Diffusiophoresis of ionic catalytic particles

TL;DR

This paper extends diffusiophoresis theory to catalytic particles with ion release, showing their velocity depends on ion flux and they migrate faster than inert particles in dilute solutions.

Contribution

It introduces a model for diffusiophoresis of catalytic particles with ion release, highlighting the impact of ion flux and electrolyte concentration on particle velocity.

Findings

Particle velocity increases linearly with ion flux (Damkohler number)

Catalytic particles migrate faster than inert ones in dilute solutions

Charged regions form around particles when ion diffusion coefficients differ

Abstract

A migration of charged particles relative to a solvent, caused by a gradient of salt concentration and termed a diffusiophoresis, is of much interest being exploited in many fields. Existing theories deal with diffusiophoresis of passive inert particles. In this paper, we extend prior models by focusing on a particle, which is both passive and catalytic, by postulating an uniform ion release over its surface. We derive an expression for a particle velocity depending on a dimensionless ion flux (Damkohler number Da) and show that a charged region is formed at distances of the order of the particle size, provided the diffusion coefficients of anions and cations are unequal. When Da becomes large enough, the contribution of this (outer) region to the particle velocity dominates. In this case the speed of catalytic passive particles augments linearly with Da and is inversely proportional to the square of electrolyte concentration. As a result, they always migrate towards a high concentration region and in dilute solutions become much faster than inert (non-catalytic) ones.