(645e) Molecular Simulation of Compressibility of Water in Carbon Nanopores

Compressibility of fluids in porous media determines the response of those media to mechanical loads, such as elastic waves propagation. Therefore, compressibility is one of the key parameters used in geophysical models for retrieving the properties of porous rocks. If the pores are in the nanometer size range, many thermodynamic properties of the fluid confined in such pores are altered, and fluid compressibility is not an exception.¹ The adsorption-ultrasonic experiments on mesoporous solids have shown that the compressibility of a fluid adsorbed in a nanopore is noticeably lower than in the same bulk fluid.² Recent molecular simulation studies of confined simple fluids, such as argon³ and nitrogen,⁴ have confirmed these observations quantitatively, also predicting a linear relation between the reciprocal compressibility and the reciprocal pore size. While the observations for nitrogen and argon fluids are of interest, it is the behavior of water under confinement that is of practical importance in many fields, including geophysics. Here we used the TIP4P/2005 water model,⁵ which is known to quantitatively predict the compressibility of bulk liquid water in the wide range of pressures.⁶ We calculated the compressibility of water confined in carbon pores of different geometries and sizes, using Monte Carlo and molecular dynamics simulations. Similar as in the studies of argon and nitrogen, we observed a significant deviation of compressibility of confined water from the bulk values. The pore size dependence, however, appeared to be different from the argon and nitrogen case, and we attribute this difference to the solvophobic instead of solvophilic confinement.

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