Soft-Core Attractive Model Fluid: Structure, Thermodynamics and Inter-colloidal Solvation Force†

Andrej Lajovic and Andrej Jamnik*
University of Ljubljana, Faculty of Chemistry and Chemical Technology, Aškerčeva 5, SI-1001 Ljubljana, Slovenia

Canonical and grand canonical Monte Carlo simulations are used to study a system of spherical particles interacting via a discontinuous potential combining a repulsive square soft core and an attractive square well. This, so-called coresoftened (CS) potential fluid is known to have both a gas-liquid critical point and a liquid-liquid critical point separating high density liquid (HDL) and low density liquid (LDL) phases. First, the spatial correlations and thermodynamic properties of homogeneous and inhomogeneous CS fluid are investigated. Using open ensemble simulation we study an equilibrium distribution of the CS fluid between the homogeneous phase and the planar pores mimicking the real porous material. The bulk radial distribution function displays discontinuities at the distances coinciding with the ranges of the successive repulsive and attractive parts in the CS potential function. The density profiles of confined CS fluid show the shapes arising from the interplay among the steric effects and the competition between the repulsive and attractive parts of the CS potential. Then, the effective force between a pair of big colloidal spheres immersed in a sea of small spheres interacting via CS potential is explored. The big-small interactions are modeled as hard core pair potentials with attractive or repulsive Yukawa tail leading to the accumulation repulsion and depletion attraction between the two colloids, respectively. For this purpose we apply a special simulation technique based on a separate sampling of the contributions arising from the Yukawa tail and hard-core (collision) parts of the big-small interaction potential to the total force between the colloidal particles.

Keywords: Monte Carlo simulation, soft-core potential, Yukawa potential, potential of mean force, solvation interaction