Structure of an aqueous RbCl solution in the gigapascal pressure range by neutron diffraction combined with empirical potential structure refinement modeling

Abstract An aqueous 3 m (=mol/kg) RbCl solution in D2O is measured at 298 K/0.1 MPa, 298 K/1 GPa, 523 K/1 GPa, and 523 K/4 GPa by neutron diffraction. The interference functions are analyzed by an empirical potential structure refinement (EPSR) modeling. The ion hydration and association and hydrogen-bonded water structure as a function of temperature and pressure are revealed in the pair correlation function, coordination number, triangular distribution, and spatial density function (3D structure). The results show that structure parameters are temperature and pressure-dependent. The second hydration layer of Rb+ and Cl− becomes evident under elevated pressure and temperature conditions. The average oxygen coordination number of Rb+ in the first hydration layer increases from 6.3 ± 1.7 at ambient pressure to 8.9 ± 2.3 at 4 GPa with decreasing coordination distance of 0.290 nm to 0.288 nm. The average oxygen coordination number of Cl− in the first hydration shell increases from 5.9 ± 1.5 at ambient pressure to 9.1 ± 2.1 at 4 GPa, and the corresponding coordination distance decreases from 0.322 nm to 0.314 nm with increasing pressure. The orientation of the water dipole in the first solvation shell of Rb+ and a central water molecule is sensitive to pressure, but that in the first solvation shell of Cl- does not change very much regardless of the thermodynamic condition. The number of contact-ion pairs Rb+-Cl− decreases with elevated temperature and increases with elevated pressure. Water molecules are closely packed, and the tetrahedral hydrogen-bonded network of water molecules no longer exists in extreme conditions.