Water Vapor\nAdsorption–Desorption Hysteresis\nDue to Clustering of Water on Nonporous Surfaces

Water vapor is continuously adsorbed onto and desorbed\nfrom all\nkinds of surfaces depending on changes in relative humidity. Adsorption–desorption\nhysteresis of water that occurs on various nonporous surfaces and\nextends down to low relative humidities has been reported for decades,\nbut remains unexplained. Here we show experimentally that such hysteresis\nis a common phenomenon on metal oxide and mineral surfaces and can\nbe divided into two distinct categories based on the wettability of\nthe adsorbent surface. Type I hysteresis occurs on more hydrophobic\nsurfaces and is associated with adsorption isotherms that behave rather\nlinearly as water saturation is approached, whereas type II hysteresis\noccurs on more hydrophilic surfaces and is associated with adsorption\nisotherms that curve steeply upward close to saturation. Our model\ncalculations strongly indicate that adsorption in both types occurs\ncluster-wise, and the type I hysteresis is caused by contact angle\nhysteresis, while type II hysteresis is associated with film formation\nclose to saturation. The understanding of water vapor adsorption and\ndesorption mechanisms may be key for explaining and quantifying physical\nand chemical interfacial phenomena in atmospheric and industrial environments.