Nanoconfinement Effects of Water on Hydrophilic and Hydrophobic Substrates at Cryogenic Temperatures

To gain insights into properties of nanoconfined water, adsorption structure, wettability, surface diffusion, and desorption kinetics of thin films have been investigated by using time-of-flight secondary ion mass spectrometry and temperature programmed desorption. The interfacial hydrogen bond plays a decisive role in both mobility and detachment of the water molecules, as revealed from comparison between hydroxyl- and perfluoroalkyl-terminated Si substrates. The interaction of water with graphite is stronger than that with ordinary hydrophobic substrates, as manifested by higher dewetting temperature and a lower desorption rate. The water molecules on graphite are reoriented upon heating; free OH groups tend to direct toward the substrate. Such an adsorption structure is thought to be responsible for the attractive force leading to higher wettability on graphite, in accordance with predictions of first principle calculations. At least three peaks are identified in temperature programmed desorption of water from graphite, which can be explained as a change in relative contributions between the water–graphite attractive interaction and the intermolecular hydrogen bonds with increasing coverage.