Capillary condensation in cylindrical nanopores

TL;DR

This study uses Monte Carlo simulations to investigate capillary condensation in cylindrical nanopores, revealing how pore size and surface attraction influence adsorption, hysteresis, and the relation to wetting phenomena.

Contribution

It provides new insights into how pore radius and surface interaction strength affect capillary condensation and hysteresis in cylindrical nanopores, linking these to wetting thresholds.

Findings

Hysteresis occurs for R>=1.7 nm and small D at R=1 nm.

Adsorption layers form before pore filling in strongly attractive cases.

The wetting threshold on cylindrical pores is about half that on planar surfaces.

Abstract

Using grand canonical Monte Carlo simulations, we have explored the phenomenon of capillary condensation (CC) of Ar at the triple temperature inside infinitely long, cylindrical pores. Pores of radius R= 1 nm, 1.7 nm and 2.5 nm have been investigated, using a gas-surface interaction potential parameterized by the well-depth D of the gas on a planar surface made of the same material as that comprising the porous host. For strongly attractive situations, i.e., large D, one or more (depending on R) Ar layers adsorb successively before liquid fills the pore. For very small values of D, in contrast, negligible adsorption occurs at any pressure P below saturated vapor pressure P0; above saturation, there eventually occurs a threshold value of P at which the coverage jumps from empty to full, nearly discontinuously. Hysteresis is found to occur in the simulation data whenever abrupt CC occurs, i.e. for R>= 1.7 nm, and for small D when R=1nm. Then, the pore-emptying branch of the adsorption isotherm exhibits larger N than the pore-filling branch, as is known from many experiments and simulation studies. The relation between CC and wetting on planar surfaces is discussed in terms of a threshold value of D, which is about one-half of the value found for the wetting threshold on a planar surface. This finding is consistent with a simple thermodynamic model of the wetting transition developed previously.