A very thin layer (5-80 nm) of gas phase, consisting of discrete bubbles with only about 40 000 molecules, is quite stable at the interface between a hydrophobic solid and water. We prepare this gas phase from either ambient air or from CO(2)(g) through a solvent exchange method reported previously. In this work, we examine the interface using attenuated total internal reflection infrared spectroscopy. The presence of rotational fine structure in the spectrum of CO(2) and D(2)O proves that molecules are present in the gas phase at the interface. The air bubbles are stable for more than 4 days, whereas the CO(2) bubbles are only stable for 1-2 h. We determine the average gas pressure inside the CO(2) bubbles from the IR spectrum in two ways: from the width of the rotational fine structure (P(gas) < 2 atm) and from the intensity in the IR spectrum (P(gas) = 1.1 +/- 0.4 atm). The small difference in gas pressure between the bubbles and the ambient (1 atm) is consistent with the long lifetime. The dimensions and curvature of a set of individual bubbles was determined by atomic force microscopy. The pressures of individual bubbles calculated from the measured curvature using the Laplace equation fall into the range P(gas) = 1.0-1.7 atm, which is concordant with the average pressure measured from the IR spectrum. We believe that the difference in stability of the CO(2) bubbles and the air bubbles is due to a combination of the much lower pressure of CO(2) in the atmosphere and the greater solubility of CO(2) in water, compared to N(2) and O(2). As expected, smaller bubbles have a shorter average lifetime than larger bubbles, and the average pressure and the curvature of individual bubbles decreases with time. Surface plasmon resonance measurements provide supporting evidence that the film is in the gas state: the thin film has a lower refractive index than water, and there are few common contaminants that satisfy this condition. Interfacial gas bubbles are not ubiquitous on hydrophobic solids: bubble-free and bubble-decorated hydrophobic interfaces can be routinely prepared.

We report a novel form of the gaseous state at the interface of water and highly oriented pyrolytic graphite (HOPG) that is induced by local supersaturation of gas. Such local supersaturation of gas next to the HOPG substrate can be achieved by (1) displacing an organic liquid with a gentle flow of water, (2) displacing cold water (approximately 0 degrees C) with a gentle flow of warm water (approximately 40 degrees C), or (3) preheating the HOPG substrate to approximately 80 degrees C before exposing it to water at room temperature. In addition to the spherical-cap-shaped nanobubbles reported by many researchers, flat (quasi-two-dimensional, pancake-like) gas layers and nanobubble-flat gas layer composites (spherical-cap-shaped nanobubbles sitting on top of the quasi-two-dimensional gas layers) were detected. These entities disappeared after the system was subjected to a moderate level of degassing (approximately 0.1 atm for 1.5 h), and they did not form when the liquids involved in the aforementioned displacing procedure (to induce local supersaturation of gas) had been predegassed (to approximately 0.1 atm). The stability and some physical properties of these newly found gaseous states are examined.