Measurements of the Force between Fluorocarbon Monolayer

On the LB surfaces, after unbound molecules were removed, the force profile did not change for at ... off forces between two such surfaces in air and ...
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Langmuir 2000, 16, 8360-8367

Measurements of the Force between Fluorocarbon Monolayer Surfaces in Air and Water Satomi Ohnishi* Department of Polymer Physics, National Institute of Materials & Chemical Research, Tsukuba, Ibaraki 305-8565, Japan

Vassili V. Yaminsky and Hugo K. Christenson Department of Applied Mathematics, Research School of Physical Sciences & Engineering, Australian National University, Canberra, A.C.T. 0200, Australia Received February 23, 2000. In Final Form: June 12, 2000 We have measured forces between fluorocarbon surfaces prepared by three different methods using heptadecafluoro-1,1,2,2,-tetrahydrodecyltriethoxysilane (FTE): LB deposition (LB), adsorption from diluted FTE solution with chloroform (FTE/CHCl3), and adsorption from undiluted FTE liquid (FTE/neat). In dry air, a van der Waals attraction was observed between the surfaces except for the FTE/neat surfaces. The Hamaker constant was found to be 7 × 10-20 J, which is comparable to that of the fused silica substrate. In saturated water vapor, a capillary bridging force was observed with FTE/CHCl3 and FTE/neat owing to the adsorption of water between the substrate and the fluorocarbon layer while a van der Waals attraction identical to that observed in dry air was detected between the LB surfaces. Only the LB surfaces were stable enough to effectively partition the adsorbed water layer beneath the fluorocarbon layer and air. In water, long-range forces from 120 to 150 nm were observed with FTE/neat while a rather shorter-range attraction within 40 nm was observed with LB and FTE//CHCl3. The change in force profile with time could be explained by a change in surface properties occurring on immersion in water. It is suggested that the greater the number of mobile molecules, the longer-range the attraction observed between the surfaces. On the LB surfaces, after unbound molecules were removed, the force profile did not change for at least 5 days and a strong attractive force at separations below 12-13 nm was observed in water. The range of this force is in agreement with other studies showing only a comparatively short-range attraction between stable and homogeneous hydrophobic surfaces. The magnitude of the attraction is close to that of the short-range force measured previously between mica surfaces coated with fluorocarbon surfactant monolayers. This suggests that the strong, short-range attraction measured between many LB surfaces is related to the force between stable hydrophobic surfaces but that the very long-range exponentially decaying force has a different origin.

Introduction The attractive forces found between hydrophobic surfaces across aqueous solutions1-7 have been the focus of considerable interest and debate for almost 20 years. Much controversy has been generated because of complications caused by changes in surface properties with time and the influence of electrolytes.8-13 To understand better the forces observed between hydrophobic surfaces, systematic * To whom correspondence should be addressed. E-mail, [email protected]; telephone, +81-298-61-6317; Fax, +81298-61-6232. (1) Israelachvili, J. N.; Pashley, R. M. Nature 1982, 300, 341. (2) Pchelin, V. A.; Yaminskii, V. V. Kolloidn. Zh. 1972, 34, 305. (3) Rabinovich, Ya. I.; Derjaguin, B. V.; Churaev, N. V. Adv. Colloid Interface Sci. 1982, 16, 63. (4) Israelachvili, J. N.; Pashley, R. M. J. Colloid Interface Sci. 1984, 98, 500. (5) Pashley, R. M.; McGuiggan, P. M.; Ninham, B. W.; Evans, D. F. Science 1985, 229, 1088. (6) Christenson, H. K. In Modern Approaches to Wettability: Theory and Applications; Schrader, M. E., Loeb, G.; Eds.; Plenum: New York, 1992. (7) Christenson, H. K.; Yaminsky, V. V. Colloids Surf. 1997, A129130, 67. (8) Claesson, P. M.; Blom, C. E.; Herder, P. C.; Ninham, B. W. J. Colloid Interface Sci. 1986, 114, 234. (9) Claesson, P. M.; Christenson, H. K. J. Phys. Chem. 1988, 92, 1650. (10) Christenson, H. K.; Fang, J.; Ninham, B. W.; Parker, J. L. J. Phys. Chem. 1990, 94, 8004. (11) Christenson, H. K.; Claesson, P. M.; Berg, J.; Herder, P. C. J. Phys. Chem. 1989, 93, 1472.

investigations of correlations between surface forces and surface properties, such as surface morphology, stability, and chemical composition, are essential. In our previous paper14 we reported on the preparation and characterization of fluorocarbon monolayer surfaces using a fused glass substrate. We used the same fluorinated silane, heptadecafluoro-1,1,2,2,-tetrahydrodecyltriethoxysilane (FTE), to make hydrophobic surfaces by three different methods. FTE was either (i) spread on the air-water interface, allowed to polymerize, and then deposited as an LB film (at surface pressures of 10 mN/ msdesignated LB), (ii) allowed to react with the silica surface in a CHCl3 solution (FTE/CHCl3), or (iii) allowed to react with the silica in the undiluted liquid state (FTE/ neat). The surfaces thus prepared were scanned by atomic force microscopy (AFM), their chemical composition were analyzed by X-ray photoelectron spectroscopy, wettability studies with water were performed, and adhesion or pulloff forces between two such surfaces in air and water were determined. On the basis of the results of these measurements, we surmised that the fluorocarbon surfaces prepared by the three different methods may be depicted schematically as shown in Figure 1 and Figure 8. AFM (12) Christenson, H. K.; Claesson, P. M.; Parker, J. L. J. Phys. Chem. 1992, 96, 6725. (13) Craig, V. S. J.; Ninham, B. W.; Pashley, R. M. Langmuir 1999, 15, 1562. (14) Ohnishi, S.; Ishida, T.; Yaminsky, V. V.; Christenson, H. K. Langmuir 2000, 16, 2722.

10.1021/la000262e CCC: $19.00 © 2000 American Chemical Society Published on Web 10/07/2000

Force between Fluorocarbon Monolayer Surfaces

Langmuir, Vol. 16, No. 22, 2000 8361 vs separation curves for two surfaces to be determined for arbitrary, smooth materials. One surface is rigidly connected to the base of the instrument via a piezoelectric drive, and the other is mounted on a cantilever spring of a piezoelectric bimorph, which acts as the force/deflection sensor. The sensor is positioned on a motorized microtranslation stage, which permits one surface to be moved with respect to the other to adjust their initial separation. A magnetic transducer with a magnet attached to the cantilever allows fine control of the separation in the range of up to several micrometers.18 The bimorph deflection and jumps occurring at the onset of spring instabilities were monitored with a computer. Pull-off forces were calculated from the measured jump-out distances and the cantilever spring constant (258 N/m). The speed at which the surfaces were made to approach and separate in the absence of any interaction between them was 6.5 nm/s for measurements in air and 25 nm/s in water and was varied from 50 to 200 nm/s for studies of hydrodynamic effects. The substrates used in the IG experiments were molten Pyrex glass spheres with radii of approximately 2 mm, formed by melting 2 mm glass rods. The radii of curvature of the spheres were measured with a microscope with a traveling reticule (estimated error ( 0.02 mm).

Results and Discussion Figure 1. Schematic drawings of the suggested structure of fluorocarbon surfaces prepared with heptadecafluoro(1,1,2,2,tetrahydrodecyl)triethoxysilane (FTE) by three different methods (see text). These were drawn on the basis of the results of XPS, AFM, contact angle, and pull-off force measurements.14

and friction force images revealed that these three different surfaces showed comparable mean roughness while the molecules of the surfaces were in different states of mobility; the molecules of FTE/neat were the most mobile of those of the three surfaces. In this paper we describe the results of force measurements with these well-characterized surfaces in dry air, humid air, and water. We also report on the evolution of the forces due to changes in surface properties with time of immersion in water. Materials and Methods Materials. The fused glass substrates, Pyrex rods or plates, were melted in a propane-oxygen flame. The molten glass was rinsed with pure water in order to remove possible ionic components from the glass surface and to avoid any organic contamination, and tested for uniform wetting by water. Heptadecafluoro(1,1,2,2,-tetrahydrodecyl)triethoxysilane was purchased from Gelest Inc. and used without further purification. Chloroform (ACS grade, Aldrich) and nitric acid were used as received. Ethanol (CSR Australia) was distilled under a nitrogen atmosphere. Phosphorus pentoxide (P2O5) (Fluka AG, Bushs SG, Switzerland) was used as a drying agent. The water was distilled and processed through a Millipore UHQ unit. Surface Preparation. LB Method. We followed the method for deposition of octadecyltriethoxysilane (OTE) monolayers on mica reported by Wood and Sharma.15,16 The pH of the subphase was adjusted to 2 with nitric acid , and a 3 mM chloroform solution of FTE was spread over the surface. The FTE monolayer on the air-water interface was kept at 10 mN/m for 30 min and then transferred onto the molten glass substrate by upward drawing (5 mm/min). Adsorption Method. FTE was adsorbed on the substrates from CHCl3 solution (30 mM) and from neat FTE liquid for 1 h. All surfaces were annealed in a dry nitrogen atmosphere at 100 °C for 2 h after deposition/adsorption. Force Measurements. Force measurements were carried out with the interfacial gauge (IG). A detailed description of this instrument has been given previously.17 The IG permits the force (15) Wood, J.; Sharma, R. Langmuir 1995, 11, 4797. (16) Wood, J.; Sharma, R. Langmuir 1994, 10, 2307. (17) Yaminsky, V. V.; Ninham, B. W.; Stewart, A. M. Langmuir 1996, 12, 836. Yaminsky, V.; Jones, C.; Yaminsky, F.; Ninham, B. W. Langmuir 1996, 12, 3531.

The Contact Deformation of the Substrate. To define the elastic properties of the surfaces and the zero of distance (D ) 0) between the outermost surfaces, we have considered the contact deformation δ of the substrate using JKR theory. 19-21

a3 )

R (F + 3πW + x6πRWF + (3πRW)2) E

(1)

Here R ) 2R1R2/(R1 + R2) is the harmonic mean of the radii of the spheres, W is the surface adhesion energy, E is the elastic modulus of the substrate, F is the external load, and a is the radius of the contact area. Moreover, the contact deformation length δ is given by

δ)

[ ()]

2 a0 a2 1R 3 a

3/2

(2)

where a0 is a for F ) 0 in eq 1.20 Using these equations, we can derive the relation between the external load F and the deformation length δ for various values of the surface energy γ and the elastic modulus E.22 Figure 2 shows theoretical curves of the relationship between δ and F for the fluorocarbon surfaces in air (γFTE-air ) 20 mN/m) and in water (γFTE-water ) 50 mN/m). These values of the surface energy were estimated using adhesion measurements in our previous paper,14 and the values of the elastic moduli we employed were about 7 × 10-13 mN/m2, which corresponds to the Young’s modulus of fused silica.23 Parts a and b of Figure 3 show experimental data for fluorocarbon surfaces (open circles) on separation in air and in water, respectively. In air, although the data are more scattered than in water, it can be seen that the measured deformation length, the external load, and the “jump-out” at 0.8 nm follow the theoretical prediction for E ) 7 × 10-13 mN/m2. In water, the experimental data were best fitted to the theoretical curve for E ) 6 × 10-13 (18) Stewart, A. M.; Christenson, H. K. Meas. Sci. Technol. 1990, 1, 1301. (19) Johnson, K. L.; Kendall, K.; Roberts, A. D. Proc. R. Soc. London, Ser. A 1971, 324, 301. (20) Maugis, D. J. Colloid Interface Sci. 1991, 150, 243. (21) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: San Diego, CA, 1992. (22) Barthel, E.; Lin, X. Y.; Loubet, J.-L. J. Colloid Interface Sci. 1996, 177, 401. (23) Weast, R. C., Ed. Handbook of Chemistry and Physics, 53rd ed.; CRC Press: Cleaveland, OH, 1972.

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Figure 2. JKR theoretical curves of the relationship between the external load F/R and the deformation length of the surfaces in air and in water for various values of the elastic modulus E: s, E ) 7 × 10-13 mN/m2; ---, E ) 6 × 10-13 mN/m2; ‚‚‚, E ) 5 × 10-13 mN/m2. The deformation length is the observed deviation from the contact position of undeformed surfaces. These values of the surface energy were estimated using adhesion measurements.14 The arrows show the contact deformation length of the surfaces in air and water in our experimental system.

Figure 3. Measured forces on separation between fluorocarbon surfaces deposited by the LB method (open circles): a, in air; b, in water. Jump-out was observed at 0.8-0.9 nm in air (a) and at 1.7-1.8 nm in water (b). Theoretical curves for various values of the elastic modulus E in the range between -0.5 and 2 nm are highlighted.

mN/m2. This enhanced elasticity suggests that the surfaces may be slightly swollen in water. The slope of the curve

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Figure 4. Measured forces on approach in dry air of fluorinated surfaces prepared by three different methods: O, LB; ], FTE/ CHCl3; 9, FTE/neat. The dotted line represents a van der Waals attraction calculated using a Hamaker constant A ) 7 × 10-20 J.

at the “jump-out” distance of 1.6 nm was also consistent with the stiffness of the cantilever (k ) 258 N/m). Thus, the elastic moduli of the surfaces in air and in water were determined by fitting to theoretical curves. In Figure 2, the contact deformation length follows the solid line in air and the broken line in water. Therefore, the contact deformation length at zero load (F/R ) 0) can be estimated as 1.3 nm in air and 2.6 nm in water. The distance axis of the force curves presented hereafter has been adjusted by these values of the contact deformation length. Forces in Dry Air. Figure 4 shows the forces in dry air between fluorinated surfaces prepared by three different methods. The force curves of the LB (open circles) and the FTE/CHCl3 (open diamonds) surfaces exhibited almost the same profiles, which show an attraction from 20 nm and an abrupt “jump-in” at 6 ( 1 nm. The duration of the jump was shorter than 2 ms (for distances smaller than 7 nm, the measured points are recorded at 2 ms intervals), while the surfaces approached 7 nm with a speed of 6-7 nm/s, which is the same as the driving speed (for distances larger than 7 nm, the measured points are shown at 0.1 s intervals), and the speed of approach increased below about 7 nm owing to the abrupt jump-in. The force profile remained reproducible as long as the surfaces were kept sealed under dry condition with P2O5, even after a high load of 10mN/m was applied. This shows that both the LB and the FTE/CHCl3 surfaces are so robust in dry air that they are not damaged by repeated contact and separation. The van der Waals (vdW) force calculated from Lifshitz theory is shown as a dotted line in Figure 4. Since the theoretical curve is in agreement with the experimental curves, the main force observed between the LB and the FTE/CHCl3 surfaces in dry air can be identified as a vdW force. The Hamaker constant A for nonretarded forces between these surfaces in air was found to be A ) 7 × 10-20 J, which is larger than the theoretical value for poly(tetrafluoroethylene) (PTFE) (3.8 × 10-20 J)24 and is comparable to that for fused silica (6.5 × 10-20 J).24 Owing to the greater value of the Hamaker constant, the vdW attraction between the surfaces approached that expected between silica substrates. The jump distance of 6 ( 1 nm is consistent with the theoretical value (about 5 nm) at which the spring becomes unstable under the effect of vdW forces (k/R ) A/6D3). The surface energy γ estimated by γ ) A/24πD02 (D0, cutoff distance = 2 Å) is 23 mN/m, which also agrees with the experimental value (22 ( 1 mN/m) obtained by adhesion measurements. (24) Hough, D. B.; White, L. R. Adv. Colloid Interface Sci. 1980, 14, 3.

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mR ) k(D0 - D) - f∆t -

Figure 5. Measured forces on approach in saturated water vapor of fluorinated surfaces prepared by three different methods (lower figure) and the speed at which they move over the range where they jump into contact (upper figure): O, LB; ], FTE/CHCl3; 9, FTE/neat. The dotted line in the upper figure represents the theoretical jump-in speed under the effect of vdW forces, calculated from eq 3 using a Hamaker constant A ) 7 × 10-20 J. In the inset of the lower figure, the force profile of LB in humid air (shifted to larger distances) is compared with that in dry air (‚‚‚ ‚‚‚ b ‚‚‚ ‚‚‚). The solid line is a fit obtained with the equation for the capillary force, eq 4, with V ) 3 × 10-15 cm3.

The force curve of FTE/neat (filled squares) is the force profile measured on first approach. Obviously, even on first approach the result is quite different from those with the other two surfaces. The attractive force is weaker and shows a different distance dependence. In addition, it was observed that the force profile changed after each contact. Hence it is suggested that the FTE/neat surfaces deform irreversibly on contact. As reported in our previous paper,9 Wilhelmy-plate wetting curves showed that a heterogeneous (irregular) region formed at the three-phase contact line only on the FTE/neat surface and that no such feature was observed with the other two samples. Also, the frictional force images showed that the molecules at the FTE/neat surface were in the most mobile state of the three surfaces. From these results we conclude that in dry air a vdW attraction is observed between the hydrophobic surfaces that are robust enough not to be susceptible to changes at the three-phase contact line (FTE/CHCl3 and LB). The force observed between the surfaces susceptible to mechanical damage (FTE/neat) is not reproducible even in dry air owing to structural changes of the fluorocarbon layers. Forces in Humid Air. Figure 5 shows the forces measured between surfaces (lower figure) in humid air and the speed at which they move over the range where they jump into contact (upper figure). The theoretical jump-in speed v under the effect of vdW forces was estimated using the equation of motion25,26 (25) Steblin, V. N.; Shchukin, E. D.; Yaminskii, V. V.; Yaminskii, I. V. Kolloidn. Zh. 1991, 53, 684. (26) Yaminskii, V. V.; Steblin, V. N.; Shchukin, E. D. Pure Appl. Chem. 1992, 64, 1725.

AR 12D2

(3)

Here m is the effective mass of the cantilever with the glass sphere calculated from the resonance frequency of the spring, R ()dv/dt) is the acceleration, D0, the initial distance, is taken at a point where no forces are detected, D is the distance between the surfaces, f is the load applied to the spring per second by the transducer, and ∆t is the time interval over which the surfaces move from D0 to D. The forces were measured after keeping the surfaces in saturated water vapor for 3 h. In humid air, force curves for all the surfaces were shifted toward larger distances, presumably because adsorbed layers of water were formed between the substrate and the fluorocarbon layer. It has been reported that in humid air Si-O-Si bonds are transformed to SiOH, which promotes the adsorption of water molecules.27,28 Since the water vapor adsorption isotherms have shown that about 20 water molecules/nm2 are adsorbed on glass fibers even at a relative vapor pressure of less than 0.3,28 the thickness of the adsorbed layer formed between the substrate and the fluorocarbon surface is expected to be several nanometers thick. The force profile of the LB surface in humid air (open circles) is in good agreement with that in air shifted 7.4 nm toward larger distances (closed circles in the inset). Since the jump-in speed is consistent with the theoretical vdW jump-in speed in dry air, the force acting between the LB surfaces can be attributed to a vdW force. Therefore, the 7.4 nm shift should correspond to twice the thickness of the adsorbed layer of water on the surface. Figure 6a shows a schematic drawing of the LB surface in humid air, based on the results of the force measurements. At distances greater than 7.4 nm, the surfaces are attracted by vdW forces acting between the surfaces consisting of the underlying silica substrate, the adsorbed water film, and the outer fluorocarbon layer. At distances smaller than 7.4 nm, the surfaces have already come into contact and the water layers (∼3.5 nm on each surface) have been squeezed out of contact area. At D ) 0, the jump-in speed is drastically reduced for all surfaces, which supports the 2.6 nm adjustment (the deformation length) of the distance axis estimated by JKR theory. In humid air, the force profile of FTE/CHCl3 is identical to that of FTE/neat. This suggests that the forces observed on both FTE/CHCl3 and FTE/neat cannot be attributed to a vdW attraction. The jump-in speed of FTE/CHCl3, which is also identical to that of FTE/neat, is much slower than that of LB, implying that these surfaces are traveling in a more viscous medium as they jump into contact. From these results, it is likely that the attraction observed with both FTE/CHCl3 and FTE/neat is due to bridging between the surfaces. The capillary bridging force is given by 21,29

F)

2πR γ cos θ πRD2 1+ V

(

)

(4)

where θ is contact angle of the bridge with the surface, γ is the surface energy of the bridge, and V is the volume of the bridging condensate. Equation 4 is valid when V is constant.21,29 Although the bridge should be a mixture of water and unbound FTE molecules on the surface,30 we (27) Schmitz, I.; Schreiner, M.; Friedbacher, G.; Grasserbauer, M. Anal. Chem. 1997, 69, 1012. (28) Pantano, C. G. Rev. Solid State Sci. 1989, 3, 379. (29) Yaminsky, V. V. Colloids Surf., A 1999, 159, 181.

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Figure 6. Schematic drawings of the proposed structure of fluorocarbon surfaces with the adsorbed water layer on the glass substrates in humid air: a, LB; b, FTE/CHCl3. The water layers on the glass substrates are separated by fluorocarbon layers for LB, while the water layers coalesce for FTE/CHCl3. The diameter of contact area (7.8 µm) and the deformation length (2.6 nm) were calculated using JKR theory. The diameter of the bridge of water layers (0.72 µm) was estimated from the volume of the bridge (V ) 3 × 10-15 cm3).

employed the surface tension of water (72 mN/m) as γ and the receding angle of water on FTE/CHCl3 surface (60°) as θ to obtain the theoretical capillary force curve. For the FTE/neat surface, the γ should be lower than the surface tension of water owing to the high concentration of FTE molecules. When we employed a γ of 40-50 mN/m and a receding contact angle of water on the FTE/neat surface θ of 38°, the value of γ cos θ for FTE/neat was the same as that for FTE/CHCl3, resulting in the same force profiles. As shown in Figure 5, the theoretical capillary force for V ) 3 × 10-15 cm3 shows a profile comparable to the experimental force curves of FTE/CHCl3 and FTE/neat. Assuming that the bridge is a column whose height is 7.4 nm (Figure 6b), the radius of the base of the column is calculated to be 0.36 µm, which is about 1/10 of the radius of contact area (3.9 µm) at zero load (F ) 0). From these results, we suggest that the attractive force observed on both FTE/CHCl3 and FTE/neat is a capillary force. Because of the presence of unreacted silane molecules on the surface and the lower surface coverage of molecules bound to the substrate (Figures 1 and 8), the fluorocarbon layers with adsorbed water easily coalesce and form a bridge between the surfaces (Figure 6b). The fluorocarbon monolayers prepared by the LB method, however, are stable enough to effectively partition the adsorbed water layer and air. Therefore, only a vdW attraction is observed between the LB surfaces. In addition, it is worth noting that these force profiles change reversibly on going from dry air to humid air, and from humid air back to dry air (except for FTE/neat). Ten minutes after the surfaces had been relocated from humid air to dry air (with P2O5), the same force profiles as shown in Figure 4 were observed on both the LB and the FTE/ CHCl3 surfaces. Forces in Water. With the LB and the FTE/CHCl3 surfaces, which have advancing contact angles of water (30) Chen, Y. L. E.; Gee, L. M.; Helm, C. A.; Israelachvili, J. N.; McGuiggan, P. M. J. Phys. Chem. 1989, 93, 7057.

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larger than 90°, we observed vapor cavitation between the surfaces on several occasions.31 The force profile for the surfaces showing vapor cavitation was a long-range attraction extending to distances beyond 100 µm on the first approach after cavities were generated. Similar force profiles were reported by Ishida et al.32 Details of these force curves for fluorocarbon surfaces with bubble formation will be described elsewhere. In this paper, we focus on the forces acting between fluorocarbon surfaces without apparent (visible) bubble formation. Parts a and b of Figure 7 show force curves measured with the FTE/CHCl3 and the LB surfaces in water after different immersion times. At 0-3 h, FTE/ CHCl3 and LB showed the same force profile, with an attraction within 40 nm. After 12 h, the force curves with these surfaces shifted toward a smaller distance and started showing different force profiles. The force curve of LB showed attractive forces at smaller distances (∼20 nm) than that of FTE/CHCl3 (∼30 nm). Subsequently, the force curve with LB did not change with time (apart from a slight shift to smaller distances) for at least 5 days, while the force curve with FTE/CHCl3 showed a repulsive force after 24 h. This evolution of the force profiles can be explained by the change of surface properties occurring in water. As shown in Figure 8, we suggest that the both the LB and the FTE/CHCl3 surfaces have unbound molecules from the beginning. However, these unbound molecules start coming off after the surface is immersed in water because CF3(CF2)7(CH2)2Si(OCH2CH3)3 is converted to CF3(CF2)7(CH2)2Si(OH)3 by exposure to water and the latter (converted) molecules show higher solubility in water. Therefore, the attraction originating in the bridging of unbound molecules between the surfaces is mainly observed after 0-3 h for both surfaces. Because hydrogen bonds form between unbound molecules and molecules bound to the FTE/CHCl3 surfaces, the mobile, unbound molecules on the FTE/CHCl3 surface remain longer than those on the LB surface, resulting in a longerrange attraction between the FTE/CHCl3 surfaces than between the LB surfaces. After 24 h or more, when most of the unbound molecules have been removed, the glass surface should be partly exposed to water in the case of FTE/CHCl3, while more than 95% of the surface area remains covered with a fluorocarbon monolayer in LB. At this stage, it is possible that the repulsive force observed with the FTE/CHCl3 surfaces is due to an electrostatic force, as observed with dimethyldioctadecylammonium bromide (DDOA) surfaces deposited on mica.33 The jump-in speed also showed differences between FTE/CHCl3 and LB. The LB surfaces keep accelerating until they come into contact while the FTE/CHCl3 surfaces apparently decelerate at about 10 nm. These results suggest that the fluorocarbon monolayer of the LB surface remains intact enough to act as a partition, the as same as in humid air, between the bulk water above the monolayer and the adsorbed water molecules beneath the monolayer. On the other hand, the adsorbed water layer of the FTE/CHCl3 surface is directly exposed to bulk water so that it can thicken more than that of the LB surface. In the case of FTE/neat, the force profiles continued to change with each measurement (Figure 7d). The force profile of the first measurement was similar to that of FTE/CHCl3 and LB after 0-3 h, which we attributed above to the bridging of unbound molecules between the (31) Christenson, H. K.; Claesson, P. M. Science 1988, 239, 390. (32) Ishida, N.; Kinoshita, N.; Miyahara, M.; Higashitani, K. J. Colloid Interface Sci. 1999, 216, 387. (33) Eriksson, L. G. T.; Claesson, P. M.; Ohnishi, S.; Hato, M. Thin Solid Films 1997, 300, 240.

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Figure 7. Measured forces on approach in water after different immersion times between fluorinated surfaces prepared by three different methods and the speed at which they move over the range where they jump into contact: a, FTE/CHCl3; b, LB; c, jump-in speed; d, FTE/neat. Open diamond, FTE/CHCl3 at 0-3 h; four panel diamond, FTE/CHCl3 at 12 h; filled diamond, FTE/CHCl3 at 24 h; open circle, LB at 0-3 h; four panel circle, LB at 12 h; filled circle, LB at 24 h; open box, FTE/neat at first measurement; four panel box, FTE/neat at second measurement; filled box, FTE/neat at third measurement. The solid line is a fit obtained with the equation for the capillary force, eq 4, for V ) 1.7-2.0 × 10-13 cm3.

outermost surfaces. For the second measurement, the repulsion observed at 120-150 nm might be associated with steric repulsive forces due to unbound or dangling molecules on the surfaces, which are pulled out of the surfaces by the separation after the first measurement. Note that the attraction is observed at the same distance as in the force profile of the first measurement. During the third measurement, and subsequently, a long-range attraction from 120 to 150 nm was observed. Using eq 4, the force profile could be fitted to a theoretical capillary force profile with V ) 1.7 to 2.0 × 10-13 cm3 (γ cosθ ) 20-25). Therefore, the long-range attraction from 120 to 150 nm can here also be attributed to a capillary force, but with a 10 times larger condensate volume than for the first measurement. Thus, unbound molecules between the surfaces may play a critical role in the observed variation of the attractive force. The greater the number of mobile molecules (including unbound molecules and dangling polymerized molecules on the surface), the more longerrange the attraction observed between the surfaces. This is due to the larger volume of the bridging condensate between surfaces. Force between Fluorocarbon Monolayers in Water. As suggested by the above, only fluorocarbon monolayers prepared by LB deposition showed sufficiently high stability in water to maintain physical separation between the bulk water phase above the fluorocarbon layer and the water layer under the fluorocarbon layer. In water,

the thickness of the adsorbed water film should be approximately the same as that estimated in humid air (7.4 nm for two surfaces). As shown in Figure 7b, after unbound molecules were removed by immersion in water for 12 h or rinsing with ethanol, an attractive force was observed at 20 nm between the LB surfaces and the force profile did not change for 5 days. Taking into account the thickness of water layers, the range of the attractive force would be 12-13 nm. To estimate the total force Ft acting between the fluorocarbon monolayers, any hydrodynamic force (repulsive) should be subtracted from the measured forces Fexp(LB).9 In addition, forces of inertia Fi acting on the spring should also be taken into account during the jump. The hydrodynamic force Fh between two spheres is given by,25,34

Fh )

3 πηR2 ∂D 2 D ∂t

( )

(5)

where η is the viscosity of water at 25 °C (890.9 mNs/m2 at 1 atm).23 The force of inertia Fi was estimated using the equation of motion, F ) mR. Figure 9a shows the force curves of Fh (crosses), Fi (Greek crosses), Fexp(LB) (open circles), and Ft (closed circles) as a function of adjusted distance. The results clearly show that the attractive force observed between fluorocarbon monolayers in water is much stronger than the vdW (34) Chan, D. Y. C.; Horn, R. G. J. Chem. Phys. 1985, 83, 5311.

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Figure 8. Schematic drawings of the proposed change in surface structure on immersion in water or rinsing with ethanol. These were drawn on the basis of the results of XPS, AFM, contact angle, and pull-off force measurements.14

attraction in air (broken line). The force curves of Ft measured for different speeds are shown in Figure 9b. The force can be fitted to a double exponential35 as shown in eq 5, where C1 ) 72 mN/m, λ1 ) 2 nm, C2 ) 2 mN/m, and λ2 ) 7 nm.

F/R ) C1 exp(-D/λ1) + C2 exp(-D/λ2)

(6)

Figure 9. (a, top) Hydrodynamic force Fh; ×, the force of inertia Fi; +, the measured force between the LB surfaces Fexp; O, the force taken into account by inertia Fexp - Fi; 4, the total surface force acting between fluorocarbon monolayers Fexp - Fi - Fh and b, as a function of adjusted distance. The broken line and the dotted line represent the van der Waals attraction in air and in water calculated using a Hamaker constant A ) 7 × 10-20 J (air) and A ) 0.8 × 10-20 J (water). (b, bottom) Total surface forces acting between fluorocarbon monolayers measured with various driving speeds: O, 380 nm/s; 4, 160 nm/s; 0, 60 nm/s; ], 44 nm/s; b, 36 nm/s. The broken line represents a fit with a power law (F/R ) -A/12D2, A ) 1.48 × 10-18 J), and the solid line represents a fit with a double exponential according to eq 5, with C1 ) 72 mN/m, λ1 ) 2 nm, C2 ) 2 mN/m, and λ2 ) 7 nm. The dotted line is the result of Monte Carlo simulations of water between hydrophobic surfaces reported by Forsman et al.36 The half-filled squares are the experimental results obtained between mica surfaces coated with LB films of a cationic, double-chain fluorocarbon surfactant by Claesson and Christenson.9

For comparison, a “van der Waals-type” power-law fit is also shown in Figure 9b (broken line). Obviously, the power law does not fit better than the double exponential even when an unreasonably large Hamaker constant (A ) 1.48 × 10-18 J) is used. The observed forces in this experiment cannot be explained by a van der Waals-type force. For another comparison, we have also plotted in Figure 9b the short-range results from Figure 7 of ref 4 obtained between mica surfaces coated with LB films of a cationic, double-chain surfactant (half-filled squares). In that case, a long-range attraction with an exponential decay constant of 16 nm was measured beyond 20 nm, but as can be seen, the short-range results are in close agreement with our present measurements. This suggests that the short-range attraction found between many different hydrophobic surfaces is of similar origin, while the long-range attraction

measured only with certain surfaces is an altogether different interaction. The distance of 12-13 nm at which attraction is first measurable is consistent with the jump-in observed between hydrophobic surfaces modified with octadecyltriethoxysilane in water.15 It follows that a strong attractive force at (less than) 12-13 nm acts between stable fluorocarbon monolayers in water. The dotted line in Figure 9b is the result of Monte Carlo simulations of water between hydrophobic surfaces reported by Forsman et al.36 suggesting that the density depression of water molecules in the slit gives rise to a strong attractive interaction. Although the predicted range of the attraction (1-2 nm) is much shorter than that in our experiments, the force profile based on the simulation seems to be

(35) Craig, V. S. J.; Ninham, B. W.; Pashley, R. M. Langmuir 1998, 14, 3326.

(36) Forsman, J.; Jo¨nsson, B.; Woodward, C. E. J. Phys. Chem. 1996, 100, 15005.

Force between Fluorocarbon Monolayer Surfaces

comparable with the experimental force profile. Owing to the water layer under the fluorocarbon monolayer, our hydrophobic surfaces (θa ) 123°) are stable, but not as rigid as the perfectly hard walls used in the simulations. Therefore, the experimental force curve shown here might include thermal fluctuation forces. Measurements of the forces between rigid hydrophobic surfaces are currently under way. Finally, we compare our results with previous works on force measurements between hydrophobized surfaces with silane reagents on silica substrate.37 For surfaces modified with silane reagents having an alkyl chain, SiRnX4-n (R ) an alkyl chain, X ) a reactive residue, n )1, 2, 3),29,38-43 it has been reported that use of silanes with one reactive residue, such as alkylmonochlorosilanes, leads to a longerrange attraction than the use of silanes with two or three reactive residues.39,40 For surfaces modified with meth(37) Christenson, H. K.; Claesson, P. M. Adv. Colloid Interface Sci. submitted. (38) Parker, J. L.; Cho, D. L.; Claesson, P. M. J. Phys. Chem. 1989, 93, 6121. (39) Parker, J. L.; Claesson, P. M. Langmuir 1994, 10, 635. (40) Parker, J. L.; Claesson, P. M.; Attard, P. J. Phys. Chem. 1994, 98, 8468. (41) Carambassis, A.; Jonker, L. C.; Attard, P.; Rutland, M. W. Phys. Rev. Lett. 1998, 80, 5357. (42) Rabinovich, Y. I.; Yoon, R.-H. Langmuir 1994, 10, 1903. (43) Rabinovich, Y. I.; Yoon, R.-H. Colloids Surf. 1994, A93, 263.

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ylsilanes, Si(CH3)nX4-n, it has been reported that use of monofunctional silanes42 gives a slightly longer-range attraction than the use of silanes with two reactive residues.44 However, in the case of the silanes having two reactive residues, such as dimethyldichlorosilane, it is considered that the silanes polymerize to form a long siloxane chain or form a loop on the substrate. Depending on the preparation method, surfaces modified with dimethyldichlorosilane show attractive forces of a range varying from 10 to 200 nm.45,46 Thus, from a review of previous results, it seems that sufaces with silane molecules in a more mobile state show longer-range forces, which agrees well with the results presented here. Surfaces showing “super” long-range attraction without cavitation,45 even modified with octadecyltrichlorosilane,42 may have similar surface conditions to what we have observed with FTE/neat. Acknowledgment. We thank Dr. K. Abe, Dr. K. Tamada, Dr. K. Ueno, and H. Minamikawa (NIMC) for helpful discussions and comments, we and acknowledge technical assistance from T. Sawkins and A. Hyde (ANU). LA000262E (44) Rabinovich, Y. I.; Derjaguin, B. V. Colloids Surf. 1988, 30, 243. (45) Yaminsky, V. V. Colloids Surf. 1997, 129-130, 415. (46) Eskilsson, K.; Ninham, B. W.; Yaminsky, V. V. Langmuir 1999, 15, 3242.