Hydrophobic Attraction between Silanated Silica Surfaces in the

Aug 29, 2012 - Indeed, many types of hydrophobic surfaces have been employed to investigate the hydrophobic attraction. .... Detailed investigations a...
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Hydrophobic Attraction between Silanated Silica Surfaces in the Absence of Bridging Bubbles Naoyuki Ishida,*,† Yasuyuki Kusaka, and Hirobumi Ushijima Flexible Electronics Research Center, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba 305-8565, Japan ABSTRACT: The interaction forces between silanated silica surfaces on which there were neither nanobubbles nor a gas phase were measured using colloidal probe atomic force microscopy (AFM). To obtain hydrophobic surfaces without attached nanobubbles, an aqueous solution was introduced between the surfaces after an exchange process involving several solvents. In the approaching force curves obtained, an attractive force was observed from a distance of 10−25 nm, indicating the existence of an additional attractive force stronger than the van der Waals attraction. In the retracting force curves, a strong adhesion force was observed, and the value of this force was comparable to that of the capillary bridging force. The data clearly showed that although the bridging of nanobubbles is responsible for long-range hydrophobic attraction, there also exists an additional attractive force larger than the van der Waals attraction between hydrophobic surfaces without nanobubbles. Both the ionic strength and the temperature of the solution had little influence on the force. The possible origin of the force is discussed on the basis of the obtained results.

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INTRODUCTION Hydrophobic attraction has long fascinated many researchers since its first direct measurement and indication that this force is of a greater magnitude than the van der Waals force by Israelachvili and Pashley in 1982.1 For almost 30 years, many studies have been performed to clarify the origin of the very strong and extraordinarily long-range nature of this force. However, the origin of the force had been an issue debated for long time because the experimentally observed forces, which were measured mainly using a surface force apparatus (SFA) and atomic force microscopy (AFM), sometimes had inconceivably long ranges, reaching up to several hundreds of nanometers. In addition, the observed forces had a puzzling variety of ranges and magnitudes, depending on the systems used. Because of the diverse results, conventional theories, such as electrostatic fluctuation between surfaces,2 density fluctuation of hydrophobic domains on surfaces,3 formation of spontaneous cavities between surfaces,4 and drainage of water from the gap between surfaces,5 have failed to successfully explain all the results obtained in a number of different experiments. Indeed, many types of hydrophobic surfaces have been employed to investigate the hydrophobic attraction. They can mainly be classified as follows:6,7 (i) mica or silica (glass) surfaces with physically adsorbed surfactants;1,8−23 (ii) Langmuir−Blodgett (LB) monolayers deposited on mica surfaces;24−34 (iii) surfaces coated with a hydrophobizing agent bonded chemically onto the surface, such as silica surfaces coated with silane coupling reagents;6,35−42 and (iv) inherently hydrophobic surfaces, such as bulk polymer surfaces.43−45 The wide variety of results for the measured hydrophobic force © 2012 American Chemical Society

indicated that the origin varied depending on the systems employed to measure the force. This is because the dependency of the range and strength of the attraction on the external conditions varied between individual systems.46 Among these systems, the origin of the forces for system iii, i.e., surfaces coated with a hydrophobizing agent bonded chemically onto the surface, has achieved the widest consensus. The chemisorbed hydrophobic surfaces usually give a very high surface contact angle. On such surfaces, very small bubbles (nanobubbles) can attach stably when the surfaces are in contact with an aqueous solution due to the strong incompatibility between the surfaces and the solutions. These nanobubbles coalesce and form a gas bridge when hydrophobic surfaces approach close to each other. Such a bridge generates a strong and long-range attraction due to the lower pressure in the bridge and the interfacial tension between the gas and the liquid phases. The existence of nanobubbles when the chemisorbed surfaces are immersed in aqueous solutions was first predicted in the mid-1990s37 and was experimentally confirmed by AFM observations in the early 2000s.47,48 Recent studies found that gaseous domains are not only cap-shaped bubbles but can sometimes form large “pancakes” that extend laterally for several hundred nanometers or even micrometers, with the height of a few nanometers.49,50 After confirmation of the existence of nanobubbles, gas bridging seems to have been widely accepted as a model that can successfully explain the long-range and strong nature of the hydrophobic attraction. Received: July 27, 2012 Revised: August 28, 2012 Published: August 29, 2012 13952

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silanated surfaces have stable and robust molecular layers, no rearrangements, desorption, or deposition of the molecules on the surface are likely to occur for such surfaces. Therefore, the force between the chemisorbed surfaces without bubbles is probably the “purer” component of the hydrophobic force that is produced by only surface hydrophobicity. A few studies have indicated that there is still an additional attractive force between robust hydrophobic surfaces on which there are no nanobubbles.6,57,58 The range of these forces is not unusually long, but it is on the order of a few tens of nanometers. Therefore, by exploring in more detail the nature of the hydrophobic attraction in the absence of nanobubbles, we would be able to approach the more essential properties of the true hydrophobic attraction, i.e., the range and causes of the force. Detailed investigations are certainly necessary to determine what occurs between highly hydrophobic surfaces in the absence of nanobubbles. In the present study, we conducted AFM measurements of the interaction forces between hydrophobic surfaces from which nanobubbles and other types of gas phases were removed in order to explore the nature and origin of the hydrophobic attraction. The dependence of the interaction forces on the surface hydrophobicity, ionic strength, and temperature of the solution was also investigated.

This model can probably also be applied to system iv, the inherently hydrophobic surfaces.45 It should also be noted that dissolved gas is found to have a significant effect on the hydrophobic attraction for systems iii and iv. Some studies revealed that the range of the attraction and its variability decrease in degassed water,51,52 although another study found no effect of degassing on the long-range interaction.6 These results show that nanobubbles are indeed responsible for the long-range attractive force, and the attachment of nanobubbles on surfaces can be greatly influenced by dissolved gases in the solution in which the surfaces are immersed. On the other hand, it has been suggested that the origin of the force is different for other systems. For system i, mica or silica surfaces with physically adsorbed surfactants, the electrostatic forces are important factors for the force, while Sakamoto et al. reported that the bubbles or gas phase introduced by the hydrocarbon chains of surfactants can cause a long-range attractive force.23 Kékicheff and Spalla first found an electrostatic effect on the force experimentally between the silica surfaces in cetyltrimethylammonium bromide (CTAB) solutions.19 They pointed out that the decay length of the attractive forces was exactly half of the Debye screening length, which can be attributed to correlations between adsorbed ion pairs on the overall neutral surfaces. Zhang et al. reported that even though the bubbles and gas phase are completely removed from the system, there is still a long-range attraction force which is much stronger than the van der Waals force.53 They suggested that the force was caused by patchy domains of surface charges due to the uneven adsorption of the surfactant molecules. These studies clearly showed that electrostatic interaction is the main origin for the surfaces hydrophobized by the surfactant adsorption. For system ii, LB monolayers deposited on mica surfaces, the reason for the long-range force still seems complicated and varied. One of the factors making analysis of this system difficult is the unstable and mobile nature of the deposited layers in aqueous solutions. Christenson et al. already found in the late 1980s that cavity bridging occurs at a minimum when the hydrophobic surfaces touch each other in the fringes of equal chromatic order (FECO) of the SFA,25 but they also found that the long-range force can act without such a bridge. Christenson and Yaminsky54 reported in the 1990s that the lateral mobility of LB layers is attributable to the long-range force. More recent studies using AFM imaging have shown that the LB monolayers are usually not even and transform into patchy bilayers when the surfaces are immersed in water. The oppositely charged domains formed by such patchy bilayers would cause a long-range electrostatic attraction between the surfaces in water.55 These results indicate that factors other than surface hydrophobicity are also important for this system. The surfaces hydrophobized with LB films might not be ideal hydrophobic surfaces for exploration of the fundamental nature of the hydrophobic attraction. While the bubble-bridging model is the most agreed upon origin for system iii, the bridging force is sometimes regarded as not the “true” meaning of the hydrophobic attraction56 because the force is not produced by the surface hydrophobicity itself but by the intervention of the bubbles attached onto hydrophobic surfaces. Therefore, it is of great interest to investigate the interaction forces that act between chemisorbed hydrophobic surfaces when such bubbles are completely removed. Because chemisorbed hydrophobic surfaces such as

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MATERIALS AND METHODS

Materials. Silica particles 8−30 μm in diameter (Tatsumori) and silicon wafers (Nilaco) were used as test surfaces. Reagent grade octadecyltrichlorosilane (OTS; Shin-Etsu Chemicals) and analytical grade toluene, chloroform, ethanol, and dimethyl sulfoxide (DMSO) (Wako Pure Chemicals) were used without further purification. Water was purified with a Milli-Q system (Millipore). Surface Hydrophobization. The silica particles were stirred for 10 min in an ethanol solution that was heated to ∼60 °C and in a 5 vol % H2O2 solution for another 10 min and then rinsed twice with warm Milli-Q pure water prior to use. The silicon wafers (Nilaco) were cut into ca. 1.3 cm × 1.3 cm substrates and sonicated in chloroform. Then, they were cleaned in a plasma cleaner (PDC-32G; Harrick Plasma) operating at 12 W under reduced air pressure for 10 min and then sonicated again in pure water for 5 min. The substrates were then sonicated in a 5 mM sodium hydroxide solution for 5 min and finally rinsed again with pure water. The cleaned silica particles and silicon substrates were hydrophobized with OTS in toluene solution. A cleaned silica particle was first attached to the top of an AFM cantilever using epoxy glue, which is inert in organic solutions (Araldite; Ciba-Geigy) to make a colloid probe. Then, the probe and the cleaned substrates were immersed in an OTS solution in toluene. After the reaction, they were washed successively with chloroform, acetone, and ethanol. They were then vacuum-dried and stored in a nitrogen atmosphere. The surface hydrophobicity was altered by varying the concentration of OTS in toluene and immersion time and was evaluated using the contact angle θ of a drop of pure water measured by the sessile drop method by using a contact angle goniometer (DM-500; Kyowa Interface Science). The root-mean-square (rms) roughness of the hydrophobized plates was in the range 0.10−0.20 nm over a 1 × 1 μm area. Force Measurements. A multimode atomic force microscope (Nanoscope IIIa; Veeco Instruments) equipped with a closed fluid cell was used to measure the interaction forces between the hydrophobized silica sphere of the colloid probe and the silica plate in an aqueous solution. The cantilevers mainly used were the stiff and rectangular silicon cantilevers (NCH; Nanosensors). The spring constant k of the cantilever was determined by measuring the resonance frequency.59 When the solution temperature was varied, the temperature was controlled with a heat stage mounted on top of the scanner (Veeco Instruments). The solution temperature was monitored with a small thermocouple inserted into the liquid cell. The deviation of 13953

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temperature was estimated to be less than ±0.1 °C at any given temperature. We used the solution-exchange method to prevent nanobubbles from attaching to the hydrophobic surfaces. In brief, in this method, a solution that can completely wet the hydrophobic surface and is miscible with water, such as ethanol, is first introduced into the AFM liquid cell and then exchanged with an aqueous solution. However, exchanging ethanol directly with water sometimes causes the opposite effect and actually increases the probability of forming nanobubbles on the surface because mixing of ethanol directly with water produces large amounts of bubbles in bulk solution, which then attach to the surfaces.58,60 Therefore, we used dimethyl sulfoxide (DMSO), which has a surface tension intermediate between those of ethanol and water, as the liquid for filling after ethanol, and this was then exchanged by water. The detailed procedure is as follows. First, ethanol was injected into the fluid cell. Then, the ethanol was replaced by a 1:1 solution of ethanol and DMSO. The solution was then replaced by DMSO, followed by a 1:1 solution of DMSO and water. The surfaces were rinsed repeatedly by flushing copious amounts of water into the cell. The utmost care was taken not to introduce any gas. Finally, an aqueous solution was injected, and the force measurements commenced. All of the solution was degassed prior to use by sonication under reduced pressure.

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RESULTS AND DISCUSSION Figure 1 shows a tapping-mode AFM image of the OTSmodified Si substrate with a contact angle of 108° after going

Figure 2. (a) Approaching and separating force curves between OTScoated surfaces with a contact angle of 108° in a 1 × 10−3 M NaNO3 solution. The inset shows a close-up plot in the short-range region of the approaching force. The solid line shows van der Waals attraction, calculated assuming a Hamaker constant of 0.8 × 10−20 J.9 The arrows indicate the points of jump-in and jump-out. (b) Approaching force between same surfaces measured in 1 × 10−3, 1 × 10−2, and 1 × 10−1 M NaNO3 solutions by using a weaker cantilever. Lines show the fitting for the DLVO calculation under constant charge (solid line) and constant potential (dashed line) conditions. The pair of lines on the right and left is the fitting for 1 × 10−3 and 1 × 10−2 M, respectively. The parameters used for the calculations were surface potential = 45 mV, Debye length = 10.0 nm (1 × 10−3 M) and 3.04 nm (1 × 10−2 M), and Hamaker constant = 0.8 × 10−20 J.

Figure 1. Tapping-mode AFM height image of an OTS-coated Si wafer surface with a contact angle of 108° in water after the bubbleremoving process.

through the process for the removal of nanobubbles. The image is almost flat, and no specific features are seen on the surface, confirming that most of the nanobubbles or gas phase was successfully removed from the surface. The approaching and retracting force curves measured between the surfaces with a contact angle of 108° in 1 × 10−3 M NaNO3 solution after going through the process for removal of nanobubbles are shown in Figure 2a. In the approaching force curve, there is clearly an attractive force that commenced to act from a distance of ∼20 nm. The attractive force was strong enough to make the particle jump-in to make contact with the substrate at ∼10 nm. Although this attractive force was quite different from the force caused by bubble bridging, which has a range on the order of 100 nm with a step on the onset of the force,6 this force is still long-range and stronger than the van der Waals attraction, as shown in the

inset of Figure 2a. After the surfaces were brought into contact, a very strong adhesion force was observed when the surfaces were retracted. The adhesion force was also strong enough to make the surfaces jump-out to return to the position where surface force values were zero. We also examined the short-range region of the forces using weak cantilevers. In Figure 2b, a close-up plot of the shortrange region of the forces using a rectangular contact-mode cantilever (CONT; Nanosensors) is shown. The force was initially repulsive in a 1 × 10−3 M solution as the surfaces approached, and then the surfaces jumped into contact in the middle of the repulsive force, indicating that the attractive force 13954

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acts between the surfaces from a distance of ∼20 nm, which is equivalent to the distance observed using the stiffer cantilever. The repulsive force also fit well to the DLVO calculated lines until the jump-in to contact occurred. This means that the repulsion is attributable to an electrostatic double-layer force because the modification of a silica surface with silanating reagents usually has little effect on the electrostatic property of the surface, as reported previously.61 On the other hand, in 1 × 10−2 and 1 × 10−1 M solutions, no repulsive forces were observed before the jump-in, although the ranges of the attractions were the same as in the 1 × 10−3 M solution. This should be because the ranges of the electrostatic repulsion in both solutions were less than those of attraction and the attractive force acted before the electrostatic repulsion. Therefore, these results imply that the attractive force acts in addition to the DLVO force. When the surface contact angle was reduced to 95°, the general characteristics of the force curves did not change significantly. As can be seen in Figure 3, the attractive force on

Figure 4. Approaching force curves between OTS-coated surfaces surface with a contact angle of 77° in 1 × 10−3 and 1 × 10−2 M NaNO3 solutions. The inset shows the fitting of the curve in 1 × 10−3 M for the DLVO calculation under constant charge (solid line) and constant potential (dashed line) conditions. The parameters used for the calculations were surface potential = 44 mV, Debye length = 9.8 nm, and Hamaker constant = 0.8 × 10−20 J.

when the surface hydrophobicity was high enough, presumably when the contact angles of the surfaces were greater than 90°. We found that the electrolyte had only a minor effect on the attraction. Figure 5 shows the ranges of the attractive forces,

Figure 3. Approaching and separating force curves between OTScoated surfaces with a contact angle of 95° in a 1 × 10−3 M NaNO3 solution. The arrows indicate the points of jump-in and jump-out. The inset shows a close-up plot in the short-range region of the approaching force. The solid line shows van der Waals attraction, assuming a Hamaker constant of 0.8 × 10−20 J.

approach was still stronger than the van der Waals force, and there was a strong adhesion force on the retracting curve. Compared to the surface with a contact angle of 108°, the range of the attractive force on approach decreased and the strength of the adhesion force also decreased. These force curves suggested that the attractive force was dependent on the surface hydrophobicity. However, when the contact angle of the surfaces was further reduced to below 90°, the force curves changed completely, as shown in Figure 4. The force curves for the surfaces with a contact angle of 77° measured in 1 × 10−3 and 1 × 10−2 M NaNO3 showed only a monotonically increasing repulsive force, which fit well to the DLVO theoretical force, and no extra attractive force was seen. In this case, the modification of a silica surface with silanating reagents also had little effect on the electrostatic properties of the surface. This result indicated that short-range hydrophobic attraction would be caused essentially

Figure 5. Dependence of the range of attractive forces between OTScoated surfaces on the electrolyte concentration of NaNO3 solutions.

which are the separation distances at which the attractive force in the approaching force curves starts to act, measured in 1 × 10−3, 1 × 10−2, and 1 × 10−1 M NaNO3 solutions for 108° and 95° surfaces. The range was obtained from about 100 force curves measured at different positions in several sets of measurements for each concentration. Because the range of the forces had no systematic dependence on the concentration beyond experimental deviation, we concluded that the electrolyte had little effect on the interaction. 13955

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force. It is worth noting that the range of the attractive force on approach and the value of the adhesion force are almost identical to the values measured between the OTS-coated surfaces with a similar contact angle (Figure 3). This suggests that the quantitative nature of the attractive force is dependent on surface hydrophobicity, even though the methods of surface preparation were different, and that the difference of charge state resulting from the methods of surface preparation has no significant effect on the attractive force. The effect of solution temperature was also investigated using a heating stage. The approaching force curves for the surfaces with a contact angle of 108° measured at three different temperatures are shown in Figure 7. Neither the range

The change in adhesion force values contact angle would be a key to understanding the origin of the strong adhesion force. Thus, we compared the measured adhesion force and the value of the capillary force Fc, assuming cavity bridge formation between the surfaces as calculated by63 Fc/R = 4πγ cos(180° − θ )

where γ is the surface tension of the solution. Table 1 shows the comparison of the average values of the adhesion forces Table 1. Comparison of the Measured Adhesion Forces between OTS-Coated Surfaces with the Calculated Capillary Force contact angle (deg)

measured adhesion force (mN/m)

capillary force (mN/m)

108 95

296.5 81.2

279.6 78.9

obtained by about 100 force curves measured at different positions and the calculated capillary forces. Clearly, both values are comparable to each other. This suggests that the adhesion force may have been caused by a cavity bridge that formed due to the high hydrophobicity of the surfaces, at least when the surfaces were brought into contact. We also measured the forces between the surfaces hydrophobized by alkoxylation of a long-chain alcohol. As mentioned above, when silica surfaces are modified with silanating reagents, particularly chlorosilanes, a negative charge usually remains on the surfaces, and this charge may possibly affect the interaction. To prevent this, we prepared hydrophobic surfaces by alkoxylation with dodecanol; hydrophobization was achieved by heating the silica surfaces in a 0.1% dodecanol solution in heptane near the boiling point (∼250 °C).62 In the force curves shown in Figure 6, there can also be seen an attractive force on approach that is larger than the van der Waals attraction and a strong adhesion force in the retraction

Figure 7. Approaching force curves between OTS-coated surfaces with a contact angle of 108° measured at 25, 32, and 40 °C. The forces were measured in a 1 × 10−3 M NaNO3 solution. The arrows indicate the points of jump-in.

nor strength of the obtained force seemed to change beyond experimental error. For other surfaces used in the present study, the solution temperature showed no significant effect on the obtained approach forces.

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DISCUSSION From the results obtained in the present study, it is clear that between chemisorbed hydrophobic surfaces in the absence of nanobubbles there is an additional attractive force stronger than the van der Waals force. This force must be the purer component of the hydrophobic attraction because the force is thought to be generated without intervening nanobubbles. This finding strengthens the recent view of the hydrophobic attraction. In their comprehensive review of the hydrophobic attraction, Christenson and Claesson7 categorized such an attraction as the short-range hydrophobic force and predicted that this type of force is indeed the “true” hydrophobic attraction. The force found in this study showed quite good agreement with their suggestion. It is particularly worth noting that the range of the obtained attractive forces, which is about 10−25 nm, is comparable to the short-range hydrophobic forces that have been found for several different systems. Such short-range hydrophobic forces between the robust hydrophobic layers such as the hydrophobic silane layers deposited on plasma-treated mica32 or glass64

Figure 6. Approaching and separating force curves between silica surfaces hydrophobized by alkoxylation with dodecanol. The contact angle of the surfaces was 95° and the forces were measured in a 1 × 10−3 M NaNO3 solution. The arrows indicate the points of jump-in and jump-out. The solid line shows van der Waals attraction, assuming a Hamaker constant of 0.5 × 10−20 J. 13956

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surfaces using LB techniques, plasma-polymerized films on mica65 surfaces, bulk polymer surfaces,44 and thiol-modified gold surfaces.41,42 Although it is not clear whether all the systems employed in the above studies were in the presence or absence of nanobubbles, this trend indicates that this range of the force has some critical importance and contains one of the keys to determining the essential properties of the hydrophobic attraction. Indeed, Meyer et al.66 insisted that such short-range attractions at separations >20 nm should be common for all types of hydrophobic surfaces and contain information about the true nature hydrophobic attraction. The observed forces are not significantly affected by the ionic strength of the solution. Actually, the lateral polymerization of the OTS molecules tends to form domain portions on the layer. This could produce some charge domains, possibly causing an electrostatic interaction. The present results, however, confirmed that this is not the case for the OTS-coated surfaces. In addition, the observation that both silanated and alkoxylated surfaces gave similar forces when the contact angles were equivalent clearly indicates that the force is not dependent on the surface charge state but on the surface hydrophobicity. This independence of the forces on the electrolyte concentration in the solution is one of the characteristics of the short-range hydrophobic force between robust hydrophobic surfaces also found in previous studies.32,42,44 Therefore, it would be reasonable to suppose that the contribution of electrostatic phenomena in terms of being the origin of the short-range hydrophobic force is minimal. The fact that the values of the adhesion force measured for the surfaces with different contact angles are comparable to those of the capillary force suggests that a vapor cavity should form between the surfaces, at least when they are brought into contact. The spontaneous formation of a vapor cavity is expected between strongly hydrophobic surfaces, particularly when the water contact angle of the surfaces is greater than 90°, as in the present case, where the vapor state is energetically more favorable for the solution adjacent to the strongly hydrophobic surface than that remaining in the liquid state. Such a large adhesion force caused by cavitation bridging has actually been observed for similar robust silanated hydrophobic layers using SFA upon contact of the surfaces.32 Although the formation of a cavity is highly possible as the origin of the strong adhesion force, it is still unclear from the present results whether the cavity had already formed when the surfaces approached each other and causes the attractive force between the approaching surfaces. Formation of a cavity during the approach of the surfaces could be possible because inherently dissolved gas that is not removed by the degassing procedure can migrate at the hydrophobic surfaces and induce the cavitation.67 Zhang et al.58 measured the interaction forces between a carbon sphere and a highly ordered pyrolytic graphite (HOPG) surface and found that even when there were no nanobubbles on the surfaces, an attractive force acted with a range of around 50 nm, which is very similar to the forces measured between the surfaces with nanobubbles. They attributed the origin of the attraction to cavitation. If a vapor cavity exists, the attractive force may be caused by bubbles remaining on the surface due to the split of the vapor cavity bridging upon retraction of the surfaces. The remaining bubbles on the surfaces in turn could have caused the attractive force when the surfaces approach again, as they observed by AFM imaging. In our case, however, the range of the obtained interaction for the nonbubble case was clearly different from

the bubble-bridging force measured previously for the same surfaces.6 Thus, we consider that bubbles are unlikely to remain on the surface due to the approach−retraction cycles of the surfaces because the distance of the attraction between the approaching surfaces was almost identical, independent of the measurement cycle. The cavity did not seem to grow during repeated approach−retraction cycles, as often observed in the case of a nanobubble-bridging force.6,68 In addition, the measurement-position dependence of the attraction was minimal, whereas the attractive force caused by the bubbles depends greatly on the measurement position.6 This indicates that the cavity, if any, should disappear after the surfaces are separated and a cavity forms at every measurement cycle. The force did not show significant dependence on temperature over the temperature range used in this experiment. This independence of the force on temperature calls into question the classical thermodynamic origin for this short-range hydrophobic attraction. When the water molecules are around the hydrophobic wall, they maintain an energetically unfavorable state. Some entropic contribution due to the energetically unfavorable state has sometimes been thought to induce an attractive force. If this is the origin of this force, however, the attractive force should be more or less dependent on the solution temperature, which was not the case in the present study. It is therefore likely that the contribution of thermodynamic phenomena to the short-range hydrophobic force is not significant, although the temperature range of the solution used in this study was rather limited.

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CONCLUSION The present study confirmed that even in the absence of nanobubbles, a long-range attractive force exists between surfaces with robust hydrophobic layers prepared by the chemisorption of a hydrophobizing reagent. The range of the force is approximately 10−25 nm, and the force is stronger than the van der Waals attraction. This force would be a true component of hydrophobic attraction because it is probably caused by the surface hydrophobicity itself and is not affected by other materials such as nanobubbles. The change in surface hydrophobicity significantly altered the range and strength of the attraction, whereas both the ionic strength and temperature of the solution had minimal influence. Because the range of this hydrophobic attraction is estimated to be on the order of a few tens of nanometers, which is not anomalous compared to the range of the bubble-bridging force, several explanations may still be applicable for the origin of this force. Although the findings of this study, i.e., the ionic strength and temperature of a solution have minor effects on the force, exclude some of the explanations for the origin of this force, there could still be several candidates. For example, an inherently dissolved gas possibly plays some role in the interactions. Although all the solutions used in the experiments were degassed prior to use, it is extremely difficult to achieve absolute degassing. It is suggested that this cluster of dissolved gases induces a reduction in the density of water molecules around it, resulting in a possible cause for the attractive force.69,70 Thus, the role of dissolved gases should be further examined carefully. Furthermore, there are some other recently presented theories that may explain this force, such as the molecular vibration of solutions, which would create an attractive force when the solution has low affinity to the surfaces.71 Further investigations are necessary to determine the origin of the force, but the present results provide useful 13957

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information for theoretical analyses for determining the origin of the true hydrophobic force.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address †

Department of Applied Chemistry and Biotechnology, Okayama University, 3-1-1 Tsushima-naka, Kita-ku, Okayama 700-8530, Japan. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS N.I. acknowledges the financial support for this work from the Core-to-Core Program, promoted by the Japan Society for the Promotion of Science (Project No. 18004).

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REFERENCES

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