(208e) Compressibility of Argon Confined in Nanopores: Effect of the Pore Geometry

Compressibility is a fundamental thermodynamic property of a fluid. When a fluid is confined in a nanopore, many of its thermodynamic properties differ from that of a fluid in the bulk. Experimental measurements of sound velocity on fluid-saturated nanoporous materials can give information on the compressibility of the fluids confined in the pores [1,2]. Those experiments showed in particular that the compressibility of confined argon is different from the bulk [2].

The experimental observations have been confirmed using Monte Carlo simulations in the grand canonical ensemble (GCMC), where the compressibility was calculated based on the fluctuation of the number of particles in the simulated pores [3,4]. These results, performed for a spherical pore model, showed that the isothermal elastic modulus, which is the reciprocal of compressibility, has a linear relationship with respect to pore size. This method has been shown to be quite accurate for spherical pores larger than 2.5 nm, but has complications for smaller pores. A spherical pore ca. 2.5 nm can accommodate only about 125 molecules. Below this number, the fluctuations become non-Gaussian, making compressibility calculations unsuitable. Cylindrical pores can surpass this difficulty by increasing the length of the pore and thus the number of molecules in the system, while keeping the fluid nanoconfined. Here we extend this method for cylindrical pores.

The model system for our simulations was Lennard-Jones argon in cylindrical silica pores of diameters ranging from 1 to 6 nm and lengths 3.4, 6.8, and 13.6 nm. We found that the isothermal modulus displays a similar linear relationship with the pore size as for the fluid confined in the spherical pore model for pores of size 3 to 6 nm. Interestingly, the calculations for the micropore size range (~2 nm or less) showed that this regularity breaks even when the system is large enough that the fluctuations are Gaussian. Therefore, the behavior in the 2 nm pore did not coincide with the linear relationship found for the larger pores, showing a qualitative difference between the confinement in meso- and micro- pores.

[1] J.H. Page, et al., Phys. Rev. E 52, 2763 (1995).
[2] K. Schappert & R. Pelster, Europhys. Lett.105, 56001 (2014).
[3] G.Y. Gor, et al., J. Chem. Phys. 143, 194506 (2015).
[4] G.Y. Gor, et al., J. Chem. Phys. 145, 164505 (2016).