Cluster-associated filling of water molecules in slit-shaped graphitic nanopores

Abstract

Water adsorption isotherms of hydrophobic activated carbon fibres (ACFs) having average pore widths, w, of 0.7 and 1.1 nm at 303 K were greatly different from each other; the adsorption isotherm of ACF with w = 0.7 nm had no clear hysteresis loop, while that of ACF with w = 1.1 nm had an explicit hysteresis loop. The structures of water adsorbed in both nanopores were studied using adsorption measurement, grand canonical Monte Carlo (GCMC) simulation, and in situ small angle X-ray scattering (SAXS). The in situ SAXS study of water in the 1.1 nm pores with the aid of GCMC simulation showed that adsorption proceeds through isolated cluster formation, whereas a mixed structure of a partial monolayer array and isolated clusters are formed on the desorption near the fractional filling, φ = 0.4. This adsorbed structure difference is the origin of the adsorption hysteresis. In contrast, both adsorption and desorption processes have the isolated cluster structure near φ = 0.4 in the case of 0.7 nm pores, agreeing with the absence of a clear hysteresis loop.