Scanning of Silicon Wafers in Contact with Aqueous CTAB Solutions

Article pubs.acs.org/Langmuir

Scanning of Silicon Wafers in Contact with Aqueous CTAB Solutions below the CMC Liset A. C. Lüderitz and Regine von Klitzing* Stranski-Laboratorium für Physikalische und Theoretische Chemie, Institut für Chemie, Technische Universität Berlin, Strasse des 17. Juni 124, D-10623 Berlin, Germany ABSTRACT: Hydrophilic silicon wafers are studied against aqueous solutions of hexadecyl trimethyl ammonium bromide (CTAB) at concentrations between 0.05 mM up to 1 mM (CMC). AFM studies show that nanobubbles are formed at concentrations up to 0.4 mM. From 0.5 mM upward, no bubbles could be detected. This is interpreted as the formation of hydrophobic domains of surfactant aggregates, becoming hydrophilic at about 0.5 mM. The high contact angle of the nanobubbles (140−150° through water) indicates that the nanobubbles are located on the surfactant domains. A combined imaging and colloidal probe AFM study serves to highlight the surfactant patches adsorbed at the surface via nanobubbles. The nanobubbles have a diameter between 30 and 60 nm (after tip deconvolution), depending on the surfactant concentration. This corresponds to a Laplace pressure of about 30 atm. The presence of the nanobubbles is correlated with force measurements between a silica probe and a silicon wafer surface. The study is a contribution to the better understanding of the short-range attraction between hydrophilic surfaces exposed to a surfactant solution.

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INTRODUCTION Surfactants can be used as stabilizers/emulsifiers (above the CMC) in the cosmetic industry. Below the CMC, surfactants find applications in flotation processes since they can be adsorbed on a hydrophilic surface rendering the surface hydrophobic.1 The surfactant morphologies found at the surface are a function of the surface charge, the type of head groups, and the hydrophobic part of the surfactant.2,3 Many studies have been performed to clarify the structure of surfactants at the surface of mica or silica surfaces above the CMC. At the silica surface, micelles or flattened bilayers of CTAB close to the CMC have been reported.4 Velegol et al.5 described the CTAB adsorbed layer at the silica surface in the presence of 0.9 × CMC and 10 × CMC solutions. At 0.9 × CMC surfactant concentration, a coexistence of spheres and short rods was observed at the silica surface, whereas wormlike micelles were observed at 10 × CMC surfactant concentration. In some cases, a transition from the wormlike micelles to a laterally homogeneous structure (interpreted as a bilayer) similar to that observed on mica occurred. Ducker et al.6 studied the adsorption of CTAB on mica at a concentration of 2 × CMC. They obtained a flat sheet CTAB morphology in the absence of salt at the mica surface. Sharma et al.7 reported that the adsorption of CTAB on mica at low concentration 10−5 M occurs patchwise. The distances between the patches was not constant, and a coexistence between patches of different heights was also observed, which was interpreted as surfactant molecules or aggregates. An increase of the concentration produced more closely packed surfactant patches. At 10−3 M © 2012 American Chemical Society

surfactant concentration, a continuous wormlike admicellar structure with reduced separation compared to previous concentrations was observed at the mica surface. A further increase of the concentration to 10−2 M produced a continuous bilayer structure at the mica surface. They demonstrated that the variation of pH with the consequent variation in surface charge density influences the structure of the adsorbed micelles. The surface charge can also be varied by surface treatment.8 Different substrates may also have different surface charges. The p.z.c for the adsorption of CTAB depends on the surface used: at the silica surface, 5 × 10−5 M was obtained, whereas the neutralization of the mica surface occurred at a lower concentration of 3.5 × 10−6 M.9 Yaminski et al.10 studied the adsorption of CTAB to a silica surface in the presence of sodium acetate using a surface force apparatus. They reported a pronounced attraction between surfaces when the CTAB concentration is increased to 5 × 10−5 M. The attraction is explained by so-called hydrophobic interactions. Hence, one mechanism to explain the hydrophobic interactions is through bridging of nanobubbles, which are present at the opposing hydrophobic surfaces.11−15 The presence of nanobubbles corresponds to a reduced density of water, which was detected at hydrophobic surfaces by neutron reflectometry.16 It is known from the literature that the nanobubbles are stable for several days at hydrophobic interfaces and that they Received: July 11, 2011 Revised: January 16, 2012 Published: January 20, 2012 3360

dx.doi.org/10.1021/la202635a | Langmuir 2012, 28, 3360−3368

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are present in solutions saturated with gas.17 So far, nanobubbles have been studied on surfaces hydrophobized by chemical pretreatment (HOPG,18 OTS18,19). The liquid phase was either water or surfactant solutions like SDS or CTAB.18 Recently, nanobubbles were imaged on ultraflat gold covered with binary self-assembled monolayers (SAMs) with variable hydrophilic/hydrophobic balance.20 Still, nanobubbles were found at the surface of SAMs with a macroscopic contact angle of 15°, but they were very tiny. Ducker17 explains the stability of nanobubbles at a hydrophobic surface by surface active contaminants. Under this assumption, the adsorption of surface active material to the nanobubble avoids the diffusion of gas out of the nanobubbles. The surface active material will stabilize the bubbles through creation of a diffusional barrier. The surface active contaminant may be adsorbed at the solid/liquid interface with the corresponding decrease of the solid−liquid interfacial tension. In that case, the liquid/vapor interfacial tension has to become extremely small to fit the observed low nanobubble contact angle (θ ≈ 16°), which leads to a flattening of the nanobubbles.17 Zhang et al.18 studied the nanobubbles in the presence of two different surfactants, hexadecyltrimethylammonium bromide (CTAB) and (SDS) sodium dodecylsulfate at 0.5 × CMC. Little or no variation in contact angle was observed for the nanobubbles present at HOPG or OTS surfaces when surfactant was added to the solution. This was later explained by the fact that the nanobubbles were already covered with some kind of surface active material (contaminants) so that the effect of surfactant on the nanobubbles was not seen.17 A further proof for stabilization of nanobubbles by contaminants was their decreased stability in surfactant solutions well above the CMC, where the contamination is solubilized within the micelles. The bubbles on a graphite surface disappeared after 15 min exposure to 5 × CMC SDS solution. A systematic investigation of the influence of the surfactant concentration on the stability of nanobubbles at a solid/liquid interface is still missing. Another open question is the hydrophilic/hydrophobic balance of silicon oxide surfaces at low CTAB concentration (below 0.5 mM). Therefore, in the present article, hydrophilic silicon oxide surfaces are studied against aqueous solutions of a broad CTAB concentration regime (0.05−0.8 mM). In this regime, the silicon oxide surface is partially hydrophobic, and nanobubbles may be present at the surface. Hence, the surfactant fulfills two tasks: (1) modification of the silicon oxide surface via physisorption and (2) the stabilization of the nanobubbles. The nanobubbles are studied via scanning force microscopy (SFM) and the results are correlated with the interactions between a silica microsphere and a planar silicon oxide surface against aqueous CTAB solutions in a Colloidal Probe AFM.

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solution H2O2/H2SO4 50:50 for 30 min, thereafter washed with MilliQ water, and then immediately used for the experiments. This method allows the creation of an oxide layer at the silicon wafer surface and renders the surface highly hydrophilic. Methods. The scanning was performed using a MFP-3D Asylum Research atomic force microscope (AFM). The images of the spherical features in liquid were obtained in iDrive tapping mode using iDrive compatible cantilevers from Asylum Research. These were gold coated with a nominal spring constant of 0.09 N/m. iDrive is a patented technique that uses Lorentz force to magnetically actuate a cantilever with an oscillating current that flows through the legs.22 This technique is recommended for imaging of extremely soft matter in liquid. The set point was adjusted to minimize the force on the sample while still tracking the surface. The tip curvature radius measured by SEM is Rtip = 15 nm ± 5 nm. A new experiment was performed for each concentration. Further, the images were flattened with a first or second order polynomial fit. The cross-section of the observed spherical features was fitted to an arc of a circle, which allows the determination of the bubble parameters (see refs 19, 23, and 24). At least 10 spherical features were analyzed (when possible) to obtain the parameters. The roughness (rms) was obtained from a 300 × 300 nm image. The force measurements between a silica particle and an oxidized silicon wafer were also performed with a MFP-3D Asylum Research AFM mounted on an inverted optical microscope (Olympus IX71). This technique is well described elsewhere.25−27 In brief, a cantilever with the colloidal probe is mounted on the AFM head and a clean silicon wafer is placed on the scanner. A laser is pointed at the end of the cantilever. The cantilever moves in the z direction, and the deflection of the cantilever while approaching the surfaces is registered by a photosensitive detector. The spring constant is determined using the thermal noise method; the typical value is 0.03 N/m. A new clean silicon wafer was used for each concentration. For the analysis, only approach curves are shown. The velocity of the approach was 600 nm/ s. All cantilevers were plasma cleaned before use. The measurements with MPF-3D were obtained at room temperature at 1 atm. The contact angles were measured with a Goniometer (OCA5) using the Sessile drop method. The clean silicon wafers were placed on the sample stage. A syringe was used to place a drop of the surfactant solution on the silicon wafer surface. The software SCA 20 determined the contact angle using the Young−Laplace fitting method.28

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RESULTS Scanning of a Silicon Wafer from 0.05 to 1 mM CTAB. Figure 1 represents the surface of a silicon oxide surface in

EXPERIMENTAL SECTION

Materials. Solutions from cetyl-trimethyl-ammonium bromide (CTAB, analytic grade, Aldrich, purity > 99%) were prepared in a range of concentrations, 0.05 mM to 0.8 mM in pure water (Milli-Q). CTAB was used as received. Surface tension measurements were made in the range of the studied concentration at 298 K with a tensiometer Krüss K11 using the ring method. The CMC of CTAB obtained was 1 mM, no minima was detected around the CMC, which indicates that no surface active contaminants were present in the CTAB used. The value of surface tension at 1 mM CMC of 36.2 mN/m correlates well with the literature value.21 Nonporous silica particles, 4.63 μm in diameter, were used for the force measurements. The silicon wafers (type-P Wacker Siltronic Burghausen) were cut and cleaned in piranha

Figure 1. AFM tapping mode of a silicon oxide surface in water. AFM images taken with a magnetic actuated cantilever; nominal spring constant, 0.09 N/m; amplitude set point, 0.265 V; set point ratio, 0.26; scan rate, 0.5 Hz; and drive frequency, 6.91 kHz.

milli-Q water. A homogeneous surface is observed in water with a surface roughness of 0.43 nm. The contact angle of the cleaned silicon wafer is close to 0°. 3361


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Figure 2. AFM tapping mode of a silicon oxide surface at 0.05 mM CTAB concentration. AFM images taken with a magnetic actuated cantilever; nominal spring constant, 0.09 N/m; amplitude set point, 0.430 V; set point ratio, 0.43; scan rate, 0.5 Hz; and drive frequency, 6.34 kHz.

was observed on flat hydrophilic silicon wafers. The surface is smooth and hydrophilic with a roughness of 0.43 nm determined in water by the iDrive tapping mode. The roughness correlates well with the values obtained in the literature.29 In the presence of 0.05−0.4 mM CTAB, some features appear at the silicon wafer surface, which resemble nanobubbles. They are spherical, at 0.5 mM flatter (micropancakes), and they vanish close to CMC (0.8 × CMC). Similar morphologies have been observed by other authors.19,20,30−34 From the phase image, one can conclude that the features are deformable at least at a concentration of 0.05 mM. The features are not present when a hydrophilic silicon wafer is imaged in water. The features with heights between 8 and 15 nm correlate well with that reported by other authors.35 At low concentration of 0.05 mM, the feature height is around 15 nm. The observed features cannot be micelles because micelles would have a diameter of 3.4 nm.5 The properties of the spherical features suggest that they are nanobubbles on the surface of a partially hydrophobic silicon wafer (see Figure 7). The presence of nanobubbles at the surface in Figure 2 is an evidence that a partial hydrophobization of the silicon wafer surface occurs due to the adsorption of surfactant. The nucleation of the nanobubbles may be produced by air dissolved in water.17 The nanobubbles have a regular shape and are stabilized by the surfactant in the solution. It can not be excluded that surface active contaminants may also be present at the bubble interface.17,36 The contaminants in ref 17 are due to the surface preparation (silanization process). Since in the

When the concentration of surfactant is increased to 0.05 mM spherical features (Figure 2), which resemble nanobubbles, are observed at the surface. At this concentration, the features diameter is 50−80 nm. In the phase image, the spherical features are also recognized since a phase drop is observed along the features indicating the different nature of features and substrate. At 0.3 mM the features are still spherical, and the diameter is around 57 nm (Figure 3). The spherical features dominate the surface topology. From this concentration, a slight phase shift between substrate and feature is observed in the phase image. At 0.4 mM surfactant concentration, a few spherical features could still be found at the surface (see Figure 4). Since the observed features resemble a spherical cap, their parameters can be obtained by fitting a cross-section to an arc of a circle (see Table 1). Figures 5 and 6 represent the adsorbed surfactant layer at concentrations 0.5 and 0.8 mM. For 0.5 mM, other types of features were found at the surface. They are flatter (height about 2 nm) than the features at lower CTAB concentration. At 0.8 mM, the roughness decreases to 0.27 ± 0.03 nm, which is lower than the original silicon wafer roughness (0.43 nm), and no features could be identified.

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DISCUSSION Nanobubbles. No features are observed at the surface of a cleaned hydrophilic silicon wafer. That correlates with the findings of ref 29 where no spontaneous formation of bubbles 3362


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surfactant concentration of 0.3 mM, the bubble height is 7.6 ± 1 nm and the radius 28 ± 4.3 nm (see Figure 3). At 0.4 mM, the bubbles are smaller, which introduces more error in the analysis. At this concentration, nanobubbles with heights of 5− 8 nm were found with a radius between 20−33 nm. The nanoscopic contact angle remains constant between 140° and 150° irrespective of the CTAB concentration (see Table 1) and is in good agreement with the literature.18,37,38 Zhang et al.19 also obtained no variation in contact angle in spite of a high polydispersity. This is in contrast to the work of Yang et al.39 where a variation of the nanoscopic contact angle of the nanobubbles was observed after adding 2-butanol surfactant. They reported a slight decrease in the height of the nanobubbles and a more pronounced decrease in width after adding surfactant, which led to a decrease of the nanoscopic contact angle. The diameters of the nanobubbles in this work are smaller than those reported by other authors18,19 because they are associated to the hydrophobic domains at the surface. They are stable at least for 30 min observation time. Yang et al.39 reported small bubbles on a hydrophobic surface with a diameter of 100 nm, whereas Kameda et al.40 reported nanobubbles with a diameter between 10 and 100 nm on a Au(111) surface. Since the size of the bubbles is in the same order of magnitude as the size of the tip, it is necessary to perform tip deconvolution. The end of the tip is spherical with a curvature radius of Rtip = 15 nm ± 5 nm obtained by scanning electron

present experiments the partial hydrophobization occurs in situ on a clean hydrophilic silicon wafer, only a tiny amount of contaminants should be present, if any. Another source of contaminant could be the tip itself.37 Borkent et al.37 proposed that contaminants coming from the gel package and deposited on the tip can precipitate on the organic surface once the tip is immersed in water for scanning. The polysiloxanes (from the gel package) could be adsorbed at the air/liquid interface (it is energetically unfavorable that polysiloxanes deposit on hydrophilic surfaces immersed in liquid20) and might be the reason for the discrepancy observed between the nanoscopic and macroscopic contact angles.37 Song et al.20 did not find experimental evidence that oligomeric siloxanes from the gel package influence the contact angle. Therefore, this source of contaminants can also be excluded from our experiments. The nanobubbles seem to be flattened at the surface (see Table 1). We propose that the nanobubbles are seated at the hydrophobic tail of the CTAB (see Figure 7) and that they label the hydrophobic domains. The size and distribution of the aggregates in the presence of nanobubbles may differ from the size and distribution of the aggregates in degassed solution since the bubbles could modify the distribution of surfactant. The bubble parameters were obtained by fitting a crosssection of the nanobubble to an arc of a circle.19,23,24 At 0.05 mM surfactant concentration, the height of the bubbles is 13 ± 2.2 nm, and the radius is 37.7 ± 4.6 nm. With increasing surfactant concentration, the bubbles become smaller. At a 3363


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Table 1. Parameters of the Nanobubbles Obtained by Fitting the Cross-Section to an Arc of a Circlea surfactant concentration, I (mM)

av height, h (nm)

av bubble radius, r (nm)

av radius of curvature, Rc (nm)

contact angle, θair (deg)

macroscopic contact angle, θwater (deg)

av nanoscopic contact angle, θwater (deg)

0.05 0.3 0.4

13.1 7.5 5.2

37.7 28.0 20.4

58.6 58.0 46.5

38.4 30.4 29.0

48.4 41.0 42.4

141.6 149.6 151.0

a

The small size of the nanobubbles at 0.4 mM CTAB concentration introduces more errors in the fitting and in the obtained parameters, but still the parameters of the nanobubbles are shown for comparison.

microscopy. The deconvoluted radius of the curvature of the nanobubbles can be calculated as follows:20 R cd = R c − R tip

According to the Laplace equation ΔP =

(1)

2σvl Rc

(2)

(ΔP, Laplace pressure; σvl, interfacial tension at the liquid− vapor interface; Rc, curvature radius), the increase in surfactant concentration CTAB and the related decrease in surface tension leads to a decrease in bubble radius at constant ΔP (see Figures 2−4). The nanoscopic contact angle througth water is much larger than the macroscopic one (see Table 1). Song et al.20 reported a correlation within the experimental errors of the macroscopic and nanoscopic contact angle of nanobubbles found on a hydrophilic sample (binary self-assembly monolayers; SAMs, θmacro = 37°). In our experiments, only the local hydrophobicity of the (hydrophobic) domains/surfactant aggregates is taken into account for the determination of the nanoscopic contact angle . In contrast, different areas including hydrophobic domains and hydrophilic areas contribute to the macroscopic contact angle. A drop in phase angle is detected at the position of the nanobubbles at low surfactant concentration (see Figure 2). At higher concentrations, the phase shift was

(Rcd, deconvoluted curvature radius of the nanobubble; Rc, convoluted curvature radius of the nanobubble; Rtip, tip curvature radius). The error propagation was stated (see Table 2) taking into account the error in fitting the nanobubble cross-section to an arc of a circle and the error due to the uncertainty in the determination of the tip radius. Note that the nanobubble parameters vary slightly after tip deconvolution and that the nanoscopic contact angle still remains constant within the experimental errors (see Table 2). The Laplace equation predicts that small bubbles will have a large internal pressure. The pressure inside the bubbles is about 30 times the atmospheric pressure for the studied concentrations. Song et al.20 obtained similar Laplace pressures for the nanobubbles. In comparison, the internal pressure of the nanobubbles in water with a diameter of about 1 μm is in the range of 1 to 1.7 atm.19 3364


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Figure 7. Schematic picture of nanobubbles (not to scale) seated on the hydrophobic tails of the surfactant molecules.

small or not detected (see Figures 3−5). Simonsen et al.41 argued that the phase change is due to the deformation of the bubbles when the tip approaches the surface. The lack in phase

shift for higher surfactant concentrations is due to the fact that the bubbles become stiffer and that their viscoelasticity approaches the one of the substrate. 3365


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Table 2. Parameters of the Nanobubbles Obtained after Tip Deconvolution; Rtip 15 nm ± 5 nm surfactant concentration, I (mM)

av height, h (nm)

av bubble radius, rd (nm)

av radius of curvature, Rcd

contact angle, θair (deg)

macroscopic contact angle, θwater (deg)

av nanoscopic contact angle, θwater (deg)

nanoscopic Δθwater(deg)

0.05 0.3 0.4

13.1 7.5 5.2

31.0 24.3 17.0

43.6 43.0 31.5

45.4 34.3 32.8

48.4 41.0 42.4

134.6 145.6 147.0

3.0 3.3 5.1

Figure 8. Force curves between a silica particle and a silicon oxide surface in the presence of 0.4 and 0.5 mM CTAB concentration.

Figure 9. Force curves between a silica particle and a silicon oxide surface in the presence of 1 mM CTAB concentration.

Correlation with Force Curves. In order to correlate the appearance of nanobubbles with interactions between hydrophilic surfaces in the presence of CTAB, force curves between a silica particle and a silicon wafer were recorded. As shown in Figure 8, they cannot be fitted with the DLVO theory at distances smaller than 20 nm. The obtained Debye length

coincides well with the theoretical one, which indicates that almost all surfactant molecules in solution are dissociated. From force measurements, we obtained that for concentrations below 0.4 mM, only attraction is observed during the whole range (data not shown). At 0.4 mM surfactant concentration, a slight repulsion is observed at long-range, which indicates a weak 3366


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the CTAB concentration. This angle verifies the hydrophobicity of the domains formed by the surfactant aggregates. It is much higher than the macroscopic contact angle of a CTAB solution droplet (about 40°), which presents an average of hydrophilic and hydrophobic areas on a silicon wafer partially covered with CTAB. The Laplace pressure within the nanobubbles is about 30 atm. With increasing CTAB concentration, the nanobubbles become smaller and less prominent. This indicates that the silicon wafer surface becomes more hydrophilic. At low CTAB concentration (0.05−0.4 mM), the surface is partially covered with hydrophobic domains where the nanobubbles can be placed. Since nanobubbles were only observed at low surfactant concentration (below 0.5 mM), they may play a role in the hydrophobic interactions. A strong attraction (jump-in) is observed at short distances (≤20 nm), which is explained by the rupture of the lamella between opposing nanobubbles seated on the hydrophobic domains. With increasing concentration, more and more hydrophilic domains, i.e., micelles or patchy bilayers, are formed until there are no bubbles observed close to the CMC (1 mM). At 0.5 mM, longrange electrostatic forces as well as steric repulsion followed by micelle/bilayer expulsion are observed in the interaction curve between the two opposing silica/silicon oxide surfaces. In the height image, only micropancakes were detected. At 1 mM, the exerted force was not high enough to expulse the adsorbed aggregates from the surface.

charge reversal (see Figure 8). At this concentration, the nanobubbles have a height varying from 5 to 8 nm. The jumpin contact takes place at a distance of about 18 nm. Under the assumption that both opposing surfaces are decorated with nanobubbles, locally a foam lamella may be formed between 2 opposing nanobubbles (5 nm height), which ruptures at a distance of about 8 nm between the outer surface. This might lead to the observed attraction. Similar arguments are published in ref 42 for the rupture of aqueous wetting films on a hydrophobic surface.43 When increasing the concentration to 0.5 mM, a strong repulsion is detected up to a distance of 10 nm. Then a jump-in contact is observed. The force curve can be fitted via the Gouy−Chapman theory for distances larger than 20 nm. The strong repulsion between 10 and 20 nm cannot be fitted by DLVO theory, which might be an indication for steric forces due to the formation of micelles or (patchy) bilayers at the silica/silicon oxide surface, respectively. Velegol et al.5 found a jump-in contact at 5 nm for the interaction between a tip and a silica surface in the presence of CTAB solution at 0.9 × CMC. In the present study, the jump-in contact occurs at 10 nm distance since the interactions are between a silica particle and a silicon oxide surface. The size of micelles in solution is about 5 nm and, at the surface, 3.4 nm.5 In both studies, the jump-in contact is due to the expulsion of micelles or bilayer patches between the surfaces. In the height image, only micropancakes were detected at this concentration. Micropancakes (thin gas layers) and micelles or patchy bilayers may coexist at this concentration. The obtained Debye lengths at 0.4 mM and 0.5 mM are 15.2 and 13.6 nm, respectively, which correlates well with the theoretical values. At 1 mM surfactant concentration, a monotonous repulsive interaction between a silica particle and a silicon wafer is obtained (see Figure 9). According to the present results, the repulsion is assumed to be caused by the approach of two opposing micelles or bilayers adsorbed on the silicon oxide/ silica surfaces. The micelles/patchy bilayers are then so closely packed and stiff that they cannot be pressed out from the surfaces. No jump-in contact is observed, and the 0 distance refers to the point of contact between opposing micelles/ bilayers. No bubbles were detected by SFM at this concentration since the silicon wafer surface is already hydrophilic. A decrease in height and radius of the bubble in the presence of surfactant with decreasing amplitude set point was reported in ref 18. Interestingly, this effect was not seen in the absence of surfactant. Therefore, in the present study, a similar amplitude set point was used to compare the variation of height of the nanobubbles with surfactant concentration. We concluded that the flattening of the nanobubbles at 0.5 mM is due to the decrease in surface tension.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +49 30 31423476. Fax: +49 30 31426602. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the DFG for financial support via the SPP 1273 Kolloidverfahrenstechnik (kL-1165/10). Vincent Craig and Gerhard Findenegg are acknowledged for the helpful discussions. Ulrich Gernert is acknowledged for the nice SEM images of the cantilevers.

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REFERENCES

(1) Fuerstenau, D. W.; Herrera-Urbina, R. In Cationic Surfactants: Physical Chemistry; Rubingh, D. N., Holland, P. M., Eds.; Marcel Dekker: New York, 1991; Vol. 37, p 408. (2) Subramanian, V.; Ducker, W. A. Langmuir 2000, 16, 4447−4454. (3) Biswas, S. C.; Chattoraj, D. K. J. Colloid Interface Sci. 1998, 205, 12−20. (4) Rutland, M. W.; Parker, J. L. Langmuir 1994, 10, 1110−1121. (5) Velegol, S. B.; Fleming, B. D.; Biggs, S.; Wanless, E. J.; Tilton, R. D. Langmuir 2000, 16, 2548−2556. (6) Ducker, W. A.; Wanless, E. J. Langmuir 1999, 15, 160−168. (7) Sharma, B. G.; Basu, S.; Sharma, M. M. Langmuir 1996, 12, 6506−6512. (8) Atkin, R.; Craig, V. S. J.; Wanless, E. J.; Biggs, S. Adv. Colloid Interface Sci. 2003, 103, 219−304. (9) Parker, J. L.; Yaminski, V. V.; Claeson, P. M. J. Phys. Chem. 1993, 97, 7706−7710. (10) Yaminski, V.; Jones, C.; Yaminski, F.; Ninham, B. W. Langmuir 1996, 12, 3531−3535. (11) Tyrrell, W. G.; Attard, P. Phys. Rev. Lett. 2001, 87, 176104. (12) Palmer, L. A.; Cookson, D.; Lamb, R. N. Langmuir 2011, 27, 144−147.

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CONCLUSIONS We reported the presence of small nanobubbles at the surface of a silicon oxide surface exposed to aqueous CTAB solutions. The effect of the surfactant is 2-fold; it can partially hydrophobize the silicon wafer surface and stabilize the nanobubbles. The hydrophobic surfactant patches present at the silicon oxide surface at low concentration (below 0.5 mM) are labeled by nanobubbles, which are imaged. The diameter of the nanobubbles varies from 30 to 60 nm (after tip deconvolution). The nanoscopic contact angle through water remains constant between 140° and 150° and is independent of 3367


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