CAPILLARY CONDENSATION MODEL WITHIN NANO-SCALE PORES STUDIED WITH MOLECULAR-DYNAMICS SIMULATION

A new capillary condensation model for nano-scale pores is proposed. T he effect of the pore wall potential on the condensation phenomenon wa s considered in the model. The critical relative pressure at which the condensation phase is formed can be related to the pore size by the m odel. The curvature dependency of the surface tension was also taken i nto account. This is a new model based on hydrostatic analysis, and it s feature is non-uniformity of the condensation phase caused by the po tential field exerted by the pore walls. We carried out adsorption sim ulations within slit-like pores in the range of 2 - 4 nm in width by u sing a Molecular Dynamics (MD) method. In the simulations, equilibrium vapor pressure for an adsorbed state was able to be calculated by cou nting the number of adsorbate particles which desorbed from the pore a nd reached a border plane with imaginary vapor phase. We used argon-li ke Ld particles as the adsorbate and the adsorbent consisted of LJ car bon-like walls. For various pore widths, we simulated the adsorption p henomena to obtain the adsorption equilibrium relation, from the state of the surface adsorption on a pore wall under a low relative pressur e to the state of the condensation under a high relative pressure. Con sequently, significant discrepancy in the critical relative pressure f or 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 o f the proposed model was examined also from the aspects of the shape o f gas-condensate interface and pressure distribution in the condensed phase, and gave fairly good agreement.