Coarse graining the dynamics of nano-confined solutes: the case of ions in clays

Abstract

We investigate the possibility of describing by a continuous solvent model the dynamics of solutes confined down to the molecular scale. We derive a Generalized Langevin Equation (GLE) for the generic motion of a solute in an external potential using the Mori-Zwanzig formalism. We then compute the corresponding memory function from molecular simulations, in the case of cesium ions confined in the interlayer porosity of montmorillonite clays, with a very low water content (only six solvent molecules per ion). Previous attempts to describe the dynamics of cesium in this system by a simple Langevin equation were unsuccessful. The purpose of the present work is not to perform GLE simulations using the memory function from molecular simulations, but rather to analyze the separation of time scales between the confined ions and solvent. We show that such a separation is not achieved and discuss the relative contribution of the ion-surface, ion-solvent and ion-ion interactions to the dynamics. On the ps time scale, the ion oscillates in a surface-and-solvent cage, which relaxes on much longer time scales extending to several nanoseconds. The resulting overall dynamics ressembles that of glasses or diffusion inside a solid by site-to-site hopping.

External Potential Modifies Friction of Molecular Solutes in Water

“…To gain insight into this puzzling finding, we extract the memory function from simulation data. For this we introduce a variant of existing methods [4,16,[27][28][29] that gives robust and reliable results for kernels and friction coefficients over a wide range of confinement potential strengths. We first observe that in the frozen limit, K ¼ ∞, Eq.…”

The Influence of Nanoporosity on the Behavior of Carbon-Bearing Fluids

“…see the recent contribution by Loganathan and Kalinichev 115 ), which include pore edges, and because of the practical importance of quantifying ion migrations in the subsurface, advanced force fields are being developed to enhance the reliability of atomistic simulations, [116][117][118] and various approaches are being developed to include hydrodynamics and multiscale properties in the calculations. [119][120][121][122] Figure 12.6 (Left) Self-diffusion coefficient for water in slit-shaped pores of width 1 nm carved out of different minerals at the same temperature. (Right) Self-diffusion coefficient for methane molecules dissolved in confined water.…”

Anomalous power law decay in solvation dynamics of DNA: a mode coupling theory analysis of ion contribution

“…Turq and coworkers made pioneering advances employing the mean spherical approximation (MSA) and their treatment provide description of the concentration dependence of electrolyte conductivity of concentration till about 1M. [2][3][4][5][6][7] Despite many successes, this treatment was not developed to treat dynamical properties like coherent dynamical structure factor and visco-elastic properties like frequency dependent viscosity. More recently, such an approach was initiated by using a mode coupling theory (MCT).…”

“…As a result, our ability to calculate such things as the transport properties of aqueous electrolyte solutions has remained somewhat imperfect even after a century of study, although notable progress has been made in recent years by efforts of many scientists, notably by Turq and co-workers. [2][3][4][5][6][7] One of the major difficulty of understanding has been the unavailability of experimental probes that could explore electrolyte dynamics at different length scales. The transport properties that we usually address are concentration dependence of the bulk, zero frequency properties, such as conductivity and viscosity.…”