(78c) Compressibility of Confined Fluid and Its Dependence on Pressure

Ultrasonic experiments allow one to measure the compressibility (or elastic modulus) of bulk solid or fluid samples. Recent ultrasonic experiments carried out on fluid-saturated nanoporous glass shed light on compressibility of a confined fluid [1]. In particular those experiments showed that the elastic modulus of confined argon is a linear function of Laplace pressure in the pores. Our calculations based on classical density functional theory and Monte Carlo simulations confirmed this observation qualitatively [2,3]. Moreover, we found that the compressibility of a confined fluid is a function of the pore size.

Here we focus on the pressure dependence of the modulus. For bulk materials, this relationship is linear over a wide range of pressures and is known as the Tait-Munaghan equation. Using transition matrix Monte Carlo simulations [4] we calculated the isothermal modulus of bulk argon and confined in silica mesopores of various sizes. Our calculations show that although the elastic modulus itself is strongly affected by confinement, the slope of the Tait-Murnaghan equation is not, and is practically a universal thermodynamic property of a fluid. In addition to revealing a new fundamental regularity for confined fluids, our results provide additional information which can be applied for the analysis of experimental ultrasonic data, and allows one to estimate the elastic parameters of an unknown porous medium.

1. Schappert, K. & Pelster, R. Influence of the Laplace pressure on the elasticity of argon in nanopores, Europhys. Lett., 2014, 105, 56001
2. Gor, G. Y. Adsorption Stress Changes the Elasticity of Liquid Argon Confined in a Nanopore, Langmuir, 2014, 30, 13564-13569
3. Gor, G. Y.; Siderius, D. W.; Rasmussen, C. J.; Krekelberg, W. P.; Shen, V. K. & Bernstein, N. Relation Between Pore Size and the Compressibility of a Confined Fluid, J. Chem. Phys., 2015, 143, 194506
4. Siderius, D. W. & Shen, V. K. Use of the grand canonical transition-matrix Monte Carlo method to model gas adsorption in porous materials, J. Phys. Chem. C, 2013, 117, 5861-5872