Diffusion and conduction in a salt-free colloidal suspension via molecular dynamics simulations

TL;DR

This study uses molecular dynamics simulations to explore how colloidal particles in a salt-free suspension diffuse, move electrophoretically, and conduct electricity, revealing key effects like counterion condensation and a conductivity maximum.

Contribution

It provides a detailed analysis of diffusion, mobility, and conductivity in salt-free colloids, highlighting the roles of counterion effects and relaxation phenomena.

Findings

Diffusion constant affected by counterion condensation and relaxation effects

Electrical conductivity shows a maximum separating different conduction regimes

Electrophoretic mobility analyzed in terms of free and condensed counterions

Abstract

Molecular dynamics (MD) simulations are used to determine the diffusion coefficients, electrophoretic mobilities and electrical conductivity of a charged colloidal suspension in the salt-free regime as a function of the colloid charge. The behavior of the colloidal particles' diffusion constant can be well understood in terms of two coupled effects: counterion 'condensation' and slowdown due to the relaxation effect. We find that the conductivity exhibits a maximum which approximately separates the regimes of counterion-dominated and colloid-dominated conduction. We analyze the electrophoretic mobilities and the conductivity in terms of commonly employed assumptions about the role of "free" and "condensed" counterions, and discuss different interpretations of this approach.