Capillary condensation in the atomic-scale slits

Capillary condensation determines fundamentally the adhesion, nucleation, adsorption and friction processes in all granular and porous materials, micromechanical devices and nanotechnology, even in living systems. Its theoretical basis-the Kelvin equation, whose cogency has long been questioned on the nanoscale, is under continuously debate. Consensus has been reached on the validity of the Kelvin equation down to 4 nm, but pores and cracks in nature do not stop their extending to the molecular dimensions. Up to now, little information, at least experimentally, is available at the molecular scale. Here we report water condensation in ultimately thin capillaries, which is capable of accommodate only one layer of water molecules. Pronounced deviations are observed under strong confinement less than 2 nm. Theoretical interpretation of changes in the solid-liquid surface energy resulting from rearranging the water structure near a particular surface complements the Kelvin equation in the ultimate limit of the atomic-scale confinement.