Adsorption of water on carbon materials: The formation of “water bridge” and its effect on water adsorption

Abstract

Water adsorption is very important for the modifications and applications of carbon materials. Some interesting phenomena in the process of water adsorption are still difficult to explain, and this necessitated our study. X-ray diffraction and X-ray photoelectron spectroscopies were used to characterize the structure and surface chemical properties of the carbon materials (namely graphite, activated carbon and coal). The water adsorption on carbon materials was analyzed by adsorption isotherms, and molecular simulation was used to explore the adsorption mechanisms. The results indicate that different structures and surface chemical properties of the carbon materials led to different adsorption isotherms. Graphite exhibited a Type III adsorption isotherm due to its compact structure and fewer oxygen-containing functional groups. The adsorption isotherms of activated carbon and coal conformed to the Type IV adsorption isotherm, but they presented dissimilar hysteresis loops. The “water bridge” was one of the important factors that affected water adsorption, and it was generated from the adsorption and charge transfer of water molecules induced by adjacent oxygen-containing functional groups on the surface. The strong hydrogen bond network formed by the interconnection of “water bridge” enhanced the water adsorption, capillary condensation, “ink bottle” effect, and adsorption/desorption hysteresis.