Properties of patchy colloidal particles close to a surface: a Monte Carlo and density functional study

TL;DR

This study combines Monte Carlo simulations and density functional theory to analyze the surface behavior of patchy colloidal particles near a wall, revealing temperature-dependent adsorption-desorption transitions and limitations of current theoretical models.

Contribution

It introduces a combined simulation and DFT approach to study surface properties of patchy particles, highlighting the importance of orientational correlations at low temperatures.

Findings

Density profile transitions from adsorption to desorption near the wall.

Theoretical models are accurate near the coexistence regime.

Simulations show additional adsorption behavior due to orientational order at low temperatures.

Abstract

We investigate the behavior of a patchy particle model close to a hard-wall via Monte Carlo simulation and density functional theory (DFT). Two DFT approaches, based on the homogeneous and inhomogeneous versions of Wertheim's first order perturbation theory for the association free energy are used. We evaluate, by simulation and theory, the equilibrium bulk phase diagram of the fluid and analyze the surface properties for two isochores, one of which is close to the liquid side of the gas-liquid coexistence curve. We find that the density profile near the wall crosses over from a typical high-temperature adsorption profile to a low-temperature desorption one, for the isochore close to coexistence. We relate this behavior to the properties of the bulk network liquid and find that the theoretical descriptions are reasonably accurate in this regime. At very low temperatures, however, an almost fully bonded network is formed, and the simulations reveal a second adsorption regime which is not captured by DFT. We trace this failure to the neglect of orientational correlations of the particles, which are found to exhibit surface induced orientational order in this regime.