The Use of Slightly Soluble, Non-Polar Solutes as Probes for Obtaining Evidence of Water Structure

Abstract

The solubilities of non-polar molecules in water have been examined extensively in recent years in attempts to explain their abnormal behavior as compared to solubility behavior for ideal and for regular solutions. The explanations for the abnormal solubility behavior have involved considerations of enthalpy and entropy changes which have been attributed to iceberg formation, the mixing of molecules, and intermolecular interactions. From their analysis, Frank and Evans(1) concluded that water forms iceberg-like structures around the solute molecules, and such behavior explains the large negative entropies of solution and the small or negative enthalpies of solution. Shinoda and Fujihira(2) have extended the regular solution theory of Hildebrand and Scott(3) to account for the solution behavior in terms of a large positive enthalpy of mixing due to the intermolecular interactions plus a large negative enthalpy of iceberg formation to give the net small or negative enthalpy of solution and the corresponding entropy of solution. They conclude that the iceberg formation of water promotes the solubility of non-polar solutes (at room temperature relative totemperatures above 120°C) and that the small solubilities are not due to entropy effects.