Effective Electrostatic Interactions in Colloid-Nanoparticle Mixtures

TL;DR

This paper develops a statistical mechanical theory to simplify modeling of colloid-nanoparticle mixtures by reducing complex multicomponent systems to effective one-component models, validated through molecular dynamics simulations.

Contribution

The authors introduce a sequential coarse-graining approach that maps complex colloid-nanoparticle systems onto a single effective colloid model, facilitating large-scale simulations.

Findings

Effective pair potential accurately predicts colloid structure.

Nanoparticles enhance electrostatic screening and destabilize suspensions.

Theory validated by molecular dynamics simulations.

Abstract

Interparticle interactions and bulk properties of colloidal suspensions can be substantially modified by addition of nanoparticles. Extreme asymmetries in size and charge between colloidal particles and nanoparticles present severe computational challenges to molecular-scale modeling of such complex systems. We present a statistical mechanical theory of effective electrostatic interactions that can greatly ease large-scale modeling of charged colloid-nanoparticle mixtures. By applying a sequential coarse-graining procedure, we show that a multicomponent mixture of charged colloids, nanoparticles, counterions, and coions can be mapped first onto a binary mixture of colloids and nanoparticles and then onto a one-component model of colloids alone. In a linear-response approximation, the one-component model is governed by a single effective pair potential and a one-body volume energy, whose parameters depend nontrivially on nano particle size, charge, and concentration. To test the theory, we perform molecular dynamics simulations of the two-component and one-component models and compute structural properties. For moderate electrostatic couplings, colloid-colloid radial distribution functions and static structure factors agree closely between the two models, validating the sequential coarse-graining approach. Nanoparticles of sufficient charge and concentration enhance screening of electrostatic interactions, weakening correlations between charged colloids and destabilizing suspensions, consistent with experiments.