Relationship between the Gas-Phase Entropies of Molecules and Their Entropies of Solvation in Water and 1-Octanol

Abstract

The entropies of solvation of ammonia, methane, and water in water are almost identical, yet their enthalpies of solvation are quite different. This suggests that, despite differences in the solute-solvent interactions, all molecules, including water, lose the same fraction of their entropy upon going from the gas phase into water and that changes in the entropy of water contribute little to the entropy of solvation. An equation containing no adjustable parameters is derived from these assumptions and found to reproduce the entropies of solvation of the hydrocarbons, alcohols, aromatic compounds, primary amines, and inert gases in water. These ideas are successfully extended to the water/1-octanol partition coefficients of the hydrocarbons. Our ideas are in conflict with prevailing ideas of the origin of entropies of solvation in water. Reasons for their validity and success are discussed. © 1980, American Chemical Society. All rights reserved.