Capillary condensation model within nano-scale pores studied with molecular dynamics simulation

A new capillary condensation model for nano-scale pores is proposed. The effect of the pore wall potential on the condensation phenomenon was considered in the model. The critical relative pressure at which the condensation phase is formed can be related to the pore size by the model. The curvature dependency of the surface tension was also taken into account. This is a new model based on hydrostatic analysis, and its feature is non-uniformity of the condensation phase caused by the potential field exerted by the pore walls. We carried out adsorption simulations within slit-like pores in the range of 2 - 4 nm in width by using a Molecular Dynamics (MD) method. In the simulations, equilibrium vapor pressure for an adsorbed state was able to be calculated by counting the number of adsorbate particles which desorbed from the pore and reached a border plane with imaginary vapor phase. We used argon-like Ld particles as the adsorbate and the adsorbent consisted of LJ carbon-like walls. For various pore widths, we simulated the adsorption phenomena to obtain the adsorption equilibrium relation, from the state of the surface adsorption on a pore wall under a low relative pressure to the state of the condensation under a high relative pressure. Consequently, significant discrepancy in the critical relative pressure for capillary condensation from the value predicted by the Kelvin model was reaffirmed, while the proposed model predicted well the critical relative pressure for condensation in nano-scale pores. The validity of the proposed model was examined also from the aspects of the shape of gas-condensate interface and pressure distribution in the condensed phase, and gave fairly good agreement.