Compressibility of Confined Fluids from Volume Fluctuations

TL;DR

This paper introduces a rapid molecular simulation method based on volume fluctuations to accurately calculate the compressibility of confined fluids like methane in nanopores, revealing significant deviations from bulk properties at small scales.

Contribution

A novel, faster simulation technique utilizing volume fluctuations for confined fluid compressibility, applicable to pore sizes up to 100 nm, improving upon Monte Carlo methods.

Findings

Compressibility increases by a factor of 4 in 3 nm pores.

Deviations from bulk compressibility decrease with increasing pore size.

Method is an order of magnitude faster than Monte Carlo simulations.

Abstract

When fluids are confined in nanopores, many of their properties deviate from bulk. These include bulk modulus, or compressibility, which determines the mechanical properties of fluid-saturated porous solids. Such properties are of importance for exploration and recovery of coal-bed methane and shale gas. We developed a new molecular simulation method for calculating compressibility of confined fluids, and applied it to methane in carbon nanopores. The method is based on volume fluctuations in the isothermal-isobaric ensemble, made possible through integrated potentials. Our method is one order of magnitude faster than the Monte Carlo approach, and allows calculations for pore sizes up to 100 nm. Our simulations predicted an increase in the fluid bulk modulus by a factor of 4 in 3 nm slit pores, and showed a gradual decrease with the increase of the pore size, so that at 100 nm, the deviation from the bulk is less than 5%.