Molecular-level simulations of chemical reaction equilibrium and diffusion in slit and cylindrical nanopores: model dimerisation reactions

Abstract

A molecular-level simulation study of the effects of confinement on chemical reaction equilibrium and diffusion in both slit and cylindrical nanopores is presented. First, the reaction ensemble Monte Carlo (RxMC) method is implemented to investigate the effects of nanopore size and geometry, and bulk pressure on the model dimerisation reaction, , in slit and cylindrical nanopores in equilibrium with a vapour-phase reservoir. After determining the reaction equilibrium concentrations in the nanopore phase from RxMC simulations, canonical molecular dynamics (MD) is implemented to study the diffusion of fluid mixtures with concentrations matching the final average concentrations from the RxMC simulations. The canonical MD simulations mimic a diffusion-limited reacting system, where it is assumed that the reaction rates are very fast relative to the diffusion, and therefore assumed that chemical equilibrium is effectively maintained and unperturbed at all times in the system. The diffusion is analysed in terms of the overall and space-dependent mean-square displacement and corresponding self-diffusion coefficients. Monomers and dimers are treated as Lennard-Jones (LJ) and two-centre LJ fluids, respectively, while the interactions of the fluids with the nanopore walls are modelled using the Steele 10-4-3 potential. The model parameters and state conditions are chosen in order to enhance reaction conversions in the nanopore phase with respect to the bulk vapour phase. The main result of this work is a relation between the space-dependent diffusion and the structure of the reacting fluids within the nanopores.