Hysteresis loop and scanning curves of argon adsorption in closed-end wedge pores.

Abstract

The hysteresis loop and scanning curves for argon adsorbed in a wedge pore with one end closed are studied with grand canonical Monte Carlo simulation. We have found multiple hysteresis loops for pores with either the narrow end or the wider end closed. In pores with the narrow end closed, adsorption and desorption exhibits a two-stage sequence of rapid change, followed by a gradual change in adsorbate density. The pore can be divided into zones of commensurate packing and junctions of incommensurate packing. A striking feature is that the sequence of these two stages is opposite for the adsorption and desorption processes. This can be explained by cohesion in the adsorbate, in which a steep condensation process is associated with the zones and a steep evaporation process is associated with the junctions between them. For pores with the wider end closed, the processes of adsorption and desorption from various zones are correlated with each other. In pores with the narrow end closed, the scanning curves trace reversibly along the segment of the isotherm, where the isotherm shows gradual change, and when the scanning curve reaches a point between the gradual change segment and the sharp change segment, the scanning curve crosses from one boundary of the hysteresis loop to the corresponding point on the other boundary. This indicates that the condensation and evaporation states are not affected by scanning but that, in scanning across the hysteresis loop, the adsorbate passes through a sequence of metastable states as the distribution of density is rearranged, without any significant change in the overall density. In contrast, for pores with the wider end closed, both the descending curve from a partially filled pore and the ascending curve are identical to the desorption branch of the corresponding pore with its narrow end closed.