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Influence of Wetting Properties on the Long-Range “Hydrophobic” Interaction between Self-Assembled Alkylthiolate Monolayers Thomas Ederth* and Bo Liedberg† Department of Chemistry, Surface Chemistry, Royal Institute of Technology, SE-100 44 Stockholm, Sweden, Institute for Surface Chemistry, Box 5607, SE-114 86 Stockholm, Sweden, and Laboratory of Applied Physics, Department of Physics and Measurement Technology, Linko¨ ping University, SE-581 83 Linko¨ ping, Sweden Received July 20, 1999. In Final Form: September 20, 1999 The effect of solid-liquid interfacial energy on the long-range attraction between self-assembled thiolate monolayers in water has been studied by direct force measurements. The solid-liquid interfacial energy was tuned by changing the properties of the solid surface: the thiolate monolayers were prepared by self-assembly of mixtures of methyl- and hydroxyl-functionalized alkylthiols onto thin gold films. The wetting properties were examined by contact angle measurements with the Wilhelmy plate method. Our results show that the shape of the long-ranged attractive force is sensitive to the advancing solid-liquid contact angle: whenever it exceeds 90° the force profiles are discontinuous and contains steps, whereas no attraction beyond the van der Waals force is observed for contact angles lower than 90°. We attribute the steps in the long-range attraction between hydrophobic surfaces to bridging of microscopic bubbles residing on the surfaces, and we conclude that the stability of these bubbles are related to macroscopic contact angles.
Introduction Over the last 2 decades, the long-range attractions observed between macroscopic hydrophobic surfaces in water and aqueous solutions have been a headache for the surface and colloid science community.1 Although studies of the wetting behavior of methylated silica by Laskowski and Kitchener already in 1969 indicated that water films on hydrophobic surfaces were unstable at certain thicknesses,2 this was not observed in direct force measurements until 12 years later, when an attraction in excess of the predicted van der Waals attraction was observed by Pashley and Israelachvili between mica surfaces rendered hydrophobic by adsorption of cationic surfactants from solution.3 This finding initiated a plethora of experiments with a multitude of experimental setups, systems, and surface preparation procedures, having in common that the surfaces were hydrophobic and that there was an extra “hydrophobic” interaction between the two surfaces in water. Surface preparation procedures include the aforementioned surfactant adsorption,3,4 LangmuirBlodgett films,5 silanization of mica6 or silica,7 plasma deposition,8 and thiol modification of gold.9 Further, results * To whom correspondence should be addressed. Present address: Physical and Theoretical Chemistry Laboratory, South Parks Road, Oxford OX1 3Q2, U.K. E-mail:
[email protected]. Fax: +44 1865 275410. Phone: +44 1865 275400. † Linko ¨ ping University. (1) Christenson, H. K. In Modern approaches to wettability: Theory and applications; Schrader, M. E., Loeb, G., Eds.; Plenum Press: New York, 1992. (2) Laskowski, J.; Kitchener, J. A. J. Colloid Interface Sci. 1969, 29, 670. (3) Pashley, R. M.; Israelachvili, J. N. Colloids Surf. 1981, 2, 169. (4) Parker, J. L.; Yaminsky, V. V.; Claesson, P. M. J. Phys. Chem. 1993, 97, 7706. (5) Claesson, P. M.; Blom, C. E.; Herder, P. C.; Ninham, B. W. J. Colloid Interface Sci. 1986, 114, 234. (6) Wood, J.; Sharma, R. Langmuir 1994, 10, 2307. (7) Parker, J. L.; Claesson, P. M.; Attard, P. J. Phys. Chem. 1994, 98, 8468. (8) Parker, J. L.; Cho, D. L.; Claesson, P. M. J. Phys. Chem. 1989, 93, 6121.
for other solvophobic systems have been reported, where instead of water, ethylene glycol10,11 or propanetriol12 have been used. Effects of temperature,7,13 dissolved gas,14-16 electrolyte,17-20 surface hydrophobicity,21,22 hydrocarbon chain length,13 neutron irradiation,15 etc. have been investigated, and it might be added that many of the studies disagree about the effects of the factors under study. Efforts to explain the various findings within a theoretical model have engaged many contributors, and the situation seems to be just as confused as on the experimental side. Briefly, the efforts to describe the experimental results have invoked structural changes in the water extending from hydrophobic surfaces,23,24 various models proposing an electrostatic origin of the interaction,25-27 metastability of the thin water films (9) Ederth, T.; Claesson, P.; Liedberg, B. Langmuir 1998, 14, 4782. (10) Parker, J. L.; Claesson, P. M. Langmuir 1992, 8, 757. (11) Tsao, Y.-H.; Evans, D. F.; Wennerstro¨m, H. Science 1993, 262, 547. (12) Boehnke, U.-C.; Remmler, T.; Motschmann, H.; Wurlitzer, S.; Hauwede, J.; Fischer, T. M. J. Colloid Interface Sci. 1999, 211, 243. (13) Tsao, Y.; Yang, S. X.; Evans, D. F.; Wennerstro¨m, H. Langmuir 1991, 7, 3154. (14) Considine, R. F.; Hayes, R. A.; Horn, R. G. Langmuir 1999, 15, 1657. (15) Craig, V. S. J.; Ninham, B. W.; Pashley, R. M. Langmuir 1999, 15, 1562. (16) Mahnke, J.; Stearnes, J.; Hayes, R. A.; Fornasiero, D.; Ralston, J. Phys. Chem. Chem. Phys. 1999, 1, 2793. (17) Kurihara, K.; Kunitake, T. J. Am. Chem. Soc. 1992, 114, 10927. (18) Christenson, H. K.; Fang, J.; Ninham, B. W.; Parker, J. L. J. Phys. Chem. 1990, 94, 8004. (19) Christenson, H. K.; Claesson, P. M.; Parker, J. L. J. Phys. Chem. 1992, 96, 6725. (20) Craig, V. S. J.; Ninham, B. W.; Pashley, R. M. Langmuir 1998, 14, 3326. (21) Pashley, R. M.; McGuiggan, P. M.; Ninham, B. W.; Evans, D. F. Science 1985, 229, 1088. (22) Hato, M. J. Phys. Chem. 1996, 100, 18530. (23) Eriksson, J. C.; Ljunggren, S.; Claesson, P. M. J. Chem. Soc., Faraday Trans. 2 1989, 85, 163. (24) Derjaguin, B. V.; Churaev, N. V. Langmuir 1987, 3, 607. (25) Attard, P. J. Phys. Chem. 1989, 93, 6441. (26) Podgornik, R. J. Chem. Phys. 1989, 91, 5840.
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bounded by hydrophobic surfaces,7,28 and adsorptiondesorption phenomena at the interfaces.29,30 Efforts to simulate the behavior of water near hydrophobic walls have been made, indicating that density depression occurs near the walls.31-33 However, the results for the interaction generally show limited agreement with experimental results. It is worthwhile to mention that due to the large qualitative and quantitative variations in published experimental data, a single explanation is unlikely to be able to account for all published results. It rather appears that the discrepancies in the observed phenomena can partly be attributed to variations in the detailed structure of the various surfaces, as suggested by Yaminsky et al.29 The interest in self-assembled monolayers (SAMs) of thiolates adsorbed onto gold surfaces has expanded quickly since the first experiments in 1983.34-37 Several thiol compounds are commercially available and many more have been synthesized in research laboratories, and applications both within fundamental and applied research have been explored. The thiolate monolayers are robust, and the surface properties can be easily changed by functionalization of the thiolates or by adsorption from mixed thiol solutions. The versatility of these surfaces makes them excellent substrates for surface force measurements, also for such measurements using the atomic force microscope (AFM).38-40 In a previous publication, we investigated some fundamental issues concerning the use of alkylthiolate SAMs on gold as macroscopic substrates for force measurements, including results for the interaction in water between methylated thiolate surfaces adsorbed onto gold as well.9 In this article, the aim is to investigate the effect of changes in the solid-liquid interfacial energy on the longrange hydrophobic interaction between rigidly attached surface films. The wetting properties were modulated by varying the ratio of hydrophobically and hydrophilically terminated thiolates adsorbed onto the gold substrates. Using this method, the contact angle with water can be controlled to within (1°. The wetting behavior of various mixtures have been studied and force measurements performed with some of these combinations. Materials and Methods Details of surface preparation procedures and their characterization have been published elsewhere,9 and only the essentials are repeated here. Surfaces for surface force measurements were produced by melting borosilicate glass rods (length 25-30 mm, diameter 2 mm) in a butane-oxygen burner to produce a sphere (27) Tsao, Y. H.; Evans, D. F.; Wennerstro¨m, H. Langmuir 1993, 9, 779. (28) Be´rard, D. R.; Attard, P.; Patey, G. N. J. Chem. Phys. 1993, 98, 7236. (29) Yaminsky, V. V.; Ninham, B. W.; Christenson, H. K.; Pashley, R. M. Langmuir 1996, 12, 1936. (30) Christenson, H. K.; Yaminsky, V. V. Colloids Surf. A 1997, 129130, 67. (31) Forsman, J.; Jo¨nsson, B.; Woodward, C. E.; Wennerstro¨m, H. J. Phys. Chem. B 1997, 101, 4253. (32) Grigera, J. R.; Kalko, S. G. Langmuir 1996, 12, 154. (33) Sakurai, M.; Tamagawa, H.; Ariga, K.; Kunitake, T.; Inoue, Y. Chem. Phys. Lett. 1998, 289, 567. (34) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (35) Ulman, A. An introduction to ultrathin organic films: From Langmuir-Blodgett to self-assembly; Academic Press: San Diego, CA, 1991. (36) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (37) Ulman, A. Chem. Rev. 1996, 96, 1533. (38) Thomas, R. C.; Tangyunyong, P.; Houston, J. E.; Michalske, T. A.; Crooks, R. M. J. Phys. Chem. 1994, 98, 4493. (39) Frisbie, C. D.; Rozsnyai, L. F.; Noy, A.; Wrighton, M. S.; Lieber, C. M. Science 1994, 265, 2071. (40) Hu, K.; Bard, A. J. Langmuir 1997, 13, 5114.
Ederth and Liedberg with a diameter of approximately 4 mm at the end of the rod. For contact angle measurements, silicon (100) wafers with native oxide, polished on both sides, were used (width 12.5 mm, length 20 mm, thickness 0.45 mm). The latter were cleaned in a mixture of 5/7 H2O, 1/7 30% H2O2, and 1/7 25% NH3 at 80 °C for 10 min. The flats and the spheres were treated identically from this point. The surfaces were mounted in an ultrahigh vacuum electronbeam evaporation system (Baltzers UMS 500P) where first a 1 nm Ti adhesion layer and further a 10 nm Au layer was deposited onto the surfaces. For the flat surfaces intended for contact angle measurements, the evaporation step was repeated to cover both sides. Immediately after removal from the vacuum chamber, the surfaces were immersed in thiol solutions. Thiols were adsorbed from 1 mM solutions in ethanol for at least 15 h. 16-Thiohexadecane (C16) (Fluka, >95%) and 16-thiohexadecanol (C16OH) (gift from Pharmacia Biosensor, g99.5%) were used without further purification. Before use, the surfaces were removed from the thiol solution, ultrasonicated in ethanol for 5 min to remove excess thiols physisorbed on the surface, and finally dried in a gentle flow of dry nitrogen. Mixed solutions of C16- and C16OHthiols were prepared from 1 mM solutions of each kind, which were then weighed to the right proportions. Thiol fractions mentioned in this paper refer to the fraction in the solution from which the monolayers were formed. For simplicity, a surface prepared from, say, an 80% C16 solution, will be referred to as an “80% surface”. The water was purified with a Milli-Q Plus 185 water purification system (Millipore) and, for surface force measurements, deaerated using a water jet pump for 2 h immediately before use. Ethanol (Kemetyl, Stockholm, 99.5%) was used as received. Contact angle measurements and surface tension measurements were performed with the Wilhelmy plate method (Kru¨ss 12 Tensiometer). For surface tension measurements a platinum plate was used. The force, F, acting on a plate immersed in a liquid is the sum of the buoyancy and the capillary force
F ) A∆Fgh + Lγlv cos θ where A is the cross-sectional area of the plate, ∆F the density difference between the air and the liquid, h the immersion depth, L the perimeter, γlv the liquid-vapor surface tension, and θ the solid-liquid contact angle. Extrapolating to zero immersion depth gives θ, provided γlv is known, and vice versa. The water surface tension was measured before each experiment and was always g72.3 mN/m. AFM imaging of the SAMs was made in tapping mode (Nanoscope III, Digital Instruments), to characterize the topography of the surfaces. All images (256 × 256 points) were acquired in air and plane-fitted with a second-order polynomial to adjust for the imperfections in the lateral motion of the scanner and the curvature of the surfaces. The real surface area was calculated using the “surface area” tool in the Nanoscope III software, which calculates this by summing the area of all triangles formed between triplets of adjacent image points. The instrument used for the surface force measurements is based on a bimorph force sensor (MASIF, Australian Scientific Instruments).41-43 The instrument uses a piezoelectric bimorph acting both as a single cantilever spring and as the force sensor, onto which one of the two surfaces is attached. The other surface is mounted onto a piezoelectric tube actuator, whose expansion is directly measured using a linearly variable displacement transducer (LVDT), to compensate for nonlinearities and hysteresis in the piezotube. The acquisition of a force-distance profile is performed by approaching the two surfaces at a constant speed (typically 10 nm/s) from 500 to 1000 nm separation until they meet in a “hard wall” contact, when the surfaces are separated again. The LVDT sensor data is used to determine the sensitivity of the bimorph (in the constant compliance region) in each approach. The data are further processed to give the force between the surfaces as a function of the distance relative to the “hard wall” contact. Thus, to obtain correct force-distance profiles, (41) Stewart, A. M. Meas. Sci. Technol. 1995, 6, 114. (42) Parker, J. L. Langmuir 1992, 8, 551. (43) Parker, J. L. Prog. Surf. Sci. 1994, 47, 205.
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the deformation of the surfaces due to the forces acting on them in the contact region must be accounted for. The best distance resolution is approximately 0.2 nm, the minimum detectable force is on the order of 10-8 N, and the normalized force resolution about 10 µN/m. Unless stated, no corrections for deformation have been made in the graphs; the point of zero separation corresponds to the location of the ”hard wall”, as selected in the analysis. All forces have been normalized with the average radius R of the spheres, where R ) R1R2/(R1 + R2) and R1 and R2 are the radii of the two spheres. The resulting normalized force F/R is related to the interaction free energy per unit area, G, between plane surfaces through the Derjaguin approximation,44 F/R ) 2πG; thus the results can be directly compared with results from measurements in the crossed-cylinder or sphere-on-flat geometries. The stiffness of the measuring spring in the surface force instrument was determined by adding a 1 g weight to the spring and measuring the static deflection. (Attard et al. pointed out that methods relying on the measurement of resonant frequency shift with added mass consistently results in larger values of the spring constant than this method,45 though to our experience the ratio between the stiffnesses, as obtained by measuring the static deflection and with the resonance method, is about 1.51.7, rather than 4, as suggested by Attard et al.) Averaged force-distance profiles were calculated by arranging the force-distance data pairs from between five and seven approaches into a single column, whereupon the whole column was sorted in distance order and a seven-point running average was used to produce the averaged force curve. Only those forcedistance profiles which are explicitly labeled so are averaged. To make the graphs less cluttered, all other data are presented as solid lines connecting the measured data points. Surface deformation is modeled with Johnson-KendallRoberts theory (JKR),46 which is a continuum mechanics model for elastic deformation. With R as above, this theory relates the radius of the flattened contact area, a, to the interfacial energy per contact area, γ,
a3 )
3(1 - ν2)R (F + 3πγR + [6πγRF + (3πγR)2]1/2) 2E
where E is Young’s modulus, ν the Poisson ratio, and F the applied load. At zero applied load, the central displacement, δ, i.e., the compression along the symmetry axis of the sphere-sphere contact, is
δ)
[
]
(1 - ν2) a2 - 4πγa R E
1/2
The pull-off force, i.e., the force required to separate the surfaces from adhesive contact, is given by -3γπR/2 in the JKR model. The electrostatic double-layer interactions were calculated by solving the Poisson-Boltzmann equation according to the procedure outlined in ref 47. Where the electrostatic contribution had to be subtracted from the total force to discriminate the attractive interaction at short separations, the interaction was assumed to be under constant charge conditions (rather than constant potential or charge regulation), since this seemed to produce rather accurate fits. It is possible that this method results in some error in the quantitative characterization of the attraction, since the resulting shape of the force profile at short separations might not be correct, but the method is more than sufficient for comparisons of the qualitative behavior.
Results Contact Angle Measurements of Water on Mixed C16/C16OH Surfaces. The advancing and receding (44) Derjaguin, B. Kolloid-Z. 1934, 69, 155. (45) Attard, P.; Schulz, J. C.; Rutland, M. W. Rev. Sci. Instr. 1998, 69, 3852. (46) Johnson, K. L.; Kendall, K.; Roberts, A. D. Proc. R. Soc. London 1971, A 324, 301. (47) Chan, D. Y. C.; Pashley, R. M.; White, L. R. J. Colloid Interface Sci. 1980, 77, 283.
Figure 1. Advancing (filled diamonds) and receding (open diamonds) contact angles of water with surfaces prepared from mixtures of C16- and C16OH-thiols of various mixing ratios.
contact angles of water with surfaces prepared from mixtures of C16- and C16OH-thiols of various mixing ratios are shown in Figure 1. (As has already been mentioned, the fractions on the abscissa are solution fractions and do not necessary reflect the composition on the surface; see the Discussion for details.) For surface force measurements, surfaces prepared from 0%, 65%, 72.5%, 80%, and 100% C16 solutions were used; 65% surfaces have both advancing and receding contact angles below 90°, 72.5% surface span 90° with one angle above and the other below 90°, whereas 80% surfaces have both the advancing and receding contact angles above 90°.48 For reference, the contact angles were measured also on pure C16 and C16OH surfaces. Advancing and receding contact angles were θa ) 110° ( 1° and θr ) 104° ( 1°, for the former type, and for the latter θa ) 24° ( 2°, and θr ) 15° ( 2°. The increase in data scatter with the hydrophilic C16OH surfaces actually corresponds to a decrease in the relative error in measured surface energy, which is proportional to the cosine of the contact angle. The corresponding relative errors in the surface energy are approximately (5% for the C16 surfaces, but a mere (2% for the C16OH case. It is worth mentioning that the geometry of the used surfaces might also be a source of scatter: when a silicon (100) wafer is cut, the edges will in general not be perpendicular to the surface but rather follow the crystal planes at 45° relative to the surface, which might result in a maximum variation of the perimeter of up to 7% for two surfaces with the same nominal dimensions. However, from looking at the scatter in the data, it seems that these variations are much smaller. Force Measurements with Mixed C16/C16OH Surfaces. Forces were measured in pure water with surfaces prepared from mixtures containing 65%, 72.5%, and 80% C16, i.e., surfaces with contact angles spanning across 90°. For comparison, results were obtained with pure C16OH and C16 surfaces as well. Surfaces prepared from 65% C16 solutions interact with electrostatic double-layer forces at large separations, indicating that the surfaces are charged (Figure 2). On the other hand, no electrostatic contribution can be seen in the data from either the 0% or 80% surfaces. (Results (48) Other fractions have been investigated as well, but these were chosen because they bound the only range where the differences in composition are reflected in changes in the force profiles between the surfaces. In the ranges 0-65% and 80-100%, no differences were observed in the long-range attractive part of the force measurements (although adhesion increases with the hydrophobicity).
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Figure 2. Normalized total interaction in water between surfaces prepared from 65% C16 solutions, accompanied with a fitted solution to the Poisson-Boltzmann equation under constant charge conditions, resulting in a surface potential of 41 mV.
Figure 3. Forces in pure water between surfaces prepared from 65% C16 (dashed lines) and 80% C16 (solid lines) solutions. The advancing contact angles of water corresponding to the 65% surfaces are 88° ( 1°, and 98° ( 1° for the 80% surfaces.
from other compositions indicate that, for surfaces with contact angles less than 90°, the charge increases with the fraction of methyl groups on the surfaces. This is a subject of current investigations and will be reported on separately.49) DLVO fits to the repulsive interactions on the 65% surfaces under constant charge conditions result in surface potentials of approximately 40-50 mV in pure water. A precise quantitative analysis of the electrostatic interaction in pure water is difficult, since the decay length of the interaction is very large, and the assignment of a region with no interaction to set the zero force level becomes unreliable. To compare the short-range behavior in the various measurements, the electrostatic repulsion was subtracted from the total interaction before proceeding. The data in Figure 3, for 65% and 80% surfaces (advancing contact angles 88° and 98°, respectively), clearly indicate that forces between surfaces with the lower contact angles do not exhibit the steplike onset of the attraction that is observed with the 80% surfaces. The range of the attraction for the 80% surfaces was in most cases 25-35 nm, but in a few cases steps were observed at separations extending up to 50 nm. Results for 72.5% surfaces (θa ) 94°) fall somewhere between the results of 65% and 80% surfaces, showing occasional steps in the attraction, but mostly without them. Figure 4 compares an averaged curve (the thick line), calculated from five force profiles without steps, with one (49) Ederth, T. Manuscript in preparation.
Ederth and Liedberg
Figure 4. Forces in pure water between surfaces prepared from 72.5% C16 solutions. The thick line is an average of five approaches with no discernible steps in the force profiles. The thin line is a single approach with a qualitatively different behavior (see text for details).
Figure 5. Bimorph deflection upon separation of the two surfaces while a cavity is bridging the surfaces. As the surfaces are separated, the attraction between the surfaces decreases as the cavity is elongated, until it finally breaks, and the deflection of the bimorph becomes independent of the separation between the surfaces. Note that the displacement is relative and is not equivalent to the surface separation. Zero at the separation axis corresponds to a separation of a few microns in this case.
single approach, where a step is present. For the latter curve, it seems that after the steplike onset, the range and shape of the curve is the same as for the curves without steps. Also, with some curves it is difficult to determine whether there is a step or not, but the range of the force still appears to be approximately the same in all cases. In accordance with the previous study dealing with purely methylated surfaces,9 the range or shape of the attractive interaction does not change systematically with time. However, between the 80% surfaces, and sometimes also with the 72.5%, a cavity is formed upon contact that is observable upon separation: if the cavity does not disappear when the surfaces are separated far enough to jump out of adhesive contact, the cavity is revealed as a coupling between the surfaces, and upon further separation the disappearance of the cavity can eventually be observed as a discontinuity in the bimorph deflection signal (Figure 5). Now, what has been observed is that the range of this coupling between the surfaces that is observable upon separation depends both on the time the surfaces have been left in contact and on the time that has passed since the water was injected (or, perhaps more relevant, since deaerating was discontinued). The longer the waiting time (either of the two times), the further the two surfaces have to be separated to make the cavity disappear. Results for the pull-off forces (the force required to separate the surfaces from adhesive contact) are unreliable
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due to the variations in roughness of the underlying gold substrates between individual samples,9 but for 65% surfaces, they were found to be in the range 110-140 mN/m, for 80% surfaces around 300 mN/m, and for the fully hydroxylated surfaces (0% C16) 2-3 mN/m. Discussion Contact Angle Measurements. The relation between solution and surface SAM composition is determined by adsorbate-adsorbate interactions within the layer and in the solution, solvent-adsorbate interactions, and surface-adsorbate interactions. The surface fraction is increased for the component with favorable surfaceadsorbate interaction and unfavorable solvent-adsorbate interaction. Where these interactions differ significantly between the adsorbing species, the SAMs might phase separate into two-dimensional domains on the surface,50,51 or the SAM composition can differ significantly from the solution composition.52 For thiolates similar to those used in this study, albeit of shorter length, Bain et al. observed that the adsorption of hydroxyl-terminated thiols from ethanol is disfavored in a mixture with methyl-terminated thiols.53,54 This is probably due to a higher affinity of hydroxyl-terminated thiols to ethanol, as compared to the interaction between ethanol and the methyl-terminated thiol. Adsorption from a different, non-hydrogen-bonding solvent (tetrahydrofurane) yields an almost linear relationship.35 The particular combination of C16 and C16OH thiolate films adsorbed from ethanol solutions used in this study has been investigated in a similar manner by Bertilsson and Liedberg, and the surface fraction of OH groups is again a nonlinear function of the corresponding solution composition.55 We did not measure the surface compositions corresponding to the used solution compositions, since the interest was mainly in the wetting properties of the surfaces, but an indirect way of accessing the surface composition is to compare the measured contact angles with models for heterogeneous surfaces. The contact angle, θ, formed by a liquid in contact with a solid is given by the Young-Dupre´ equation,
γlv cos θ ) γsv - γsl where γlv, γsv, and γsl are the interfactial free energies per area for the liquid-vapor, solid-vapor, and solid-liquid interfaces, respectively. This holds also with the kind of heterogeneous surfaces used here. The easiest way of modeling the contact angle with the heterogeneous surfaces is by interpolation between the two homogeneous surfaces. Assuming that the energy contribution from one particular species present on the surface is direcly proportional to the relative surface area occupied by that species, the weighted sum of the cosines of the contact angles with the molecules present on the surface gives an estimate of the resulting contact angle. The resulting relation is the Cassie equation,56 (50) Stranick, S. J.; Parikh, A. N.; Tao, Y.-T.; Allara, D. L.; Weiss, P. S. J. Phys. Chem. 1994, 98, 7636. (51) Tamada, K.; Hara, M.; Sasabe, H.; Knoll, W. Langmuir 1997, 13, 1558. (52) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M. Langmuir 1992, 8, 1330. (53) Bain, C. D.; Evall, J.; Whitesides, M. J. Am. Chem. Soc. 1989, 111, 7155. (54) Bain, C. D.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1989, 5, 723. (55) Bertilsson, L.; Liedberg, B. Langmuir 1993, 9, 141. (56) Cassie, A. B. D. Discuss. Faraday Soc. 1948, 3, 11.
Figure 6. The cosines of the advancing (filled diamonds) and receding (open diamonds) contact angles with water on the mixed SAMs. The thin lines represent the Cassie prediction calculated from the contact angles for single-component surfaces, where the surface composition is assumed to be equal to the composition in the adsorbate solutions (solid line for advancing, dashed line for receding contact angles). The thick line represents the Cassie prediction in a situation where the surface composition of the methyl-terminated thiol is assumed to be 10% higher than in the solution.
cos θ ) ROH cos θOH + RCH3 cos θCH3 where R are the surface fractions of the functional groups and θ the contact angles for the pure surfaces of each kind. With cos θOH ) cos(24°) and cos θCH3 ) cos(110°), the Cassie equation gives the results indicated by the thin solid lines in Figure 6, where the cosines of the contact angles in Figure 1 are shown, rather than the contact angles themselves. The receding angles follow the Cassie relation fairly well, whereas the advancing angles do not. Following the results of previous studies,53-55 we assume that the adsorption of hydroxyl-terminated thiols is disfavored at low concentrations and in addition make the rather crude assumption that the ratio between the solution and surface fractions is constant in the range covered by the data in Figure 6. The thick line in Figure 6 represents a situation where the methyl surface fraction is 10% higher than the solution fraction, and all measured surface energies fall between this line and the line formed by the Cassie equation. Here, the Cassie equation resulted in lower contact angles than those measured, but keeping in mind that the Cassie equation will predict larger contact angles than a model taking the intermolecular surface interactions into account,57 we conclude that in this situation the deviation from the Cassie equation is not because it ignores details of the lateral interactions but rather because the surface fractions are different from the solution fractions and the ”fit” procedure above should not be totally inappropriate in determining an upper limit for the deviation of the surface composition from the solution composition. The contact angle hysteresis is somewhat enhanced for the mixtures, as compared to the methylated and the hydroxylated surfaces (Figure 7), but the interpretation of this difference is not at all obvious. Surface roughness and surface heterogeneities are believed to be factors affecting the contact angle hysteresis, but there is no reason to believe that the substrate roughness differs between the samples. Regarding heterogeneities, both (57) Israelachvili, J. N.; Gee, M. L. Langmuir 1989, 5, 288.
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Figure 7. Contact angle hysteresis for the investigated SAM surfaces.
Figure 8. Atomic force microscope image of a SAM-coated gold surface. The granularity is solely caused by the underlying polycrystalline metal surface.
theoretical58 and experimental59 studies indicate that surface phase-separation on the nanometer scale does not contribute noticeably to the hysteresis. (For domains smaller than 30 nm, it is suggested that surface heterogeneities do not even affect the contact angles.59) The grains of the underlying gold surface set an upper limit for the domain size in the SAMs under study (not implying that neighboring grains cannot have the same composition, but merely indicating that domain growth is hindered): the AFM image in Figure 8 reveals that the largest grains on these surfaces have sizes of up to 40 nm, but the peak in the rms corrugation length (as obtained from the power spectrum density of the images) is in the 5-15 nm range. From these numbers, it is questionable whether any singlecomponent domains formed on the surface would be large enough to make a significant contribution to the hysteresis. Besides, in a previous study it was concluded that mixtures of C16 and C16OH do not even form single-component domains on the surfaces; at a 1:1 mixing ratio there was no evidence of lateral hydrogen bonding on the surface.55 Despite the lack of direct evidence, we infer from this that the increased hysteresis for the mixtures is not caused by heterogeneities in the form of single-component domains, but rather an effect of the mixing of the two components (58) Neumann, A. W.; Good, R. J. J. Colloid Interface Sci. 1972, 38, 341. (59) Imabayashi, S.-I.; Gon, N.; Sasaki, T.; Hobara, D.; Kakiuchi, T. Langmuir 1998, 14, 2348.
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on the surface, where, for example, increased orientational freedom of the hydroxyl groups, permitting reorganization of the surface, could account for the hysteresis.60-62 Force Measurements. The results show that an unmistakable qualitative change in the forces is occurring whenever the (advancing) solid-liquid contact angle passes 90°. For higher values of the contact angle, the forces are purely attractive but have a steplike force onset and a shape and range inconsistent with a van der Waals force. For lower contact angles, there is a DLVO-type interaction, where a van der Waals force is superimposed upon an electrostatic double-layer interaction. In line with the suggestion of Parker et al.,7 we attribute the steps in the force profiles to bridging of microscopic bubbles on the surfaces, and the subsequent growth of them after bridging causes the attraction between the surfaces at decreasing separations. Even though our data does not provide direct evidence of the presence of bubblessimplying that, for example, cavity formation upon approach, due to metastability of the water film, could be a cause of the observed stepssthe results are consistent with other work supporting this conclusion. Several recent studies have shown that glass surfaces rendered hydrophobic with silanes7,63 and phenyl groups64 show similar behavior in water or aqueous salt solutions and also in other polar solvents.12 Carambassis et al. observed bubbles on their surfaces after immersion into water and were able to distinguish the interaction between surfaces covered with bubbles from interactions between the bare surfaces, the former showing steps in the force profiles, but not the latter.63 In a study using propanetriol, Boehnke et al. showed that when the contacting of propanetriol with the solvophobic surfaces was made in a vacuum rather than in air, the steps that were observed after contacting in air had disappeared.12 Similar effects have been observed in ellipsometric studies, where the scattering of light from hydrophobic surfaces is reduced if the surfaces are immersed in ethanol, which is subsequently replaced with water, rather than immersed directly into water.65 All these observations confirm that bubbles are formed whenever the hydrophobic surfaces pass the air-water interface upon filling the measuring chamber with water. Yet another study reports steplike attractive force profiles between silica surfaces modified with phenyl grops.64 Although the authors do not mention the possibility of bridging bubbles, the data is to a large extent consistent with our observations: the range of the forces was found to be 40 nm with a 30% deviation, and the shape of the force profile differs from one run to another. Notwithstanding this evidence, it is worth keeping in mind that cavities form between the surfaces while in contact, and the cavity thus formed during the aquisition of the first force curve might cause the formation of bubbles on the surfaces when the surfaces are separated and the bridging cavity disappears. The cavities grow with both increasing contact time and, if the contact time is kept constant, with the duration of the experiment, proving that they contain gases dissolved in the water, diffusing into the cavity. Since the formation of a cavity is expected (60) Evans, S. D.; Sharma, R.; Ulman, A. Langmuir 1991, 7, 156. (61) Ulman, A.; Evans, S. D.; Schnidman, Y.; Sharma, R.; Eilers, J. E.; Chang, J. C. J. Am. Chem. Soc. 1991, 113, 1499. (62) Kacker, N.; Kumar, S. K.; Allara, D. L. Langmuir 1997, 13, 6366. (63) Carambassis, A.; Jonker, L. C.; Attard, P.; Rutland, M. W. Phys. Rev. Lett. 1998, 80, 5357. (64) Hu¨ttl, G.; Heger, K.; Klemm, V.; Theissig, J.; Wagner, W.; Mu¨ller, E. Fres. J. Anal. Chem. 1999, 363, 206. (65) Tiberg, F. Personal communication.
Influence of Wetting Properties
when two hydrophobic surfaces are brought into contact,66 and also considering that complete degassing of the water is virtually impossible, making an experiment where they do not form does not seem possible. Even if a wealth of experiments indicate that the cause of the steplike attractions is bridging of bubbles between the surfaces, it is not well-understood what makes these bubbles stable, and despite the similarities in qualitative behavior in the force measurements referred to above, the range of the forces varies from some tens of nanometers (this study) up to more than 200 nm (Parker’s and Boehnke’s works), in fact, it seems that the detailed nature of the surfaces determines the properties of these bubbles. If the size of the bubbles is on the order of the range of the attractions, in our case 50 nm at the most, the Laplace pressure inside them is very high, and consequently, they are expected to dissolve into the liquid. Ljunggren and Eriksson estimated the lifetimes of bubbles in water with radii in the 10-100 nm range to be less than 100 µs, 67 and they have also shown that bubbles attached to a flat surface, where the contact angle is greater than 90°, are not thermodynamically stable.68 However, the real surfaces used in this study, as well as the silanated glass/ silica surfaces used in other studies, are far from ideal but are impaired by roughness and chemical microheterogeneities. The granularity of the SAM surface (or, more precisely, the underlying polycrystalline gold layer) is evident from Figure 8. Surface imperfections, topographic as well as chemical, provide sites for bubble nucleation, where microscopic bubbles can attach to or grow from or where microscopic contact angles below 90° are possible, while still keeping the contact angles on a macroscopic level above 90°. More, contaminants in the gold film tend to gather along grain boundaries, and the adsorption of the hydrophobising agent is expected to be less favorable there, thus producing patches where the three-phase line could be pinned, or where the equilibrium contact angle can be less than that found at the gold surface. Note that a lower local equilibrium contact angle reduces the curvature of the bubbles and thus the Laplace pressure. The recent observations that charged submicrocavities are stable in water and, further, that the concentration of them is increased close to hydrophobic surfaces69 might also account for the observations. Unfortunately, there is nothing in our data that makes a discrimination between the alternative stabilizing mechanisms possible. It cannot be that the area increase caused by the roughness as such stabilizes the bubbles; if the real surface area of the solid beneath the bubble is larger than the nominal area due to the rougness, the free energy change in forming a bubble is larger than what is expected over a flat (ideal) surface. Now, the real surface areasas determined by AFMsis but a mere 2% larger than the nominal. The corresponding change in surface free energy can be expressed as an uncertainty in contact angle, using Wenzel’s modification of Young’s equation for contact angles on rough surfaces,70
γlv cos θ )
Areal (γ - γsl) A sv
where Areal is the real surface area and A is the geometric (66) Yushchenko, V. S.; Yaminsky, V. V.; Shchukin, E. D. J. Colloid Interface Sci. 1983, 96, 307. (67) Ljunggren, S.; Eriksson, J. C. Colloids Surf. A 1997, 129-130, 151. (68) Eriksson, J. C. E.; Ljunggren, S. Langmuir 1995, 11, 2325. (69) Bunkin, N. F.; Kiseleva, O. A.; Lobeyev, A. V.; Movchan, T. G.; Ninham, B. W.; Vinogradova, O. I. Langmuir 1997, 13, 3024. (70) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988.
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Figure 9. Normalized force profiles for measurements in water using surfaces prepared from 65% C16 solutions and fully hydroxylated surfaces (0% C16). Each data set in the figure is the average of between five and seven approaches. The 0% C16 curve is hidden behind one of the 65% curves. The slight variations in shape between the data for the surfaces prepared from 65% solutions is probably caused by the procedure where the electrostatic contribution was subtracted from the total force. The solid line is a calculated nonretarded van der Waals force, corresponding to a Hamaker constant of 3.9 × 10-20 J.
surface area. The contact angle error caused by roughness for a 100% C16 surface is then (with θ ) 110°) approximately 0.5°, which is within the measurement error for the contact angle measurements. Since the macroscopic contact angle seems to determine the force behavior, this effect must be negligible and cannot be used as an argument for an increased bubble stability. What about surface microheterogeneities? The change in free energy associated with the formation of a bubble on the surface is different depending on whether the bubble is formed over methyl-terminated areas or over a homogeneously mixed 80% C16 area. If the SAM is phaseseparated into domains rather than homogeneously mixed, it is conceivable that bubbles on the surfaces could be stabilized over hydrophobic patches on the surfaces. However, the distinct change in the force curves around the 90° contact angle provides further support to the view that the SAMs are either not phase separated into nanometer-sized domains or, if they are, their presence is not crucial for the formation of the bubbles on the surfaces. Since the formation of the monolayer is independent of the subsequent contact angle measurement, such domains would still be present at contact angles below 90°, albeit smaller in size, or fewer, and the change associated with the passing of 90° would be gradual. Further, the microscopic structure is bound to be different in the 100% and 80% cases, but this change is not reflected in a significant difference in the force curves, while the contact angles with water are clearly different, which also suggests that the microscopic structure is of minor importance. Disregarding for a moment the distinct qualitative change in the shape of the attraction on going above the 90° contact angle, we also observe that the range of the interaction does not differ significantly between fully hydroxylated surfaces and 65% methylated surfaces. Figure 9 compares data from several force curves of each kind (each curve in the figure is an average of five to seven individual curves from each experiment), and there appears to be no significant differences in the interaction. The deviation of one of the 65% curves is probably a result of either the constant charge assumption in the subtraction
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procedure, whereby the electrostatic contribution to the total force was removed, or due to variations in roughness of the surfaces in the two experiments, resulting in different degrees of deformation. Included in the figure is also a calculated nonretarded van der Waals force, corresponding to a Hamaker constant of 3.9 × 10-20 J.71 Deformation effects have been neglected in calculating the data in Figure 9, and taking those into account would shift the 65% curves toward even smaller separations. The deformation of ideally smooth surfaces with the surface energy of a 65% surface, approximately 35 mJ/ m2,72 as calculated using the elastic properties of glass at zero applied load, results in a central displacement of 3.5 nm. However, the finite roughness of the surfaces renders this number improbable, and the deformation is very likely to be smaller.73 The JKR prediction for the pull-off force for these surfaces is 320 mN/m, while the measurements never resulted in pull-off forces in excess of 140 mN/m. The fact that force measurements between surfaces with contact angles below 90° do not show any variation in the strength of the long-range attraction with varying interfacial energy suggests that there are no additional longrange forces whose magnitudes are directly dependent on, and increase with, the contact angle. (Short-range repulsions caused by removal of water molecules strongly bound to the hydroxyl groups on the surfaces have not been considered, but are likely to change with the contact angle.) The adhesion is not unaffected by the changing surface composition, as was indicated by the results for the pulloff forces: increasing from 2 to 3 mN/m for 0% surfaces to 110-140 mN/m for 65% surfaces. Since the interfacial properties are tuned by exchanging methyl groups for hydroxyl groups at the hydrocarbon-water interface, the accompanying change in van der Waals interaction is negligible. It effectively corresponds to changes in the thicknesses of the hydrocarbon and water films on the order of 1 Å on each surface (note that the metal substrates interact across water with a Hamaker constant that is approximately 100 times larger than that for hydrocarbons74). It has been suggested that the long-range ”hydrophobic” interaction can be subdivided into two parts, one part that is directly dependent on the wetting properties of the surfaces and one part that is not directly affected by changes in the wetting of the surfaces but rather depends on the mobility of surface species, adsorption-desorption processes, or on the molecular details in some other way.22 Clearly, the case currently at hand presents a situation where there is no true long-range “hydrophobic” force at all, i.e., a force that is directly proportional to the “hydrophobicity” of the surfaces. Hato studied the interaction between LB films of controlled hydrophobicity, where the outermost layer comprises methyl- and hydroxyl-terminated alkyl chains in various ratios.22 Despite the similarities in composition, (71) This value falls between the Hamaker constants expected for interactions between glass surfaces in water, and gold surfaces in water, 0.8 × 10-20 J and 9 × 10-20 J, respectively. We pointed out in an earlier publication that the calculation of the van der Waals force using Lifshitz theory is difficult in this system, due to the presence of sulfur and hydroxyl monolayers, which are not readily included in a continuum theory, and also due to the uncertainty in the thickness of the underlying metal layer (this is due to limited resolution of the thickness monitor used in the evaporation). For details, see Ederth, T.; Claesson, P.; Liedberg, B. Langmuir 1998, 14, 4, 4782. (72) Carlsson, C. Unpublished data. (73) Fuller, K. N. G.; Tabor, F. R. S. Proc. R. Soc. London A 1975, 345, 327. (74) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: London, 1991.
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as compared to the substrates used in this study, the results are of a distinctly different character. The interactions between the LB films were, generally, much more long-ranged and also followed a double-exponential law (see Figures 2 and 5 in ref 22). To the best of our knowledge, there are no reports of steplike discontinuities in the attractions between LB films or in measurements between adsorbed surfactant layers, but they rather show continuoussand usually very long-rangedsattractions. This would be understandable if the attachment of a bubble is related to a surface imperfection; where the adsorbed molecules are relatively free to move along the surface, lattice imperfections are not stable and can be repaired. These observations support the claims that long-range interactions between surfaces prepared by methods where the adsorbates are not rigidly attached to the surfaces can be related to surface mobility or shifts in adsorption/ desorption equilibria as the separation is changed.29,30 Admittedly, our results still leaves open the possibility of a true ”hydrophobic” force at contact angles above 90°, but such an interaction is inaccessible with the surfaces used in this study. Conclusions We have used direct force measurements to investigate the influence of interfacial energy changes on the longrange attractive interactions between self-assembled thiolate monolayers in water. The results from the force measurements show that whenever the solid-liquid contact angle is larger than 90°, the attractive force profiles have steplike onsets, and the shape and range of the force profiles show random variations. When the contact angle is lower than 90°, the interaction is consistent with a van der Waals interaction. For surfaces with contact angles with water in the range 24°-88°, no differences in the long-range forces were detected. The excess attractions recorded with hydrophobic surfaces do not vary systematically with the contact angles and can be referred to as ”hydrophobic” interactions only in the sense that the effect is present only for contact angles above 90°. We conclude that the steplike onset of the force is caused by bubbles adhering to the surfaces, bridging the two at separations between 25 and 50 nm. The mechanism by which the bubbles are stabilized is not known, but their stability is closely related to the macroscopic contact angle of the surfaces. Since the formation of a cavity between two solvophobic surfaces is expected, cavities will make the formation of these bubbles possible at each contact (or separation, to be precise). Attributing the long-range interaction to bridging of such bubbles, as we have, it seems that thiolate SAM surfaces in aqueous solutions will be of rather limited use for force measurements whenever the advancing contact angle is higher than 90°. In this range the excess attraction varies in an unsystematic manner, and changes in the force profiles revealing variations in the detailed properties of the surfaces cannot be unambiguously discerned. Acknowledgment. We thank P. Claesson for valuable discussions and critical reading of the manuscript. This project was financially supported by the Swedish Natural Science Research Council (NFR) and the Swedish Research Council for Engineering Sciences (TFR). LA9909650
