Compressibility of a Simple Fluid in Cylindrical Confinement: Molecular Simulation and Equation of State Modeling

Fluids confined in nanoporous materials exhibit thermodynamic properties that differ from the same fluid in bulk. Recent experiments and molecular simulations suggested that the isothermal compressibility is among these properties. The compressibility determines the elastic response of a fluid to mechanical impact, and in particular, the speed of acoustic wave propagation through it. Knowledge of the compressibility of fluids confined in nanopores is needed for understanding the elastic wave propagation in fluid-saturated nanoporous media, such as hydrocarbon-bearing shales. Molecular simulations allow for the prediction of the elastic properties of a confined fluid but require computationally expensive calculations for each system and pore size. Therefore, there is interest for a more straightforward model that can predict the elastic properties of a confined fluid as a function of the external pressure and confining pore size. Such models can be based on an equation of state (EOS) for a confined system. Here, we explore a possibility for a generalized van der Waals EOS for confined fluids to predict the compressibility. We also calculate the elastic properties of argon confined in silica nanopores from grand canonical Monte Carlo simulations. We obtain comparable adsorption isotherm predictions of the EOS and simulations at various pore sizes and temperatures without changing any other parameters. We then see how the predictions of the elastic properties from simulations compare to the EOS and find reasonable agreement. Additionally, we vary the solid-fluid interaction parameters in both the EOS and molecular simulations to represent solids other than silica and see how the elastic moduli depend on the other properties of confining pores related to the interaction strength. This work is a step toward a quantitative description of wave propagation in fluid-saturated nanoporous media.