Comparative study of anomalous size dependence of charged and neutral solute diffusion in water

TL;DR

This study compares the size-dependent diffusion of charged and neutral solutes in water, revealing distinct behaviors and origins of diffusivity maxima linked to the Levitation effect and ionic interactions.

Contribution

It demonstrates that the mechanisms behind diffusivity maxima differ for charged and neutral solutes, highlighting the roles of ionic and nonionic interactions and their dependence on solute size.

Findings

Neutral solutes show a nonmonotonic diffusion maximum at a fixed size.

Charged solutes exhibit a size-dependent shift in diffusion maxima with increasing attraction.

The origin of diffusivity maxima differs: Levitation effect for neutral, ionic interactions for charged.

Abstract

In this work, we perform a comparative study of the size dependence of diffusion of charged and neutral solutes in water. The neutral solute in water shows a nonmonotonicity in the size dependence of diffusion. This is usually connected to the well known Levitation effect where it is found that when solute diffuses through the transient solvent cages then for attractive solute-solvent interaction and for a particular size of the solute there is a force balance which leads to the maximum in diffusion. Similar maximum in diffusion of charged solutes has also been observed and connected to Levitation effect. However, earlier studies of ionic diffusion connects this nonmonotonicity to the interplay between hard sphere repulsion and Coulombic attraction. In this work, we show that although the size dependence of both charged and neutral solutes have a nonmonotonicity, there is a stark difference in their behaviour. For charged solute with increase in attraction the maximum shifts to higher solute sizes and has a lower value whereas for neutral solute it remains at the same place and has a higher value. We show by studying the ionic and non-ionic part of the potential that for larger solutes it is the nonionic part which dominates and for smaller solutes the ionic part and the is a transition between them. As the charge on the solute increases, this transition takes place at larger solute sizes which leads to the shift in the diffusivity maxima and reduction of the peak value. We show that although the charged solutes also explore the solvent cage even before we reach the size which Levitates due to Coulombic attraction the diffusion value drops. Thus the origin of diffusivity maxima in charged and neutral solute diffusion is different.