Cluster Structures of Supercritical CH₄ Confined in Carbon Nanospaces with in Situ High-Pressure Small-Angle X-ray Scattering and Grand Canonical Monte Carlo Simulation

Abstract

The cluster structures of supercritical CH4 confined in slit-shaped carbon micropores was investigated using in situ small-angle X-ray scattering (SAXS) and the cluster analysis for grand canonical Monte Carlo (GCMC) simulation. The pore size distribution of micropores was determined from the fitting of the GCMC-simulated adsorption isotherm to the experimental one for high-pressure CH4; the pore size distributions of two kinds of activated carbon fibers (ACFs) were very narrow, and their average widths are 0.6 and 1.1 nm. The density fluctuation changes with fractional filling from in situ SAXS and GCMC simulation evidenced explicitly the formation of clusters of high concentration in the micropores. CH4 molecules in smaller pores of 0.6 nm form highly concentrated clusters even in low fractional filling. Fewer clusters are formed in the case of wider pores of 1.1 nm.