Hydrogen-bonded network of hydration water around model solutes

Abstract

Clustering of water molecules in the hydration shells of spherical structureless solutes was studied in dependence on thermodynamic state, solute radius R_sp and strength U₀ of water–solute interaction. Two qualitatively different clustering states of hydration water have been found: an “ordered” state with a hydrogen-bonded (H-bonded) network, which includes most of the hydration water, and a “disordered” state with small H-bonded clusters of hydration water. The transition from the ordered to disordered state occurs upon increasing temperature and decreasing pressure. This percolation transition is rounded due to the finite solute size and occurs in some temperature (pressure) interval. A finite-size scaling was applied to determine the transition temperature T_∞ in the limit R_sp → ∞. Strengthening of the water–solute interaction strongly enhances the stability of the ordered state: the transition temperature increases by about 35 °C, when U₀ decreases by 1 kcal mol⁻¹. At T > T_∞ and fixed U₀, the stability of the H-bonded water network increases upon decreasing solute size.