Ion solvation under gigapascal pressure

Abstract

Ion solvation in a range of gigapascal (GPa) pressure is of great significance for high-pressure chemical synthesis and circulation of matter within the Earth’s interior. We perform neutron scattering (NS) experiments and molecular dynamics simulations of deuterated aqueous solutions of MCl (M = Li, Na, K, Rb, and Cs) at 0.1 MPa and 0.7 GPa/298 K. An empirical potential structure refinement method analyzes the NS data. Upon compression to 0.7 GPa, the outer-shell water molecules enter the nearest neighbor of ions and the solvated ion clusters become denser. The hydration factor and static hydration number, based on the orientation distribution of the water dipole in the first solvation shell, show that compression weakens the strength of ionic hydration. Compression suppresses the diffusion of ions, particularly those of structure-breaking ions. The average residence time of water molecules indicates that under compression, the exchange rate of water molecules in the solvation shell of the structure-making ion (Li+) and the bulk water molecules is faster. In contrast, the effect of pressure on the exchange rate of water molecules in the solvation shell of the boundary ion (Na+) and the structure-breaking ions (K+, Rb+, Cs+) and the bulk water molecules can be ignored.