  When a fluid is confined to a nanopore, its thermodynamic properties differ
from the properties of a bulk fluid, so measuring such properties of the
confined fluid can provide information about the pore sizes. Here we report a
simple relation between the pore size and isothermal compressibility of argon
confined in these pores. Compressibility is calculated from the fluctuations of
the number of particles in the grand canonical ensemble using two different
simulation techniques: conventional grand-canonical Monte Carlo and
grand-canonical ensemble transition-matrix Monte Carlo. Our results provide a
theoretical framework for extracting the information on the pore sizes of
fluid-saturated samples by measuring the compressibility from ultrasonic
experiments.
