(520g) Temperature Dependence of the Elastic Moduli of Confined Liquid Argon

The elastic modulus is a fundamental property of a fluid that is a measure the resistance of a fluid to being compressed due to an applied pressure. The knowledge of elastic moduli of a fluids confined in nanopores is needed for predictions of fluid flow and wave propagation in nanoporous media. The modulus of a confined fluid depends on a number of properties, such as the pressure in the pore, the size and geometry of the pore, and the surface properties of the pore walls [1-3].

Recent ultrasonic experiments have explored the temperature dependence of the elastic modulus and shown that the modulus of confined liquid argon changes linearly with temperature for a range of at least 10 degrees Kelvin [4]. Here we study the temperature dependence of the elastic modulus of confined liquid argon calculated from molecular simulations. Ultrasonic experiments typically operate under adiabatic conditions, which will provide the isentropic elastic modulus. One of the challenges is relating the experimentally determined isentropic modulus with the isothermal modulus, obtained by molecular simulations. Instead of directly simulating in isentropic conditions, we calculate the heat capacity ratio and isothermal modulus from simulations at a constant temperature, which then enables calculating the isentropic modulus.

We performed molecular simulations of argon in spherical nanopores with diameters between 3 and 8 nanometers, with temperatures between 76K and 87.3K. We operated in the grand canonical ensemble, which is typically used to simulate adsorption. Statistical mechanical relationships based on the fluctuations of the number of particles and potential energy were used determine the isothermal modulus, thermal expansion coefficient, and constant volume heat capacity. We used these values to calculate the constant pressure heat capacity using the generalized Mayer’s equation. The ratio of heat capacities was used to calculate the isentropic modulus. We showed that the modulus is linear with temperature with a slope that is consistent for all simulated pore sizes and agrees well with the experimental data. We also explored how temperature affects the other thermodynamic properties for the confined fluid.

[1] G. Y. Gor, Langmuir, 30(45), 13564 (2014)

[2] G. Y. Gor, D. W. Siderius, V. K. Shen, N. Bernstein, J. Chem. Phys. 145(16), 164505 (2016)

[3] C. D. Dobrzanski, M. A. Maximov, G. Y. Gor, J. Chem. Phys. 148, 054503 (2018)

[4] K. Schappert & R. Pelster, J. Phys. Chem. C. 122, 5537 (2018)