Sorption strains and their consequences for capillary condensation in nanoconfinement

Abstract

We employ Monte Carlo simulations in a semi-grand canonical ensemble to investigate the impact of pore deformation on capillary condensation in nanoconfined fluids. The fluid is composed of ‘simple’ spherically symmetric molecules of the Lennard-Jones type. These molecules are confined to a slit pore, where the pore walls consist of a single layer of atoms distributed according to the (100) plane of the face-centred cubic lattice. The atoms are bound to their equilibrium lattice sites by harmonic potentials such that they can depart to some extent from these sites on account of their thermal energy and the interaction with the fluid molecules. Under experimentally realistic conditions, our results show that, upon filling with fluid, the effective average pore size first increases, and then drops sharply at capillary condensation. The pore eventually expands again when the density of the confined liquid-like phase is further enhanced. Compared with the ideal case of perfectly rigid substrates, deformability of the pore causes capillary condensation to shift to higher bulk pressures; that is, the liquid-like phases are destabilised relative to the confined gas-like phases.