Water vapor adsorption-desorption hysteresis due to clustering of water on nonporous surfaces

Water vapor is continuously adsorbed onto and desorbed from all kinds of surfaces depending on changes of relative humidity. Adsorption-desorption hysteresis of water that occurs on various nonporous surfaces and extends down to low relative humidities has been reported for decades but remains unexplained. Here we show experimentally that such hysteresis is a common phenomenon on metal oxide and mineral surfaces and can be divided into two distinct categories based on the wettability of the adsorbent surface. Type I hysteresis occurs on more hydrophobic surfaces and is associated with adsorption isotherms that behave rather linearly as water saturation is approached, whereas type II hysteresis occurs on more hydrophilic surfaces and is associated with adsorption isotherms that curve steeply upward close to saturation. Our model calculations provide strong indication that adsorption in both types occurs cluster-wise, and the type I hysteresis is caused by contact angle hysteresis while type II hysteresis is associated with film formation close to saturation.