On the applicability of density dependent effective interactions in cluster-forming systems

TL;DR

This study evaluates the effectiveness of coarse-grained pair potentials in modeling ultrasoft colloidal systems, revealing their limitations at higher densities where local density effects become significant.

Contribution

It demonstrates the density-dependent transferability issues of effective pair potentials in cluster-forming colloidal systems and highlights the need to incorporate local density effects.

Findings

Good agreement at low densities with homogeneous distributions

Deterioration of accuracy at higher densities with polydisperse clusters

Local density considerations improve coarse-grained modeling

Abstract

We systematically studied the validity and transferability of effective, coarse-grained, pair potentials in ultrasoft colloidal systems. We focused on amphiphilic dendrimers, macromolecules which can aggregate into clusters of overlapping particles to minimize the contact area with the surrounding (implicit) solvent. Simulations were performed for both the monomeric and coarse-grained model in the liquid phase at densities ranging from infinite dilution up to values close to the freezing point. For every state point, each macromolecule was mapped onto a single interaction site and the effective pair potential was computed using a coarse-graining technique based on force-matching. We found excellent agreement between the spatial dendrimer distributions obtained from the coarse-grained and microscopically detailed simulations at low densities, where the macromolecules were distributed homogeneously in the system. However, the agreement deteriorated significantly when the density was increased further and the cluster occupation became more polydisperse. Under these conditions, the effective pair potential of the coarse-grained model can no longer be computed by averaging over the whole system, but the local density needs to be taken into account instead.