How increasing pressure affects the ion hydration structure and shell properties at ambient temperature

This work deals with the effect of increasing pressure at 298.15 K on the structure and hydration shell properties of ions in infinitely diluted solutions. Results were obtained from NPT Monte Carlo simulations at various pressures, from 1 atm up to 8000 atm, for some alkali metal, alkaline earth and halide ions in TIP4P water. As pressure increases, the ion-O and the ion-H radial distribution functions (rdfs) are subjected to changes to differing extents depending on both the charge and the size of the ion. The first peak of the ion-O rdf is shifted to a shorter distance from the ion for Cs+, Br− and I−, while in the other cases there is no evidence of shortening. In contrast, for the alkaline earth ions, the most prominent effect is the decrease in the height of the first peak. Minima positions of the ion-O rdfs were used to define the first and the second hydration shells at a given pressure. Water dipole orientation with respect to the radial direction was examined, showing that at higher pressures the ion-dipole interaction becomes less attractive than at 1 atm. A much less favorable orientation was found for waters in the first shell of halide ions. Shell properties were computed from definite integrals of the ion-O rdfs, such as the coordination number and the shell contribution to the excess volume. For the first hydration shell, apart from Ca2+, there is a significant increase in the coordination number upon increasing pressure. This effect becomes more important the larger the ion size is and this is very significant for alkali metal and halide ions. In the case of I− the gap observed between 4 katm and 5 katm reflects the striking effect of increasing pressure on the shell definition. The coordination number remains almost constant when using an alternative boundary for the shell. This was suggested by the radial distribution of water-dipole orientations. Shell excess volume contributions are discussed by examining their dependence on pressure. Electrostriction is shown for the first shell, while the second shell's contribution to the excess volume is positive. At a higher pressure, the shell electrostrictive volume per water molecule is always less than at 1 atm. The greatest effect is shown for the first shell of alkaline earth. The effect of a different shell boundary is examined on the shell quantities of halide ions.