Elasticity of Confined Simple Fluids from an Extended Peng-Robinson Equation of State

Thermodynamic properties of fluids in nanopores are altered by confinement, and equations of state (EOS) for bulk fluids are not able to predict them. We utilized a recent EOS based on the Peng-Robinson EOS, which takes into account the effects of confinement, to derive an analytical expression for the elastic properties of the fluid, isothermal bulk modulus, or compressibility. We calculated the modulus of liquid argon confined in model spherical nanopores of various sizes. We compared the predictions based on the EOS to calculations of the bulk modulus directly from Monte Carlo simulations in the grand canonical ensemble (GCMC). The bulk modulus of argon in mesopores predicted by the EOS appeared consistent with the GCMC predictions, showing in particular linear dependence on Laplace pressure and on the inverse pore size. Furthermore, the EOS appeared capable of predicting the modulus of the fluid in micropores, where GCMC predictions failed. Our result is a step toward developing an equation of state for confined fluids, which can be used for predictions of elasticity.