Effect of pore geometry on the compressibility of a confined simple fluid

Abstract

Fluids confined in nanopores exhibit properties different from the properties of the same fluids in bulk; among these properties is 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.

Solvation pressure in spherical mesopores: Macroscopic theory and molecular simulations

“…The internal pore diameters used in DBdB equations were recalculated from the external diameters in the range of 2-10 nm, the difference is explained in details in Ref. 48. A comparison of the isotherms is shown in Figure 2.…”

Elastocapillarity in nanopores: Sorption strain from the actions of surface tension and surface stress

“…(26), are not known. We have inspected the density data, based on grand canonical Monte Carlo simulation for another simple fluid, argon, in cylindrical silica pores, in which the fluid-fluid and solid-fluid interactions are represented by the Lennard-Jones potential [139]. The decrease in the mean density of the liquid with diminishing pore radius suggests a negative superficial excess of matter, aboutˆ BL I = −1.7 × 10 −6 mol/m 2 or −0.12 dense-packed atomic monolayers.…”

Elastic properties of confined fluids from molecular modeling to ultrasonic experiments on porous solids

“…One other relationship that was examined by Dobrzanski et al was how the elastic modulus depends on the size of the pores. Molecular simulation and DFT works [22,59,62], summarized in Sections II B, and IV, have shown that the bulk modulus of a subcritical fluid in confinement has a nearly linear relationship with reciprocal pore size, i.e., K ∝ 1/d. Eq.…”