Water Interface: From

14069

2007, 111, 14069-14072 Published on Web 11/30/2007

Transient Behavior of the Hydrophobic Surface/Water Interface: From Nanobubbles to Organic Layer Julio Martinez and Pieter Stroeve* Department of Chemical Engineering and Materials Science, UniVersity of California DaVis, 1 Shields AVenue, DaVis, California 95616 ReceiVed: September 4, 2007; In Final Form: NoVember 14, 2007

We report the formation and subsequent change of the water-depleted layer at a hydrophobic surface/water interface. With water as the solvent, surface plasmon resonance measurements indicate time dependent evolution of two separate states. The first state is the water-depleted layer, and it is characterized by a layer of nanobubbles on the surface and is short-lived in time (order of 10 min). The second state is a final equilibrium state, which occurs in approximately 30 h, where a layer is formed with organic characteristics. If, instead of water, an aqueous solution is exposed to the hydrophobic surface, the evolution from nanobubbles to an organic like layer shows dependency on the surface energy of the liquid media.

Introduction The existence and nature of the layer nearby a hydrophobic surface immersed in an aqueous solution is a controversial topic in the surface-science community. Several research groups have observed a layer composed of nanobubbles by atomic force microscopy and neutron reflection.1-5 On the other hand, some studies cannot corroborate the layer’s presence under similar experimental conditions.6,7 Furthermore, others report an organic layer rather than nanobubbles.8,9 Molecular dynamic simulations indicate the presence of a region at a hydrophobic surface which is known as the water-depleted layer (WDL) where the density of water molecules gradually decreases from a bulk value to virtually no water molecules at the surface.10 Models that incorporate the WDL have been used for fitting neutron and X-ray reflectivity data for hydrophobic surface/water systems.11-13 Present research is focused on providing an explanation for these different and sometimes conflicting observations.14,15 In this work, we study the formation of the WDL on a hydrophobic self-assembled monolayer (SAM) in an aqueous solution by using surface plasmon resonance (SPR). We experimentally observe that initially the WDL is formed on the SAM. However, the composition of the WDL layer changes slowly in time into a layer with organic character. To our knowledge, this is the first report in which the WDL is shown to have transient behavior supporting the existence of both a nanobubbles and an organic layer. Experimental Section Material and Methods. Undecanethiol, 99+% sodium sulfate, and n-butanol were purchase from Sigma-Aldrich. Water of 18 MΩ/cm or better was used in this research. Ethanol 200 proof (ETH) from Gold Shield Chemical was employed. Pure gold (99.999%) was deposited onto clean LaSFN9 glass slides (Schott, Germany) by thermal evaporation in an Edwards AUTO * Corresponding author. E-mail: [email protected].

10.1021/jp077110d CCC: $37.00

306 under a vacuum of 1 × 10-5 Torr. The rate of gold deposition was between 0.2 and 0.4 Å/s. The self-assembled monolayer of undecanethiol on gold (hydrophobic SAM) was formed by exposing the gold covered LaSFN9 glass slides to a 5 mM undecanethiol/ethanol solution for 18 h.16 After SAM formation on the gold, the surfaces were rinsed with large amounts of ethanol and dried with nitrogen. Three solutions were prepared: 30% v/v ethanol in water (ETH:30), 0.88M Na2SO4 in water (W-SALT), and 10 mM n-butanol in water (n-BTOH). These solutions were employed through this research. Experimental Method. The SPR unit, equipped with a He/Ne LASER (wavelength 632.8 nm) and flow cell, has been described previously.16 The flow cell (sample holder), solution containers, tubing, and connectors were meticulously cleaned before the experiments. Solutions were pumped into the SPR flow cell (volume less than 1 mL) at a rate of 5 mL/min with a peristaltic pump in such a way that air bubbles were avoided in the system. Three sets of data were collected. The first set was performed with the hydrophobic surface initially exposed to ETH. Then, the water was introduced into the SPR cell and continuously flushed for 20 min to remove any trace of ETH from the system. Finally, water was circulated in a closed loop to avoid evaporation. The second set followed the previous steps, but the SAM was initially exposed to the ETH:30 solution, and then the W-SALT solution was introduced into the sytem. Finally, the third set included the exchange of the solution exposed to the SAM from ethanol to n-BTOH. The total volume of the circulating solution in the closed loop was equal to 1 L. Modeling of the normalized reflectivity versus angle data was carried out with the WINSPALL 2.0 program (Max-Planck Institute for Polymer Research, Mainz, Germany) where the refractive index of each layer was fixed and the thickness was selected as the estimated parameter. Refractive indexes for solutions were measured with an Abbe 60 refractometer using a sodium lamp as the light source. The measured refractive index of the ETH:30 solution was equal to 1.349. The W-SALT © 2007 American Chemical Society

14070 J. Phys. Chem. B, Vol. 111, No. 51, 2007

Letters

Figure 2. Change of normalized reflectivity measured for ETH:30, W-SALT case. RSAM stands for the reflectivity of SAM/ETH:30 at 57° (2° degree less than the minimum reflectivity angle for SAM/ETH: 30). The inset provides detail at the time of W-SALT injection.

Figure 1. (A) Change of normalized reflectivity measured for ETH, water case. RSAM stands for the reflectivity of SAM/ETH at 56.6° about 1° less than the minimum reflectivity angle. Note change in the scale. (B) Reflectivity change as function of laser incident angle for the final equilibrium condition in (A). The dashed curve for no WDL is calculated using the measured thicknesses of the gold and SAM layers. The inset provides a close detail of the SAM/water minimum plasmon angle.

solution (0.88M Na2SO4) was selected because its refractive index exactly matches the one for ETH:30 for the sodium line as the light source. Because ETH:30 and W-SALT are largely composed of water, we assume that at 632.8 nm both refractive indexes are the same and equal to 1.346.17 Thicknesses equal to 45 ( 3 nm for Au and 1.5 ( 0.1 nm for the SAM were measured by SPR. All experimental runs were performed at a temperature of 24 ( 1 °C. Result and Discussion We study the evolution of the WDL on the SAM by the change of the normalized reflectivity at a fixed angle resulting from the exchange of the solvent from ETH to pure water (Figure 1A). The observed initial drop for the normalized reflectivity in Figure 1A is due to the change of the refractive index of ethanol to that of water (1.357 and 1.331, respectively17). An apparent equilibrium plateau is reached within

minutes after continuous pure water injection and is steady for 10 min. However, more careful inspection indicates that the normalized reflectivity is not steady over much longer times. The normalized reflectivity changes slowly over many hours and increases to reach a final equilibrium reflectivity value of about 0.53 after 30 h. The pure water flowing in the loop is not absolutely sealed from the environment. For a period of 30 h, an extremely small amount of organic material from the air could enter the flow system by diffusion through the tubes and the connectors into the flowing water and be transported into the sample holder. To determine if indeed the increase in reflectivity is related to the adsorption of organic material, the reflectivity as a function of angle at the final equilibrium conditions for SAM/water is plotted in Figure 1B and compared to the calculated curve for the same system assuming no WDL. The observed higher plasmon angle for the SAM/water curve indicates the adsorption of layer with an average refractive index larger than water,18 suggesting the existence of layer with organic characteristics as previously reported.8,9 The calculated R/RSAM for SAM/water with no WDL is 0.42, which turns out to be larger than R/RSAM for the plateau shown in Figure 1A, which supports the formation of a nanobubbles layer at the initial stage of the water introduction.14 The absence of nanobubbles on graphite for ethanol solutions with concentration larger than 20% v/v in water has been observed by AFM.19 For our system, SPR data corroborates such observation on SAM. The change of normalized reflectivity at a fixed angle by changing the solvent exposed to the SAM from ETH:30 to W-SALT solution is shown in Figure 2. Due to the fact that the refractive index of the background solution is maintained constant, a change in the SPR response is associated with the formation of a new layer.18 The mixing of ETH:30 with W-SALT solution at the initial stage results in the increment of background refractive index20 observed in Figure 2 as a peak right at the beginning of the W-SALT injection into the SPR flow cell. Then, the normalized reflectivity decreases, which is associated with the formation of a layer with a refractive index smaller than the background solution.18 This new layer indicates the formation of nanobubbles onto the SAM.

Letters

Figure 3. Reflectivity change as function of laser incident angle at the equilibrium condition for SAM/water and SAM/W-SALT cases. Fitting to the experimental data is represented by the solid line assuming a refractive index of 1.37 for the adsorbed organic layer.

However, the composition of the WDL is not steady because the normalized reflectivity continuously increases reaching its maximum about 10 h after W-SALT injection. Surprisingly, the equilibrium value is 1.028 with R/RSAM larger than 1 observed 60 min after solution exchange. The resulting R/RSAM indicates that the average refractive index of the WDL is larger than W-SALT,18 and it can be explained by the adsorption of organics on the SAM.9 The effect of salt solutions on a nanobubble layer has been reported by Zhang and co-workers14 who showed no substantial change with respect to the water case. However, they did not report the exchange of the hydrophobic surface/ethanol system directly by a salt solution. The increment observed for ETH:30/W-SALT and ETH/water systems from a minimum R/RSAM value is explained by the presence of organic material, which progressively adsorbs on the SAM presumably contaminants picked up by the media over time of the experiment.8,9,15 Experimental data indicate the formation of a nanobubbles layer that precedes the adsorption of organic contaminants. However, the presence of nanobubbles at the final equilibrium conditions cannot be discarded. For example, adsorbed bovine serum albumin has been observed to coexist with nanobubbles.21 The upper limit for the refractive index of an organic layer adsorbed on a hydrophobic surface has been reported as ∼1.37.9 Assuming this refractive index for our adsorbed layer at the equilibrium, thicknesses of 1 nm for W-SALT and 5.6 nm for water cases (Figure 3) are obtained, which are larger than the 0.6 nm of the adsorbed organic layer on polystyrene thin film observed by NR.15 Maccarini et al. reported the influence of aqueous salt solution/hydrophobic SAM systems on the WDL. They observed a larger WDL thickness than that for water/SAM case which it was associated with a decrease of the affinity of aqueous salt solutions to the SAM due to the increment of the surface energy induced by the addition of salt to water.22 The surface energy for W-SALT is calculated from experimental data23 as 75 mJ/m2 at 25 °C or about 3% larger than pure water surface energy at the same temperature. The larger surface energy of W-SALT can explain the absence of a steady nanobubbles layer for W-SALT/SAM case because the energy cost of keeping organics in solution would be increased compare with water/SAM case. The addition

J. Phys. Chem. B, Vol. 111, No. 51, 2007 14071

Figure 4. Change of normalized reflectivity as function of time for SAM/water, SAM/W-SALT, and SAM/n-BTOH cases after the minimum plateau region. Thus time ) 0 min is the time at which the normalized reflectivity starts to steadily increase after the minimum plateau value. Note that the R/RSAM ∼ 0.92 at time ) 0 for SAM/ W-SALT case. Arrows indicate the corresponding R/RSAM axis.

of a surface-active species to the solution exposed to the hydrophobic SAM has also been explored. AFM studies indicate that nanobubbles exist in such situations.14,24 For example, the reduction of the nanobubbles sizes have been observed for a 20% volume of 2-butanol in water solution. In our work, solution exchange from ethanol to n-BTOH in the flow cell gives a minimum plateau associated with the nanobubbles layer (see Supporting Information). However, the initial rate of change from the minimum R/RSAM was the slowest for the n-BTOH case among all studied cases, the fastest being the W-SALT/ SAM case (Figure 4). The slow rate of adsorption onto the SAM is probably due to an increase of the hydrophobic interactions between the liquid media and species in solution because of the lower surface energy of the 10 mM n-BTOH (71 mJ/m2 at 25 °C25). In this research, both layers are observed rather than either the nanobubbles or the organic layer as previously reported. Previously, sample preparation was introduced as a factor that influences the presence or absence of either nanobubbles14 or organic contamination.15 Due to the very long experimental time scale, the fluid flowing in the closed loop can pick up contaminants from the air due to diffusion through the tubing and the connections in the loop. The plateau observed in Figure 1A can be mistakenly taken as equilibrium, with the WDL composed by nanobubbles, if the experiment is terminated after 10 min. This point is important because conclusions regarding the WDL may be also influenced at what time the data are collected. Conclusions Long-time SPR reflectivity data provide evidence of the formation of a WDL composed of nanobubbles. The WDL exhibits transient behavior by very slowly evolving into an organic-like layer. The rate of this evolution is increased by an increment of the surface energy of the solution exposed to the hydrophobic SAM which indicates the nature of the hydrophobic interactions for the reported observations. The findings reported in this work provide for the first time a link between previously contradictory reports and sheds insight to the formation and characterization of the WDL. Acknowledgment. We gratefully acknowledge the financial support from the Manuel Lujan Jr., Neutron Scattering Center

14072 J. Phys. Chem. B, Vol. 111, No. 51, 2007 through the CARE program. The Manuel Lujan Jr., Neutron Scattering Center is a national user facility funded by the United States Department of Energy, Office of Basic Energy SciencesMaterials Science. At the time that this research was completed, the Manuel Lujan Jr., Neutron Scattering Center was managed by the University of California under contract No. W-7405ENG-36. P.S. thanks Prof. Dr. Wolfgang Knoll, of the MPI for Polymer Research at Mainz, Germany, for the invaluable advice that helped the author to construct two SPR instruments in his laboratory at UC Davis. This advice has given the author the unique opportunity to conduct SPR research in the past 10 years. Supporting Information Available: The reflectivity change as a function of laser incident angle at the equilibrium condition for the SAM/ ETH:30, SAM/W-SALT, and hydrophilic SAM/ ethanol-water cases; the change of normalized reflectivity as function of time measured for the n-BTOH case. Supporting information is available for free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Steitz, R.; Gutberlet, T.; Hauss, T.; Klo¨sgen, B.; Krastev, R.; Schemmel, S.; Simonsen, A. C.; Findenegg, G. H. Nanobubbles and Their Precursor Layer at the Interface of Water Against a Hydrophobic Substrate. Langmuir 2003, 19, 2409-2418. (2) 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, 2573-2575. (3) Jeon, S.; Desikan, R.; Tian, F.; Thundat, T. Influence of Nanobubbles on the Bending of Microcantilevers. Appl. Phys. Lett. 2006, 88, 103118. (4) Simonsen, A. C.; Hansen, P. L.; Klo¨sgen, B. Nanobubbles Give Evidence of Incomplete Wetting at a Hydrophobic Interface. J. Colloid Interface Sci. 2004, 273, 291-299. (5) Agrawal, A.; Park, J.; Ryu, D. Y.; Hammond, P. T.; Russell, T. P.; McKinley, G. H. Controlling the Location and Spatial Extent of Nanobubbles Using Hydrophobically Nanopatterned Surfaces. Nano Lett. 2005, 5, 1751-1756. (6) Takata, Y.; Cho, J. H. J.; Law, B. M.; Aratono, M. Ellipsometric Search for Vapor Layers at Liquid-hydrophobic Solid Surfaces. Langmuir 2006, 22, 1715-1721. (7) Mao, M.; Zhang, J. H.; Yoon, R. H.; Ducker, W. A. Is There a Thin Film of Air at the Interface Between Water and Smooth Hydrophobic Solids? Langmuir 2004, 20, 1834-1849. (8) Evans, D. R.; Craig, V. S. J.; Senden, T. J. The Hydrophobic Force: Nanobubbles of Polymeric Contaminant? Physica A 2004, 339, 101105.

Letters (9) McKee, C. T.; Ducker, W. A. Refractive Index of Thin, Aqueous Films between Hydrophobic Surfaces Studied Using Evanescent Wave Atomic Force Microscopy. Langmuir 2005, 21, 12153-12159. (10) Mamatkulov, S. I.; Khabibullaev, P. K.; Netz, R. R. Water at Hydrophobic Substrates: Curvature, Pressure, and Temperature Effects. Langmuir 2004, 20, 4756-4763. (11) Doshi, D. A.; Watkins, E. B.; Israelachvili, J. N.; Majewski, J. Reduced Water Density at Hydrophobic Surfaces: Effect of Dissolved Gases. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 9458-9462. (12) Schwendel, D.; Hayashi, T.; Dahint, R.; Pertsin, A.; Grunze, M.; Steitz, R.; Schreiber, F. Interaction of Water with Self-assembled Monolayers: Neutron Reflectivity Measurements of the Water Density in the Interface Region. Langmuir 2003, 19, 2284-2293. (13) Mezger, M.; Reichert, H.; Scho¨der, S.; Okasinski, J.; Schro¨der, H.; Dosch, H.; Palms, D.; Ralston, J.; Honkima¨ki, V. High-resolution in Situ X-Ray Study of the Hydrophobic Gap at the Water-Octadecyl-Trichlorosilane Interface. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 18401-18404. (14) Zhang, X. H.; Maeda, N.; Craig, V. S. J. Physical Properties of Nanobubbles on Hydrophobic Surfaces in Water and Aqueous Solutions. Langmuir 2006, 22, 5025-5035. (15) Seo, Y. S.; Satija, S. No Intrinsic Depletion Layer on a Polystyrene Thin Film at a Water Interface. Langmuir 2006, 22, 7113-7116. (16) Martinez, J.; Talroze, R.; Watkins, E.; Majewski, J. P.; Stroeve, P. Templating Polypeptides on Self-Assembled Hemicylindrical Surface Micelles. J. Phys. Chem. C 2007, 111, 9211-9220. (17) Longtin, P. J.; Fan, C. H. Precision Laser-Based Concentration and Refractive Index Measurement of Liquids. Microscale Therm. Eng. 1998, 2, 261-272. (18) Jung, L. S.; Campbell, C. T.; Chinowsky, T. M.; Mar, M. N.; Yee, S. S. Quantitative Interpretation of the Response of Surface Plasmon Resonance Sensors to Adsorbed Films. Langmuir 1998, 14, 5636-5648. (19) Zhang, X. H.; Wu, Z. H.; Zhang, X. D.; Li, G.; Hu, J. Nanobubbles at the Interface of HOPG and Ethanol Solution. Int. J. Nanoscience 2005, 4, 399-407. (20) Padova, J. Ion-Solvent Interaction in Mixed Solvents. Part III. The Molar Refraction of Electrolytes. Can. J. Chem. 1965, 43, 458-462. (21) Wu, Z.; Zhang, X.; Zhang, X.; Li, G.; Sun, J.; Zhang, Y.; Li, M.; Hu, J. Nanobubbles Influence on BSA Adsorption on Mica Surface. Surf. Interface Anal. 2005, 37, 797-801. (22) Maccarini, M.; Steitz, R.; Himmelhaus, M.; Fick, J.; Tatur, S.; Wolff, M.; Grunze, M.; Janecˇek, J.; Netz, R. R. Density Deletion at SolidLiquid Interfaces: a Neutron Reflectivity Study. Langmuir 2007, 23, 598608. (23) Abramzon, A. A.; Gaukhberg, R. D. Surface Tension of Salt Solutions. Russian J. Appl. Chem. 1993, 66, 1473-1480. (24) 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, 7072-7077. (25) Harkins, W. D.; Wampler, R. W. The Activity Coefficients and The Adsorption of Organic Solutes. I. Normal Butyl Alcohol in Aqueous Solution by the Freezing Point Method. J. Am. Chem. Soc. 1931, 53, 850859.