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.

Black carbon (BC) from fuel combustion is an effective light absorber that contributes significantly to direct climate forcing. The forcing is altered when BC combines with other substances, which modify its mixing state and morphology, making the evaluation of its atmospheric lifetime and climate impact a challenge. To elucidate the associated mechanisms, we exposed BC aerosol to supersaturated vapors of different chemicals to form thin coatings and measured the coating mass required to induce the restructuring of BC aggregates. We found that studied chemicals fall into two distinct groups based on a single dimensionless parameter, χ, which depends on the diameter of BC monomer spheres and the coating material properties, including vapor supersaturation, molar volume, and surface tension. We show that when χ is small (low-volatility chemicals), the highly supersaturated vapor condenses uniformly over aggregates, including convex monomers and concave junctions in between monomers, but when χ is large (intermediate-volatility chemicals), junctions become preferred. The aggregates undergo prompt restructuring when condensation in the junctions dominates over condensation on monomer spheres. For a given monomer diameter, the coating distribution is mostly controlled by vapor supersaturation. The χ factor can be incorporated straightforwardly into atmospheric models to improve simulations of BC aging.

Fluids confined in nanopores are ubiquitous in nature and technology. In recent years, the interest in confined fluids has grown, driven by research on unconventional hydrocarbon resources -shale gas and shale oil, much of which are confined in nanopores. When fluids are confined in nanopores, many of their properties differ from those of the same fluid in the bulk. These properties include density, freezing point, transport coefficients, thermal expansion coefficient, and elastic properties. The elastic moduli of a fluid confined in the pores contribute to the overall elasticity of the fluid-saturated porous medium and determine the speed at which elastic waves traverse through the medium. Wave propagation in fluid-saturated porous media is pivotal for geophysics, as elastic waves are used for characterization of formations and rock samples. In this paper, we present a comprehensive review of experimental works on wave propagation in fluid-saturated nanoporous media, as well as theoretical works focused on calculation of compressibility of fluids in confinement. We discuss models that bridge the gap between experiments and theory, revealing a number of open questions that are both fundamental and applied in nature. While some results were demonstrated both experimentally and theoretically (e.g. the pressure dependence of compressibility of fluids), others were theoretically predicted, but not verified in experiments (e.g. linear scaling of modulus with the pore size). Therefore, there is a demand for the combined experimental-modeling studies on porous samples with various characteristic pore sizes. The extension of molecular simulation studies from simple model fluids to the more complex molecular fluids is another open area of practical interest.

Unmineable coalbeds are a promising source of natural gas and can act as a receptacle for CO2 sequestration. This is because they are composed of extensive nanoporous systems, which allow for significant amounts of methane or CO2 to be trapped in the adsorbed state. The amount of the fluid confined in the coal seams can be determined from seismic wave propagation using the Gassmann equation. However, to accurately apply the Gassmann theory to coalbed methane, the effects of confinement on methane in these nanoporous systems must be taken into account. In this work, we investigate these effects of confinement on supercritical methane in model carbon nanopores. Using Monte Carlo and molecular dynamics simulations, we calculated the isothermal elastic modulus of confined methane. We showed that the effects of confinement on the elastic modulus of supercritical methane are similar to the effects on subcritical fluids: (1) the elastic modulus of the confined fluid is higher than in bulk; (2) for a given pore size, the modulus monotonically increases with pressure; and (3) at a given pressure, the modulus monotonically increases with the reciprocal pore size. However, these effects appeared much more pronounced than for subcritical fluids, showing up to seven-fold increases of the modulus in 2 nm pores. Such a significant increase should be taken into account when predicting wave propagation in methane-saturated porous media.

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.