The adsorption of methane and water, and their coexisting state in carbon micropores were investigated by X-ray scattering measurements. The mechanisms of single-component adsorption of water and methane in the carbon micropores proceeded by cluster formation and micropore filling, respectively. The X-ray scattering of the coexisting state reveals that a mesoscopic phase separation occurred in the carbon micropores at 111 K because of the weak interaction between water and methane. At this low temperature, water molecules could not reform hydrogen bonds even with further adsorption of methane molecules. The X-ray scattering measurements provided detailed information on the coexisting state and can be used to study the temperature dependence of the phase behavior of the methane–water system during methane hydration formation in nanospaces.

Understanding the kinetics of water adsorption/desorption on activated carbon is significant for chemical applications in which competitive adsorption of water occurs. In this study, we investigated the water adsorption process by determining the adsorption rate constant using the novel pressure feedback method (PFM), which measures the rate of adsorption directly with high precision (∼10 nmol s–1) by controlling the introducing and outgassing flow rates. The PFM was used to investigate water vapor adsorption kinetics on activated carbon fibers (ACFs) of different pore sizes and to obtain a correlation of rate constant with pore size. The systems show good agreement with the stretched exponential model. The adsorption rate constants were found to be lower for ACFs with a larger pore width (average pore width; w = 1.03 nm) as compared with those for smaller pore systems (w = 0.57, 0.72 nm).

The density and intermolecular structure of water in carbon micropores (w = 1.36 nm) are investigated by small-angle X-ray scattering (SAXS) and X-ray diffraction (XRD) measurements between 20 K and 298 K. The SAXS results suggest that the density of the water in the micropores increased with increasing temperature over a wide temperature range (20–277 K). The density changed by 10%, which is comparable to the density change of 7% between bulk ice (Ic) at 20 K and water at 277 K. The results of XRD at low temperatures (less than 200 K) show that the water forms the cubic ice (Ic) structure, although its peak shape and radial distribution functions changed continuously to those of a liquid-like structure with increasing temperature. The SAXS and XRD results both showed that the water in the hydrophobic nanospaces had no phase transition point. The continuous structural change from ice Ic to liquid with increasing temperature suggests that water shows negative thermal expansion over a wide temperature range in hydrophobic nanospaces. The combination of XRD and SAXS measurements makes it possible to describe confined systems in nanospaces with intermolecular structure and density of adsorbed molecular assemblies.

The intermolecular structure of C2H5OH molecules confined in slit-shaped graphitic micropore of activated carbon fiber was investigated by in situ X-ray diffraction (XRD) measurement and reverse Monte Carlo (RMC) analysis. The pseudo-3-dimensional intermolecular structure of C2H5OH adsorbed in the micropores was determined by applying the RMC analysis to XRD data, assuming a simple slit-shaped space composed of double graphene sheets. The results were consistent with conventional Monte Carlo simulation; e.g., bilayer structure formed by hydrogen bonds among C2H5OH adsorbed at low fractional filling. The RMC method based on experimental XRD data may be a useful tool to estimate the 3-dimensional structure of adsorbed phase confined in pores.

The density and intermolecular structure of water in carbon micropores (w = 1.36 nm) are investigated by small-angle X-ray scattering (SAXS) and X-ray diffraction (XRD) measurements between 20 K and 298 K. The SAXS results suggest that the density of the water in the micropores increased with increasing temperature over a wide temperature range (20–277 K). The density changed by 10%, which is comparable to the density change of 7% between bulk ice (Ic) at 20 K and water at 277 K. The results of XRD at low temperatures (less than 200 K) show that the water forms the cubic ice (Ic) structure, although its peak shape and radial distribution functions changed continuously to those of a liquid-like structure with increasing temperature. The SAXS and XRD results both showed that the water in the hydrophobic nanospaces had no phase transition point. The continuous structural change from ice Ic to liquid with increasing temperature suggests that water shows negative thermal expansion over a wide temperature range in hydrophobic nanospaces. The combination of XRD and SAXS measurements makes it possible to describe confined systems in nanospaces with intermolecular structure and density of adsorbed molecular assemblies.