Capillary condensation in deformable mesopores: wetting versus nanomechanics

Abstract

We employ grand canonical ensemble Monte Carlo simulations to investigate the strain experienced by a nanoscopic slit pore when this pore fills with fluid material. Both solid substrates of our model system consist of a single layer of solid atoms bound to their equilibrium lattice sites by a harmonic potential such that these atoms are thermally coupled to molecules of a fluid phase confined between them. Parameters are tuned such that they represent an experimental situation in which pentane is adsorbed by mesoporous silica. Our focus is on strain isotherms, that is the net deformation of the solid as fluid material is imbibed by the pore. At low pressures prior to pore filling, strain isotherms are dominated by wetting characteristics of the fluid–solid interface whereas nanomechanical properties of the pore may be deduced quantitatively from high-pressure portions of the strain isotherm after the pore is completely filled with fluid. To that end we introduce a thermodynamic analysis of the high-pressure portion of the sorption isotherm that permits us to determine the elasticity of the confining solid material in terms of a so-called pore-load modulus which is also experimentally accessible.

Keywords:

• phase transition
• capillary condensation
• sorption strain
• Monte Carlo simulation

Acknowledgements

We thank Oskar Paris (Montanuniversität Leoben) and Peter Fratzl (Max-Planck-Institut für Kolloid- und Grenzflächenforschung) for many fruitful discussions, a most enjoyable collaboration and for providing the experimental data presented in Figure 1. M. Schoen is grateful for a discussion with Pablo G. Debenedetti (Princeton University) during the ‘8th Liblice Conference on the Statistical Mechanics of Liquids’ which led to an improved derivation of Equation (5.17).