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Long-Range Force of Attraction between Solvophobic Surfaces in Water and Organic Liquids Containing Dissolved Air† Robert F. Considine and Calum J. Drummond* CSIRO Molecular Science, Bag 10, Clayton South, Victoria, 3169 Australia Received April 20, 1999. In Final Form: August 25, 1999 An atomic force microscope has been employed to measure the force of interaction between a micronsized colloidal sphere and a flat plate, both coated with a copolymer of perfluoro(2,2-dimethyl-1,3-dioxole) and tetrafluoroethylene (Teflon AF1600) in water, glycerol, formamide, ethylene glycol, ethylammonium nitrate, formic acid, ethanol, methanol, diiodomethane, 1-bromonaphthalene, hexadecane, and hexane. A long-range force of attraction was measured in water and, with the exception of the n-alkanols and n-alkanes, all the organic liquids. The results indicate that there is a macroscopic long-range attraction between solvophobic surfaces that has a different origin from that of the hydrophobic interaction observed at the molecular level. The results also indicate that there is a “solvophobic force” that is not due to either orientational ordering propagated by hydrogen bonds, electrostatic (or polarization) effects, or condensates of “loosely attached” surface material; all mechanisms that have been invoked previously to explain the macroscopic “hydrophobic force”. The force curves measured in the organic liquids provide a new perspective which is consistent with the hypothesis that submicroscopic air bubbles adhering to the macroscopic surfaces are responsible for a long-range attraction.
Introduction Hydrophobic interaction drives the self-assembly of lipids and surfactants and the folding of polypeptide chains into compact globular protein structures.1,2 At the molecular level, hydrophobic interaction is considered to be associated with the unique properties of water and is difficult to directly measure.1 Cecil3 was the first to suggest the use of macroscopic surfaces (silanated small glass beads) as a model system to study hydrophobic interaction. It is now well established, through direct measurement, that a long-range force of attraction exists between macroscopic hydrophobic surfaces in water.4-13 To date, the origin of this macroscopic “hydrophobic force” has not been unequivocally identified and it may not operate at the molecular level.6-21 Here an atomic force microscope * Author for correspondence. E-mail: calum.drummond@ molsci.csiro.au. † This work was first presented on Nov 12, 1998 at the Thomas W. Healy Symposium on Colloid and Surface Chemistry held at The University of Melbourne, Australia. (1) Tanford, C. The Hydrophobic Effect; Formation of Micelles and Biological Membranes, 2nd ed.; Wiley: New York, 1980. (2) Evans, D. F. Langmuir 1988, 4, 3-12. (3) Cecil, R. Nature 1967, 214, 369-370. (4) Israelachvili, J.; Pashley, R. Nature 1982, 300, 341-342. (5) Pashley, R. M.; McGuiggan, P. M.; Ninham, B. W.; Evans, D. F. Science 1985, 229, 1088-1089. (6) Rabinovich, Y. I.; Derjaguin, B. V. Colloids Surf. 1988, 30, 243251. (7) Christenson, H. K.; Claesson, P. M. Science 1988, 239, 390-392. (8) Kurihara, K.; Kunitake, T. J. Am. Chem. Soc. 1992, 114, 1092710933. (9) Tsao, Y.-H.; Fennell Evans, D.; Wennertrom, H. Science 1993, 262, 547-550. (10) Parker, J. L.; Claesson, P. M.; Attard, P. J. Phys. Chem. 1994, 98, 8468-8480. (11) Craig, V. S. J.; Ninham, B. W.; Pashley, R. M. Langmuir 1998, 14, 3326-3332. (12) Carambassis, A.; Jonker, L. C.; Attard, P.; Rutland, M. W. Phys. Rev. Lett. 1998, 80, 5357-5360. (13) Considine, R. F.; Hayes, R. A.; Horn, R. G. Langmuir 1999, 15, 1657-1659. (14) Eriksson, J. C.; Ljunggren, S.; Claesson, P. M. J. Chem. Soc., Faraday Trans. 2 1989, 85, 163-176. (15) Attard, P. J. Phys. Chem. 1989, 93, 6441-6444.
(AFM) has been employed to measure the force of interaction between a micron-sized colloidal sphere and a flat plate,22 both coated with a fluoropolymer, in water and a wide range of organic liquids. In this work, macroscopic surfaces are covered with a high molecular weight copolymer of perfluoro(2,2-dimethyl-1,3-dioxole)andtetrafluoroethylene(TeflonAF1600), where the ratio of dioxole to tetrafluoroethylene repeat units is 2:1.23-25 This surface covering was chosen because Teflon AF1600 has a surface free energy of 16.4 ( 1.4 mN m-1, which is very close to the theoretical lower limit for a solid organic polymer.25 The Teflon AF1600 surface is amorphous, smooth (root-mean-square roughness of 0.5 nm and some features up to 4 nm in height, as determined by AFM contact-mode imaging over a 1.0 µm2 area), chemically inert to most liquids, electroneutral, and mechanically strong (the Teflon AF1600 elastic modulus of 950-2150 MN m-2 23 ensures that surface deformation is a minor effect in the interpretation of the forceseparation curves). It has been shown elsewhere25 that the force of attraction between two Teflon AF1600 surfaces across air is well described by the Lifshitz theory for retarded van der Waals interaction.26 Teflon AF1600 is an ideal material for investigating a “solvophobic effect”. (16) Miklavic, S. J.; Chan, D. Y. C.; White, L. R.; Healy, T. W. J. Phys. Chem. 1994, 98, 9022-9032. (17) Yaminsky, V. V.; Ninham, B. W. Langmuir 1996, 12, 49694970. (18) Ljunggren, S.; Eriksson, J. C. Colloids Surf. 1997, 129-130, 151-155. (19) Ruckenstein, E. J. Colloid Interface Sci. 1997, 188, 218-223. (20) Christenson, H.; Yaminsky, V. V. Colloids Surf. 1997, 129-130, 67-74. (21) Yaminsky, V. V. Colloids Surf. 1997, 129-130, 415-424. (22) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Nature 1991, 353, 239-241. (23) Resnick, P. R. Mater. Res. Soc. Symp. Proc. 1990, 167, 105-110. (24) Lowry, J. H.; Medlowitz, J. S.; Subramanian, N. S. Opt. Eng. 1992, 31, 1982-1985. (25) Drummond, C. J.; Georgaklis, G.; Chan, D. Y. C. Langmuir 1996, 12, 2617-2621. (26) Dzyaloshinskii, I. E.; Liftshitz, E. M.; Pitaevskii, L. P. Adv. Phys. 1961, 10, 165-209.
10.1021/la9904713 CCC: $19.00 © 2000 American Chemical Society Published on Web 10/28/1999
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Figure 1. Normalized force curves, obtained on surface approach, for the homointeraction of a Teflon AF1600 covered sphere (of radius, R) and flat plate across polar liquids at 21 °C. Hard-wall contact is defined as a surface separation of zero. Advancing and receding contact angles, θA (θR), for sessile drops of the liquids on a flat Teflon AF1600 surface in air at 21 °C, and liquid-air surface tensions, γLV, at 20 °C are provided in the figure.
Experimental Section The force measurements were performed on an AFM (Nanoscope IIIa; Digital Instruments, CA) equipped with a fluid cell. The use of the AFM to quantitatively measure the force of interaction between a sphere and a flat plate has been reported elsewhere.22,25 The cantilever spring constants, k ) 0.33 ( 0.05
N m-1, and the piezo (z) sensitivity were calibrated from the fundamental resonance frequency27 and interferometric methods,28 respectively. Small glass spheres (Duke Scientific; radii in the range 4-9 µm, as determined by optical microscopy) attached (27) Cleveland, J. P.; Manne, S.; Bocek, D.; Hansma, P. K. Rev. Sci. Instrum. 1993, 64, 403-405.
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Figure 2. Normalized force curves, obtained on surface approach, for the homointeraction of a Teflon AF1600 covered sphere (of radius, R) and flat plate across dispersive liquids at 21 °C. The force curve shown for hexadecane was not always observed. Some other experiments with hexadecane, where different covered spheres and flat plates were used, exhibited a short-range “repulsion”, instead of a jump-in, prior to hard-wall contact. One possible explanation could be that, with time, hexadecane penetrates the Teflon AF1600 and forms a composite layer that is deformable. θA (θR) and γLV values are provided in the figure. to v-shaped cantilevers, and oxidized silicon wafers, were reacted with (heptafluoroisopropoxy)propylmethyldichlorosilane (Hu¨ls Petrarch Systems) vapor, at room temperature and pressure, for approximately 30 min and then left for at least 1 day in a dustfree environment. The fluorosilanated glass spheres were then coated with Teflon AF1600 (Dupont Polymer Products, Wilmington, DE) by casting them in a solution of Teflon AF1600 (0.1 wt %) in perfluoro-2-butyltetrahydrofuran (Fluoroinert liquid FC75; 3M Australia Pty. Ltd). This coating was allowed to dry for at least 1 day and then a second cast coat was applied. The fluorosilanated wafer was coated with Teflon AF1600 by casting a thin film from the same solution (0.1 wt %). The coated surfaces were left to dry at room temperature for at least 3 days before AFM experiments were performed. Water was deionized and then passed through a Milli-Q filtration system prior to use; the water surface tension was 72.6 at 21 °C. The organic liquids were all of high purity and were purchased from a range of commercial sources, with the exception of ethylammonium nitrate (structure confirmed by NMR and melting point (12 °C); water content 620 ( 50 ppm by Karl Fischer titration) which was synthesized in-house. The rate of surface approach was 2 µm s-1 in the low-viscosity liquids. For ethylammonium nitrate, ethylene glycol and glycerol the scan rates were 600, 50 nm s-1, and 80 nm s-1, respectively. The slower scan rates were employed to minimize hydrodynamic effects.
Results and Discussion Representative force versus separation curves for the approach of the two Teflon AF1600 surfaces across water and 11 organic liquids are shown in Figures 1 and 2. The liquids can be classified into two broad groups. The first group comprising water, glycerol, formamide, ethylene glycol, ethylammonium nitrate (a low-melting fused salt), formic acid, ethanol, and methanol (Figure 1) possess high (28) Jaschke, M.; Butt, H.-J. Rev. Sci. Instrum. 1995, 66, 12581259.
polarity and are associative liquids. Surfactant selfassembly has been observed in water, glycerol, formamide, ethylene glycol, and ethylammonium nitrate.2,29 Evans has concluded that the driving force for surfactant selfassembly is a physical process that occurs in polar multiple hydrogen bonding liquids that have a high cohesive energy density.2 The second group of solvents are dispersive (van der Waals) liquids30 and include diiodomethane, 1-bromonaphthalene, hexadecane, and hexane (Figure 2). Most of the force curves exhibit a discontinuity during the approach where the force gradient exceeds the AFM cantilever spring constant, dF/dD > k (hexane is the exception; see below). In other words, the surfaces experience a strong attraction as they move toward one another. If the systems were governed purely by van der Waals interaction, then the jump-to-contact distances (Dj) on approach would be less than 6 nm, since for a sphere on flat plate Dj ) (AR/3k)1/3 where A is the nonretarded Hamaker constant and R is the radius of the sphere. Hamaker constants were calculated from Lifshitz theory by utilizing the procedure popularized by Hough and White.25,26,29-31 As a relative guide, the Hamaker constants for Teflon AF1600 homointeraction across water, diiodomethane, 1-bromonaphthalene, hexadecane, and hexane are 3.6 × 10-21, 2.7 × 10-20, 1.5 × 10-20, 1.7 × 10-21, and 6.3 × 10-22 J, respectively. The force curves measured in the n-alkanol and n-alkane systems, as shown in Figures 1 and 2, are in reasonable (29) Wa¨rnheim, T.; Jo¨nsson, A. J. Colloid Interface Sci. 1988, 125, 627-633. (30) Drummond, C. J.; Chan, D. Y. C. Langmuir 1997, 13, 38903895. (31) Hough, D. B.; White, L. R. Adv. Colloid Interface Sci. 1980, 14, 3-41.
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Figure 3. Normalized force curve, obtained on surface separation, for the homointeraction of a Teflon AF1600 covered sphere (of radius R, noted in Figure 1) and flat plate in ethylammonium nitrate at 21 °C. The scan rate was 600 nm s-1. For the eight systems that had a vapor cavity, the necking process meant that there was no linear “hard-wall” compliance region on the retraction curve that could be employed to derive the interaction force and zero separation location. We used the “hard-wall” compliance obtained on approach, and assumed that the piezo hysteresis was negligible, to obtain the force curves on surface separation, in the presence of a bridging vapor cavity. The assumption of negligible piezo hysteresis was based on measurements on the interaction of the native small glass sphere on an oxidized silicon wafer. The maximum range of the hysteresis was found to be less than 1% of the total scan size; e.g., with a 300 nm scan the maximum difference between the extend and retract compliance data was 2 nm.
accord with expectations for van der Waals interaction. The jump-to-contact distances are all within a few nanometers of the predicted location. Minor divergence in the experimental jump-to-contact-distances may be attributed to the low Hamaker constants, thermal and/or vibrational noise effects, and the fact that the Teflon AF1600 surfaces are not molecularly smooth. The attractive force measured in all the liquids, other than the n-alkanols and n-alkanes, greatly exceeds the expected van der Waals attraction. For the systems where a long-range “solvophobic force” is present, there is no link between the Hamaker constants and the jumptoward-contact distances. Many theories have been proposed to account for the long-range attraction between hydrophobic macroscopic surfaces in water.4-21 We can now unequivocally say that there is a long-range attraction between solvophobic surfaces in liquids that is not peculiar to water or polar liquids with multiple hydrogen bonds. It is clear that the macroscopic long-range attraction between solvophobic surfaces has a different origin from that of the hydrophobic interaction observed at the molecular level. In addition, our results indicate that there is a “solvophobic force” that is not due to either electrostatic (or polarization) effects,9,15,16,20 orientational ordering propagated by hydrogen bonds,14 or condensates of “loosely attached” surface material.21 The force curves that feature a long-range attraction have, in general terms, a profile very similar to those measured when large air bubbles (200-400 µm) interact with hydrophobic silanated glass spheres (5 µm).32 Similar force curves, measured for fluorosilanated surfaces (advancing contact angle >100°; sphere (10 µm)-flat plate geometry) approaching across water, have been interpreted in terms of the attraction at long range and the soft repulsion prior to hard-wall contact being a result of a submicroscopic bubble on a surface attaching to the other surface and then laterally spreading.12 Similar force curves (32) Preuss, M.; Butt, H.-J. Langmuir 1998, 14, 3164-3174.
Figure 4. (A) Normalized force of adhesion, (F/R)adh, between a Teflon AF1600 covered sphere and flat plate in liquid versus the receding contact angle, θR, of a macroscopic drop of the liquid on a Teflon AF1600 surface in air. (B) Jump-in distance on approach, Dj-in, as a function of the distance to which the surfaces jump out to once the work of adhesion has been exceeded, Dj-out, for the systems that exhibit a “solvophobic force”. The values of sphere radius, R, used for each of the experiments are the same as shown for the corresponding liquids in Figures 1 and 2. Each symbol corresponds to a different liquid: water (9), ethylene glycol ([), glycerol (b), diiodomethane (2), 1-bromonaphthalene (0), ethylammonium nitrate (4), formamide (O), formic acid (]), ethanol (gray 4), hexadecane (gray O), methanol (gray 0), and hexane (gray ]). Error bars correspond to the standard deviation of at least 10, and generally >20, measurements made at a minimum of two different locations on the flat plate. When the bars are not evident they are smaller than the symbol size. The solid line is to guide the eye.
have also recently been reported for two polystyrene latex spheres interacting across water, which display a longrange attraction that is consistent with surface-adherent submicroscopic bubbles being responsible for the attraction.13 Whether surface adherent submicroscopic bubbles can exist is a moot point. In classical theories, submicroscopic bubbles are metastable.10,18 They are expected to dissolve very rapidly because the pressure inside them is much higher than that in the surrounding liquid. Nevertheless, we support the view that the most likely explanation for the long-range attraction is submicroscopic bubbles adhering to the surfaces.10,12,13 Consistent with this view, we observed micron-size surface-adherent bubbles in all the systems that exhibited a long-range attraction (force curves were not measured in the vicinity of visible bubbles).12 These micron-sized bubbles were generally generated upon fluid injection, and commonly remained stable for the duration of the measurement
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process. There have been a number of previous reports, e.g., refs 33 and 34, that the magnitude of the long-range attraction between macroscopic hydrophobic surfaces in water is reduced upon de-aeration. We have recently observed that there is a monotonic decrease in the surface separation at which the onset of the “solvophobic force” of attraction occurs between two Teflon AF1600 surfaces, as water is progressively de-aerated (R.F.C.; C.J.D.; Hayes, R., in preparation). These experimental observations are consistent with the notion that the origin of the longrange attraction observed for surfaces in a nonwetting liquid may be bridging by surface-adherent submicroscopic bubbles. All the systems that display the long-range attraction on approach (Figures 1 and 2) also exhibit force profiles that are consistent with the presence of a bridging vapor cavity upon separation. An example of a force curve obtained on separating the two Teflon AF1600 surfaces is provided in Figure 3. In these types of systems, the force as a function of increasing surface separation can be explained in terms of lateral contraction of the vapor bridge across the surfaces, and necking of the cavity, followed by rupture.7 The normalized force of adhesion is plotted as a function of the receding contact angle, θR, in Figure 4A for all the liquids. In terms of the adhesion behavior, the liquids can be split into three groups. The first group comprises the n-alkanols and n-alkanes where both the advancing contact angle, θA, and θR < 80° and the adhesion is governed by the van der Waals attraction, the elastic properties of the material, and the surface roughness. The second group contains the liquids where θA > 80° and 80° > θR > 63° and the adhesion is unexpectedly controlled by a vapor cavity. The third group of liquids has both θA and θR > 90°, within experimental error, and the adhesion is governed by a vapor cavity, as expected from theory.7 If small bubbles are the origin of the long-range attraction on approach, then we may expect to see a correlation between the jump-in distance on approach, Dj-in, and the distance to which the surfaces jump out to (33) Meagher, L.; Craig, V. S. J. Langmuir 1994, 10, 2736-2742. (34) Craig, V. S. J.; Ninham, B. W.; Pashley, R. M. Langmuir 1999, 15, 1562-1569.
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once the work of adhesion, governed by the vapor cavity, has been exceeded, Dj-out.7 A plot of Dj-in as a function of Dj-out for the eight liquids that display a long-range attraction on approach is shown in Figure 4B. The trend which is evident lends further support to the claim that small bridging bubbles are responsible for the long-range “solvophobic force”. Here the results suggest that the “solvophobic force” begins to manifest itself when θA > ca. 80°. There is not a well-defined correlation between solvophobicity, as estimated from the contact angles, and the separation at which the long-range attraction occurs during approach. This is considered to be due to the liquids varying in levels of dissolved air (nitrogen and oxygen are nonpolar gases and, in general terms, their solubilities increase as the solvent polarity decreases) and having different surface tensions, vapor pressures, densities, and viscosities. All these physical properties can influence the size and stability of surface-adherent bubbles. A better understanding of the nature of the macroscopic “solvophobic force” can be utilized to enhance the flotation of minerals35 and to control the rheology of concentrated colloidal dispersions.36 Finally, the identification of a general long-range force of attraction between solvophobic surfaces, and the effect of the associated cavitation on the adhesion between surfaces, have ramifications for the development of nonstick (ultralow surface free energy) materials for application in both aqueous, including biological, and nonaqueous environments.37 Acknowledgment. R.F.C. is the recipient of an Australian Post-Graduate Research Award. We also thank The University of South Australia and the Australian Cooperative Research Centre for Water Quality and Treatment for support. LA9904713 (35) Pugh, R. J.; Rutland, M. W.; Manev, E.; Claesson, P. M. Int. J. Miner. Process. 1996, 46, 245-262. (36) Leong, Y. K.; Boger, D. V.; Scales, P. J.; Healy, T. W. J. Colloid Interface Sci. 1996, 181, 605-612. (37) Drummond, C. J.; Chan, D. Y. C. Langmuir 1996, 12, 33563359.
