Sedimentation of binary mixtures of like- and oppositely charged colloids: the primitive model or effective pair potentials?

TL;DR

This study investigates sedimentation equilibrium in binary mixtures of charged colloids using simulations and theory, revealing a re-entrant layering effect influenced by charge ratios and effective interactions.

Contribution

It demonstrates that the lifting and layering effects can be explained by an effective pairwise interaction model, extending previous theories to both like- and oppositely charged colloids.

Findings

Re-entrant lifting and layering phenomena observed.

Effective pairwise interactions explain sedimentation behavior.

Both repulsive and attractive colloid mixtures analyzed.

Abstract

We study sedimentation equilibrium of low-salt suspensions of binary mixtures of charged colloids, both by Monte Carlo simulations of an effective colloids-only system and by Poisson-Boltzmann theory of a colloid-ion mixture. We show that the theoretically predicted lifting and layering effect, which involves the entropy of the screening ions and a spontaneous macroscopic electric field [J. Zwanikken and R. van Roij, Europhys. Lett. {\bf 71}, 480 (2005)], can also be understood on the basis of an effective colloid-only system with pairwise screened-Coulomb interactions. We consider, by theory and by simulation, both repelling like-charged colloids and attracting oppositely charged colloids, and we find a re-entrant lifting and layering phenomenon when the charge ratio of the colloids varies from large positive through zero to large negative values.