Interplay between Wetting and Filling of Argon Adsorption in Slit Pores with Different Surface Energies Transition from Filling in Micropores to Capillary Condensation in Mesopores

Abstract

to describe the adsorption in pores having widths of less than 1.5 nm at temperatures less than the bulk critical point temperature. In mesopores, another phenomenon, capillary condensation to a liquidlike adsorbate, or "condensate", occurs (Do, D. D. Adsorption Analysis: Equilibria and Kinetics; World Scientific: London, 1983). The distinction between micropore filling and capillary condensation is not unambiguous. It is implicitly assumed in the literature that there is a strong attraction between the adsorbent and a given adsorbate, resulting in the formation of an adsorbed film which thickens as pressure increases. As the pressure approaches the bulk coexistence pressure, the adsorbed film on a planar surface becomes infinitely thick, a feature known as (complete) wetting. In this work, we provide a clear distinction between filling and condensation and also show how wetting behavior (nonwetting/partial wetting/continuous wetting) affects filling and condensation in slit pores and determine conditions under which filling occurs even when the surfaces are nonwetting or partial wetting. We have carried out extensive simulations of argon adsorption in slit pores to investigate the effects of substrate strength, pore size, and temperature on the transition from nonfilling to filling and condensation. From the analysis of the simulation results, we define filling as the phenomenon whereby a pore is filled before the formation of an adsorbate layer on the surface and capillary condensation as one in which adsorbate layers are formed on the pore walls prior to condensation. A parametric map or phase diagram of wetting/filling is constructed to show the interplay between the adsorbent strength, temperature, and pore size, which delineates the various regions of filling and capillary condensation.

Experiments of Vapor–Liquid Phase Transition of Fluids Confined in Nanopores: Implications on Modeling

“…A good discussion on the CPF compared to the capillary condensation according to molecular simulation is available. 47 As described in the previous section about the PCP, the critical pore size for capillary condensation at a constant temperature has the same physics as the PCP for condensation in pores with a constant size, so that this volume-filling mechanism is also true for adsorption at temperatures beyond the PCP. No capillary condensation occurs during CPF.…”

“…When fluids are adsorbed in pores with sizes below the critical size, such as those present in microporous adsorbents, no successive layers are formed so that fluids enter the pores through a volume-filling mechanism, also known as continuous pore filling (CPF). A good discussion on the CPF compared to the capillary condensation according to molecular simulation is available . As described in the previous section about the PCP, the critical pore size for capillary condensation at a constant temperature has the same physics as the PCP for condensation in pores with a constant size, so that this volume-filling mechanism is also true for adsorption at temperatures beyond the PCP.…”