In situ small-angle X-ray scattering study of cluster formation in carbon micropores

The in situ small-angle X-ray scattering (SAXS) of water and CCl4 adsorbed on pitch-based activated carbon fibers (ACF) were measured with different relative pressures at 303 K. We extended the Debye-Bueche theory to the three-phase mixed system (carbon, adsorbed phase, and vacant space) to obtain the average dimension of adsorbed phase la. The zero-angle intensity of X-ray scattering I0 and la gave direct information on the shape and size of molecular assemblies in micropore. CCl4 molecules fill uniformly the micropores, on the other hand, water molecules fill the micropores via the formation of cluster-like assemblies comprising about 12 molecules, irrespective of pore width. In the small pore system (w=0.75 nm) the cluster size was unchanged with fractional filling φ. On the other hand, in the wider pore system (w=1.13 nm), the cluster grew with φ. It seems that the the difference in the size and shape of water clusters in the adsorption and desorption processes should lead to the adsorption hysteresis. Thus, the I0, la, and average dimension of vacant space lv from in situ SAXS measurements may give the direct information on molecular assemblies in micropores.