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.