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