Structural correlations in highly asymmetric binary charged colloidal mixtures

TL;DR

This study investigates the structural correlations in highly asymmetric binary charged colloidal mixtures, revealing the necessity of a shifted Gaussian attraction in effective interactions and highlighting the importance of entropic microion effects.

Contribution

It introduces a refined effective interaction model including a Gaussian attraction to accurately describe macroion correlations in asymmetric mixtures.

Findings

Small-big correlation peak is smaller than big-big and small-small peaks.

An additional Gaussian attraction is needed in the effective potential.

Entropic microion interactions significantly influence macroion correlations.

Abstract

We explore structural correlations of strongly asymmetric mixtures of binary charged colloids within the primitive model of electrolytes considering large charge and size ratios of 10 and higher. Using computer simulations with explicit microions, we obtain the partial pair correlation functions between the like-charged colloidal macroions. Interestingly the big-small correlation peak amplitude is smaller than that of the big-big and small-small macroion correlation peaks, which is unfamiliar for additive repulsive interactions. Extracting optimal effective microion-averaged pair interactions between the macroions, we find that on top of non-additive Yukawa-like repulsions an additional shifted Gaussian attractive potential between the small macroions is needed to accurately reproduce their correct pair correlations. For small Coulomb couplings, the behavior is reproduced in a coarse-grained theory with microion-averaged effective interactions between the macroions. However, the accuracy of the theory deteriorates with increasing Coulomb coupling. We emphasize the relevance of entropic interactions exerted by the microions on the macroions. Our results are experimentally verifiable in binary mixtures of micron-sized colloids and like-charge nanoparticles