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%.