A Grand Canonical Monte Carlo Study of Adsorption and Capillary Phenomena in Nanopores of Various Morphologies and Topologies: Testing the BET and BJH Characterization Methods

Abstract

We report a Grand Canonical Monte Carlo simulation study of Ar adsorption at 77 K in silica nanopores having various morphologies/topologies. Both the morphological and topological disorders are shown to significantly affect the capillary condensation phenomenon. In the case of an ellipsoidal pore, we observe that the filling mechanism is similar to that of a cylindrical pore having the same section area but with a lower condensation pressure. We show that the adsorption/desorption hysteresis loop is asymmetrical for the pore with constrictions while it is symmetrical for the regular cylindrical pore. Moreover, the Ar adsorption isotherm for the constricted pore reproduces the main features of that for the fully disordered Vycor‐like porous matrix. The results for the different pore geometries (having no direct interface with the gas reservoir) indicate that the desorption occurs through cavitation at a pressure driven by the smallest void size. We also consider the validity of the BET and BJH methods for the different porous matrices. Except for the Vycor‐like matrix, the BET surface assessed from Ar adsorption isotherm at 77 K always significantly overestimates the intrinsic surface of the pore (even for a planar surface). The disagreement between the BET surface and the geometrical surface is found to increase as the confinement increases (cylindrical pore) and/or the shape of the pore becomes asymmetrical (ellipsoidal pore). Interestingly, the best agreement between the BET and the geometrical surface is found in the case of the pore with a constriction, i.e. a system that exhibits a surface with both negative and positive curvature regions. This idea is supported by the results for the Vycor‐like matrix, which has a distorted surface (many negative curvature regions combined with positive curvature regions): for this disordered porous matrix, the BET surface is found to underestimate the intrinsic surface. Finally, we show that the pore size determined using the BJH method always underestimates the pore size, in agreement with previous experimental and simulation studies.

Simple Phenomenological Model for Phase Transitions in Confined Geometry. 2. Capillary Condensation/Evaporation in Cylindrical Mesopores

“…54,55,56,57,58,59 It can be shown with similar calculations to those described above that cavitation takes place at a pressure close to the limit of stability of the metastable liquid phase (spinodal limit). Such a nucleation process occurs at very low pressure, P d , leading to a very large hysteresis loop.…”

“…Recent simulation and theoretical results in disordered and constricted mesopores have shown that these extended defects are responsible for the nonverticality of both adsorption/desorption branches in condensation/evaporation isotherms. 24,25,26,55,56 We shall demonstrate that with the present model, the equilibrium evaporation branch is naturally located close to the pressure corresponding to hysteresis loop closure point. We tested the model by comparing its performances first with experimental data for argon measured at constant pore size but with varying temperature and second with constant temperature experimental results measured for a variety of MCM pore sizes.…”

Mesoscale texture of cement hydrates

“…These values are, by far, larger than those measured in adsorption experiments (1,9,11,40,41), reporting a wide range of S sp values from 50 to 200 m 2 =g, which depend on the initial w/c. Whereas it is wellestablished that adsorption experiments underestimate S geo sp by at least 20% (37,42), the span of experimental data depending on the adsorbate (in most cases, nitrogen at 77 K and water at 300 K) is traditionally considered the result of C-S-H structure, because contrary to water, the nitrogen accesses only part of the porosity in C-S-H (typically not the layered nanotexture of C-S-H) (43). Indeed, the Jennings colloidal model (44,45) assumes two local densities with pore features designed to justify the difference in water and N 2 measurements.…”

A preliminary study on the pore characterization of Lower Silurian black shales in the Chuandong Thrust Fold Belt, southwestern China using low pressure N2 adsorption and FE-SEM methods

“…During the adsorption process, the shape is assumed to be cylindrical and the value of a is 1 (Coasne et al, 2004). The statistical thickness (t) was calculated by the Harkinse Jura model for N 2 adsorption (de Boer et al, 1963).…”