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Adsorption on nanoporous matrices is characterized by a pronounced hysteresis loop in the adsorption isotherm, when the substrate is loaded and unloaded with adsorbate, the origin of which is a matter of immense debate in the literature. In this work, we report a study of argon adsorption at 85 K on nonconnecting nanopores with one end closed to the surrounding where the effects of different pore cross sections fabricated by electron beam lithography (EBL) are investigated. A polymethylmethacrylate (PMMA) resist is deposited on the electrodes of a sensitive quartz crystal microbalance without degradation of the resonance quality factor or the long-term and short-term stabilities of the device even at cryogenic temperatures. Four different pores' cross sections: circular, square, rectangular, and triangular, are produced from EBL, and the isotherms for these pore shapes exhibit pronounced hysteresis loops whose adsorption and desorption branches are nearly vertical and have almost the same slopes. No difference is observed in the hysteresis loops of the isotherms for the pores with triangular and square cross sections, whereas the hysteresis loop for the pore with circular cross sections is much narrower, suggesting that they are more regular than the other pores. All of these observations suggest that the hysteresis behavior resulted mainly from microscopic geometric irregularities present in these porous matrices.

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.