Generalized isobaric-isothermal ensemble: application to capillary condensation and cavitation in heterogeneous nanopores

The influence of pore size and chemical heterogeneity on the adsorption/desorption hysteresis loop of a Lennard-Jones fluid confined within simple models of heterogeneous cylindrical nanopores is investigated. The thermodynamic pressure, or grand potential density, is calculated by a new method which was previously developed for mesopores (a few nanometers) to allow the introduction of chemical heterogeneity and further improved in this work for micropores (down to molecular diameters). The efficiency of the algorithm is first verified on simple cases. Secondly, we focus on the pore size effect. It is shown to have a weak influence on the capillary condensation phenomenon which always occurs around the same value of the thermodynamic pressure. On the other hand, the desorption pressure (corresponding to the liquid fracture or cavitation threshold) exhibits a linear dependence on the inverse pore radius. Finally, the effect of chemical heterogeneity, which is characterized by the relative amplitude (< 40%) and wavelength (< 4 nm), is investigated. It is shown to strongly influence the desorption branch of the hysteresis loop. As expected, liquid fracture is favoured by large amplitude heterogeneity. However, for a given amplitude, atomic-scale heterogeneity has a minor influence, whereas the longest wavelengths considered (4 nm) destabilize the confined liquid. It is proposed to correlate this with the typical size of the bubble critical nucleus.