pubs.acs.org/Langmuir © 2010 American Chemical Society
The Relationship Between Nanobubbles and the Hydrophobic Force Lauren A. Palmer,† David Cookson,‡ and Robert N. Lamb*,† †
Department of Chemistry, The University of Melbourne, Melbourne, Australia, and ‡Australian Synchrotron, Clayton, Victoria 3168, Australia Received July 26, 2010. Revised Manuscript Received October 29, 2010
The formation of nanobubbles on hydrophobic self-assembled monolayers has been examined in a binary ethanol/ water titration using small angle X-ray scattering (SAXS) and atomic force microscopy (AFM). The AFM data demonstrates a localized force effect attributed to nanobubbles on an immersed hydrophobic surface. This evidence is arguably compromised by the possibility that the AFM tip actually nucleates nanobubbles. As a complementary noninvasive technique, SAXS has been used to investigate the interfacial region of the immersed hydrophobic surface. SAXS measurements reveal an electron density depletion layer at the hydrophobic interface, with changing air solubility in the immersing liquid, due to the formation of nanobubbles.
Introduction Atomic force microscopy (AFM) measurements often reveal stable, nanoscale air inclusions on otherwise smooth hydrophobic and hydrophilic surfaces such as modified mica surfaces and highly oriented pyrolytic graphite (HOPG) surfaces.1-6 These inclusions, referred to as nanobubbles, are found to be up to 30 nm high with radii of curvature in the range of 100-300 nm.5,7,8 A variety of methods for producing nanobubbles observable with AFM have been reported1-3,7 such as heating the substrate, pressurizing water with gases such as carbon dioxide,6 or using ethanol/water exchange.6,7 The last of these techniques is relatively straightforward, as it requires only room temperature and pressure. When bulk ethanol is displaced by water, a thin layer of ethanol remains on the substrate. Gases are more soluble in ethanol than in water, so when the ethanol combines with the water, there is a local supersaturation of gases at the hydrophobic surface. This supersaturation is reported to be responsible for the formation of nanobubbles as measured by AFM.7,9 *To whom correspondence should be addressed. E-mail: rnlamb@ unimelb.edu.au. (1) Lou, S.-T.; Ouyang, Z.-Q.; Zhang, Y.; Li, X.-J.; Hu, J.; Li, M.-Q.; Yang, F.-J. Nanobubbles on Solid Surface Imaged by Atomic Force Microscopy. J. Vac. Sci. Technol., B 2000, 18(5), 2573–2575. (2) Zhang, X. H.; Zhang, X.; Sun, J.; Zhang, Z.; Li, G.; Fang, H.; Xiao, X.; Zeng, X.; Hu, J. Detection of Novel Gaseous States at the Highly Oriented Pyrolytic Graphite-Water Interface. Langmuir 2007, 23(4), 1778–1783. (3) Zhang, X. H.; Zhang, X. D.; Lou, S. T.; Zhang, Z. X.; Sun, J. L.; Hu, J. Degassing and Temperature Effects on the Formation of Nanobubbles at the Mica/Water Interface. Langmuir 2004, 20(9), 3813–3815. (4) Tyrrell, J.; Attard, P. Atomic Force Microscope Images of Nanobubbles on a Hydrophobic Surface and Corresponding Force-Separation Data. Langmuir 2002, 18(1), 160–167. (5) Zhang, L.; Zhang, Y.; Zhang, X.; Li, Z.; Shen, G.; Ye, M.; Fan, C.; Fang, H.; Hu, J. Electrochemically Controlled Formation and Growth of Hydrogen Nanobubbles. Langmuir 2006, 22(19), 8109–8113. (6) Yang, S.; Dammer, S. M.; Bremond, N.; Zandvliet, H. J. W.; Kooij, E. S.; Lohse, D. Characterization of Nanobubbles on Hydrophobic Surfaces in Water. Langmuir 2007, 23(13), 7072–7077. (7) Zhang, X. H.; Maeda, N.; Craig, V. S. J. Physical Properties of Nanobubbles on Hydrophobic Surfaces in Water and Aqueous Solutions. Langmuir 2006, 22 (11), 5025–5035. (8) Attard, P.; Moody, M. P.; Tyrrell, J. W. G. Nanobubbles: The Big Picture. Physica A 2002, 314(1-4), 696–705. (9) Hampton, M. A.; Nguyen, A. V. Nanobubbles and the nanobubble bridging capillary force. Adv. Colloid Interface Sci. 2010, 154, 30–55. (10) Holmberg, M.; K€uhle, A.; Garnæs, J.; Mørch, K.; Boisen, A. Nanobubblt Trouble on Gold Surfaces. Langmuir 2003, 19(25), 10510–10513.
144 DOI: 10.1021/la1029678
There has been some suggestion, however, that nanobubble formation is actually a byproduct of the AFM measurement method10-12 and that the resultant anomalous long-range “hydrophobic” force is due to the existence of these AFM-induced inclusions.4,9,13 The unusually long-range attraction exceeds the van der Waals interaction by at least 1-2 orders of magnitude.14,15 This highlights the need for an independent measurement of nanobubble formation. Small angle X-ray scattering (SAXS) is a noninvasive technique that has previously been used to examine the formation of air inclusions on immersed superhydrophobic sol-gel films in situ.16 In a transmission geometry, X-rays with wavelengths of 1-2 A˚ pass through a solid/liquid interface, or when dry, a solid/air interface. All X-ray scatter is driven by inhomogeneity of charge density, within the bulk of some material or at the material’s interface. A change in X-ray scatter seen from a dry material upon immersion can be attributable to a change in electron density contrast between solid/air (unwetted surface) and solid/liquid (wetted surface). This change then becomes a method for detecting the presence of air at an interface.16 Hydrophobic surfaces can be created using thiol self-assembled monolayers (SAMs) on a gold substrate. Hydrophobic surfaces created using self-assembled monolayers have good reproducibility and are robust. Unlike a surfactant based system, SAMs are chemically bonded to the surface so there is no adsorption equilibria involving, for example, surfactant molecules in the solvent phase. (11) Yakubov, G. E.; Butt, H.-J.; Vinogradova, O. I. Interaction Forces between Hydrophobic Surfaces. Attractive Jump as an Indication of Formation of “Stable” Submicrocavities. J. Phys. Chem. B 2000, 104(15), 3407–3410. (12) Steitz, R.; Gutberlet, T.; Hauss, T.; Kl€osgen, B.; Krastev, R.; Schemmel, S.; Simonsen, A.; Findenegg, G. Nanobubbles and Their Precursor Layer at the Interface of Water Against a Hydrophobic Substrate. Langmuir 2003, 19(6), 2409– 2418. (13) Ishida, N.; Higashitani, K. Interaction Forces Between Chemically Modified Hydrophobic Surfaces Evaluated by AFM - The Role of Nanoscopic Bubbles in the Interactions. Miner. Eng. 2006, 19(6-8), 719–725. (14) Teschke, O.; de Souza, E. F. Measurements of Long-Range Attractive Forces between Hydrophobic Surfaces and Atomic Force Microscopy Tips. Chem. Phys. Lett. 2003, 375(5,6), 540–546. (15) Teschke, O.; de Souza, E. F. Electrostatic Response of Hydrophobic Surface Measured by Atomic Force Microscopy. Appl. Phys. Lett. 2003, 82(7), 1126–1128. (16) Zhang, H.; Lamb, R. N.; Cookson, D. J. Nanowetting of Rough Superhydrophobic Surfaces. Appl. Phys. Lett. 2007, 91(25), 254106.
Published on Web 12/09/2010
Langmuir 2011, 27(1), 144–147
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Article
In this study, hydrophobic surfaces were prepared by selfassembly of a 1-octadecanethiol monolayer on a gold-coated surface (water contact angle: 109°). Both AFM and SAXS were used as complementary techniques to study the evolution of nanobubbles at the solid/liquid interface.
Experimental Section Materials. Ethanol, sulfuric acid, and 30% hydrogen peroxide were used as supplied. Milli-Q water was used throughout. Gold (99.99%) and chromium were obtained from Proscitech. 1-Octadecanethiol was obtained from Fluka. Solutions of 1-octadecanethiol (1 mM) were prepared using ethanol as the solvent. Solutions with a range of ethanol/water fractions (by weight) were prepared and left to equilibrate for at least 1 day before being used as immersing liquids for AFM and SAXS. Wettability Measurements. Contact angles were measured using a Dataphysics contact angle system OCA 15 plus, at ambient temperature. A CCD camera was used to capture the contact angle of the liquid droplet. The software used for the calculation of the contact angles was SCA 20. Sphere Attachment to the Cantilever. Unsharpened cantilevers were obtained from Digital Instruments. A Dimension 3100 atomic force microscope (Veeco), was used to place tungsten spheres, with a radius of approximately 12 μm, onto V-shaped tips with super strength epoxy glue. The glue was allowed to harden before 4-5 nm of chromium and 20-30 nm of gold were deposited on each side of the cantilever (including the tungsten colloid probe) using an EMITECH K575 sputter coater apparatus. Deposition of chromium and gold (99.99%) on both sides of the cantilever was carried out to prevent substantial bending of the tips. With the tungsten sphere attached, the spring constant of the cantilevers was approximately 240 pN/nm. The cantilever spring constant was determined using the Sader method. Calculation of Sphere Radius. The radius of the sphere was calculated using a light microscope and a calibration grid, with 10 μm spacing, as a reference. This method allows for the calculation of the sphere radius with an estimated error of less than 10%. Atomic Force Microscopy Sample Preparation. Commercial 35 mm glass disks (Asylum Research) were used as the substrate. The glass disk was cleaned thoroughly using piranha solution (3:1 mixture of sulfuric acid and hydrogen peroxide) for at least 1 h. The substrate was gold-coated by first depositing 4-5 nm of chromium followed by 60-70 nm of gold using a sputter coater. As AFM measurements are sensitive to substrate roughness, efforts were made to minimize the inherent root mean square (rms) roughness of the gold-coated surface, which is typically 0.75 nm over 1 μm2. This was achieved by heating the gold-coated glass substrates in a SEM fan-forced oven for 3 min at 200 °C and then 5 min at 100 °C, after which they were brought to room temperature, producing an ultimate rms roughness of 0.3 nm over 1 μm2. The gold substrate and colloid probe were rendered hydrophobic by placing them in a 1 mM 1-octadecanethiol solution for 18 h. After this time, the substrate was rinsed with ethanol and dried with nitrogen. Force Measurements. Force measurements were collected on an Asylum Research atomic force microscope using the software MFP3D Igor Pro (version 5.0.3.0). An open fluid cell was used for all measurements. DC force measurements were determined with modulation superimposed on the vertical motion. To avoid the hydrodynamic force, the approach and withdrawal speed of the cantilever to the surface was kept to a minimum (less than 2 μm/s). The data was analyzed using the software package MFP3D. The zero deflection of the cantilever was determined by the slope. Force measurements were collected for various ethanol/water mixtures, ranging from pure ethanol to 80% water. Langmuir 2011, 27(1), 144–147
Figure 1. X-ray beam incident on dry kapton, gold-coated kapton, and gold/1-octadecanethiol-coated kapton. Scatter from the kapton comes from both bulk inhomogeneity of electron density as well as the solid/air interface. Coating the surface with gold greatly increases the contribution of the surface scatter. Coating the gold layer with 1-octadecanethiol mitigates this enhanced scatter by reducing the solid/liquid density contrast.
SAXS Measurements. The substrate (50 μm thick kapton film) was gold-coated using the same technique as for the glass substrates. Kapton produces significant but reproducible SAXS from its bulk and to a lesser extent from its surface. Coating kapton with gold greatly increases the SAXS signal attributable to its surface because the inherent surface roughness of the film substrate is transferred to the gold layer. This enhances the “visibility” of the surface homogeneity with a significantly higher electron density contrast. Figure 1 demonstrates how this and the subsequent addition of the 1-octadecanethiol coating modify the X-ray scatter. The gold-coated kapton substrates were rendered hydrophobic by placing them in a 1 mM 1-octadecanethiol solution for 18 h. After this time they were rinsed with ethanol and allowed to dry in air. For the immersion SAXS measurements, a cell was constructed from two sheets of 50 μm thick kapton separated and sealed by a 1 mm Teflon gasket. The immersing liquid could be injected into the cell and expelled through two holes above and below the region irradiated by X-rays. One of the kapton windows had the hydrophobic coating on the inward-facing side, and the other was left uncoated. The cell could then be flushed and filled with varying ethanol/water solutions without changing its position relative to the incident X-ray beam. Small variations in the gold coating of the kapton led to significant differences in scattering signal from one point of the film to the next. To mitigate this, multiple SAXS measurements were made at different points on a given film and averaged to give a single “representative” SAXS profile for each immersion test. It should be noted that the measurement positions on a given sample were kept constant from one immersion test (with different ethanol/water ratios) to the next. The X-ray beam irradiated a spot 120 60 μm2 in size, with sufficient distance (500 μm) between each measured point to ensure that there was no overlap between irradiated points. All measurements were done with an X-ray wavelength of 1 A˚ on the SAXS instrument at the Australian Synchrotron. The SAXS profiles were measured as 2D scattering images, which were then integrated radially to give 1D plots of intensity versus q. The quantity q is related to the scattering angle j and irradiating wavelength λ by q = 4π sin(j/2)/λ. All SAXS intensities quoted are given in units of cm-1, a unit of scattering intensity calibrated from the X-ray scatter from a known thickness of pure water. All measurements of dry and immersed surfaces were corrected for differences in X-ray transmission seen between the empty and liquid-filled cell. DOI: 10.1021/la1029678
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Figure 2. Contact angle data for varying ethanol/water fractions for the surface 1-octadecanethiol.
Figure 3. Approach curves for SAM of 1-octadecanethiol on gold surface for varying ethanol/water mixtures using the third V shaped cantilever (k = 240 pN/nm, sphere radius 12.5 μm), T = 20 °C. Solutions are air saturated.
Results and Discussion Force measurements were performed on 1-octadecanethiol in pure ethanol through to approximately 80% water in ethanol using a titration based system (Figure 2). Force measurements could not be performed above 80% water due to the hydrophobic nature of the thiol. The different ethanol/water mixtures had contact angles ranging from 0° to about 100°, depending on the surface (Figure 2), and were carefully titrated in to the sample cell to ensure that entrained air was not introduced to the coated interface. The force measurements demonstrated an attractive interaction that not only exceeded the range of typical van der Waals interactions (5-10 nm) but manifested itself at separations up to 120 nm.17 Force versus separation curves of the surface of 1-octadecanethiol are shown in Figure 3. The separation refers to the distance between the spherical tip and the top of the sample upon approach to the surface. In pure ethanol, there was a very weak jump-in, and only a weak adhesion was observed. There are no long-range repulsive forces in ethanol demonstrating that nanobubbles have not yet formed. The ethanol solvent was then replaced by pumping in a solution of 10% water in ethanol, the interaction force dramatically increased. The jump-in however was sudden and did not demonstrate behavior predicted by a typical van der Waals interaction curve. In general, the strength of the attractive interaction increased with increasing water content in the solvent when the same surface and tip were used. At 80% water, large deviations (17) Christenson, H. K.; Claesson, P. M.; Berg, J.; Herder, P. C. Forces between Fluorocarbon Surfactant Monolayers: Salt Effects on the Hydrophobic Interaction. J. Phys. Chem. 1989, 93(4), 1472–1478.
146 DOI: 10.1021/la1029678
Figure 4. Adhesive force of 1-octadecanethiol in various ethanol/ water fractions.
from the typical van der Waals interaction were observed, with jump-ins occurring at separations > 100 nm. The compliance slope after contact was generally linear for concentrations of less than 70% water in ethanol. At 70% water in ethanol, the compliance slope deviates from linearity, suggesting the surface is changing from a hard material to a soft material. Figure 4 shows that the adhesion interactions for 1-octadecanethiol exhibited an increase in the pull-off distance, suggesting that the cantilever probe preferred to remain on the substrate. The AFM data presented is consistent with earlier studies done on other types of hydrophobic surfaces.1-6 As with earlier measurements, the question arises as to whether the AFM probe is in fact nucleating nanobubbles. To demonstrate the independent existence of nanobubbles in a noninvasive fashion, SAXS was used to image nanobubbles on a self-assembled monolayer of 1-octadecanethiol. The SAXS profiles measured from the hydrophilic (gold coating only) and hydrophobic (gold/1-octadecanethiol coating) show clear differences both in a dry and an immersed state (Figure 5). While the gross differences between the gold and gold/thiol coatings in a dry state are explained in Figure 1, the differences between the dry and immersed profiles are subtler. At low q, a small but significant drop in SAXS intensity is seen upon immersion of both the hydrophilic and hydrophobic surfaces in pure ethanol. Pure ethanol wets both these surfaces and is therefore reducing scattering contrast at the solid/liquid interface. It should be noted that this effect is much smaller for the uncoated kapton, as its uncoated surface provides a smaller percentage of its total SAXS signal. The profiles in Figure 5 represent the aggregate of a number of profiles (>10) measured at different but consistent positions on each sample. To track the evolution of nanobubbles at the surface of the coatings, a “representative intensity” from each aggregate profile was plotted against the composition by weight of the immersing liquid (Figure 6). This “representative intensity” was the average profile intensity within the q-range region shown in Figure 5. The choice of this region was a compromise between maximizing the wetting effect signal and avoiding the lowest q values, where low-q instrumental noise starts to dominate. The changes in SAXS intensity shown in Figure 6 are shown for both the hydrophilic and the hydrophobic surfaces. The electron density Fliquid for ethanol/water mixtures increases with dilution from 2.7 1023 electrons/cm3 for pure ethanol to 3.3 1023 electrons/cm3 for pure water. The portion of the total measured scattering intensity attributable to the coating/air or coating/ liquid interface is proportional to the quantity (Fcoating - Fliquid)2, where Fcoating is the effective electron density of the coating surface. Langmuir 2011, 27(1), 144–147
Palmer et al.
Figure 5. SAXS profiles of dry kapton (A), dry kapton þ gold (B),
and dry kapton þ gold þ 1-octadecanethiol (C). The immersion profiles (dotted lines) differ at low q due to changes caused by wetting by the pure ethanol, while at high q the immersion profiles diverge due to bulk scatter from the immersing liquid.
Figure 6. Average intensity from q = 0.004 to 0.0045 A˚-1 plotted against the immersion liquid composition for both gold and gold/ 1-octadecanethiol-coated kapton surfaces. Each plotted point is derived from 10 or more irradiated positions (0.1 0.06 mm beam size) on each coated sample. The error bars represent the standard error ((2 standard deviations) derived from these averages.
In the case of a dry surface, Fliquid ∼ 0 (air has negligible electron density), which makes the interface scatter proportional to Fcoating2 alone. The immersing liquids used in this study had lower bulk electron densities than the surface coatings (i.e., 0 < Fliquid < Fcoating). As a result, bringing any of the ethanol/water mixtures in direct wetting contact to the surface should reduce the magnitude of (Fcoating - Fliquid)2 with a commensurate reduction in the measured scatter. As expected, the addition of ethanol into the dry cell decreases the measured scattering intensity for both the hydrophilic and hydrophobic coated surfaces. Differences in scattering from the two surfaces become apparent, however, as the percentage of water in the solvent mixture increases. The gold-coated kapton remains wetted for all ethanol/ water mixtures, and its scattering intensity continues to decrease after the first drop caused by initial wetting with pure ethanol. The continued intensity reduction with increasing water ratio is driven by an increase in Fliquid, which in turn reduces the quantity (Fcoating - Fliquid)2. Langmuir 2011, 27(1), 144–147
Article
Figure 7. X-ray beam incident on dry and immersed gold/1octadecanethiol-coated kapton. Surface scatter from the hydrophobic surface is suppressed when wetted by pure ethanol. However, with increasing water content in the wetting liquid, gas accumulating at the interface increases the electron density contrast across the interface, thereby increasing the surface scatter.
By contrast, the scattering intensity of the gold/1-octadecanethiol coating, rather than continuing to drop, starts increasing again almost immediately with increasing water content in the immersing liquid. This can only be explained by an increase in the effective electron density contrast. As the solid surface remains unchanged, this change in contrast must be due to nanobubbles displacing the liquid at the liquid/solid interface (Figure 7). SAXS measurements on 1-octadecanethiol demonstrate that the formation of nanobubbles is independent of an AFM tip probing the hydrophobic surface. It should be noted that the SAXS data cannot determine the morphology of the air at the hydrophobic surface, distinguishing say between nanobubbles or larger patches of air coverage (micropancakes2). The formation of nanobubbles at 5-10% water, detected by SAXS, correlates with the increase in the attractive interaction measured with AFM. It is suggested that the formation of nanobubbles on hydrophobic surfaces is responsible for the so-called hydrophobic force.
Conclusion It has been shown that the formation of nanobubbles at a hydrophobic surface during ethanol/water titration is not purely an artifact of AFM measurements. SAXS demonstrates that the formation of nanobubbles on 1-octadecanethiol is measurable at water concentrations as low as 5-10%. The SAXS measurements correlate with the deviation from the van der Waals interactions seen in the AFM measurements, and are consistent with the formation of nanobubbles on the self-assembled monolayer 1-octadecanethiol. It is therefore suggested that the formation of nanobubbles is responsible for the so-called long-range hydrophobic force. To measure true hydrophobic forces, the interaction forces between two immersed hydrophobic interfaces needs to be measured in the absence of nanobubbles. Acknowledgment. This research was undertaken on the SAXS/WAXS beamline at the Australian Synchrotron, Victoria, Australia. L.A.P. would like to acknowledge the scientific support and assistance provided by Paul Mulvaney, Nigel Kirby, Irving I. Liaw, Jacky K. L. Cho, and Alex H. F. Wu during this work. DOI: 10.1021/la1029678
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