The Real Reason Why Oil and Water Don't Mix

Abstract

The majority of introductory chemistry and organic chemistry textbooks state that oil and water don't mix because of enthalpic effects. These texts generally make the argument that the mixing process is endothermic, reasoning that the water-water hydrogen bonds that must be broken in order to accommodate the solute are much stronger than the subsequent solvent-solute dipole-induced dipole intermolecular forces that are formed. In fact, in most cases the mixing process is exothermic, so the immiscibility of the two liquids must be explained by a loss of entropy in the system. The widely accepted model explaining the hydrophobic effect invokes the formation of icelike clathrate hydrate "cages" around nonpolar solute molecules. Water molecules at the surface of these relatively rigid clathrate structures are strongly hydrogen-bonded to one another. The formation of these solvent "cages" explains why both Delta H and Delta S are negative for the solution process, and the endergonicity of solvation is thus due to entropy and not enthalpy. Authors should remove from their textbooks the incorrect enthalpic/hydrogen-bond explanation for the hydrophobic effect. Because aspects of the correct entropic/clathrate "cage" explanation lie beyond the scope of introductory or organic chemistry courses, it may be wisest to omit any detailed physical explanation of the "like dissolves like" phenomenon. If the overall format of the text permits, a brief discussion of solvation entropy effects might be included in the section dealing with the immiscibility of oil and water