On the microscopic origin of the hysteresis loop in closed end pores – Adsorbate restructuring

Abstract

We present here compelling evidence for adsorbate restructuring as the microscopic origin of hysteresis in closed end pores. Our argument is based on molecular simulations in the grand canonical (GCE) and mesocanonical (MCE) ensembles, for argon adsorbed at 87K in slit mesopores of different topologies (open end, closed end and closed pores). The MCE isotherms have sigmoidal van der Waals type loops, while the GCE isotherms exhibit hysteresis loops, which are confined between the gas-like and liquid-like spinodal points. One interesting feature, not previously recognized, is that the condensation in the MCE isotherms for pores of different topologies is identical, and it occurs at the same chemical potential, which is the coexistence chemical potential of two phases in equilibrium: the liquid-like condensate and the gas-like phase. At the onset of the condensation, the liquid-like condensate is a thin liquid bridge formed by taking molecules from the metastable adsorbed film, resulting in a thinner and stable adsorbed film. As more molecules are added into the pore, the liquid bridge thickens along the axial direction and both menisci advance towards the pore mouth. As the menisci approach the pore mouth, the liquid condensate is progressively restructured to become more cohesive.When the pressure is reduced in the grand canonical ensemble (open system), a lower pressure is required to disrupt this cohesive structure, resulting in hysteresis in the closed end pores. The classical theories, which conclude that isotherms for closed end pores must be reversible, are in error because they assume that the interfacial energy parameter (the product of the surface tension and the liquid molar volume) has the same value as in a bulk liquid.

Comparative study on ionic dissolution and structural changes of montmorillonite, kaolinite, and muscovite during interfacial reactions with oxalic acid solution

“…The isotherms of the kaolinite and muscovite corresponded to those of type-II (Gao et al, 2016), which are generally attributed to the nonporous or macroporous materials. The hysteresis loop can reflect the information about pore distribution and morphology within the specimen (Phadungbut et al, 2016;Toncón-Leal et al, 2021). After the montmorillonite reacted with oxalic acid, the slope of hysteresis loop increased, indicating the pore morphology changed from slit pores between flaky particles to irregularly internal voids (Sing, 1982).…”

Unveiling the Molecular Origin of Vapor-Liquid Phase Transition of Bulk and Confined Fluids

“…( 5) The density at Point BS1 is greater than that at the point just before sudden condensation of the GCE isotherm. This is because the minute size of the dosing cell used in the MCE simulation allows the adsorption system to control a much narrower undulating zone between the adsorbed layers and the gas-like core to be substantially smaller [38], requiring a greater chemical potential to build up the adsorbed layer for condensation.…”

An improved model for graphitized thermal carbon black: crystallite stacking model

“…It has been known by many experimentalists [15] that the adsorption of molecules on graphite surface at higher loadings (above third layer) may be interfered by the capillary condensation in the confined spaces. To this date, the condensation inside graphitic pores has been studied with simulation of slit carbon pores [16][17][18].…”