Structural mechanism of water adsorption in hydrophobic micropores from in situ small angle X-ray scattering

The in situ small-angle X-ray scattering (SAXS) of water adsorbed on pitch-based activated carbon fibers (ACF) of pore width w=0.8 and 1.1 nm were measured at 303 K and different relative pressures. The SAXS spectrum of ACF of w=1.1 nm in the direction of desorption did not overlap that in the adsorption direction, showing hysteresis. The density fluctuation data from Ornstein–Zernike (OZ) analysis for the SAXS profiles indicated that water molecules form great clusters in the course of adsorption, but desorption proceeds through uniform molecular evaporation.