Compressibility of Confined Fluids from Volume Fluctuations

When fluids are confined in nanopores, many of their properties deviate from bulk. These include bulk modulus, or compressibility, which contributes to the mechanical properties of fluid-saturated porous solids. Such properties are of importance for exploration and recovery of coal-bed methane and shale gas. We developed a new molecular simulation approach for calculating compressibility of confined fluids, and applied it to methane in carbon nanopores. The method is based on volume fluctuations in the isothermal-isobaric ensemble, made possible through integrated potentials. Our method is more than an order of magnitude faster than the Monte Carlo approach, and allows calculations for pore sizes up to 100 nm. Our simulations predicted an increase in the fluid bulk modulus by a factor of 4 in 3 nm slit pores, and showed a gradual decrease with the increase of the pore size, so that at 100 nm, the deviation from the bulk is less than 5%.