Hydrophobic Attraction Measured between Asymmetric Hydrophobic

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Hydrophobic Attraction Measured between Asymmetric Hydrophobic Surfaces Naoyuki Ishida, Kohei Matsuo, Koreyoshi Imamura, and Vincent S. J. Craig Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b04246 • Publication Date (Web): 28 Feb 2018 Downloaded from http://pubs.acs.org on March 4, 2018

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Langmuir

(Article)

Hydrophobic Attraction Measured between Asymmetric Hydrophobic Surfaces

Naoyuki Ishida1*, Kohei Matsuo1, Koreyoshi Imamura1 and Vincent S. J. Craig2

1 Division of Applied Chemistry, Graduate School of Natural Science and Technology, Okayama University, 3-1-1 Tsushima-naka, Kita-ku, Okayama 700-8530, Japan 2 Department of Applied Mathematics, Research School of Physics and Engineering, The Australian National University, Canberra, ACT 2601, Australia

ACS Paragon Plus Environment

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ABSTRACT The interaction forces between silica surfaces modified to different degrees of hydrophobicity were measured using colloidal probe atomic force microscopy (AFM). A highly hydrophobic silica particle was prepared with octadecyltrichlorosilane (OTS) and the interaction forces were measured against silica substrates modified to produce surfaces of varying hydrophobicity. The interaction forces between the highly hydrophobic particle and a completely hydrophilic silicon wafer surface fitted well to the DLVO theory, indicating that no additional (non–DLVO) forces act between the surfaces. When the silicon wafer surface was treated to produce a contact angle of water on surface of 40°, an additional attractive force that is longer-ranged than the van der Waals force was observed between the surfaces. The range and magnitude of the attractive force increases with the contact angle of water on the substrate. Beyond the effect on the contact angle, the hydrocarbon chain length and the terminal groups of hydrophobic layer on the substrate only have a minor effect on the magnitude of the force, even when the substrate is terminated with polar carboxyl groups, provided the hydrophobicity of the other surface is high.

KEYWORDS: hydrophobic attraction, hydrophobicity, asymmetric surface, atomic force microscopy, interaction force measurement


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INTRODUCTION Among surface forces, the hydrophobic attraction is one of the most important nonspecific interactions. It was almost 35 years ago that the hydrophobic attraction was measured directly for the first time using the surface force apparatus (SFA) and it was shown that this force is greater in magnitude than the van der Waals (vdW) force.1 Since then, this force has been the subject of numerous studies using SFA2-10 and atomic force microscopy (AFM).11-18 A number of contributions have made significant progress19-21 in understanding the puzzling hydrophobic attraction, which had been observed over inconceivably large distances, reaching up to several hundreds of nanometers.4, 17 The long-range nature of the force is now explained as arising from multiple origins. The bridging of surface nanobubbles is considered to be the main cause of the very long-range attraction when the surfaces are very hydrophobic, as exhibited by chemisorbed surfaces with hydrocarbon layers. The existence of stable nanobubbles on the surfaces in contact with an aqueous solution was first implicated in the mid-1990s6 and was later experimentally confirmed by AFM observations.22-23 On the other hand, when hydrophobic surfaces are prepared by physical adsorption of surfactants, an electrostatic attraction may arise from the interaction of domains of surfactant aggregates that produce patches with different charge properties.18, 24-25 An attraction of similar origin is applicable for hydrophobic surfaces produced by Langmuir-Blodgett (LB) monolayers, as LB monolayers are usually not even and transform into patchy bilayers when the surfaces are immersed in water.9 Recent studies into the hydrophobic attraction have come to focus on the “pure” component of the force, i.e. the force caused by the surface hydrophobicity itself without any other effects like nanobubbles or patchy charges. The pure hydrophobic attraction is not unusually long-ranged,


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generally having a range of less than 10–15 nm.20, 26 It is suggested that the strength of the hydrophobic attraction exponentially decays with the separation distance. In many cases the decay length of the force between solid surfaces has been found to lie in the range of 0.3–1.0 nm,27 but longer decay lengths up to about 2.0 nm have also been measured for highly hydrophobic surfaces.10 On the other hand, Tabor et al.28 measured the forces between fluorocarbon oil droplets, with the refractive index matched to that of water. This enabled the strength of the vdW attraction to be minimized the pure hydrophobic attraction to be measured. They observed a force that was ~5 nm in range and had a decay length of 0.3 nm, which is relatively short-ranged considering the very high hydrophobicity of the oil droplet. This difference in the range of the force for solid-solid and liquid-liquid systems implies that the mechanism underlying the “pure” hydrophobic attraction could vary depending on the nature of the interface. Despite these detailed studies, however, the origin the pure hydrophobic attraction still remains unresolved. Most of the studies on the hydrophobic force between solids have been conducted between symmetric surfaces with same degree of hydrophobicity, with relatively few studies utilizing surfaces of different (asymmetric) hydrophobicity surfaces. However, interactions between asymmetric hydrophobic surfaces should be equivalently important, since the interactions between asymmetric surfaces appear in a wide variety of industrial and natural phenomena. The examples include not only traditional technologies such as heterogeneous aggregation of particles, permeation of particles through filter materials and particle attachment onto bubbles in flotation, but also recently developed technologies like nanoparticle patterning on substrates and attachment of particles to oil droplets in Pickering emulsions. It is also of particular importance in biological fields, such as attachment of drugs and drug delivery materials to proteins and membranes and the behavior of nanoparticles interacting with cells. In these applications, a fundamental


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understanding of interactions between surfaces in aqueous environments is critical. Therefore in the present study, we focus on the interactions between surfaces with a range of hydrophobicity. The main aim of this study is to investigate the relationship between the hydrophobicity of surfaces, and the properties (magnitude and range) of the force. In order to explore the mechanism of the hydrophobic force, it is of great importance to understand how the hydrophobicity of surfaces influences the force. Nevertheless, this relationship is yet to be clearly elucidated. The advantage of measuring the interaction between asymmetric hydrophobic surfaces is that we can use surfaces with a wider variety of combinations of hydrophobicity. If the surfaces are prepared in a controlled manner, the interactions between such surfaces are expected to provide much information on how hydrophobicity of surfaces affects the nature of the force. Further, changes in the chemical properties of the surface such as the terminal functional group, whilst controlling for the hydrophobicity of the surface, should provide insight into the mechanism of the force. Herein, we conducted AFM measurements of the interaction forces between model hydrophobic surfaces with asymmetric hydrophobicity as an attempt to reveal the origin and mechanism of the pure hydrophobic attraction. Forces were measured between a silica particle of fixed hydrophobicity and a plate which had the hydrophobicity varied, using surface modification by silanizing reagents. We also prepared hydrophobic surfaces with different terminal groups and investigated how this impacts the interaction forces.

MATERIALS AND METHODS


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Materials. Silica particles 8–20 µm in diameter (Tatsumori, Japan) and silicon wafers (Nilaco, Japan) were used as surfaces. Chemical structures of the reagents used for surface hydrophobization are shown in Fig. 1. Silane coupling reagents octadecyltrichlorosilane (OTS), butyltrichlorosilane

(BTS),

3-phenylpropyltrichlorosilane

(PPTS)

and

3-methacryloxypropyltriethoxysilane (MPTES) were purchased from Shin-Etsu Chemicals, Japan and used as received. 10-Carboxydecylphosphonic acid (CDPA) was purchased from Dojindo Chemicals, Japan and used without further purification. Analytical grade toluene, chloroform, and ethanol (Wako Pure Chemicals, Japan) were used without further purification. All aqueous solutions were prepared with ultrapure water purified with a Milli-Q system (Millipore, Japan).

Figure 1. Chemical structures of (a) octadecyltrichlorosilane (OTS); (b) butyltrichlorosilane (BTS); (c) 3-phenylpropyltrichlorosilane

(PPTS);

(d)

3-methacryloxypropyltriethoxysilane

(MPTES);

(e)

10-Carboxydecylphosphonic acid (CDPA).


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Surface Modification. The silica particles were washed in stirred warm ethanol for 10 min and then in a stirred 5 vol% H2O2 aqueous solution for another 10 min, and then washed in a stirred 5 mM sodium hydroxide solution and rinsed several times with warm ultrapure water prior to use. The silicon wafers were cut into ca. 1.3 cm × 1.3 cm substrates and sonicated in chloroform. Then, the wafers were cleaned in a plasma cleaner (PDC-32G; Harrick Plasma, United States) operating at 12 W in the presence of water vapor for 10 min. They were then sonicated in a 5 mM sodium hydroxide solution for 5 min and finally sonicated in ultrapure water for 10 min. After washing, the wafers were dried under a stream of nitrogen. The silica particles were hydrophobized with OTS. The cleaned silica particles were added to a 1 mM OTS solution in toluene and stirred for 1 h before separation from the solution by filtration. Then the particles were washed by stirring in chloroform several times and vacuum-dried. A hydrophobized silica particle was attached to the top of an AFM cantilever using epoxy resin (Araldite; Nichiban, Japan) to make a colloid probe. The silicon substrates were modified with OTS, BTS, PPTS, MPTES and CDPA. When the chlorosilanes (OTS, BTS or PPTS) were used, the cleaned substrates were immersed in a solution of the appropriate chlorosilane in toluene. After the reaction, they were sonicated successively in chloroform and ethanol, and dried under a stream of nitrogen gas. The surface hydrophobicity was altered by varying the solution concentration of chlorosilane in toluene and the time of immersion in the solution, when using OTS and BTS. Hydrophobization with PPTS was conducted using a 1 mM solution and 1 h of reaction time. When the ethoxysilane MPTES was used, the substrates were exposed to the vapor of MPTES for 6 h at room temperature in a sealed glass container. After


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treatment, the surfaces were sonicated in ethanol to remove unbound MPTES. All the substrates modified with the silanes were annealed at 120°C for 1 h in a drying oven after hydrophobization. For the modification with CDPA, the wafers were immersed in a 1 mM CDPA solution in ethanol for 6 h at room temperature and sonicated in ethanol after the reaction. The surface hydrophobicity of the substrates was evaluated by advancing contact angle θs against water, which was measured by sessile drop method using a custom-made contact angle goniometer. The contact angle of the hydrophobized particles θp was estimated to be 108° from the contact angle of a silicon wafer hydrophobized in the same manner. We also checked θp using the method of Preuss and Butt29 and found that the measured angle was 108° ± 3°. Since this value is almost the same as that for a silica surface covered with close-packed OTS,30 we estimated that the particles were covered with a monolayer of close-packed OTS. Force Measurements. Interaction forces were measured between a colloid probe and a substrate using a multimode atomic force microscope (Nanoscope IIIa; Veeco Instruments, United States). The force measurements were conducted using a closed fluid cell and the solutions were degassed prior to use by sonication under reduced pressure for at least 30 min. Silicon nitride probes with triangular cantilevers (NP-S; Nanosensors, Switzerland) were used to measure relatively weak forces whereas rectangular silicon cantilevers (NCH; Nanosensors, Switzerland) for tapping-mode imaging were used to measure strong attractive forces. The spring constant k of the cantilever was determined by measuring the resonance frequency with and without a particle on the tip.31 Surface Characterization. Surface imaging of the hydrophobized substrates was performed by tapping mode AFM (Nanoscope IIIa; Veeco Instruments, United States) under ambient conditions. The thickness of the hydrophobic layer was measured using an ellipsometer (LTE; Gaertner


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Scientific, Germany) in air. The surface potential of the substrates was estimated using a zeta potential analyzer (ELSZ; Otsuka Electronics, Japan) with the flat plate measurement attachment.

RESULTS Surface Hydrophobicity and Interaction Force.

Figure 2 shows typical force curves F

normalized by probe radius R measured on approach between symmetric hydrophobic surfaces against separation distance h. The measurements were conducted in 1 mM, 10 mM and 0.5 M aqueous NaCl solution. The particle and the substrate were hydrophobized with OTS in the same batch and are estimated to have same contact angles, measured on the flat to be 108°. The interaction in each solution clearly shows an attractive force that commenced to act from a distance of approximately 10 nm. The gradient of the attractive force was sufficient to cause the particle to jump into contact with the substrate from ca. 6 nm. The electrostatic double-layer force expected to be observed in the 1 mM and 10 mM solutions cannot be seen observed, because the stiff cantilever used to measure strong attractive forces prevents the detection of the relatively weak electrostatic force. (This is also the case for the other measurements using stiff cantilevers.) After the surfaces were brought into contact, a very strong adhesion force was also observed when the surfaces were separated. The vdW attraction force calculated using the planar 5-layer (silica– OTS–water–OTS–silica) model is also shown in the same figure for comparison. The measured attractive force is clearly longer-range and stronger than the vdW attraction, indicating the existence of an additional attraction, generally known as the hydrophobic attraction, between the surfaces. These attractive forces have almost the same interaction range as those measured in a previous study,26 in which the nanobubbles were successfully removed from the surface. Further,


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we have applied the bubble-removing process developed in ref. 26 to the present systems and measured interaction forces. The interaction forces were almost the same after the bubble removing process, indicating they arise from the hydrophobic nature of the surface without the intervention of nanobubbles. The hydrophobic attraction is often empirically fitted to an exponentially decaying function27 as

F/R = –C exp (–h/h )

(1),

where h0 is a characteristic decay length and C is a constant correlated to the interfacial energy between the surfaces. The average value of h0 obtained by fitting this equation for this surface was 1.65 nm, which is in the range of the value (0.3–2.0 nm) normally measured between solid hydrophobic surfaces.10

Figure 2. Approaching force curves between OTS-coated surfaces that have same contact angle (108°) measured in 1 mM, 10 mM, and 0.5 M NaCl solutions. The solid line shows vdW attraction, calculated


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using the planar 5-layer (silica–OTS –water–OTS–silica) model. The arrows indicate the jump-in of the surfaces. The inset shows the data for 1 mM plotted on a semi-logarithmic scale with the fitting (dashed line) using eq. (1), assuming C = –203.6 mN/m and h0 = 1.6 nm.

The interaction forces of a hydrophilic, unmodified silica substrate (θs < 5°) with the OTS-hydrophobized particle (θp = 108°) fitted well to DLVO theory as shown in Fig. S1, which consistent with previous results for the hydrophobic-hydrophilic surfaces.5,32 This indicates that there is no additional attraction in the forces when one surface is completely hydrophilic. On the other hand, for substrates of greater hydrophobicity, the interaction forces with the OTS-hydrophobized particle are very different as presented in Fig. 3 where the vdW force is shown for comparison. For the slightly hydrophobic surface with θs = 40°, an attractive force stronger than the vdW force was seen on approach of the surfaces. By increasing θs, the attractive forces are seen to become stronger and longer-ranged. These forces indicate that when the hydrophobicity of one surface is high, the hydrophobic attraction can act even if the hydrophobicity of the other surface is low, even as low as θs = 40°. For each substrate, the force measured in a 0.5 M solution was almost identical to that in a 1 mM solution as shown in Fig. S2, indicating electrolyte has only minor effect on the interaction.


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Figure 3. Approaching force curves between an OTS-coated particle with θp = 108° and OTS-coated substrates with θs = 40°, 65°, 86° and 95° measured in a 1 mM NaCl solution. The arrows indicate the points of jump-in of the probe. The solid line shows vdW attraction, calculated using the planar 5-layer (silica– OTS–water–OTS–silica) model. The arrows indicate the jump-in of the surfaces.

The forces were also measured for the substrates hydrophobized with BTS (C4) in order to examine the effect of the hydrocarbon chain length on the interaction. Figure 4 shows the force curves for substrates of various hydrophobicity of the substrates with the OTS-hydrophobized particle (θp = 108°). Also in this case, the hydrophobic force acts even when one of substrates is not highly hydrophobic and is longer-ranged and stronger with increasing θs. This trend in the forces is quite similar to those for the substrates hydrophobized with OTS which implies the characteristics of the hydrophobic force are independent of the length of carbon chain (when controlled for the contact angle). More quantitative analysis is provided in the later section. Also in this case, the force measured in a 0.5 M solution was almost identical to that in a 1 mM solution for


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each substrate, as shown in Fig. S3.

Figure 4. Approaching force curves between an OTS-coated particle with θp = 108° and BTS-coated substrates with θs = 60°, 80° and 94° measured in a 1 mM NaCl solution. The solid line shows vdW attraction, calculated using the planar 5-layer model. The arrows indicate the jump-in of the surfaces.

Effect of Terminal Groups. We also investigated how the terminal groups of the silane chains affect the attractive forces. For the substrates modified with PPTS and MPTES, θs values against water were measured to be 85° and 62° respectively. These values are much less than 95° for the surface modified with the fully packed BTS (hydrocarbon-terminated), obviously due to the less hydrophobic phenyl and methacryloxy groups. AFM images of the modified substrates (Fig. S4) show smooth surfaces, suggesting that a monolayer of MPTES and PPTS was formed on the


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surfaces. Force curves obtained using the MPTES and PPTS substrates are shown in Figure 5a and b respectively, measured against the OTS-modified silica particle. The vdW attraction is also shown for comparison. On approach an attractive force acting from a separation distance of 5–10 nm was observed for both cases. The attractive forces are also longer in range and stronger than the vdW force, indicating the presence of the hydrophobic force. The forces in 1 mM and 0.5 M solutions were almost identical, indicating electrolyte does not have any effect on the interaction as observed for the OTS substrate. These force curves are compared with those for BTS-hydrophobized substrates that have contact angles similar to the PPTS and MPTES-modified substrates. As shown in the same figures, the interaction forces for the MPTES and PPTS when compared to the BTS surface with the same contact angle gave similar force curves.

Figure 5. Approaching force curves measured between an OTS-coated silica particle with θp = 108° and


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substrates modified with (a) PPTS (θs = 85°) and (b) MPTES (θs = 62°). Forces measured in 1 mM and 0.5 M NaCl solutions are shown. For the comparison, the forces for the BTS surfaces that have close θs to both substrate are also shown. The solid line shows vdW attraction, calculated using the planar 5-layer model. The arrows indicate the jump-in of the surfaces.

The substrate was also modified with CDPA, a phosphonic acid with carboxyl group. The value of θs for the CDPA-modified surface was 56°. The thickness of the CDPA layer was measured to be 1.0 nm, which is comparable to the CDPA monolayer on ZnO wafer.33 As the AFM observation of the surface gave a smooth image (Fig. S4), we can confirm that a smooth monolayer of CDPA is formed on the silica substrate. The surface potential of the CDPA-modified surfaces measured in a 1 mM NaCl solution without pH adjustment was –38 mV. This value is slightly different from the value for the untreated silica substrate, which is –45 mV in solution without pH adjustment. This difference indicates that the surface charge is caused by the dissociation of the carboxyl groups. Although the water molecules are expected to make bonds with these terminal carboxyl groups, the CDPA-modified surface is moderately hydrophobic because the relatively long hydrocarbon chain (C9) of CDPA increases the surface hydrophobicity. Here we note that the surface-water interfacial energy will be dependent both on specific short-range interactions between the terminal groups and water and the van der Waals interactions, which probe some distance into the interface. As presented in Fig. 6, an attractive force stronger than the vdW attraction was measured between the CDPA-modified substrate and the OTS-modified silica particle, despite the polarity of the terminal carboxyl group. The force was found to be quite similar to that for the BTS-modified substrate with θs = 60°, which has the most similar contact angle to the CDPA surface, shown in the


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same figure. Changing the electrolyte concentration did not affect the interaction force, as the forces were almost the same in a 1 mM and a 0.5 M NaCl solution. These results clearly show that even when one surface has a polar terminal group, the hydrophobic attraction can act as it does between moderately hydrophobic and highly hydrophobic surfaces.

Figure 6. Approaching force curves measured between an OTS-coated silica particle with θp = 108° and substrates modified with CDPA measured in 1 mM and 0.5 M NaCl solutions. For the comparison, the forces for the BTS surfaces with θs = 60° is shown. The solid line shows vdW attraction, calculated using the planar 5-layer model. The arrows indicate the jump-in of the surfaces.

Dependence of Force on Substrate Hydrophobicity. In order to evaluate the dependency of the forces on θs more quantitatively, we fitted the force curves for all surfaces to eq. (1). The value of h0 and C obtained is plotted against θs for all the systems in Fig. 7a and b. For each substrate a


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datum point is presented which is obtained from fitting and averaging the forces obtained at 30 different locations in two independent measurements. Both parameters were seen to be variable, as shown by the error bars in the plots denoting the standard deviation. It is worth noting that the deviation was largest for surfaces with intermediate to moderately high hydrophobicity, namely about θs = 80°–95°. This means that the range and magnitude of the force depends largely on the experimental batch and the measurement place. Nevertheless, the average value of h0 and C essentially increases with θs, indicating that the attractive forces become longer in range and stronger with increasing hydrophobicity of the substrate, noting that the one surface is unchanged across all measurements. A small transition in the slope of h0 versus θs occurs at θs > 80–90°, whilst C increases somewhat linearly with θs. It is noteworthy that irrespective of the carbon chain length (OTS or BTS) and the terminal groups (BTS, PPTS, MPTES or CDPA), h0 and C fall on a master curve when plotted against θs. These results confirm that the hydrophobic attraction depends greatly on the contact angle for the systems presented, irrespective of the carbon chain length and the terminal groups. The average value of h0 for each θs was within a range between 0.8–1.7 nm, corresponding to the range (0.3–2.0 nm) normally measured between solid hydrophobic surfaces. The parameter C is often correlated with the interfacial tension γs, as the interaction energy between planar surfaces per unit area W = –2γs calculated by F = 2πRW for symmetric cases. However, the obtained γs from C (γs = C/4π) in Fig. 7b was about 10 mN/m for symmetric surfaces (θs = θp = 108°), which is much lower than the expected γs for a hydrocarbon surface which is about 50 mN/m.34 This discrepancy might be partly due to the roughness of the surfaces.


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Figure 7. Dependence of the fitting parameter h0 (a) and C (b) of the attractive forces to eq. (1) on θs. The average value of the forces obtained at 30 different points of the substrates in two independent measurements for one substrate is shown in each plot. The error bars denote the standard deviations for each data.

The adhesion force measured in a 1 mM NaCl solution is plotted against θs in Fig. 8. The fundamental feature of the plot is that the force was strongly dependent on θs, and the carbon chain length and the terminal groups has only minor influence as observed for the forces measured on approach as characterized by h0 and C. The deviation of the data seems smaller than that for h0 and C and a clearer transition in the gradient can be seen at around θs = 80°, indicating the force increases more steeply at higher θs, that is when the surfaces are more hydrophobic.


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Figure 8. The adhesion forces plotted against θs. The average value of the forces obtained at 30 different points of the substrates in two independent measurements for one substrate is shown. The error bars denote the standard deviations for each data. The dashed line denotes the capillary force obtained by eq. (2) assuming θ1 = 72° (θp = 108°).

DISCUSSION In the previous results and Fig. S1 of present study, the hydrophobic force was not observed between a highly hydrophobized surface and a completely hydrophilic silica surface. On the other hand, when one surface is slightly hydrophobized to θs = 40°, the hydrophobic attraction is revealed in the force curve. We want to point out here that this result is in contrast to our previous results for the symmetric silica surfaces hydrophobized with OTS,26, 35 In the symmetric case, the hydrophobic attraction becomes unstable or disappears at much higher contact angle, about 80°– 90°. This difference indicates that in the asymmetric systems, the high hydrophobicity of one


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surface can promote the hydrophobic attraction to the extent that it is present even when the hydrophobicity of the other surface is low. Interestingly, this result is quite similar to those for air bubble-solid surface systems. Fielden et al. found a hydrophobic force between an air bubble and a silica surface hydrophobized by dehydroxylation with θ = 30°.36 Shi et al. measured a hydrophobic attraction between an air bubble and a hydrophobized mica surface of θ = 45°.37 They also evaluated the decay length of the force to be approximately 0.8 nm, which is comparable to that for the attractive force obtained for OTS-modified substrate with θs = 40° in this study. From this coincidence, one may suspect that these attractive forces are possibly caused by nanobubbles remaining on the surfaces. However, we are excluding this possibility, as the range of the attractive force observed here is less than 10 nm in most cases, whereas the height of nanobubbles is generally more than 10 nm.38 Further, the attraction measured on approach was almost identical independent of measurement cycle, whereas the force often becomes longer-ranged by the repeated approach-retraction cycles in the case of a nanobubble-bridging force, due to the coalesce and growth of the bubbles.17, 39 Additionally, although there is certain degree of fluctuation in the range and magnitude of the forces the dependence on the location of the measurement is not as large as when the attractive force is due to nanobubbles.17 The hydrocarbon chain length and the nature of the terminal groups on the chemisorbed layer did not determine the range and the magnitude of the attraction – other than their influence on the contact angle. In this regard the interactions obtained for the CDPA surface are particularly important, as the carboxyl groups can hydrate strongly and such hydration is expected to prevent observation of the hydrophobic force, as is the case for hydrophilic silica (Fig. S1). Since CDPA can bond covalently to oxide surfaces,40 an attractive force is not likely due to the lateral fluctuations of the adsorbed molecules.41 The phenyl and the methacryloxy groups can also


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interact more strongly with water molecules than hydrocarbon groups,42 but the force is dependent mostly on θs and the surfaces having similar θs produce similar attractive forces. This independence of the force from the property of the surface molecules when controlled for contact angle emphasizes that the high hydrophobicity of one surface dominates the hydrophobic force and renders the influence of nanoscopic properties of the molecules on the other surface only minor. Attractive forces between asymmetric surfaces are sometimes related to an electrostatic force. When two surfaces that have surface potentials of different magnitude interact an attractive double-layer forces can occur even if they have the same sign, by interacting close to the constant surface potential condition.43 Alternatively, if some charged domains are created by the lateral polymerization of the silane molecules on the surfaces, it could also induce an electrostatic interaction.13 These explanation cannot explain the present results, as all the attractive forces measured here were almost unchanged with changing electrolyte concentration. Additionally, the attractive forces were present at high concentration of electrolyte, which screens out electrostatic interactions. This independence of the forces on the electrolyte concentration is one of the characteristics of the hydrophobic attraction between robust, chemisorbed hydrophobic surfaces reported previously.7, 12, 26 Therefore, it would be reasonable to suppose that the contribution of electrostatic phenomena to the attractive forces in the present case is only minor. In contrast, the strong dependence of the forces on the surface hydrophobicity suggests the formation of a vapor cavity between the surfaces as a possible candidate for the origin of the interaction. This is because the strength of the capillary force due to bridging is principally determined by the macroscopic contact angle of the interacting surfaces. The actual formation of a bridging cavity has sometimes been supposed in SFA measurements by the deformation of fringes


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of equal chromatic order (FECO) due to changes of refractive index, though there have not been conclusive observations. Recently, Faghihnejad and Zeng44 observed the formation of a bridging cavity at a separation distance of < 20 nm between polystyrene surfaces, although it is difficult to distinguish whether the cavity was formed by spontaneous cavitation or by the coalescence of preexisting nanobubbles. The fluctuating nature of h0 and C values shown in Fig. 7a and b could be due to the stochastic nature of cavity formation. Ducker and Mastropietro45 have also pointed out the variability in the range of the attraction between silanated glass surfaces between experimental batches and attributed it to cavitation. The formation of a vapor cavity might be sensitive to unavoidable disturbances such as microscopic surface roughness and small vibrations as well as inherently stochastic. In which case, the variability in the range and strength of the force depending on the experimental batch and the location on the substrate can be understood. Particularly, the large deviation of h0 and C at θs = 80°–95° mentioned above could be attributed to the formation of cavity becoming more difficult with decreasing θs. The measured adhesion force can be fitted to the capillary force Fc between contacting particle and flat plate that have different contact angles θ1 (particle) and θ2 (plate). An analytical expression of Fc for a liquid bridge between a sphere of radius R and a flat plate approximating the meniscus curvature as circular is given as46 Fc /R=2πγL cos θ1 +cos θ2

(2),

where γL denotes the surface tension of liquid. This equation is directly applicable to a vapor bridge in water by assuming θ1 = θp – 180° and θ2 = θs – 180°. In Fig. 8, the calculated capillary


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forces are shown as a dashed line when θ1 is assumed to be 72° (= 180°–108°). At about θs > 80° (i.e. θ2 < 100°), the calculated forces fit well to the obtained adhesion forces, suggesting the formation of a bridging cavity, at least when the surfaces are brought into contact, in this contact angle region. In contrast, at about θs < 80° the adhesion forces were observed in the retracting force, whereas the calculation does not give any adhesion at θs < 72°. Indeed, eq. (2) gives negative adhesion force when θ2 is more than 108° (i.e. θs < 72°) because the concave shape of the meniscus required to give rise to an attractive force cannot be formed geometrically with this boundary condition. Thus, the adhesion force is not likely due to the bridging cavity at θs < 72°. It is possible that the meniscus has a contact angle different from that expected from the macroscopic value on the surface due to its microscopic size, noting that surface nanobubbles often have larger contact angles than the macroscopic value.22, 47 However, considering the fact that h0 and C values at about

θs < 80° have less fluctuating nature than those for higher θs, it is implied that there could be another mechanism for the attraction at small θs,. In this case the adhesion may simply be due to van der Waals forces and the lack of a repulsive hydration force. It is also worth noting that the present results are in quite good agreement with a recent molecular dynamics simulation of interactions between surfaces with different contact angles conducted by Kanduć and Netz48. From the simulation results, they derived a correlation between the contact angle of the interacting surfaces (θ1 + θ2) and the state of the interaction. The cavitation-induced attraction was observed when (θ1 + θ2)/2 > 90°. Whilst reducing the contact angles leads to a hydration repulsion between the surfaces, they pointed out that there is an intermediate “dry adhesion without cavitation” regime between the cavitation and the hydration repulsion regimes. In the dry adhesion, all of the water is expelled into the bulk by at close contact.


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The dry adhesion is predicted to occur when (θ1 + θ2)/2 < 90°, which is corresponding to θs = 72° in our system. This agrees well with θs = 70°~80° at which some transition was observed in the adhesion force in the present result. Moreover, our result that the attractive force began to act when

θs = 40°, which is (θ1 + θ2)/2 = 74°, corresponds well with their prediction of a lower limit of the dry adhesion, (θ1 + θ2)/2 > 67°~83°. This agreement suggests that for the present systems attraction is expected when the surfaces are brought into contact. The agreement between experiment and simulation on the adhesion, however, does not explain directly the origin of the obtained attractive force on the approach. There is discrepancy in the range of the attraction between the simulation and experimental results, and this should be carefully considered. Moreover, the dry adhesion in the simulation only describes an adhesion after the surfaces are brought into contact and attraction between the distant surfaces has not been found yet when θs is not high. More detailed investigation is required to explain the attractive force, particularly when θs is not high. All the attractive forces obtained in the present study have the range of 5–10 nm, which is consistent with most of the other studies of the “pure” hydrophobic attraction. Given the high surface energy of hydrophobic surfaces in water, the presence and magnitude of the attraction between hydrophobic surfaces is unsurprising, but the range of the interaction is surprising. How does one hydrophobic surface sense the presence of another hydrophobic surface when the surfaces are separated by a distance of 5–10 nm? In the presence of electrolyte, electrolytic interactions are highly screened and can therefore be ruled out for the experiments presented here. The effect of water structure has been considered but the radial distribution function of liquid water shows no features beyond 1.5 nm.49 However variations in the density of water are not the


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only order structure that should be considered. We believe a recent study on the orientational ordering of water may be important in understanding the range of the hydrophobic attraction. Chen et al.50 recently investigated changes in the orientational order of water molecules in aqueous electrolyte solutions using femtosecond elastic second harmonic scattering. They found that the change in the orientational order of water due to ions in solution propagates out to about 8 nm. Although this does not suggest a mechanism for the hydrophobic force, the length scale is quite noteworthy as it shows that the orientational structure of water extends to far greater distances than the radial distribution function. If a similar perturbation of water structure were caused by hydrophobic surfaces and propagated over the same length scale, it would explain how one hydrophobic surface is able to sense the presence of another at distances of 5–10 nm. One might then posit that orientational restructuring of the water may provide the mechanism by which the surfaces reach the lower energy state in which they are in contact. If such a mechanism operates on biological molecules, it would implicate water in directing substrates to the hydrophobic pockets of enzymes over substantial distances. We suggest that theoretical investigations of the orientational ordering of water and how it is perturbed by macroscopic hydrophobic or hydrophilic surfaces may provide valuable insights into the hydrophobic attraction.

Summary The interaction forces between hydrophobized silica surfaces that have different hydrophobicity were measured using colloidal probe AFM. The interaction forces between the highly hydrophobic particle and a completely hydrophilic substrate fitted well to the DLVO theory, indicating that no additional forces act between the surfaces. On the other hand, when the silicon


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wafer surface was slightly hydrophobized and the contact angle of the surface was 40°, an additional attractive force that is longer-ranged than the van der Waals force was observed between the surfaces. The range and strength of the attractive force increases with increasing contact angle of the substrate. Both hydrocarbon chain length and terminal groups of hydrophobizing reagent on the substrate have only minor effect on the attraction, when controlled for the substrate hydrophobicity as measured by the contact angle. This implies that when one surface is highly hydrophobic, this promotes the hydrophobic attraction such that it is observed even against a mildly hydrophobic surface. The nanoscopic properties of the molecules on the other surface play only a minor role byeong their influence on the contact angle. Despite the mechanism(s) responsible for the hydrophobic attraction remaining unresolved, we believe our results will be an important basis to explore the origin of the hydrophobic attraction theoretically. As well, our results provide useful information to predict the behavior of colloidal materials with dissimilar hydrophobicity, which is fundamentally important in many industrial and natural phenomena.

Supporting Information Force curves between OTS-coated particle and unmodified silica, force curves for OTS and BTS-coated substrates at different salt concentrations, AFM image of substrates

Acknowledgements N. I. acknowledges financial support by KAKENHI (Grant Numbers 25420803 and 15KK0238) from the Japan Society for the Promotion of Science and by a research


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grant from The Information Center of Particle Technology, Japan.


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20. Meyer, E. E.; Rosenberg, K. J.; Israelachvili, J., Recent progress in understanding hydrophobic interactions. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15739-15746. 21. Tabor, R. F.; Grieser, F.; Dagastine, R. R.; Chan, D. Y. C., The hydrophobic force: measurements and methods. Phys. Chem. Chem. Phys. 2014, 16, 18065-18075. 22. Ishida, N.; Inoue, T.; Miyahara, M.; Higashitani, K., Nano Bubbles on a Hydrophobic Surface in Water Observed by Tapping-Mode Atomic Force Microscopy. Langmuir 2000, 16, 6377-6380. 23. 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. 24. Miklavic, S. J.; Chan, D. Y. C.; White, L. R.; Healy, T. W., Double-layer forces between heterogeneous charged surfaces. J. Phys. Chem. 1994, 98, 9022-9032. 25. Kekicheff, P.; Spalla, O., Long-Range Electrostatic Attraction between Similar, Charge-Neutral Walls. Phys. Rev. Lett. 1995, 75, 1851-1854. 26. Ishida, N.; Kusaka, Y.; Ushijima, H., Hydrophobic Attraction between Silanated Silica Surfaces in the Absence of Bridging Bubbles. Langmuir 2012, 28, 13952-13959. 27. Hammer, M. U.; Anderson, T. H.; Chaimovich, A.; Shell, M. S.; Israelachvili, J., The search for the hydrophobic force law. Faraday Discuss. 2010, 146, 299-308. 28. Tabor, R. F.; Wu, C.; Grieser, F.; Dagastine, R. R.; Chan, D. Y. C., Measurement of the Hydrophobic Force in a Soft Matter System. J. Phys. Chem. Lett. 2013, 4, 3872-3877. 29. Preuss, M.; Butt, H.-J., Measuring the Contact Angle of Individual Colloidal Particles. J. Colloid Interface Sci. 1998, 208, 468-477. 30. Tillman, N.; Ulman, A.; Schildkraut, J. S.; Penner, T. L., Incorporation of phenoxy groups in self-assembled monolayers of trichlorosilane derivatives. Effects on film thickness, wettability, and molecular orientation. J. Am. Chem. Soc. 1988, 110, 6136-6144. 31. Cleveland, J. P.; Manne, S.; Bocek, D.; Hansma, P. K., A nondestructive method for determining the spring constant of cantilevers for scanning force microscopy. Rev. Sci. Instrum. 1993, 64, 403-405. 32. Kaggwa, G. B.; Nalam, P. C.; Kilpatrick, J. I.; Spencer, N. D.; Jarvis, S. P., Impact of Hydrophilic/Hydrophobic Surface Chemistry on Hydration Forces in the Absence of Confinement. Langmuir 2012, 28, 6589-6594. 33. Zhang, B.; Kong, T.; Xu, W.; Su, R.; Gao, Y.; Cheng, G., Surface Functionalization of Zinc Oxide by Carboxyalkylphosphonic Acid Self-Assembled Monolayers. Langmuir 2010, 26, 4514-4522. 34. Fowkes, F. M., Additivity of Intermolecular Forces at Interfaces. I. Determination of the Contribution to Surface and Interfacial Tensions of Dispersion Forces in Various Liquids. J. Phys. Chem. 1963, 67, 2538-2541. 35. Soga, Y.; Imanaka, H.; Imamura, K.; Ishida, N., Effect of surface hydrophobicity on short-range hydrophobic attraction between silanated silica surfaces. Adv. Powder Technol. 2015, 26, 1729-1733. 36. Fielden, M. L.; Hayes, R. A.; Ralston, J., Surface and Capillary Forces Affecting Air Bubble −Particle Interactions in Aqueous Electrolyte. Langmuir 1996, 12, 3721-3727. 37. Shi, C.; Cui, X.; Xie, L.; Liu, Q.; Chan, D. Y. C.; Israelachvili, J. N.; Zeng, H., Measuring Forces and Spatiotemporal Evolution of Thin Water Films between an Air Bubble and Solid Surfaces of Different Hydrophobicity. ACS Nano 2015, 9, 95-104. 38. Alheshibri, M.; Qian, J.; Jehannin, M.; Craig, V. S. J., A History of Nanobubbles. Langmuir 2016, 32, 11086-11100. 39. Yakubov, G. E.; Butt, H.-J.; Vinogradova, O. I., Interaction Forces between Hydrophobic


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Surfaces. Attractive Jump as an Indication of Formation of “Stable” Submicrocavities. J. Phys. Chem. B 2000, 104, 3407-3410. 40. Cattani-Scholz, A., Functional Organophosphonate Interfaces for Nanotechnology: A Review. ACS Appl. Mater. Interfaces 2017, 9, 25643-25655. 41. Yaminsky, V. V.; Ninham, B. W., Hydrophobic force: lateral enhancement of subcritical fluctuations. Langmuir 1993, 9, 3618-3624. 42. Suzuki, M.; Shigematsu, J.; Fukunishi, Y.; Kodama, T., Hydrophobic Hydration Analysis on Amino Acid Solutions by the Microwave Dielectric Method. J. Phys. Chem. B 1997, 101, 3839-3845. 43. Israelachvili, J. N., Intermolecular and Surface Forces. Third ed.; Academic Press: Boston, 2011. 44. Faghihnejad, A.; Zeng, H., Hydrophobic interactions between polymer surfaces: using polystyrene as a model system. Soft Matter 2012, 8, 2746-2759. 45. Ducker, W. A.; Mastropietro, D., Forces between extended hydrophobic solids: Is there a long-range hydrophobic force? Curr. Opin. Colloid Interface Sci. 2016, 22, 51-58. 46. Butt, H.-J.; Kappl, M., Surface and Interfacial Forces. Wiley-VCH: Weinheim, 2010. 47. 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. 48. Kanduč, M.; Netz, R. R., From hydration repulsion to dry adhesion between asymmetric hydrophilic and hydrophobic surfaces. Proc. Natl. Acad. Sci. U.S.A. 2015, 112, 12338-12343. 49. Soper, A. K., The radial distribution functions of water and ice from 220 to 673 K and at pressures up to 400 MPa. Chem. Phys. 2000, 258, 121-137. 50. Chen, Y.; Okur, H. I.; Gomopoulos, N.; Macias-Romero, C.; Cremer, P. S.; Petersen, P. B.; Tocci, G.; Wilkins, D. M.; Liang, C.; Ceriotti, M.; Roke, S., Electrolytes induce long-range orientational order and free energy changes in the H-bond network of bulk water. Sci. Adv. 2016, 2, e1501891.


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Figure 1. Chemical structures of (a) octadecyltrichlorosilane (OTS); (b) butyltrichlorosilane (BTS); (c) 3phenylpropyltrichlorosilane (PPTS); (d) 3-methacryloxypropyltriethoxysilane (MPTES); (e) 10Carboxydecylphosphonic acid (CDPA). 109x67mm (300 x 300 DPI)


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Figure 2. Approaching force curves between OTS-coated surfaces that have same contact angle (108°) measured in 1 mM, 10 mM, and 0.5 M NaCl solutions. The solid line shows vdW attraction, calculated using the planar 5-layer (silica-OTS-water-OTS-silica) model. The arrows indicate the jump-in of the surfaces. The inset shows the 1 mM data plotted on a semi-logarithmic scale with the fitting (dashed line) using eq. (1), assuming C = -203.6 mN/m and h0 = 1.6 nm. 122x117mm (300 x 300 DPI)


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Figure 3. Approaching force curves between an OTS-coated particle with θp = 108° and OTS-coated substrates with θs = 40°, 65°, 86° and 95° measured in a 1 mM NaCl solution. The arrows indicate the points of jump-in of the probe. The solid line shows vdW attraction, calculated using the planar 5-layer (silica-OTS-water-OTS-silica) model. The arrows indicate the jump-in of the surfaces. 119x115mm (300 x 300 DPI)


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Figure 4. Approaching force curves between an OTS-coated particle with θp = 108° and BTS-coated substrates with θs = 60°, 80° and 94° measured in a 1 mM NaCl solution. The solid line shows vdW attraction, calculated using the planar 5-layer model. The arrows indicate the jump-in of the surfaces. 119x115mm (300 x 300 DPI)


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Figure 5. Approaching force curves measured between an OTS-coated silica particle with θp = 108° and substrates modified with (a) PPTS (θs = 85°) and (b) MPTES (θs = 62°). Forces measured in 1 mM and 0.5 M NaCl solutions are shown. For the comparison, the forces for the BTS surfaces that have close s to both substrate are also shown. The solid line shows vdW attraction, calculated using the planar 5-layer model. The arrows indicate the jump-in of the surfaces. 122x58mm (300 x 300 DPI)


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Figure 6. Approaching force curves measured between an OTS-coated silica particle with θp = 108° and substrates modified with CDPA measured in 1 mM and 0.5 M NaCl solutions. For the comparison, the forces for the BTS surfaces with θs = 60° is shown. The solid line shows vdW attraction, calculated using the planar 5-layer model. The arrows indicate the jump-in of the surfaces. 119x115mm (300 x 300 DPI)


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Figure 7. Dependence of the fitting parameter h0 (a) and C (b) of the attractive forces to eq. (1) on θs. The average value of the forces obtained at 30 different points of the substrates in two independent measurements for one substrate is shown in each plot. The error bars denote the standard deviations for each data. 122x58mm (300 x 300 DPI)


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Figure 8. The adhesion forces plotted against θs. The average value of the forces obtained at 30 different points of the substrates in two independent measurements for one substrate is shown. The error bars denote the standard deviations for each data. The dashed line denotes the capillary force obtained by eq. (2) assuming θ1 = 72° (θp = 108°). 119x115mm (300 x 300 DPI)


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For Table of Contents Only 80x40mm (300 x 300 DPI)


Hydrophobic Attraction Measured between Asymmetric Hydrophobic

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