Langmuir 1991, 7, 3154-3159
3154
Interactions between Hydrophobic Surfaces. Dependence on Temperature and Alkyl Chain Length Yi-hua Tsao, S. X. Yang, and D. Fennel1 Evans* Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455
HBkan Wennerstrom Division of Physical Chemistry, T h e Chemical Center, University of Lund, P.O.Box 124, S-22100 Lund, Sweden Received July 8, 1991. I n Final Form: August 14, 1991 Attractive forces between two hydrophobized mica surfaces immersed in water were measured using the surface force apparatus (SFA) over the temperature range 25-50 O C . Monolayers of dihexadecyldimethylammonium (DHDA), dioctadecyldimethylammonium (DODA), or dieicosyldimethylammonium (DEDA)surfactants were deposited onto mica by dipping cleaved mica into a cyclohexane solutioncontaining the surfactant. The SFA measurementsconfirmthe presence of a very long range attractive force reported in previous studies. A surprising result is that the magnitude of this attractive force depends on the surfactant chain length and changes dramatically with temperature. The molecular structure of the monolayer in air and in water at 25 OC was obtained from atomic force microscopy (AFM) images. The monolayers are uniform and continuous over microns, and the spacing between individual methyl groups was determined. For aqueous solutions, the differences in the AFM images for the three surfactants correlatewith the differencesin the SFA curves. These observationsmay explain the disagreement between the published force curves from different laboratories. Analysis of the observed temperature dependence leads to the following main conclusions: (1) the magnitude of the force depends critically on the state of the hydrocarbon chains on the surface. A very long range force is observed only when the chains show crystallineorder; (2) the force has a stronger temperature dependencethan that associated with hydrophobic interaction between molecules or the hydrocarbon--watersurface tension; (3) cavitation mechanisms are ruled out; and (4) we still do not understand the origin of this anomalous long-range attractive force between hydrophobic surfaces in water. Introduction Of all the interaction forces detected between surfaces in simple liquids, perhaps the most intriguing and confusing is the very long range strongly attractive force observed between hydrophobic surfaces in That a strong attractive interaction should exist between such surfaces followsdirectly from considerations of the surface free energies, but the range and magnitude of the effect is surprising. Forces have been measured up to surface separations of 90 nm4with magnitudes that are 1-2 orders larger than those predicted by continuum theory. It is a debated issue whether this deviation from continuum behavior should be attributed to molecular properties of the liquid or to surface interactions. One explanation is based on water structural effect^.^ The surface is assumed to induce order into the water molecules in close proximity. This order decays into the liquid and gives rise to a long-range interaction with the same decay length as the order profile. Another proposal is that Debye screened dipole-dipole correlations possessing an anomalously large amplitude6J give rise to the long-range attractive force. A more qualitative suggestion is that formation of vapor cavities between the hydrophobic (1) Israelachvili, J. N.; Pashley, R. M. Nature 1982, 300,341. (2) Pashley, R. M.; McGuiggan, P. M.; Ninham, B. W.; Evans, D.F. Science 1985,229, 1088. (3) Christenson,H. K.; Fang, J.; Ninham, B. W.; Parker, J. L. J.Phys. Chem. 1990, 94, 8004, and references therein. (4) Claesson,P. M.;Christenson,H. K. J. Phys. Chem. 1988,92,1650. (5) Eriksson,J. C.;Ljunggren, S.;Claesson,P. M. J.Chem. Soc., Faraday Trans. 2 1989,85, 163. (6) Podgornik, R. J. Chem. Phys. 1989,91, 5840. ( 7 )Attard, P. J. Phys. Chem. 1989,93, 6441.
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surfaces, which can be rationalized on thermodynamic grounds, plays some role in generating the force.8 None of these explanations accounts for all of the experimental observations. However, there is also considerable ambiguity in the measurements. While the same qualitative picture emerges from measurements from different laboratories, they do not agree quantitatively. The magnitude and the range of the force seems to depend on the material comprising the hydrophobic monolayer, its method of deposition onto mica, and the electrolyte concentration in the bulk. There is clearly a need for experimental studies that establish unambiguously how the force depends on external parameters and properties of the hydrophobic monolayer. In the present paper we report a study where the surface forces have been measured for neutral monolayers formed by depositing cationic surfactants of three different chain lengths, dihexadecyldimethylammonium(DHDA),dioctadecyldimethylammonium (DODA),and dieicosyldimethylamonium (DEDA), onto the negatively charged mica surface. Measurements were made at three different temperatures (-25,40, and 50 "C). In addition, we present atomic force microscopy images of the surfactants adsorbed on mica. Images obtained in air and in water at 25 "Cprovide the first molecular characterization of these hydrophobic monolayers. Neither temperature dependence nor molecular structure information has been reported previously, and as we shall see, they do provide interesting information about the hydrophobic surface force. (8) Christensen, H. K.; Claesson, P. M. Science 1988, 239, 390.
0 1991 American Chemical Society
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Materials and Methods The dihexadecyldimethylammonium acetate (DHDAA) and dioctadecyldimethylammonium acetate (DODAA) salts were prepared by eluting correspondentbromide salts in methanol on ion-exchange resin (Fisher REXYN 201) in acetate form.2 The acetate salts were than recrystallized in ether. Dieicosyldimethylammonium bromide (DEDABr) was a gift from Professor R. Moss and was used as received. Cyclohexaneis of analytic grade and was used without further purification. Millipore water was further processed using a Water Prodigy (from Labconco) and showed high resistivity (>18 Q-cm). The water was then heated with gentle stirring under low pressurefor several hours to remove dissolved gases. The monolayers were prepared by dipping mica, mounted on a flat stainless steel disk for atomic force microscopy measurements or glued, using Shell Epon Resin 1004F, onto cylindrical lenses, radius R 1-2 cm for surface forces measurements,into cyclohexane solutions containing 2 X lo4 M DHDAA, DODAA, or DEDABr. The solubilities of DODAA and DEDABr in cyclohexane at room temperature are very low, so the solution was heated gently without boiling in order to dissolve and deposit DODAA or DEDABr onto-micasurfaces. The surfactant-coated surfaces were rinsed in warm cyclohexane to dissolve any excess surfactant and then dried. Monolayers of DHDA, DODA, and DEDA on mica were directly visualized by atomic€orcemicroscopy (AFM; NanoScope 11, Digital Instruments) at 25 "C. The AFM apparatus was operated at a minimum force on all the surfaces investigated. A 0.7 pm size scanner was used and calibrated with highly oriented pyrolytic graphite (HOPG; Union Carbide) and muscovite mica (Union Mica Corp.). Microfabricated Si3N4 probes (Digital Instruments) were used. The spring constant of the cantilever was 0.38 N/m. Submolecular resolution was obtained. The measurements were carried out with a surface force apparatus (SFA) similar to that designed by Tabor9 and Israelachvili,lobut with a number of mechanical modifications. Two molecularly smooth silvered mica sheets of identical thickness were mounted in a cross-cylindrical configuration in the surfaceforce apparatus. The surface separationwas measured with an interferometric technique by observing fringes of equal chromaticorder (FECO). The surfaceseparation can be resolved down to 1A. A variable double-leaf spring" was used in our SFA modified to carry out measurements at different temperatures. The temperature was controlled in an air bath using a Tronac Inc. precision temperature controller with heating tapes attached to the wall of the air bath chamber. A thermocouplecontained in a stainless steel shell was inserted into the SFA to monitor the temperature. A desired temperature can be achieved within 2 h with temperaturefluctuationsless than 0.1 "C. A piezocrystal was installed in the upper mount to control the surface separation precisely. No double-layer repulsions were observed at large surface separation with the surfactant monolayers deposited on mica over the temperature range we studied. We concluded that the surfaces were neutral by use of the dipping method and did not recharge when immersed into water. The mica surfacesjumped into contact when the slope of the force curve equaled or slightly exceeded the spring constant of the variable spring under purely attractive forces. The spring constant divided by radius curvature K / R , assumed to equal d(F/R)/dD, was plotted versus the distanceD where the surfaces began to jump. Each reported curve represents at least two separate series of measurements.
Results Figure 1compares AFM images of mica in air and water. The dimensions measured from the images are in accordance with those obtained from X-ray data. Each high ~
~~
(9) Tabor, D.; Winterton, R. H.S.h o c . R. SOC.London, A 1969,312, 435. (10)Israelachvili.J. N.: Adams. G. E.J. Chem. SOC..Furudav Trans 1 1978, 74, 975. (11) Israelachvili,J. N.; Pashley, R. M. J. Colloid Interface Sci. 1984, I
98,500.
,
Figure 1. (a, top) AFM image of muscovite mica in air at 25 "C.
(b, bottom) AFM image of mica in water at 25 "C. Each high spot corresponds to three oxygen atoms on the basal plane.
spot in the image corresponds to three oxygen atoms in the basal plane of mica. In water, there is some indication that the three oxygens associated with each "bump" can be detected. Analyses of X-ray data establish that the area per charged site in mica is 50 A2. Figure 2 shows images of a monolayer of DHDA adsorbed on mica in air. In Figure 2a, a highly regular structure is observed. Parts b and c of Figure 2 show higher resolution images of DHDA in air. Analysis of the surface profiles establishesthat individual methyl groups can be imaged. Figure 3 displays images of DHDA, DODA, and DEDA in water. With DODA and DEDA, we detect peaks in the surface profiles corresponding to individualmethyl groups. This suggests that the hydrocarbon chains remain in the frozen state. However, with DHDA we were unable to obtain the molecular resolution expected for an ordered array of hydrocarbon chains. Instead we detect a nearly structureless surface more characteristic of a melted or liquidlike hydrocarbon chain. Measurements along the surface profiles of Figure 3a allow us to calculate an area per molecule of 52 f 2 A2(n = 60) for DHDA. Similar measurements on DODA and DEDA give values of 51 f 1(n = 49) and 47 f 1A2 ( n =
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3156 Langmuir, Vol. 7, No. 12,1991
Figure 2. (a, top) AFM image of DHDA monolayer adsorbed on mica in air a t 25 "C. Each high spot re resents individual The periodical DHDA molecules with an area of 50 f 2 ordering of the DHDA molecules reflects electrostatic interaction between the negative-charged mica and the positive-charged surfactant. (b, middle) Three-dimensional view of area of 100 A X 100 of DHDA monolayer on mica in air; individual methyl group can be discerned. (c, bottom) High-resolution AFM image of DHDA monolayer on mica in air.
12.
Figure 3. (a, top) AFM image of DHDA monolayer on mica in water at 25 O C . The image showsa decrease in structure compared to that in air (Figure 2a). (b) and (c) AFM images of (b, middle) DODA and (c, bottom) DEDA monolayers indicate that surfactant chains remain in frozen or crystalline state on mica in water at 25 "C.
38). Thus, the area per surfactant molecule and the higher order arrays are dictated mainly by the electrostatic
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Interactions between Hydrophobic Surfaces 0
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Figure 4. Interactions between DHDA, DODA, and DEDA monolayers in water obtained from surface force measurements at 25 OC. The monolayer contact position was set to be D = 0. At 25 O C , the attractive forces between DODA and DEDA monolayers are identical and stronger than those for DHDA. 0
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Figure 6. Interactions between DHDA, DODA, and DEDA monolayers in water at 50 OC. The dotted line represents van der Waals attractive force calculated by using Hamaker constant H = 1.8 X lo-%J. Comparison between the force curves for DEDA at 25 and 50 OC establishesthat the attractive interactions are similar, while those for DHDA and DODA decrease with increasing temperature and approach the van der Waals theoretical limit. Table I. Summary for Biexponential Fit of Surface Forces Measurements between Dihexadecyldimethylammonium (DHDA), Dioctadecyldimethylammonium (DODA), and Dieicosyldimethylammonium (DEDA) Monolayers at Various Temperatures
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DODA
interaction between anion sites on mica and the positive charge on the surfactant head groups. Figure 4 shows plots of d(F/R)/dDversus D determined from surface force measurements at ambient temperature for mica surfaces covered with a neutralizing layer of the three surfactants. According to the Derjaguin approximation,12the force between two flat surfaces is given by (dF/dD)/BrR. Twoeffects are apparent from the data. There is indeed a long-range attractive force, measurable up to 50-nm separation, with no observable repulsive double-layer force. There is also a significant difference in the measured force curves at ambient temperatures; those for DEDA and DODA agree within experimental error while the DHDA curve shows a substantially weaker attractive force. At long range the forces between DHDA and DODA/DEDA differ by roughly a factor of 3 while at shorter range the difference increases and reaches a factor of 10. The observed curves are not singly exponential, but they can be fitted to a double exponential with characteristic decay lengths of 2 and 25 nm, respectively. The same decay lengths have been used to fit the data for DHDA and DODA/DEDA. We consider the double-exponentialform to be a convenient but not a unique representation of the data. In Figure 5 we show the results of the same experiments performed on DODA and DHDA at a higher temperature of 40 "C. The qualitative features of the force remain at this temperature, but there are major quantitative changes. The range of the force has decreased, as reflected by the shorter decay lengths of 1.7 and 15 nm for the two ex-
DEDA
25 40 50 25 40 50 25 50
0.5 0.3 0.3 16.0 10.0 0.3 16.0 8.0
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ponentials, respectively. The preexponential factors have also decreased. Note that the conclusion of a change in both the range and the magnitude of the force depends on the biexponential representation. We cannot rule out that a fit to a different functional form would result in solely a change in amplitude. The temperature dependence of the force becomes even more apparent at 50 "C. In this case, Figure 6 reveals that the extreme long range characteristic of the force has been lost for DHDA and DODA. For DEDA, however, the measured attractive forces were only slightly lower than what was measured at 25 "C.At shorter range we also see that the force is equal for the two surfactants DHDA and DODA, in marked contrast to the behavior at room temperature. The fitted parameters in the biexponential expression for the force are summarized in Table I. Discussion Relation to Previous Studies. There have been several previous studies of long-range attractive forces arising from the interaction of dialkyldimethylammonium monolayers separated by ~ a t e r . ~In~these ~ J ~studies the hydrophobic surface is generated by equilibrium adsorption from solution2or by deposition using the LangmuirBlodgett t e ~ h n i q u e s . ~In J ~this study we have used yet another method where the surfactant is adsorbed on the (13) Claesson, P. M.; Clom, C. 2.;Herder, P. C.; Ninham, B. W. J . Colloid Interface Sci. 1986, 114, 234.
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3158 Langmuir, Vol. 7, No. 12, 1991
mica from an apolar cyclohexane solution. It appears that the cationic surfactants exchange quantitatively with the potassium ions of the mica surface. The AFM images establish that the monolayer remains intact when immersed in water. Surfactant-free media eliminate the possibility that the long-range attractive forces are generated by absorption of additional surfactant from solution onto monolayers, as suggested in ref 4. In the two previous studies of DODA monolayers, it was found that a double-exponential form was required to fit the data in accordance with the present study. However, the deduced decay lengths do not agree fully. Claesson et al.l3 worked with slightly charged surfaces, leading to a long-range double-layer repulsion which partly masks the presence of the long-range tail of the attraction. With electrolyte present, there is probably also a genuine change in the long-rangeforce. Claesson and Christenson4utilized another technique for measuring the force, which yields FIR directly. This measurement gives a stronger weight to the long-range part of the force, as can be seen by comparing the relative weights of the two exponentials in FIR and in d(F/R)/dD shown in Table I. A numerical differentiation of the data in ref 4 yields values of d(F/ R)/dD that largely agree with our data in the range 150250 A, but show a somewhat weaker attraction a t longer range. The fact that a long-range attractive interaction has been detected in several laboratories using different preparation techniques is reassuring. However, the differences in the precise magnitude of the measured forces have been perplexing since it was unclear whether the discrepancy reflected inherent differences in sample preparation or experimental difficulty in measuring this weak long-range force. State of the Alkyl Chains in the Monolayer. The combined observations from the AFM imaging and the temperature-dependent force measurements strongly indicate that the state of the alkyl chains on the surface is a crucial factor in determining the force. The AFM images of the adsorbed monolayers in air show that they are uniform and continuous over areas extending for several microns. These observations verify a previously tentative hypothesis about the structure of these monolayers. When immersed into water, AFM images show that DODA and DEDA monolayers change only slightly and show the long-range ordering expected for a crystalline array of frozen hydrocarbon chains. However, images of the shorter chained DHDA reveal that chain packing is less ordered than that for the other two compounds. These images obtained at ambient temperatures correlate in an interesting way with the observed forces at the same temperature (25 "C). As seen in Figure 4, the force curves are very similar for DODA and DEDA while the attraction is substantially weaker for DHDA. At the higher temperature of 50 "C, it is DHDA and DODA that show a similar behavior while the long-chain DEDA still yields a strong attraction which is virtually unaffected by the change in temperature. We conclude that there is a direct relation between the weakening of the force on increasing temperature and the change in the state of the hydrocarbon chains in the monolayer. The very strong long-range attractive force measurable up to separations of 50 nm only occurs when there is a crystalline order of the chains. As this order is lost on raising the temperature, the force gradually decreases until the conventional van der Waals force becomes dominant at long range.
That a melting of the chains can be expected in the temperature range studied is supported by comparison with the chain transition temperatures for the corresponding surfactants. These are DHDAA, 34 "C; DHDABr, 44 "C; DODABr, 54 "C; and (CzzH&DABr, 70"C;14DODAA and DEDABr are estimated to be 44 and 62 "C, respectively. Temperature Dependence of the Force. The trend in Figures 4 - 6 shows that the force decreases dramatically in strength as the temperature increasesover the relatively moderate range 25-50 "C. This observation provides new insight into this long-range attractive force. If the formation of cavities, directly or indirectly, were involved in generating the force, one would expect that its range would increase with increasing temperature. Cavity formation can be seen as a boiling point depression, and on approaching 100 "C, the range of the force should be very large. The observed decrease in the force with increasing temperature indicates that formation of cavities is not a significant phenomenon in generatingthe measured long-range attractive force. At 25 "C the force in the DHDA sytem is 1-2 orders of magnitude larger than the expected van der Waals force. At 50 "C the force has decreased to the extent that it is no longer clear what is the hydrophobic contribution and what is the van der Waals contribution. In fact the data for DHDA at 50 "C can be fitted to a van der Waals force d(F/R)/dD = H/3D3 over the entire measured range using H = 1.8 X J (see Figure 6). This value is intermediate between the estiJ for mica-water-mica and mated value of H = 2.2 X H=5X J for hydrocarbon-water-hydrocarbon. We do not expect any significant differences in H for the different surfactant systems, and it is indeed found that the force is the same within experimental accuracy for DODA and DHDA a t 50 OC in the range D = 4-8 nm. This observation supports the conclusion that here one is seeing a van der Waals force. After one of the experiments employing DHDA a t 50 "C, the SFA was cooled to room temperature and the longrange attractive force observed a t 25 "C was obtained. This observation suggests that the hydrophobic DHDA monolayer remains intact a t 50 "C. At larger separations there is a long-range attractive force that dominates over the van der Waals force for DODA. From the perspective of the Lifshitz theory this is in fact a remarkable observation. Asymptotically, a van der Waals force should dominate and deviations are expected a t shorter range. For DODA we see the opposite, namely, a van der Waals force at shorter range, while at long range another force dominates. The Lifshitz theory contains the asymptotic form of all normal attractive fluctuation forces, and the force curve at 50 "C highlights the veryunusual behavior of this surface force when viewed from a continuum point of view. Comparison with Other Hydrophobic Interactions. We have seen above that the long-range interaction between hydrophobic surfaces depends strongly on the state of the hydrocarbon chains on the surface. How does this correlate with more established manifestations of the conventional hydrophobic effect? For interactions between individual apolar molecules in an aqueous solution, the question of frozen versus fluid chains is irrelevant since the effective interaction arises mainly from the strong cohesion in water. Consequently, (14)Okahata, Y.;Ando, R.;Kunitake, T.Ber. Bunsenges Phys. Chem. 1981,85, 789.
Interactions between Hydrophobic Surfaces
the particular nature of the solute is not a crucial factor, and both flexible and rigid hydrocarbons show a hydrophobic interaction of similar magnitude. In all our force measurements we believe that there is a hydrocarbon monolayer exposed to the water. On thermodynamic grounds we then expect that the interaction energy at contact should be of similar magnitude for the hydrophobic contribution. Yet we observe large differences in the forces at long range. This leads to the conclusion that the forces of the hydrophobic interaction do not necessarily have a long-rangecharacter. Thus, there seems to be no firm basis for making conclusions about the range of the hydrophobic interaction between apolar molecules on the basis of the surface force measurements.
Conclusions Based on the SFA observations and on the AFM images, we can arrive at a number of qualitative conclusionsabout the molecular mechanism leading to the observed behavior: (i) The monolayer is uniform and continuous over areas that extend over microns. Differences in the state of the hydrocarbon chains in the monolayer correlate with measured forces.
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(ii) The force cannot be understood by simply considering a uniform structureless hydrocarbon surface exposed to water. Existing theories of the force assume an idealized surface as the starting point. (iii) The observed temperature dependence of the force is not consistent with a cavitation mechanism of the force. (iv) At 50 "C the DHDA monolayer interaction force has decreased to the extent that the conventional van der Waals force tends to dominate. We find it a significant observation that the van der Waals force dominates more a t short range than at long range for the distances studied. Even though the present study indicates some directions in which we can look for a solution, the urgent problem af finding the explanation for the apparent long-range attractive force between hydrophobic surfaces remains to be solved.
Acknowledgment. Support by NIH Grant GM 34341 and the Center for Interfacial Engineering, an NSF Engineering Research Center, is gratefully acknowledged. Registry NO. DHDAA, 71326-37-9;DODAA, 13308-45-7; DEDABr, 31500-63-7.
