Nanobubbles at the Interface between Water and a Hydrophobic Solid

Abstract

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.

Activated drying in hydrophobic nanopores and the line tension of water

“…On strongly hydrophobic surfaces, measurements have focused on the shape of nanobubbles. State-of-the-art investigations have not evidenced a variation of the contact angle with the bubble size (40,44,45), corresponding to an upper amplitude of about 100 pN for the line tension of water. Therefore, we compare our results with the systematic study by Weijs et al (42) performed for LennardJones fluids, although the water/silane/silica system investigated here is chemically different.…”

Partial Wetting of Aqueous Solutions on High Aspect Ratio Nanopillars with Hydrophilic Surface Finish

“…Although surface nanobubbles have mainly been studied using techniques like AFM that are not applicable to nanostructured surfaces, there are also a few reports on the use of ATR-FTIR. 22,23 They have been observed on a wide range of surfaces characterized by contact angles ranging from about 105…”

Effect of impurities in description of surface nanobubbles

“…Over the years AFM techniques have been the most popular method in studying these surface nanobubbles [1][2][3][4][5]. Depending on the conditions that lead to their formation, different behaviors of the nanobubbles have been found by these studies: e.g., their spherical cap-like shape and chances of deviation from that shape [6][7][8], merging of two adjacently located nanobubbles [6,9], disappearance of nanobubbles in case the water is degassed [10], possible reappearances by exchange of solvents [7,[11][12][13][14][15] or increase of temperature [11], or electrolysis [9,16] etc. The different relevant issues pertaining to the formation and behavior of surface nanobubbles are well summarized in a very recent review by Hampton and Nguyen [17].…”

“…Investigations report nanobubbles to remain stable for over days when left undisturbed [12,15]. Simple calculations of the Laplace pressure (Δp) for a nanobubble of radius R b =100 nm (at the OTS-silicon-water interface, for example, with σ lg = 0.072N/m), however, gives an estimate of…”

“…This influx then balances the gas outflux from the nanobubble, leading to bubble stability. Other explanations include possible lowering of surface tension for large curvatures on small scales [23,24], the oversaturation of liquid with gas in the vicinity of nanobubbles [15], the effect of induced charges in the Debye layer developed around the bubble interface [25], etc. A recent study by Borkent et al [8] suggests that contaminations also have a strong effect on the contact angle.…”