Letter pubs.acs.org/Langmuir
Molecular Layer of Gaslike Domains at a Hydrophobic−Water Interface Observed by Frequency-Modulation Atomic Force Microscopy Yi-Hsien Lu,†,‡,§ Chih-Wen Yang,† and Ing-Shouh Hwang*,† †
Institute of Physics, Academia Sinica, Nankang, Taipei 115, Taiwan Nanoscience and Technology Program, Taiwan International Graduate Program, Institute of Physics, Academia Sinica, Taipei 115, Taiwan § Department of Physics, National Taiwan University, Taipei 106, Taiwan ‡
S Supporting Information *
ABSTRACT: It was numerically predicted that dissolved gas particles could enrich and adsorb at hydrophobic−liquid interfaces. Here we observe nucleation and growth of bright patches of ∼0.45 nm high on the graphite surface in pure water with frequency-modulation atomic force microscopy when the dissolved gas concentration is below the saturation level. The bright patches, suspected to be caused by adsorption of nitrogen molecules at the graphite−water interface, are composed of domains of a rowlike structure with the row separation of 4.2 ± 0.3 nm. The observation of this ordered adlayer might underline the gas segregation at various water interfaces.
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INTRODUCTION The contact of water with solid surfaces is ubiquitous, and the phenomena at the water−solid interfaces are of fundamental and extreme importance to a great number of physical, chemical, biological, industrial, and environmental processes. However, the microscopic picture about how water meets a solid surface remains controversial. For water in equilibrium with air in standard conditions, the concentrations of dissolved nitrogen and oxygen are about 0.5 and 0.2 mM, corresponding to the mole fraction of ∼10−5 and ∼0.5 × 10−5, respectively. Due to the low concentrations of gases dissolved in water, the role of gases in various water interfaces is usually not considered or simply ignored. Nevertheless, several studies have indicated that the concentration of the dissolved gases may affect interfacial properties. For example, stability of colloids1 and emulsions,2 the range of the hydrophobic attraction,3 the electrical conductivity of salt solutions,4 the efficiency of flotation,5 and the condition of boundary slip in flowing water6 can be influenced by the amount of dissolved gases. Therefore, there is a need to investigate the role of gases at the water−solid interfaces. It has been known that the effect of dissolved gases is much more important for the hydrophobic−water interfaces than the hydrophilic−water interfaces. Many studies using atomic force microscopy (AFM) reported observation of nanobubbles on different hydrophobic surfaces in water when the gas concentration was near the saturation level or above.7−17 Several of them also showed evidence that nanobubbles contain gas molecules. That means gas molecules dissolved in water can © 2012 American Chemical Society
accumulate at hydrophobic−water interfaces. On the other hand, a numerical study using molecular dynamics simulations showed that dissolved gas particles could enrich and adsorb on hydrophobic surfaces.18 In this work, we would like to report our experimental observation of gaslike domains at the interface of HOPG and water. We also find that the adsorption can occur when the dissolved gas concentration is significantly below the saturation level.
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EXPERIMENTAL SECTION
The AFM images shown in this work were taken using a PicoSPM II microscope from Agilent Technologies, which is equipped with an open liquid cell and an environmental isolation chamber. About 400 μL of DI water is injected into the liquid cell, corresponding to water depth of ∼2.2 mm. The system has been modified for operation with both the tapping mode and the frequency-modulation (FM) mode. A schematic for operation with frequency-modulation is shown in Figure S1 in the Supporting Information. The AFM images are taken with Si cantilevers (PPP-NCHR from Nanosensors) with spring constants 20−40 N/m. The resonance frequency and quality factor in water are 120−150 kHz and 8−12, respectively. In our experiments, highly ordered pyrolytic graphite (HOPG) is used as the substrate because it is moderately hydrophobic (contact angle of ∼83°)19 and can be easily cleaved to expose a clean and atomically flat surface. The HOPG sample has a size of 20 mm × 20 mm (ZYB grade from Momentive), and is cleaved right before each AFM experiment. All water used is purified with Milli-Q systems Received: April 24, 2012 Revised: August 16, 2012 Published: August 17, 2012 12691
dx.doi.org/10.1021/la301671a | Langmuir 2012, 28, 12691−12695
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When the scanning is resumed at t ∼ 210 min, the surface is already ∼35% covered by the bright patches (Figure 2a). Now the bright patches become more stable against the AFM scanning. It clearly indicates that nucleation and growth of the bright patches are spontaneous processes at this interface, rather than induced by the AFM probe. Figure 2b and c shows two higher-resolution images of the area, which reveal that the bright patches are composed of domains of a rowlike structure. Three different row orientations corresponding to the threefold symmetry of the HOPG substrate are clearly seen, indicating good registry of the adlayer with the substrate. The row spacing and the height of the patches are measured to be 4.2 ± 0.3 and 0.45 nm, respectively. We have measured the tip−sample interactions on a bright patch and on a sample surface away from any bright patch by detecting the resonance frequency shift of the AFM cantilever vs the sample displacement (Figure 3). It has been derived that the frequency shift, Δf, is proportional to the force gradient (-∂Fts/∂z), where Fts is the interaction force between the tip and the sample.20,21 Since no frequency shift is detected at large tip−sample separations, we can conclude that there is no detectable long-range electrostatic interaction, suggesting little electrical charge on the bright patches as well as on the graphite surface. At small tip−sample separations (
