Effect of Pore Geometry on the Compressibility of a Confined Simple Fluid

TL;DR

This study uses Monte Carlo simulations to analyze how pore geometry affects the compressibility of confined liquid argon, revealing regularities linked to pore shape and size that inform experimental analysis.

Contribution

It introduces a detailed simulation-based analysis of fluid compressibility in nanopores, highlighting the influence of pore geometry and size on elastic properties.

Findings

Fluid modulus in spherical pores depends logarithmically on vapor pressure.

Modulus in spherical pores is linearly related to reciprocal pore size.

Results at higher temperature confirm regularities and show differences between geometries.

Abstract

Fluids confined in nanopores exhibit properties different from the properties of the same fluids in bulk, among these properties are the isothermal compressibility or elastic modulus. The modulus of a fluid in nanopores can be extracted from ultrasonic experiments or calculated from molecular simulations. Using Monte Carlo simulations in the grand canonical ensemble, we calculated the modulus for liquid argon at its normal boiling point (87.3~K) adsorbed in model silica pores of two different morphologies and various sizes. For spherical pores, for all the pore sizes (diameters) exceeding 2~nm, we obtained a logarithmic dependence of fluid modulus on the vapor pressure. Calculation of the modulus at saturation showed that the modulus of the fluid in spherical pores is a linear function of the reciprocal pore size. The calculation of the modulus of the fluid in cylindrical pores appeared too scattered to make quantitative conclusions. We performed additional simulations at higher temperature (119.6~K), at which Monte Carlo insertions and removals become more efficient. The results of the simulations at higher temperature confirmed both regularities for cylindrical pores and showed quantitative difference between the fluid moduli in pores of different geometries. Both of the observed regularities for the modulus stem from the Tait-Murnaghan equation applied to the confined fluid. Our results, along with the development of the effective medium theories for nanoporous media, set the groundwork for analysis of the experimentally-measured elastic properties of fluid-saturated nanoporous materials.