Solubility of Polar and Non-Polar Aromatic Molecules in Subcritical Water: The Role of the Dielectric Constant

Liquid water at temperatures above the boiling point and high pressures, also known as pressurized hot water, or subcritical water (SBCW), is an effective solvent for both polar and non-polar organic solutes. This is often associated to the decrease of water's dielectric constant at high temperatures, apparently allowing water to behave like an organic solvent. The decrease of the solubility at high pressures, in turn, is explained by a mild increase of the dielectric constant of water. Nevertheless, the relationship between the dielectric constant of water, hydration, and the solubility of polar and non-polar molecules in SBCW, remains poorly understood. Here, we study through molecular dynamics, the hydration thermodynamic parameters and the solubility of non-polar and polar aromatic model systems, for which a solubility increase in SBCW is observed. We show that the temperature dependence of the hydration free energy of the model non-polar solutes is nonmonotonic, exhibiting a solute size independent maximum at ~475 K, above which hydration becomes entropically favorable and enthalpically unfavorable. The monotonic increase of the solubility, separated here in hydration and vaporization or sublimation components of the pure liquid or solid solute, respectively, is, in turn, related to the temperature increase of the latter, and only to a minor extent with the decrease of the hydration free energy above ~475 K, via the hydration entropy. A solubility increase or decrease is also found at high pressures for different solutes, explained by the relative magnitude of the hydration and the vaporization or sublimation components of the solubility. For the model solid polar system studied, the hydration free energy increases monotonically with the temperature, instead, and the solubility increase is caused by the decrease of the sublimation component of the solubility. Thus, despite of the observed increase of the hydration free energy with pressure, related to the entropic component decrease, our results indicate that the dielectric constant plays no significant role on the solubility increase of non-polar and polar solutes in SBCW, opposite to the dielectric constant picture. The structure of water next to the solutes is also investigated and a structural enhancement at room temperature is observed, resulting in significantly stronger pair interactions between a water molecule and its third and fourth nearest water neighbors. This structural and energetic enhancement nearly vanishes, however, at high temperatures, contributing to a positive hydration entropy.