers, this bond is weak). The −OH (in most cases), −CHO (in aldehydes), and >NH groups form two hydrogen bonds with water. The amino group −NH2 forms three hydrogen bonds with water.