On the hysteresis of adsorption and desorption of simple gases in open end and closed end pores

Abstract

This paper presents a comprehensive computer simulation study of the microscopic mechanisms of adsorption and desorption in uniform sized pores. Our specific aim is to elucidate the origin of hysteresis, especially in those pores having one end closed to the bulk gas surroundings. These pores, despite their simplicity, capture many fundamental aspects of how molecules adsorb and are restructured in pores, which results from the interplay between a number of fundamental processes: (1) molecular layering, (2) clustering, (3) capillary condensation and evaporation and (4) molecular ordering. We have found that pore size, surface affinity and temperature are the most important parameters influencing these processes. The inter-relationship between them is highly significant in determining the possible existence of a hysteresis loop. Two classes of loop have been identified: (1) a condensation and evaporation loop and (2) a restructuring loop. Our simulations show that the origins of hysteresis stem from the following causes: (1) Different curvatures of the interface separating the adsorbed layer and the gas-like phase during adsorption and desorption. This always occurs in open ended pores when the temperature is below the critical hysteresis temperature and, most interestingly, is manifested in closed end pores only when the surface affinity is very weak;(2) Low temperatures, where the adsorbate become solid-like as filling progresses in both open and closed end pores;(3) Restructuring of the condensed phase in both open and closed end pores of specific pore size where the adsorbate changes from a liquid-like state to a solid-like state.