Size-Dependent Maximum in Ion Conductivity:  The Levitation Effect Provides an Alternative Explanation

Abstract

We propose an alternative explanation of the size-dependent maximum in ion mobility in water in terms of the levitation effect, which accounts for the observed size-dependent maximum in the mobility of guest diffusion in porous media. In this explanation, the size at which the maximum occurs is related to the structure of the void space of the water; at the mobility maximum, the diffusant passes smoothly through necks connecting voids, and its potential energy shows minimum fluctuations. Molecular dynamics simulations of charged spheres of varying sizes are used to support this hypothesis. As in the levitation effect, the friction coefficient, the potential energy fluctuations, and the activation energy are found to be minima for particles with maximum self diffusivities similar to the guest diffusion in zeolites. Wavelength-dependent self diffusivities indicate a monotonic and oscillatory dependence, respectively, on wavenumber k for anomalous (AR) and linear regimes (LR). These are associated with single and biexponential decay of the incoherent intermediate scattering function.

Non-monotonic size dependence of diffusion and levitation effect: A mode-coupling theory analysis

“…Yashonath and co-workers have connected this behaviour to levitation effect [8][9][10][11][12][13][14]. They have found that when a solute size is approximately similar to the size of the zeolitic pore or in case of pure solvent, the size of the neck of the solvent cage then the solute particle feels a symmetric attraction from all directions and thus levitates through the pore and diffuses faster.…”

“…The solute tends to spend longer time bound to one side of the neck or one solvent particle (in case of LJ solvent), which leads to lower diffusion constant. In experiments, simulations and also in theoretical studies such maximum in diffusivity for ion diffusion in polar solvent has been found [12] [13][14][15][16] [17]. All these studies show that the non monotonic size dependence of diffusion is a generic phenomena independent of the nature of the solvent.…”

“…This study is done in order to understand the non monotonic size dependence of diffusion of small solutes which was earlier found in simulation studies [9]. In the earlier simulation and experimental studies the diffusion of small solute particles in porous zeolite structures [13][17], pure solvents [9] and ionic medium [14] have predicted that the non monotonic size dependence of diffusion is a generic phenomena, independent of the nature of the solvent.…”

Hydrogen bonded structure and dynamics of liquid-vapor interface of water-ammonia mixture: An ab initio molecular dynamics study

“…27,[71][72][73][74] In the present work, we have performed such an analysis of voids present in the bulk and interfacial regions by using the method of Voronoi polyhedra construction. The details of this method can be found in Ref.…”

The influence of polarizability and charge transfer on specific ion effects in the dynamics of aqueous salt solutions

“…[6][7][8][9][10][11][12] The mobility of the ions is influenced by the interactions with the first solvation shell and the dielectric friction created by long-ranged electrostatic interactions. Strong interactions between the ions and the first solvation shell create a diffusing ion that carries tightly bound water a) srick@uno.edu molecules, a solvent-berg.…”