Thermodynamics and Modeling of Sorption Isotherms

Abstract

An extended version of the thermodynamic computation model for sorption isotherms and capillary condensation is introduced. With the combination of the excess surface work (ESW) and disjoining pressure approach sorption isotherms for various pore size distributions of cylindrical and slit-like mesopores are successfully modeled. The main idea is that during adsorption on pore walls the approaching films will collapse and capillary condensation proceeds, while evaporation is described with a modified Kelvin equation. In both cases adsorption and desorption, the surface tension dependency on the pore size is considered.

Surface Area Determination of Porous Materials Using the Brunauer–Emmett–Teller (BET) Method: Limitations and Improvements

“…As noted above, the BET method relies on the accurate determination of monolayer loading from adsorption isotherm data alone, and the accurate determination of the surface area is subject to the choice of linear regions obtained from the adsorption isotherms. To aid in finding this loading, we also investigated the use of the excess sorption work (ESW) method. , The ESW method − is based on the thermodynamics of gaseous adsorption in porous solids and requires transforming the adsorption isotherm into the excess sorption work function as a function of loading, which helps in directly identifying the monolayer capacity by locating the first local minimum. We tested the applicability of the ESW method for computing the surface areas of MOFs.…”

Capillary Condensation of Single- and Multicomponent Fluids in Nanopores

“… a Note: Surface tension of pure nitrogen and argon are from Adolphs, and those of CO 2 and n -C 5 H 12 are from the Dortmund Data Bank (DDB). For single-component fluids, P con / P sat (calculated in other work) denotes the calculated results with the original Kelvin equation, whereas that for multicomponent fluid represents the calculated results by Shapiro’s modified Kelvin equation (Shapiro and Stenby).…”

“…Bulk surface tension γ ∞ of N 2 at 77 K and Ar at 87 K are 8.88 and 12.73 mN/m, respectively Figure plots the surface tension of CO 2 + n -C 5 H 12 at bulk conditions varying with temperature, which is calculated from the algorithm demonstrated in Figure .…”

“…Shapiro and Stenby proposed a modified Kelvin equation by considering the real gas effect and extended it for the multicomponent fluids. Adolphs modified the Kelvin equation by incorporating both the pore size effect on surface tension and the molecule–wall dispersion force, concluding that the modified model is valid to describe the adsorption isotherm with hysteresis.…”

“…Generally, the existing modifications to the Kelvin equation can be classified into three categories: the inclusion of the pore size effect on surface tension, the direct adjustments to the pore radius by accounting for the thickness of the adsorbed phase, , and the consideration of the proximity effect of the solid surface on fluid potential. , However, the assumption of ideal gas, which is obviously invalid in nanopores, , has rarely been considered in the existing modifications. In addition, most efforts are only made for single-component fluids.…”