  The thermodynamic properties of gases have been understood primarily through
phase diagrams of bulk gases. However, observations of gases confined in a
nanometer space have posed a challenge to the principles of classical
thermodynamics. Here, we investigated interfacial structures comprising either
O2 or N2 between water and a hydrophobic solid surface by using advanced atomic
force microscopy techniques. Ordered epitaxial layers and cap-shaped
nanostructures were observed. In addition, pancake-shaped disordered layers
that had grown on top of the epitaxial base layers were observed in
oxygen-supersaturated water. We propose that hydrophobic solid surfaces provide
low-chemical-potential sites at which gas molecules dissolved in water can be
adsorbed. The structures are further stabilized by interfacial water. Gas
molecules can agglomerate into a condensed form when confined in a sufficiently
small space under ambient conditions. The ordering and thermodynamic properties
of the confined gases are determined primarily according to interfacial
interactions. The crystalline solid surface may even induce a solid-gas state.
