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 O 2 or N 2 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.G ases exist throughout the universe and are essential in daily life as well as science and technology. Gases are generally defined as molecules that have boiling points below room temperature, such as the small nonpolar molecules N 2 , O 2 , He, and Xe. Gases are vapor under ambient conditions because van der Waals (VDW) interactions among gas molecules are much weaker than thermal energy. Condensing gas molecules into a liquid or solid state, based on the phase diagrams of bulk gases, requires high pressures or cryogenic techniques. However, numerous puzzling observations regarding gases confined in a small space have been reported. For example, gases have been observed to accumulate in a cap-shaped space on a nanometer scale at solid-water interfaces, mainly hydrophobic-water interfaces, under ambient conditions 1-13 . The cap-shaped structures are generally considered interfacial nanobubbles (INBs) that feature gas molecules in their vapor (gaseous) phase. However, their thermodynamic stability, nature, nucleation, and other properties and behaviors remain unclear. Theoretical prediction has indicated that gases inside a bubble of nanometer size should dissolve into the surrounding water in a short time 14 because of its high internal pressure, P in , which can be described using the Young-Laplace equation,where P 0 is the liquid pressure (approximately 1 atm in most laboratory conditions), C is the surface tension of the interface between liquid and gas, and r is the radius of the bubble (see Supplementary Note 1). Numerous atomic force microscopy (AFM) observations have shown that INBs are stable for hours or days [1][2][3][4][5][6][7][8][9][10][11][12] , which are at least 10-11 orders of magnitude longer than the theoretical lifetime estimated based on the Young-Laplace equation 9 ....

Thermally-nucleated self-assembly of water and alcohol into stable structures at hydrophobic interfaces

“…We note that the row-like structures reported here bear some resemblance with recent reports of longitudinal nanobubbles forming at the surface of HOPG in pure water, allegedly from dissolved gas molecules 26 27 . Nanobubbles can be excluded here considering the scale of the smallest features observed.…”

AFM Study of Surface Nanobubbles on Binary Self-Assembled Monolayers on Ultraflat Gold with Identical Macroscopic Static Water Contact Angles and Different Terminal Functional Groups

“…45,46 Similarly, the adsorption of contamination by (allegedly) 5 condensed gas is either not present or indiscriminate. 32 These data are in line with the 6 universality of the contact angle according to the model by Zhang and Lohse that predicts a 7 universal contact angle that solely depends on the gas oversaturation. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 …”

High-Resolution Characterization of Preferential Gas Adsorption at the Graphene–Water Interface

“…3a) and stiffness ( Supplementary Fig. 3b) acquired with PeakForce mode, the cap-shaped nanostructures on graphene-covered regions exhibited a darker contrast in stiffness than the other regions, resembling the INBs reported elsewhere 10,11,17,18 . The force curves measured on the cap-shaped structures also exhibited 5 characteristics that were very similar to those of INBs [17][18][19] ( Supplementary Fig.…”

“…However, typical domain sizes at the HOPG-water interface are on the order of tens of nm 17,18,[20][21][22] , much smaller than those observed at the graphene-water interface, which are on the order of hundreds of nm or larger. Similar striped patterns of large domains were also reported for graphene in air 23 , and recently they were concluded to be responsible for friction anisotropy measured on graphene 24 .…”

“…In addition, the attractive hydrophobic interactions between dissolved gas molecules (in particular, gas clusters) and a hydrophobic surface may facilitate gas adsorption. Thermodynamically, hydrophobic surfaces may provide low-chemicalpotential sites at which gas molecules dissolved in water can be adsorbed 17 .…”