Density Functional Study on Enhancement of Modulus of Confined Fluid in Nanopores

Abstract

Elastic modulus in fluid is a crucial thermodynamic property due to its close relation to the viscoelastic behaviors in dynamics and the pore size analysis in porous materials. Especially, elasticity in nanoconfined fluid, which exhibits significant deviation from that of bulk fluid, has gained a multitude of applications in industrial and engineering fields. In this work, general expressions of bulk and shear moduli for inhomogeneous fluids have been derived based on Hooke's law. In the bulk phase, the obtained moduli reduce exactly to those obtained from thermodynamic fluctuation theory. In addition, both moduli have been expressed as functionals of fluid density functions. Besides, the Cauchy relation and Voigt notation for inhomogeneous elasticity have been first obtained. The validity of the obtained moduli has been verified through their comparison with grand canonical Monte Carlo simulation. Our further calculations suggest that the bulk modulus of confined fluid is inhomogeneous across the pore and the averaged bulk modulus is significantly enhanced by the confinement. In addition, the pore size dependence of the bulk modulus and the related Tait−Murnaghan equation have also been investigated in confined argon. It is found that in small pores, the modulus and Tait−Murnaghan equation show obvious oscillation with pore size. For larger pores, however, these properties reduce linearly to the values in the bulk case.

Elastic properties of confined fluids from molecular modeling to ultrasonic experiments on porous solids

“…They found that the elastic modulus has large deviations in the pore from the average value and can have large negative spikes. The negative modulus has been found to relate to the gas-liquid or liquid-solid transitions, which can be stabilized by confinement in nanopores [60]. The calculated average value of the isothermal modulus is consistent with other similar theoretical predictions, and in particular with the data from Dobrzanski et al [62] obtained for argon in silica pores by GCMC using Eq.…”

“…Showing that multiple differing methods of molecular modeling (i.e., MD, GCMC, and DFT) and use of various thermodynamic ensembles are able to predict the same values for the elastic properties of the confined fluid. [44,57,60,162] Note that these theoretical results have practical implications, in particular they suggest the pore-size dependent correction for parameters for the Gassmann equation often used by practitioners. The dependence of fluid compressibility on the pore surface properties could be important for processes such as enhanced oil recovery, or carbon dioxide sequestration, which cause the surface modifications of the geological porous media [172,173].…”

“…Application of DFT for calculation of local elastic properties was further used by Sun et al who modeled argon in slit and later in spherical pores [58][59][60]. They formed the expressions for elastic moduli based directly on Hooke's law.…”

“…58-61. Finally, Sun et al [60] obtained an average of this modulus in the pore over the width of the pore d…”

Compressibility of a Simple Fluid in Cylindrical Confinement: Molecular Simulation and Equation of State Modeling

“…It is worth noting that a recent work showed that the average of the local moduli of confined liquid argon calculated using cDFT matches the elastic moduli calculated from the GCMC simulations using eq 4. 93 Although the agreement between the predictions for the elastic modulus from molecular simulation and from EOS is only qualitative, it is still an important milestone. The lack of quantitative agreement is not unexpected; the primary reason for the deviation may be the difference of fluid−fluid and solid−fluid interaction potentials.…”