Depletion induced isotropic-isotropic phase separation in suspensions of rod-like colloids

TL;DR

This study uses Monte Carlo simulations to explore phase separation in rod-like colloid suspensions induced by depletion forces from non-adsorbing polymers, analyzing phase boundaries, critical points, and interfacial tension.

Contribution

It provides a detailed phase diagram and critical behavior analysis for rod-polymer mixtures, comparing simulation results with free volume theory predictions.

Findings

Phase separation occurs at high depletant concentrations.

Phase boundaries are estimated using grand canonical ensemble and finite-size scaling.

Interfacial tension near the critical point follows a power-law behavior.

Abstract

When non-adsorbing polymers are added to an isotropic suspension of rod-like colloids, the colloids effectively attract each other via depletion forces. We performed Monte Carlo simulations to study the phase diagram of such rod-polymer mixture. The colloidal rods were modelled as hard spherocylinders; the polymers were described as spheres of the same diameter as the rods. The polymers may overlap with no energy cost, while overlap of polymers and rods is forbidden. Large amounts of depletant cause phase separation of the mixture. We estimated the phase boundaries of isotropic-isotropic coexistence both, in the bulk and in confinement. To determine the phase boundaries we applied the grand canonical ensemble using successive umbrella sampling [J. Chem. Phys. 120, 10925 (2004)], and we performed a finite-size scaling analysis to estimate the location of the critical point. The results are compared with predictions of the free volume theory developed by Lekkerkerker and Stroobants [Nuovo Cimento D 16, 949 (1994)]. We also give estimates for the interfacial tension between the coexisting isotropic phases and analyse its power-law behaviour on approach of the critical point.