Size-dependent maximum in ion conductivity: the levitation effect provides an alternative explanation.

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.