Large Attractive Depletion Interactions in Soft Repulsive-Sphere Binary Mixtures

TL;DR

This paper investigates how soft repulsive interactions in binary mixtures influence depletion forces, revealing that softness can significantly enhance attraction between large particles and potentially alter phase behavior.

Contribution

It introduces a theoretical and simulation analysis of depletion interactions in soft-sphere mixtures, highlighting the impact of softness on effective potentials and phase transitions.

Findings

Softness increases the attractiveness of depletion potentials.

Metastable fluid-fluid transition becomes more prominent in soft-sphere mixtures.

Soft particles may stabilize phase transitions absent in hard-sphere systems.

Abstract

We consider binary mixtures of soft repulsive spherical particles and calculate the depletion interaction between two big spheres mediated by the fluid of small spheres, using different theoretical and simulation methods. The validity of the theoretical approach, a virial expansion in terms of the density of the small spheres, is checked against simulation results. Attention is given to the approach toward the hard-sphere limit, and to the effect of density and temperature on the strength of the depletion potential. Our results indicate, surprisingly, that even a modest degree of softness in the pair potential governing the direct interactions between the particles may lead to a significantly more attractive total effective potential for the big spheres than in the hard-sphere case. This might lead to significant differences in phase behavior, structure and dynamics of a binary mixture of soft repulsive spheres. In particular, a perturbative scheme is applied to predict the phase diagram of an effective system of big spheres interacting via depletion forces for a size ratio of small and big spheres of 0.2; this diagram includes the usual fluid-solid transition but, in the soft-sphere case, the metastable fluid-fluid transition, which is probably absent in hard-sphere mixtures, is close to being stable with respect to direct fluid-solid coexistence. From these results the interesting possibility arises that, for sufficiently soft repulsive particles, this phase transition could become stable. Possible implications for the phase behavior of real colloidal dispersions are discussed.