Assessment of scaled particle theory predictions of the convergence of solvation entropies

Entropy convergence is the experimental observation that the hydration entropies of families of non-polar solutes cross one another and converge at a distinct temperature above the boiling point of water. Entropy convergence has subsequently received significant theoretical and molecular simulation interest to interpret its molecular origin. Classic scaled particle theory has enjoyed success in describing entropy convergence for cavity-like, hard sphere solutes in water despite the fact it only considers water's equation-of-state and effective hard sphere diameter while neglecting liquid state inter-molecular correlations. This stands in difference to traditional interpretations of the hydrophobic effect that invoke water's three-dimensional structure when describing aqueous solutions of non-polar moieties. Here we investigate the origins of entropy convergence in classic scaled particle theory. We demonstrate convergence results from the theory's unphysical prediction that the surface tension of the solvent against a hard, flat interface exhibits a maximum as a function of temperature, indicative of a surface entropy that changes sign from negative to positive values with increasing temperature. In addition, we find that classic scaled particle theory can predict convergence like behavior in an organic liquid for which the phenomenon is unexpected. (C) 2020 Elsevier B.V. All rights reserved.