1650
J. Pkys. Chem. 1988, 92, 1650-1655
mind the approximations employed to arrive at these values, one cannot find great differences between the results of the present work and those of ref 1, although the physical state of hexadecane is very different in the two cases.
coefficients for the distribution of a solute between gas and solid phases can easily be determined by using the simple experimental arrangement of Figure 1 and a relatively simple mathematical model leading to the general eq 30. These coefficients can be practically useful in various laboratory and industrial operations.
Conclusion Partition ratios, partition coefficients, and mass transfer
Acknowledgment. We are thankful to Mrs. Margaret Barkoula for her assistance.
Very Long Range Attractlve Forces between Uncharged Hydrocarbon and Fluorocarbon Surfaces in Water Per M. Claessont and Hugo K. Christenson* Department of Applied Mathematics, Research School of Physical Sciences, Australian National University, GPO Box 4, Canberra, ACT 2601, Australia (Received: July 13, 1987; In Final Form: October 2, 1987)
A new method has been used to carry out accurate measurementsof the hydrophobic interaction between uncharged hydrocarbon and fluorocarbon monolayer-coated surfaces in a surface force apparatus. The measurable range of the attraction between these deposited monolayers extends to a separation of 80 nm in conductivity water and can be approximated by an exponential function with two decay lengths of 2-3 and 13-16 nm, respectively. There is only a slight difference in the strength of the interaction between the fluorocarbon and hydrocarbon surfaces, in contrast to the significant difference in hydrophobicity as measured by the advancing contact angle of water. We suggest that the much shorter range attractive forces measured between monolayers adsorbed from solution are due to the formation of partial bilayers. We speculate that the very long range hydrophobic interaction found here is related to the metastability of water films between very hydrophobic surfaces.
Introduction The interfacial free energy of water against hydrocarbon is much larger than expected from continuum theories of van der Waals forces. This indicates that some additional force-the “hydrophobic interaction”-operates between hydrocarbon surfaces in water. Similarly, the self-assembly of amphiphilic molecules, including many of biological importance, as well as the conformations adopted by enzymes and other proteins in solution, are discussed in terms of a hydrophobic interactiod.2 between nonpolar solute molecules in water. All this, of course, gives no indication of the range of such an interaction and it has often k e n discussed as a “contact force” or a “hydrophobic b ~ n d ” . ~ , ~ We will here present results that show that the range of the hydrophobic interaction between macroscopic surfaces is much greater than previously thought. Under certain conditions it is measurable at surface separations of 90 nm (compared to perhaps 10 nm for a typical van der Waals interaction in a liquid medium) and it exceeds the van der Waals force by 2 orders of magnitude over the greater part of this range. One of the first indications of such a range was found by Blake and K i t ~ h e n e rwho , ~ estimated a maximum range of 64 nm for the hydrophobic interaction by observing the rupture of air bubbles next to a hydrophobic surface. The first direct measurements of the interaction, by Israelachvili and Pashley,6 showed an exponentially decaying attraction with a decay length of 1.0 nm and a measurable range of only 10 nm. They measured the forces between mica surfaces with adsorbed hexadecyltrimethylammonium ions (CTAB solution), and the observed attraction appeared to be independent of electrolyte concentration in the range lo4 to 5 X lo-* M. Subsequent measurements by Pashley et al.’ gave a range of 15 nm (exponential decay length of 1.4 nm) between monolayers of dihexadecyldimethylammonium acetate (DHDAA) adsorbed from solution at 2.5 X M. Claesson et al.* extended the measurable range further to 30 nm in con‘Permanent address: Institute for Surface Chemistry, Box 5607, s-114 86 Stockholm, Sweden, and Department of Physical Chemistry, Royal Institute of Technology, s-100 44 Stockholm, Sweden.
ductivity water (3 X M in univalent electrolyte) between dimethyldioctadecylammonium bromide (DDOA) monolayers deposited on mica as Langmuir-Blodgett films. In this case an exponential function with two decay lengths, 1.2 and 5.5 nm, respectively, was found to best fit the experimental data. Addition of KBr to 0.01 M reduced the weaker decay to 4.5 nm whereas the stronger, short-range decay was unaffected. Recent measurements of Rabinovich and Derjaguing show the existence of a very long range attractive force between methylated silica surfaces in aqueous solution. In spite of all this experimental data the origin of the hydrophobic interaction remains shrouded in mystery. At present there is not even any theoretical justification for the exponential functions generally used to fit the experimental results. One interpretation has been that hydrophobic surfaces induce small changes in the adjacent dynamic water structure. The attraction would result from the free energy gain on removing water molecules from the interlayer to bulk.1° There have been suggestions that the hydrophobic interaction is an attractive doublelayer force between surfaces of unequal charge. The functional form and relatively weak dependence on electrolyte concentration6,8 tend to rule this out. Yet another possibility that has been considered is that cavitation, or the spontaneous formation of vapor cavities (1)
Tanford, C . The Hydrophobic Effect, 2nd ed.; Wiley: New York,
1980.
(2) Franks, F. In Water: A Comprehensive Treatise; Franks, F., Ed.; Plenum: New York, 1973; Vol. 4, Chapter 1. (3) Kauzmann, W. Adv. Protein Chem. 1959,14,1. (4) Nemethy, G.; Scheraga, H. A. J. Phys. Chem. 1962, 66, 1773. (5) Blake, T. D.;Kitchener, J. A. J. Chem. Soc., Faraday Trans. 1972, 68, 1435. (6) Israelachvili, J. N.; Pashley, R. M. J. Colloid Interface Sei. 1984, 98, 500. ( 7 ) Pashley, R. M.; McGuiggan, P. M.; Ninham, B. W.; Evans, D. F. Science (Washington, DC) 1985, 229, 1088. (8) Claesson, P.M.; Blom, C. E.; Herder, P. C.; Ninham, B. W. J. Colloid Interface Sei. 1986, 114, 234. (9) Rabinovich, Ya. I.; Derjaguin, B. V. Colloids Surf.,in press. (10) Christenson, H. K.; Claesson, P. M.; Pashley, R. M. Proc. Indian Acad. Sei., Chem. Sei. 1987, 98, 379.
0022-3654/88/2092-1650$01.50/00 1988 American Chemical Society
Forces between Uncharged Hydrophobic Surfaces between hydrophobic surfaces, may be the cause of the hydrophobic Recent, systematic studies have shown that cavitation does indeed occur in these systems but that it does not itself give rise to the type of attraction measured.13 We will, however, suggest that the two phenomena are related-that the hydrophobic attraction is a result of the proximity of the system to the phase transition involved in cavitation. The hydrophobic interaction has in most cases been extracted from the measurements by determining the gradient of the force between curved surfaces and, where applicable, subtracting out the repulsive double-layer interaction. Unfortunately, the subtraction of a double-layer force often relies on guesswork as to the form of the interaction at small separations where it is swamped by the hydrophobic interaction. This is especially the case with the low electrolyte concentrations and low surface charges often encountered in monolayer systems. One important aim of this paper is to present a new method of extracting the force between two surfaces from the experiment. We have used a new technique based on that used by Chan and Horn in a recent study of the viscosity of thin films,14 whereby two surfaces are made to approach each other with a constant driving speed. We have taken their method and reversed the treatment-instead of using a known force law to extract the viscosity, we assume that the viscosity in the interlayer region is equal to its bulk value with the slip plane a t D = 0 and obtain a force law. W e have checked the results of this “drainage” method against those obtained from conventional methods.15 All direct measurements of the hydrophobic interaction to date have been carried out with hydrocarbon surfaces. Fluorocarbon surfaces should provide a useful comparison with a significantly more hydrophobic system (as measured by the advancing contact angle of water). We have managed to obtain hydrophobic monolayer surfaces of both hydrocarbon and fluorocarbon surfactants with immeasurably small surface charges. In other words, we avoid the uncertainty of having to subtract out a double-layer force and are left with a purely attractive force law.
Experimental Section Surface Preparation. The surfaces were prepared by Langmuir-Blodgett deposition of surfactant monolayers on molecularly smooth sheets of muscovite mica. The fluorinated surfactant was the double-chain cationic N-(a-trimethylammonioacetyl)0,O’bis( l H , 1H,2H,2H-perfluorodecyl) L-glutamate chloride, obtained from SOGO Ltd., Japan, and used as received. The proton N M R spectrum was found to be consistent with the given structure, and the surfactant gave one spot only with thin-layer chromatography (solvent = 2-propanol/acetone/ammonia, 67:30:30; Rf value = 0.38). Dimethyldioctadecylammonium bromide (DDOA) from Eastman was obtained in recrystallized form. The monolayers were spread in an all-Teflon trough on water from an Elga UHQ water purification unit fed with deionized and distilled water, bypassing the reverse osmosis stage. Deposition was carried out at a surface pressure of 20 mN/m for the fluorocarbon surfactant and 25 m N / m for the DDOA. The advancing (e,) and receding contact angles of water on the hydrophobed mica surfaces were 113O and 50-60’ (fluorocarbon surfactant) and 93O and 50’ (DDOA). The hydrophobed surfaces were mounted as crossed cylinders in a surface force apparatus15 and immersed in deaerated water from the Elga U H Q unit. All measurements were carried out a t room temperature. Force Measurements. Surface forces are usually measured by determining the deflection of a spring on which one of the surfaces is mounted (the conventional force-measuring t e c h n i q ~ e ) . ’In ~ (1 1) Rabinovich, Ya. I.; Derjaguin, B. V.; Churaev, N. V. Adv. Colloid Interface Sei. 1982, 16, 63. (12) Shchukin, E. D.; Amelina, E. A,; Yaminsky, V. V. Colloids Surf. 1981, 2, 221. (13) Christenson, H. K.; Claesson, P. M. Science (Washington, DC),in press. (14) Chan, D. Y.C.; Horn, R. G. J. Chem. Phys. 1985, 83, 5311. (15) Israelachvili, J. N.; Adams, G. E.; J . Chem. SOC.,Faraday Trans. 1 1978, 74, 915.
The Journal of Physical Chemistry, Vol. 92, No. 6,1988 1651 regimes where the gradient of the force exceeds the spring constant the system is unstable and this method cannot be used. Increasing the spring constant extends the measurable regime but leads to a loss of sensitivity. Instead, one often resorts to the gradient method, which is based on determining the onset of the unstable regime for varying values of the spring constant, Le., those points where dF/dD = k. To obtain the force it is then necessary to integrate some function that provides a reasonable fit to the data. In the drainage method the two surfaces are driven toward each other with a constant driving speed. At large separations they will approach one another with a uniform velocity, but at smaller separations the spring on which one of the surfaces is mounted will start to deflect as the surfaces begin to feel a force. There are two major contributions to the total force Ft. The first is the equilibrium surface force F,, and the second is a hydrodynamic force Fhdue to the viscosity q of the medium. The hydrodynamic force is given byI4
+
where Rh = [(1/2)(1/R1 lR2)]-’ is the harmonic mean of the two principal radii of curvature of the surfaces R, and R2 and RB= (R1R2)’/* is the geometric mean radius. The surface force is consequently given by
The force thus obtained between the curved surfaces is proportional to the free energy of interaction G between parallel, flat surfaces according to the Derjaguin approximation:I6
F, = 2aR,G
(3)
The above assumes that the relative velocity of the surfaces at all times is sufficiently low that equilibrium forces operate between the surfaces. Inertial and acceleration effects are also ignored. Chan and Horn found deviations in the hydrodynamic force of up to 10% from the theoretically expected at separations below 50 nm in nonpolar liquids of slightly higher viscosity than water.14 On the other hand, using a different method Israelachvili found no experimentally significant deviation from the bulk viscosity value for water between (hydrophilic) mica surfaces down to separations of 5 31m.l’ We have endeavored to keep the hydrodynamic force as small as possible in order to minimize the effects of any deviations from theory. This is most easily achieved by using a low driving speed (typically 0.5-2.5 nm/s). Experimentally, the surfaces are driven together with a ramp generator giving a linear voltage change with time across a piezoelectric crystal on which one of the surfaces is mounted. The mica surfaces are silvered on the back sides, and this gives rise to a series of fringes of equal chromatic orderl8 when white light is passed through the system. The fringe pattern together with the output from a digital timer is recorded with a video camera at the exit slit of the spectrometer and subsequently analyzed with a video micrometer (Colorado Video) to give the surface separation at any instant during the constant driving speed approach. The surface separation may be determined to 0.2-0.3 nm, and the time resolution is 0.02 s. Given the speed and separation of the surfaces, the hydrodynamic force is calculated from eq 1 and the surface force obtained by subtracting this from the total force (eq 2) as determined by the deflection of the spring.
Results Hydrocarbon Surfaces. The distance vs time plots (drainage curves) obtained when two mica surfaces coated with monolayers of dimethyldioctadecylammonium bromide (DDOA) are brought together with a constant driving speed are shown in Figure 1. The filled circles were measured with one surface mounted on a weak spring and the filled squares by using a very stiff spring (see figure caption). As can be seen, one observes an accelerated rate of (16) Derjaguin, B. V. Kolloid Zh. 1934, 69, 155. (17) Israelachvili, J. N. J . Colloid Interface Sci. 1986, 110, 263. (18) Israelachvili, J. N. J. Colloid Interface Sci. 1973, 44, 259.
1652 The Journal of Physical Chemistry, Vol. 92, No. 6, 1988 140 I
Claesson and Christenson
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Figure 1. Surface separation as a function of time (drainage run) as two mica surfaces coated with dimethyldioctadecylammonium (DDOA) ions are brought together with a constant driving speed in conductivity water. Filled circles show the results obtained when one of the surfaces is mounted on a comparatively floppy spring of spring constant k / R = 1.5 X lo4 N/m2 (normalized by the radius of curvature R of the surfaces). Under the influence of the hydrophobic attraction there is an increase in actual speed at separations below 70-80 nm. The solid line shows the curve expected in the absence of surface forces (with the same driving speed), in which case a noticeable slowing down of the surfaces due to viscosity effects occurs in the last 20 nm. The thick solid line shows the curve expected if a van der Waals attraction only (Hamaker constant = 2 X 1W20J) acted between the surfaces. The difference between this and the actual results provides a very graphic illustration of the range of the hydrophobic interaction. The filled squares are the results obtained with a stiff spring of k / R lo7 N/m2, in which case a deviation from the practically straight line relationship expected in the absence of surface forces (solid line) is noticeable only in the last 10 nm.
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Figure 3. Force (normalized by the radius of curvature of the surfaces) as a function of separation between two DDOA-coated surfaces in water.
Shown are forces obtained from the drainage run illustrated in Figure 1 (filled circles), from another drainage run with a slightly larger spring constant ( k / R = 2.2 X lo4 N/mZ, open circles), and forces measured with a conventional means (filled and open squares). The solid line shows the van der Waals attraction in the nonretarded approximation, calculated with a Hamaker constant of 2 X J (equal to that of mica across water).
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Figure 2. Hydrodynamic force (open triangles), total measured force
(open circles), and surface force (filled circles) as a function of surface separation calculated from the drainage run shown as the left-hand curve in Figure 1. There is an experimentally significant attraction coming in at about 70 nm. approach a t separations below 70 and 10 nm, respectively. The curves expected in the complete absence of surface forces (solid line) and by assuming that a van der Waals attraction only is operating (thick solid line) are indicated in the figure. On the scale of Figure 1 these two cases can hardly be distinguished for the run with the stiff spring. Obviously, the results can only be explained by the presence of an attractive force of much greater range and magnitude than the van der Waals force. Figure 2 shows the total force, the hydrodynamic force, and the surface force calculated from the filled circles of Figure 1. Figure 3 shows the attractive forces obtained by using the drainage method with two different spring constants (one of which is shown in Figures 1 and 2) together with the results from conventional force measurements. The agreement between the
Figure 4. Force as calculated from drainage runs between DDOA-coated surfaces in water on a semilogarithmic scale. The results of the two drainage runs of Figure 3 are shown with the same symbols. Also shown are results of a drainage run using a very stiff ( k / R 10' N/m2) spring
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(filled squares). The hydrophobic attraction appears to have a long-range exponential decay of 13 nm and a short-range decay of 3 nm. The solid line are the results of ref 8, obtained with similar surfaces but by using the gradient method (see Introduction) and subtracting a weak doublelayer repulsion. The inset shows the same values plotted on a log-log scale. The hydrophobic attraction roughly approximates a power law with an exponent of -2.3 in the range 3-40 nm but falls off more rapidly with distance beyond this. different methods certainly does not leave much to be desired, and the assumptions made in analyzing the drainage data appear to be vindicated. There is no sign of any repulsion, which indicates that any surface charge must be very low, probably less than one charge per 1000 nm2. There is an experimentally significant attraction coming in at about 70 nm. For comparison, the nonretarded van der Waals force calculated from Lifshitz theory is shown as a solid line. The Hamaker constant has been taken as that for mica across water (2 X J), and the curve gives an upper bound for the strength of the interaction. The results of
Forces between Uncharged Hydrophobic Surfaces
The Journal of Physical Chemistry, Vol. 92, No. 6, 1988 1653
D (nm) 40 60
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Figure 5. Slope of the force between two mica surfaces coated with the
fluorinated surfactant N-(a-trimethylammonioacetyl)0,O'-bis(lH,lH,2H,2H-perfluorodecyl) L-glutamate chloride in water, as measured by the gradient method (see text). The two points for each value of the gradient of the force represent lower and upper bounds for the onset of the instability. The two straight lines are least-squares fits to two separate sets of points. the two drainage runs are plotted on a semilogarithmic scale in Figure 4 together with the results obtained by using a stiff spring. The error is much larger in this case due to an uncertainty in the exact value of the spring constant and the difficulty in accurately determining the speed of the surfaces at small separations. It is also in this regime that any deviations in viscosity, boundary conditions for liquid flow, etc. might influence the results obtained from the drainage runs. At long range the hydrophobic attraction appears to decay exponentially with a decay constant of 13 nm, but at smaller separations the attraction starts to increase more rapidly and corresponds to an exponential decay of 2-3 nm. Alternatively, a power-law fit would give an exponent of -2.3 (see inset) over most of the range, although the force decreases more rapidly beyond about 40 nm. The solid line shows the fit to the experimental data obtained in previous measurements by using the gradient method with DDOA-coated surfaces carrying a weak surface charge.* The adhesion between the surfaces at contact was found to be rather variable from experiment to experiment (200-500 mN/m). The reason for this is not clear, but the long-range force was always independent of the variations in the adhesion. Fluorocarbon Surfaces. The gradient method was used to determine the slope of the attractive force between the fluorocarbon-coated surfaces. We found it very difficult to determine the exact onset of the instabilities, especially for the weaker spring constants. The results are given in Figure 5, where the two points for each value of (l/R)(dF/dD) indicate lower and upper limits for the jump positions. The points appear to lie on two straight lines on the semilogarithmic scale of Figure 5, although more points would be desirable to establish this with more certainty. Some of these results have been published in a recent review.1° A great number of drainage runs with different spring constants was carried out with the fluorocarbon surfaces. In principle it should be possible to use one run with a weak spring constant (giving maximum sensitivity) to extract the force down to contact. In practice, the difficulty of accurately determining the speed at small separations means that one has to use a stiffer spring which, because it responds less to the attractive force, gives a smaller net speed at a given separation. The weak, long-range attraction obtained from the drainage curves is shown in Figure 6 on a linear scale. Also shown are the results of one drainage run and conventional force measurements obtained in a different experiment with different surfaces. Once again the drainage method compares favorably with the other techniques. As with the hydrocarbon surfaces no detectable double-layer force is present and the attraction is experimentally significant at 80-90 nm from contact! The results of five runs with different spring constants are shown on a semilogarithmic plot in Figure 7 . The attraction appears to decay exponentially with a decay length of 16 nm and a preexponential factor of -2.2 at long range but becomes more
Figure 6. Force measured between two fluorocarbon surfactant coated
surfaces in water, plotted on a linear scale. Shown are results obtained from various drainage runs with k / R = 4.3 X lo3 N/m2 (open circles), k / R = 7.5 X lo3N/m2 (filled circles), and k / R = 3.7 X lo4 N/m2 (filled triangles). Also indicated are the results of one drainage run ( k / R = 1.0 X lo4 N/m2, crosses) and conventional force measurements ( k / R = 1.3 X lo4 N/m2, filled squares) from a different experiment with different surfaces. There is an experimentally significant attraction coming in at a separation of 80-90 nm.
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Figure 7. Force between fluorocarbon surfactant coated surfaces on a
semilogarithmic scale. The results of different drainage runs are shown, including those of Figure 5 , with k / R = 4.3 X 10' (open circles), k / R = 7.5 X lo3 (filled circles), k / R = 3.7 X lo4 (filled triangles), k / R = 2.8 X lo5 (open triangles), and k / R lo7 N/m2 (filled squares). As with the hydrocarbon surfactant the hydrophobic interaction appears to approximate an exponential with two decay lengths of 2-3 and 16 nm. The solid line is the result of an integration of the fit to the long-range results presented in Figure 5 (decay length of 9 nm). The inset shows the drainage results on a log-log scale. In the range 5-50 nm the hydrophobic attraction can be approximated by a power law with exponent
-
of -2.1.
attractive below about 20 nm. From about 6 to 50 nm the attraction looks somewhat like a power law with an exponent of -2.1 (see inset). As with the hydrocarbon surfaces the attractive force decays more rapidly a t larger separations. Alternatively, a short-range exponential attraction with a decay length of 2-3 nm, when added to the long-range exponential, would give a reasonable fit to the experimental data. The solid line in Figure 7 shows the result of integrating at least-squares fit to the four points with the weaker spring constants in Figure 5 . As can be seen, the results do not agree very well with the results of the drainage measurements and the conventional
1654 The Journal of Physical Chemistry, Vol. 92, No. 6, 1988
force measurements. The decay length is 9 nm and the preexponential factor -6.4 mN/m. We believe that the rather poor agreement is due to the difficulty of accurately determining the jump position for a weakly decaying attraction and the absence of data at larger separations (the present force-measuring spring cannot be made sufficiently weak). The adhesion was in this case also variable but was in the range 200-300 mN/m, which, depending on how one treats the occurrence of surface deformations,1+21corresponds to an interfacial energy of 15-30 mJ/m2 for the fluorocarbon-water interface. This is substantially less than one might expect (56 mJ/m2 for a perfluorohexane-water interface22). This may be related to the fact that numerous, discrete vapor cavities form between the surfaces close to contact and prevent the surfaces from coming into molecular contact because water is trapped between the ca~ities.’~,’~
Discussion It appears that every new experimental study of the hydrophobic interaction increases the exponential decay length that fits the measured forces. The decay length has gone from 1.O nm6 to 1.4 nm7 and then 5.5 nm.8 We are now faced with the somewhat embarrassing task of justifying an increase in the long-range decay to about 15 nm. However, we believe that our results speak for themselves to a large extent. There can be no mistake concerning the presence, range, and magnitude of the attraction, given the number of ways in which we have reproducibly measured these forces. Also, the range of the interaction we are observing tallies with that estimated by Blake and Kitchener5 and, more importantly perhaps, with the results of recent experiments of Derjaguin and R a b i n o ~ i c h .They ~ have measured a hydrophobic attraction M KCI between methylated silica surfaces (6, = looo) in extending to a surface separation of 60 nm with a long-range exponential decay length of 12-13 nm. The rather large difference compared to earlier measurements8 on the same system (DDOA) warrant some comment. One clear difference is that in this case the surfaces carry a much smaller charge and there is no detectable double-layer force. We thus avoid the uncertainty of subtracting this repulsion, which would have swamped a weak hydrophobic attraction. We note that the deposition pressure used here (25 mN/m) is lower than that employed in the earlier study (35 mN/m). In a recent investigation of the interaction of one hydrophobic surface deposited at this higher pressure and one mica surface, it was shown that the charge on the hydrophobic surfaces is positive.23 Apparently, the surface charge is very sensitive to the exact deposition conditions and at high surface pressures the density of surfactant molecules is sufficient to give a positively charged surface. Our results show clearly that the hydrophobic interaction does not decay as a single exponential but that an exponential function with two decay lengths provides a good fit over the entire range, possibly including the adhesion at contact. Alternatively, it is possible to postulate a power-law dependence with an exponent of -2.1, to -2.3, but it is then necessary to let the exponent increase in magnitude at very large separations, above 40-50 nm. This is similar in effect to the retardation of van der Waals forces. It is clear that we at present have no theoretical basis for choosing one particular functional form over another to fit the measured data. The “double” exponential does, however, give a simple form. A rather surprising result to emerge from our measurements is that the attraction is of similar range and magnitude for both the hydrocarbon and the fluorocarbon surfaces. Although the receding contact angles of water on the two surfaces are similar, the advancing contact angles are quite different, 93’ and 113O, (19) Israelachvili, J. N.; Perez, E.; Tandon, R. K. J. Colloid Interface Sci.
-.
19110. 78. -260. ---, --
(20) Muller, V. M.; Yushchenko, V. S.;Derjaguin, B. V. J . Colloid Interface Sci.1983, 92,92. (21) Christenson, H. K. J. Colloid Interface Sci.,in press. (22) Handa, T.; Mukerjee, P. J . Phys.Chem.1981, 85,3916. (23) Claesson, P. M.; Herder, P. C.; Blom, C. E.; Ninham, B. W. J. Colloid InterfaceSci.1987, 118,68.
Claesson and Christenson respectively, and the results seem to indicate a rather weak dependence on surface hydrophobicity as measured by the advancing contact angle. If we look at previous measurements of the hydrophobic attraction and attempt a correlation with the advancing contact angle of water (6,) on the surfaces, we find a rather bewildering picture. Dihexadecyldimethylammonium acetate (DHDAA) with 6, = 9 4 7 shows a comparatively short-range hydrophobic interaction. The early work with CTAB has recently been shown to be misleading due to use of a contaminated sample,24and 8, on purified CTAB is 95’ as opposed to the earlier quoted 64°.6 Recent force measurements by usz5show a stronger attraction between CTAB monolayers than previously obtained, although substantially smaller than for the DDOA surfaces considered in this study, even though 6, is the same. The contact angle, however, is a relation between the surface energies of macroscopic surfaces. These are contact energies and it is not obvious that there should be any simple relation to the range of the hydrophobic attraction. If we for a moment disregard the exact values of the contact angles and look at some other aspects of these results, some trends do start to emerge. Firstly, the hydrophobic surfaces giving the most long-range attraction are all composed of double-chain surfactants deposited as Langmuir-Blodgett films. Also, these surfaces either are almost uncharged, giving no detectable double-layer repulsion (as in this study), or carry only a very slight surface charge, as with previous experiments with DDOA.8 By contrast, the surfactants showing weak attractive forces have all been adsorbed from solution. This might well create a less ordered and less tightly packed surface than that obtained by Langmuir-Blodgett deposition. DHDAA, for example, was quoted as having an area per head group of 0.66 nm27-considerably larger than the 0.50-0.55 nm2 found with DDOA. There may, however, be another way in which the method of preparing the surfaces affects the strength of the interaction apart from any influence of chain packing. If is almost impossible to be. certain that adsorption from solution is limited to the formation of an adsorbed monolayer. A weakly adsorbed second layer of low density cannot be ruled out. It has, in fact, been found that bilayer formation does eventually occur with DHDAA at the same concentration as that used in measurements of the hydrophobic interaction.’ If we accept that adsorption from solution gives a strongly adsorbed hydrophobic monolayer as well as some tendency toward bilayer formation, the quoted results make sense when examined together. The measured hydrophobic interaction is of significantly shorter range for surfaces of CTAB and DHDAA than for the Langmuir-Blodgett monolayers, where there is no possibility of additional surfactant adsorption. Because the second layer of surfactant molecules is easily pressed out, the adhesion at contact is large and similar to what is measured between the Langmuir-Blodgett surfaces. Let us now briefly consider the key question of the origin of the attraction. Firstly, the hydrophobic interaction cannot be an electric double layer effect-the distance dependence does not agree with the Poisson-Boltzmann theory for dissimilpr surfaces.26 It is also highly improbable that the two identically prepared surfaces would be of opposite charge in each and every experiment. It has been shown t h e ~ r e t i c a l l y ~that ~ - ~a~refined treatment of the electrostatic fluctuation interactions between uncharged surfaces with adsorbed ions (including ion size, image charge effects, ion-ion correlations, and dispersion interactions) cannot explain the hydrophobic attraction. The additional attraction in the range of interest here ( > l o nm) resulting from such effects (24) Pashley, R. M.; McGuiggan, P. M.; Horn, R. G.; Ninham, B. W. J . Colloid Interface Sci., in press. (25) Claesson, P. M.; Christenson, H. K. unpublished results. (26) Devereux, 0. F.; de Bmyn, P. L. Interaction of Plane-ParallelDouble Layers;MIT Press: Cambridge, MA, 1963. (27) Kjellander, R.; Marcelja, S. Chem.Scr.1985, 25, 112. (28) Attard, P.; Kjellander, R.; Mitchell, D. J. Chem.Phys.Lett.1987, 139, 219. (29) Attard, P.; Kjellander, R.; Mitchell, D. J.; Jonsson, B., to be published.
J. Phys. Chem. 1988, 92, 1655-1664 corresponds to an effective Hamaker constant of 4 X J, compared to approximately 6 X J for hydrocarbon across water and 2 X J for mica across water. That charge fluctuations of neutral surfaces are an unlikely candidate is supported experimentally by the numerous cases where no long-range attraction is observed between uncharged or weakly charged surfaces that are not h y d r o p h o b i ~ . ~ @The ~ ~ above notwithstanding we have recently obtained some indications that an increased ionic strength decreases the range of the hydrophobic interaction measured between surfactant monolayer surfaces. This point, which is complicated by the fact that we also observe an increased charge density with increasing electrolyte concentration, will be addressed in a forthcoming publication. No hydrophobic attraction acts between surfaces composed of mixtures of hydrophobic and hydrophilic groups. Examples include surfaces of ethylene oxide groups3' and surfaces with adsorbed tetraalkylammonium ions.3z This indicates that a longrange hydrophobic attraction exists only between homogeneous hydrophobic areas larger than some critical size. For surfaces with 0, > 90° the range and magnitude of the attraction are rather insensitive to the exact value of the contact angle, provided the surfaces have been prepared under identical conditions. This suggests strongly that the hydrophobic interaction is related to the metastability of water films between hydrophobic surfaces."-13,33 The system is close to a liquid-vapor phase transition,
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and small density fluctuations might well lead to an attraction of sufficient magnitude to explain the results. If this interpretation is correct, the important parameter is the difference between the surface energy of the solid against the vapor and against the liquid. For an isolated, macroscopic surface this difference is related to the contact angle but the proximity of the two hydrophobic surfaces may cause a change in the relative magnitudes of the surface energies. In other words, the difference between stability and metastability in such thin films is not necessarily directly related to a macroscopic contact angle of 90'. An alternative and often invoked explanation is that the hydrophobic attraction is due to water structure. This appears less and less likely as the range and decay of the attraction is increased by more accurate measurements. We are observing an attraction at a separation in excess of 300 molecular diameters! Given the present state of theories, it seems as if the best recourse is to push on with experiments. Which we will do. Acknowledgment. We acknowledge helpful advice from R. G. Horn. We are indebted to B. W. Ninham for many valuable discussions and to V. A. Parsegian for stimulating correspondence. We are grateful to our other colleagues in the Department of Applied Mathematics for their interest and support of this work. P.M.C. acknowledges a travel grant from the Swedish Board for Technical Development (STU). Registry No. DDOA, 3700-67-2; N-(a-trimethylammonioacety1)O,O'-bis(lH,lH,2H,2H-perfluorodecyl)-~-glutamatechloride, 88185-
(30) Marra, J.; Israelachvili, J. Biochemistry 1985, 24, 4068. (31) Claesson, P. M.; Kjellander, R.; Stenius, P.; Christenson, H. K. J . Chem. Soc., Faraday Trans. I 1986, 82, 2735. (32) Claesson, P. M.; Horn, R. G.; Pashley, R. M. J. Colloid InferfaceSci. 1984, 100, 250.
38-0. (33) Yushchenko, V. S.;Yaminsky, V. V.; Shchukin, E. D. J . Colloid Interface Sei. 1983, 96, 307.
Binding Energies for AI Atom Association Complexes with Simple Alkenes and Arenest S. A. Mitchell,* B. Simard, D. M. Rayner, and P. A. Hackett* Laser Chemistry Group, Division of Chemistry, National Research Council of Canada, 100 Sussex Drive, Ottawa, Canada, K I A OR6 (Received: July 13, 1987)
A1 atom association reactions with simple alkenes and arenes in the gas phase are investigated by time-resolved resonance fluorescence excitation of ground-state A1 atoms following pulsed visible laser photolysis of trimethylaluminum in a gas cell. Ar buffer gas pressure effects on the reaction rates are observed and interpreted in terms of collision-complex lifetimes in termolecular reactions. The limiting high Ar pressure bimolecular rate constants are near the gas kinetic values, implying negligible activation energies and large Arrhenius preexponential factors for these reactions. For reactions involving truns-2-butene, tetramethylethylene, benzene, toluene, and o-xylene, an equilibration is observed between free AI atoms and A1 atoms bound in complexes with the reactant molecules. Equilibrium constants for the association reactions are obtained from an analysis of kinetic data at different pressures of reactant. Binding energies are derived from observations of the temperature dependence of the equilibrium constant in the range 283-333 K, or by estimating the standard entropy change for the association reaction. Evidence is presented which indicates that monoligand complexes are formed in all cases. A1 atom binding energies (kcalmol-I) are reported for C2H2(>13), CzH4(>16), 1-butene (>15), trans-2-butene (14.2 f l), tetramethylethylene (13.5 f l), 1,4-cyclohexadiene (>14), benzene (11.7 A l), toluene (14.1 & l), and o-xylene (14.3 A 1). Kinetic and thermochemical results are discussed in terms of alternate bonding schemes for AI-alkene and Al-arene complexes.
Introduction Doublet spin multiplicity, monoethylene complexes of Al(3s23p1) atoms have recently been prepared in low-temperature rare gas' and hydrocarbon2 matrix supports and studied by ESR spectroscopy. According to the analysis of Kasai,' the unpaired electron resides mainly in a p-orbital of A1 but is delocalized onto the CzH4 molecule because of a bonding interaction between the A1 p-orbital and the **-antibonding molecular orbital of CzH4. +Issued as NRCC No. 28176.
0022-3654/88/2092-1655$01.50/0
It was suggested that this bonding interaction accounts for the stability of the Al[C2H,] adduct, which was characterized as a a-complex with the A1 atom bound symmetrically below the C2H4 molecular plane. Alternative bonding schemes, involving formation of a charge-transfer complex, Al+[C2H4]-, a AI-C a-bonded radical, AI-CH2-CHz, or an aluminocyclopropane (1) Kasai, P. H. J. Am. Chem. SOC.1982, 104, 1165. (2) Howard, J. A.; Mile, B.; Tse, J. S.;Morris, H. J . Chem. Soc., Da[fon Trans. 1987, 83, 3701.
Published 1988 by the American Chemical Society
