Varying the charge of small cations in liquid water: Structural, transport, and thermodynamical properties

Abstract

In this work, we show how increasing the charge of small cations affects the structural, thermodynamical, and dynamical properties of these ions in liquid water. We have studied the case of lanthanoid and actinoid ions, for which we have recently developed accurate polarizable force fields, and the ionic radius is in the 0.995-1.250 Å range, and explored the valency range from 0 to 4+. We found that the ion charge strongly structures the neighboring water molecules and that, in this range of charges, the hydration enthalpies exhibit a quadratic dependence with respect to the charge, in line with the Born model. The diffusion process follows two main regimes: a hydrodynamical regime for neutral or low charges, and a dielectric friction regime for high charges in which the contraction of the ionic radius along the series of elements causes a decrease of the diffusion coefficient. This latter behavior can be qualitatively described by theoretical models, such as the Zwanzig and the solvated ion models. However, these models need be modified in order to obtain agreement with the observed behavior in the full charge range. We have thus modified the solvated ion model by introducing a dependence of the bare ion radius as a function of the ionic charge. Besides agreement between theory and simulation this modification allows one to obtain an empirical unified model. Thus, by analyzing the contributions to the drag coefficient from the viscous and the dielectric terms, we are able to explain the transition from a regime in which the effect of viscosity dominates to one in which dielectric friction governs the motion of ions with radii of ca. 1 Å.

Solvent Structure around Lanthanoid(III) Ions in Liquid DMSO As Revealed by Polarizable Molecular Dynamics Simulations

“…55,56 The dipole polarizabilities of 7 The parameters of the short range non electrostatic (vdW) interaction potential ǫ ii and r 0 ii for La 3+ and Lu 3+ have been obtained by a trial and error procedure starting from a reasonable ansatz reported by OPLS force field 58 and optimizing the "homonuclear" vdW parameters of the Ln 3+ ion in order to reproduce geometry of Ln 3+ (DMSO) n clusters obtained by the ab initio calculations in the gas phase. This procedure leaves unchanged the original Amoeba vdW parameters for the Oxygen atom therefore preserving the transferability of the resulting FF.…”

Hydration properties of Cm(iii) and Th(iv) combining coordination free energy profiles with electronic structure analysis

“…[1][2][3][4] Molecular dynamics simulations have been employed to explore the ion-water distance, coordination number, ionic diffusion, and first shell water exchange dynamics. [5][6][7][8][9][10][11] Exchange dynamics is crucial to characterize the hydration structure and transport properties 12 of highly charged ions and it is thus important to take into account the coexistence of different coordination numbers. 13,14 Density Functional Theory (DFT)-based molecular dynamics simulations of ion hydration in liquid water can at the same time describe the interactions from first principles and explicitly consider bulk properties.…”

Microscopic understanding of ion solvation in water

“…Above the transition, the dielectric friction mechanism originating from dipolar ordering should play a critical role in ion dynamics. Therefore, we consider that the regimes below and above the transition should correspond to the Stokes and dielectric friction regimes, respectively [34].…”