Thermodynamic perturbation coefficients for confined alkanes via Monte Carlo simulations

Abstract

Modeling adsorption has been a challenge for more than a century. Different approaches within different scales have been proposed: from empirical models to equations of state, from classical Density Functional Theory to molecular simulations. Particularly equations of state are of interest for industrial applications. They are usually based on the assumption that the confinement effect can be simply added as a Helmholtz free energy contribution to the fluid–fluid Helmholtz energy. To verify this hypothesis, we propose a new conceptual framework to model the solid–fluid adsorption process, in which the reference fluid is a confined hard-chain, and the perturbation system contains the dispersion interactions among the fluid segments. Two strategies are employed: Barker–Henderson and Weeks–Chandler–Andersen. The solid material is conceived as an implicit wall imposing an external potential, a 10-4-3 Steele potential, on the fluid within a slit pore. The fluid–fluid interactions are described by a Mie potential. Applying Configurational-Bias Monte Carlo (CBMC) simulations, we compute the first- and second-order perturbation coefficients. Our findings show minimal confinement influence on the first perturbation coefficient. The second perturbation coefficient exhibits more complex behaviors, with divergences for short chains at high densities and long chains at low densities. These differences are due to preferred orientations and density peaks near confinement walls.