title: Interface-induced ordering of gas molecules confined in a small space.
abstract: 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. Here we show that gas molecules can agglomerate into a condensed form when confined in a sufficiently small space under ambient conditions. The crystalline solid surface may even induce a solid-gas state when the gas-substrate interaction is significantly stronger than the gas-gas interaction. The ordering and thermodynamic properties of the confined gases are determined primarily according to interfacial interactions.
authors.affiliation: 1] Institute of Physics, Academia Sinica, Nankang, Taipei 115, Taiwan, R.O.C. [2] Nanoscience and Technology Program, Taiwan International Graduate Program, Institute of Physics, Academia Sinica, Taipei 115, Taiwan, R.O.C. [3] Department of Physics, National Taiwan University, Taipei 106, Taiwan, R.O.C.
authors.forename: Yi-Hsien
authors.initials: YH
authors.lastname: Lu
authors.affiliation: Institute of Physics, Academia Sinica, Nankang, Taipei 115, Taiwan, R.O.C.
authors.forename: Chih-Wen
authors.initials: CW
authors.lastname: Yang
authors.affiliation: Institute of Physics, Academia Sinica, Nankang, Taipei 115, Taiwan, R.O.C.
authors.forename: Chung-Kai
authors.initials: CK
authors.lastname: Fang
authors.affiliation: Institute of Physics, Academia Sinica, Nankang, Taipei 115, Taiwan, R.O.C.
authors.forename: Hsien-Chen
authors.initials: HC
authors.lastname: Ko
authors.affiliation: Institute of Physics, Academia Sinica, Nankang, Taipei 115, Taiwan, R.O.C.
authors.forename: Ing-Shouh
authors.initials: IS
authors.lastname: Hwang