Combined influence of pore size distribution and surface hydrophilicity on the water adsorption characteristics of micro- and mesoporous silica
Adsorption processes are ubiquitous in nature as well as of great technological importance for gas separation, purification, storage and thermally driven heat pumps. This has led to a strong interest in the fundamental mechanisms governing adsorption phenomena and their exploitation to tailor adsorption systems for specific applications. In particular, the adsorption of water on porous silica exhibits remarkable properties due to the strong polarity of the adsorbate and moderate hydrophilicity of the adsorbent. It is generally accepted that the adsorption of water vapor on porous silica depends upon the concentration of surface silanols and the pore size. In fact, materials with ordered mesopores and a well-defined pore size have been used as model systems to demonstrate that water adsorption occurs predominantly through capillary condensation. While the pore surface chemistry is modified to become more hydrophilic after filling of the pores in these materials, the overall shape of the water adsorption isotherm (Type V) is not significantly affected in subsequent adsorption measurements. The present contribution shows that conservation of the isotherm shape is a unique phenomenon related to ordered mesopores but doesn't apply to materials with a more complex pore structure. For materials with wider pore size distributions including micropores, a synergistic effect of surface hydroxylation and pore size leads to a dramatic change in the water adsorption isotherm after the first adsorption/desorption cycle. In fact, we demonstrate that the water cycling capacity at relative pressures below the onset of capillary condensation increases significantly in these systems. These results contribute to a fundamental understanding of water adsorption in complex systems and have important implications in applications such as adsorption heat pumps where a large water cycling capacity in a specific relative pressure window is required.