Water vapour adsorption in highly porous carbons as seen by small and wide angle X-ray scattering

The adsorption of water vapour in a high surface area commercial carbon, as-received and after different surface chemical treatments was investigated as a function of relative humidity by three separate techniques. While gravimetry and small and wide angle X-ray scattering measurements are in reasonable agreement, volumetric observations yielded isotherms with substantially lower values for the water uptake. In each case, the results are sensitive to the surface chemistry of the sample. Several weeks were required to establish equilibrium, with strikingly different results being obtained for measurements carried out on shorter time scales. The slow kinetics of the uptake accounts for most of the discrepancies among the results of the different techniques. Immersion in water caused significant macroscopic swelling of the carbon matrix over a time scale comparable to the adsorption equilibrium time.