J. Phys. Chem. 1989, 93, 1472-1478
1472
Forces between Fluorocarbon Surfactant Monolayers: Salt Effects on the Hydrophobic Interaction H. K. Christenson,* Department of Applied Mathematics. Research School of Physical Sciences, Australian National University, Canberra, A.C.T. 2601, Australia
P. M. Claesson, J. Berg, and P. C. Herder Department of Physical Chemistry, Royal Institute of Technology, S-100 44 Stockholm, and Institute for Surface Chemistry, Box 5607, S - 114 86 Stockholm, Sweden (Received: March 31, 1988; In Final Form: July 11, 1988)
Further studies of the hydrophobic attraction between fluorocarbon surfaces in water have been carried out, including (i) a comparison between surfaces prepared by Langmuir-Blodgett deposition and by adsorption from solution using the same surfactant and (ii) the effect of added electrolyte. In general, the surfaces prepared by deposition show the longest range of the attraction-measurable at 80-90 nm with an exponential decay length of about 15 nm at separations above 25 nm. In the case of adsorbed monolayers incomplete adsorption or additional adsorption to the hydrophobic monolayer surfaces often leads to an effective attraction that is of considerably shorter range. Under optimal conditions, however, adsorbed monolayers give an attraction of a similar range. In both c a w the interaction becomes more attractive at small separations and appears to decay exponentially with a decay length of 2-3 nm below about 15 nm. The deposited fluorocarbon monolayers are unstable in sodium or potassium salt solutions but remain intact in tetrapentylammonium bromide. With increasing concentration of this salt the range of the hydrophobic interaction decreases as more and more ions adsorb to the surfaces. The short-range interaction seems to be less sensitive to the presence of electrolyte-the adhesion at contact shows a much less dramatic decrease with increasing electrolyte concentration. Nevertheless, the measured electrolyte dependence of the hydrophobic interaction is at least partly due to the surfaces becoming less hydrophobic themselves, as reflected by their cavitation behavior.
Introduction The presence of a long-range attractive interaction between hydrophobic surfaces in water has only recently been convincingly demonstrated experimentally. There are some observations'** predating the advent of accurate direct force measurements, but in the wake of the polywater debacle the existence of long-range interactions between surfaces in water has not been a popular concept. This is especially so because no satisfactory theoretical interpretation of the hydrophobic attraction between macroscopic surfaces has been put forth. The hydrophobic effect is, however, a commonly accepted term for the unusual properties of aqueous solutions of nonpolar molecules which, among other things, provide the driving force for amphiphile aggregati01-1.~~~ The large positive free energy of transfer from bulk to water of such species results mainly from an unfavorable entropy term. Small, nonpolar solute molecules induce the formation of surrounding, cagelike dynamic structures or "clathrates" of water. Nonpolar molecules in water attract each other because the net ordering effect around a pair in close proximity is smaller than when they are separated. This hydrophobic interaction between solute molecules is of much greater magnitude than the van der Waals interaction. At a macroscopic hydrophobic surface no clathrate formation is possible and hydrogen bonds in the water must be broken adjacent to the surface.5 Over a sufficiently small temperature range (close to rmm temperature) the free energy of transfer from bulk to water of nonpolar species increases with temperature (because of the unfavorable entropy term), while the energy required to create a hydrophobic surface in water decreases (the entropy change is here favorable). In other words, the energetics (1) Laskowski, J.; Kitchener, J. A. J. Colloid hrerface Sci. 1969, 29,670. (2) Blake, T. D.; Kitchener, J. A. J. Chem. SOC.,Faraday Trans. I 1972, 68, 1435. (3) Tanford, C. The Hydrophobic Effecr, 2nd ed.; Wiley: New York, 1980. (4) Tanford, C. Proc. Indian Acad. Sci. (Chem. Sci.) 1987, 98, 343. ( 5 ) Lee, C. Y.; McCammon, J. A.; Rossky, P. J. J. Chem. Phys. 1984,80, 4448.
0022-365418912093-1472$01SO10
of a hydrophobic surface and a hydrophobic solute in water are quite different. In a series of recent experiments we have shown that the first measurements of the hydrophobic interaction between macrampic surfaces were in fact gross underestimates of the true range of the forces involved. Those first experiments were performed with monolayers adsorbed from solution and appeared to show an exponential attraction with a decay length of 1 nm6 or 1.5 nm,' with the force measurable at separations of up to 15 nm. Later measurements with weakly charged surfaces prepared by Langmuir-Blodgett deposition of surfactants showed that the hydrophobic attraction is not a simple exponential function of surface separation but shows two separate decay lengths- a short-range one of 1.2 nm and a long-range decay of 5.5 nm.* We have recently performed experiments with uncharged Langmuir-Blodgett films of both hydrocarbon and fluorocarbon surfactants and have obtained some quite startling result^.^*'^ The attraction was measurable out to separations of 70-90 nm, as opposed to 5-10 nm for a conventional van der Waals force. It could be fitted reasonably well with an exponential function with two decay lengths, one short of 2-3 nm and one long of 13 (hydrocarbon surfaces) or 16 nm (fluorocarbon surfaces). We do not claim any theoretical justification for the exponential forms-they merely describe the interaction quite accurately. Very similar results have been obtained by Derjaguin and Rabinovich for the interaction of hydrophobed silica surfaces in water." A complication in the interaction of macroscopic hydrophobic surfaces across water is the cavitation that occurs at small surface separations. For surfaces having contact angles of water above (6) israelachvili, J. N.; Pashley, R. M. J. Colloid Inrerface Sci. 1984, 98, 500. (7) Pashley, R. M.; McGuiggan, P. M.; Ninham, B. W.; Evans, D. F. Science (Washington, D.C.) 1985, 229, 1088. (8) Claesson, P. M.; Blom, C. E.; Herder, P. C.; Ninham, B. W. J. Colloid Interface Sci. 1986, 114, 234. (9) Christenson, H. K.; Claesson, P. M.; Pashley, R. M. Proc. Indian Acad. Sci. (Chem. Sci.) 1987, 98, 379. (IO) Claesson, P. M.; Christenson, H. K. J. Phys. Chem. 1988, 92, 1650. (1 1) Rabinovich, Ya. I.; Derjaguin, B. V. Colloids Sur/. 1988, 30, 243.
0 1989 American Chemical Society
Fluorocarbon Surfactant Monolayers 90° a vapor cavity bridging the surfaces at small separations is the thermodynamically stable state, as considered by Yushchenko et al.I2 Between hydrocarbon surfaces having advancing contact angles (e,) of 93-95O cavitation occurs only after the surfaces are separated from molecular c o n t a ~ t . ~Presumably ~ ~ * ~ ~ the separation process provides the necessary activation energy for cavity formation. With the more hydrophobic fluorocarbon surfaces (e, = 113O), however, cavities form spontaneously as soon as the surfaces come into ~ o n t a c t . ~ It * ’follows ~ from the above that the long-range attraction measured between these surfaces is, strictly speaking, a nonequilibrium force, although it is reproducible. At finite separations the energy barrier toward cavity formation is prohibitively large. As expected, the force between the surfaces in the presence of a cavity is of longer range but has a slower decay. At contact they are the same. In the present paper we have attempted to shed further light on the nature of hydrophobic forces between macroscopic surfaces. In view of the very large differences found between our measurements using Langmuir-Blodgett films and previous results with monolayers adsorbed from solution we have carried out a comparative study. We have prepared surfaces using both methods with the same surfactant, a fluorinated, double-chain quarternary ammonium compound. The results show that the very long-range attraction that occurs between the Langmuir-Blodgett films may be equalled by that between adsorbed layers under conditions that may, however, be difficult to achieve in practice. Previous information on the salt dependence of the hydrophobic attraction between surfaces has been somewhat contradictory,&8 and we have now studied the effects of different electrolytes up to M on the interaction measured between these fluorocarbon surfaces. It turned out that the monolayer surfaces were in many cases unstable in salt solution, as we could confirm with contact-angle measurements. Only in electrolytes with very large cations such as tetrapentylammonium ions were the deposited monolayers reasonably stable, although considerable ion adsorption at the hydrophobic surfaces occurred. We hope, however, that our results will finally put to rest any claims that the hydrophobic interaction is a double-layer attraction.
Materials and Methods The hydrophobic surfaces were prepared by using the double-chain cationic surfactant N-(a-(trimethy1ammonio)acetyl)0,O’-bis-( 1H , 1H,2H,2H-perfluorodecyl)-~-glutamate chloride, henceforth referred to as “fluorocarbon surfactant”. This was obtained from Sogo Ltd., Japan, and used as received. The substrate surface, muscovite mica, was purchased from Brown Mica Co., Sydney. The potassium bromide and sodium chloride were analytical reagents from May & Baker and BDH Chemicals, respectively. The tetrapentylammonium bromide was purum grade from Fluka AG, the tetramethylammonium bromide from Merck, and the tetrapropylammonium bromide from Eastman Kodak. All salts were used as received. The water was purified either with an Elga UHQ unit fed with deionized and distilled water (bypassing the reverse osmosis cartridge) or by a MilliQ unit.’ The filters in the Millipore unit were replaced by surfactant-free 0.2-wm Zetapore filters. The water used in the surface force experiments was deaerated for several hours with a water-jet pump, usually overnight. The fluorocarbon-surfactant-coated surfaces were prepared either by Langmuir-Blodgett (LB) deposition or by adsorption from solution. The LB deposition, onto mica substrate surfaces glued to silica disks, was carried out at a constant surface pressure of 20 mN/m in an all-Teflon trough. The area per molecule at the air-water interface and the mica surface was about 0.6 nmz.l4 Aqueous fluorocarbon surfactant solutions of a concentration of M were prepared by sonication for about 20 min. This (12) Yushchenko, V. S.; Yaminsky, V. V.; Schchukin, E. D. J . Colloid Interface Sci. 1983, 96, 307. (13) Christenson, H. K.; Claesson, P. M. Science (Washington, D.C.)1988, 229, 390. (14) Herder, P. C.; Berg, J.; Claesson, P. M., Christenson, H. K., to be published.
The Journal of Physical Chemistry, Vol. 93, No. 4, 1989 1473 1201
Concentration ( M I
Figure 1. Advancing and receding contact angles of a range of salt solutions on fluorocarbon-surfactant-coatedmica. The surfaces were prepared by Langmuir-Blodgett deposition at a surface pressure of 20 mN/m. For KBr the advancing and receding contact angle is represented by A and A, respectively. The corresponding symbols are for tetramethylammonium bromide 0 and 0, for tetrapropylammonium bromide and 0,and for tetrapentylammonium bromide and 0 .
+
cloudy solution containing single-walled and multiwalled vesicle^'^ was then injected into the surface force apparatus to give the appropriate surfactant concentration. Forces between the fluorocarbon surfaces were measured with a surface force apparatus.16 Briefly, the surfaces are mounted as crossed cylinders, and the surface separation is determined to within 0.1-0.2 nm with multiple-beam interferometry. The force N from the deflection is determined with a detection limit of of a double-cantilever spring on which one of the surfaces is mounted. The measured force F, normalized by the geometric mean radius of the surfaces R is proportional to the free energy of interaction of flat surfaces Gr according to F,/R = 27Gf
(1)
Strongly attractive forces were measured by using the gradient method,17 based on determining the separation at which the gradient of the force law dF,/dD equals the spring constant. At this point the mechanical spring system encounters an instability, and the surface jumps to the next stable position of the force curve. The precision involved in these measurements is less than when measuring the force directly, particularly when the second derivative of the force is small. The gradient of the force between crossed cylinders is related to the pressure between flats Pf by dF,/dD = -27Pf (2) Contact angles of electrolyte solutions on fluorocarbon surfaces prepared by LB deposition on freshly cleaved mica surfaces were measured with a goniometer. All measurements were carried out at room temperature.
Results Contact-Angle Measurements. It was found that both the advancing (e,) and receding (e,) contact angles decreased with concentration for all studied electrolytes (Figure 1). The effect was most dramatic with salts like potassium bromide, where the advancing contact angle fell from 113 O (e,) in pure water to about 90’ at lo4 M (KBr). With increasing concentration of electrolyte the surfaces became gradually less hydrophobic. For instance, the first time the contact angle with 0.1 M KBr was determined, values of 42O (e,) and 20’ (e,) were obtained. Subsequent measurements on the same spot values of = l o o and Z O O , about the same as for bare mica. By dipping the whole surface into pure water, one could establish that this spot had turned permanently hydrophilic while the rest of the surface remained hydrophobic. (15) Kunitake, T.; Okahata, Y.; Yasunami, S . J . Am. Chem. SOC.1982, 104, 5547. (16) Israelachvili, J. N.; Adams, G. E. J . Chem. SOC., Faraday Trans. I 1918, 74, 975. (17) Horn, R. G.; Israelachvili, J. N. J . Chem. Phys. 1981, 75, 1400.
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The Journal of Physicul Chemistry. Vol. 93, No. 4, 1989
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Figure 2. Normalized farce between fluorocarbonsurfaces prepared by Langmuir-Blcdgett deposition in a range of tetrapentylammonium bromide solutions as a function of surface separation. The surface sep aration is calculated from monolayer contact at D = 0, which is 4 nm (2 nm per surface)out from bare mica contact. The symbols represent forces in the following solutions: 0,conductivity of water; A, 4 x 10” M;., 1.5 X IO4 M, 0, IO-’ M, 0 , I0-l M. The solid lines are force curves calculated from DLVO theory, assuming identical surfaces,and an additional exponential attraction. The parameters used far these calculations are given in Table I!. J represents the psitian from which the attractive force pulls the surfaces into contact; the jump position. 120 100
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Figure 3. Normalized farce between fluorocarbon-surfactant-coated surfaces in 1.5 X IO4 M Pe,NBr solutions. The surface separation is calculated from monolayer contact at D = 0, which is 4 nm out from bare mica contact. The shaded areas represent forces calculated from DLVO theory assuming dissimilar surfaces interacting at constant potential. The upper and lower boundary of each region is calculated by setting the nonretarded Hamaker constant to 0 and 2 X IO-” J (the value for mica-watermica). respectively. J marks the jump position. Apparently, the potassium ions exchange with the cationic fluorocarbon surfactant molecules on the negatively charged mica surface. The same effect occurs for deposited dimethyldioctadecylammonium ions on mica? The reduction in contact angle observed with the tetraalkylammonium salts is smaller and decreases with increasing size of the ion (Figure 1). With tetrapentylammonium bromide the advancing contact angle remains larger than 90° even a t 0.1 M. The relative stability of fluorocarbon surfaces in tetrapentylammonium bromide was the reason we chose to study electrolyte effects with this salt. Surface Forces. Lnngmuir-Blodgeff Deposition. The interactions measured between fluorocarbon-coated mica surfaces immersed in a range of tetrapentylammonium bromide (Pe4NBr) solutions are summarized in Figure 2 and shown in more detail for two concentrations in Figures 3 and 4. The surface separation is calculated with respect to monolayer contact, which should be approximately 4 nm outside mica-mica contact. A purely attractive force, measurable a t separations of 80-90 nm, was observed in conductivity water, as reported previously in ref 10. This attraction decays exponentially with a decay length of about 16
a
Figure 4. Normalized force between fluorocarbon-surfactant-ooated surfaces in lWzM Pe,NBr solutions. The surfaceseparation is calculated from monolayer contact at D = 0, which is 4 nm out from bare mica contact. The shaded areas represent forces calculated from DLVO theory assuming dissimilar surfaces interacting at constant potential. The upper and lower boundary of each region is calculated by setting the nanretarded Hamaker constant to 0 and 2 X IW’O J, respectively. The surface potential combinations used for calculating the double-layer farces are ZSS/lS mV (thc upper shaded area) and 255/10 mV (the lower shaded area). No agreement between DLVO theory and experiments is found. J indicates the measured jump position. At smaller separations the measured force is strongly attractive. Note the semndary minimum between 20 and 40 nm, which is not predicted by theory. nm a t separations beyond about 25 nm. At smaller separations the attraction becomes steeper and appears exponential with a decay length of 2-3 nm below 15 nm.lo The absence of a double-layer repulsion indicates that the surface must be virtually uncharged. By contrast, a repulsive interaction was always present after addition of Pe,NBr. In IO4 M Pe,NBr the force was repulsive down to 30 nm, a t which point a force maximum occurred, and the force became attractive a t slightly smaller separations (see Figures 2 and 3). On increasing the concentration to M the repulsive force maximum was shifted to about 25 nm. Closer in the force was strongly attractive. By M Pe,NBr (Figures 2 and 4) a secondary minimum was observed a t a separation of 25 nm, whereas the force maximum had shifted inward to 9 nm. The solution was subsequently diluted in situ by a factor of about 200, upon which the range of the repulsion increased and the force maximum shifted out to a separation of 40 nm, greater than that at 1P M. For all concentrations the exact magnitude and p i t i o n of the repulsive maximum was found to vary slightly between subsequent force measurements even a t the same contact pasition. The cavitation behavior’l of the surfaces changed with electrolyte concentration. In the more dilute solutions cavitation occurred spontaneously as the surfaces came into contact. At 10-l M, however, detectable cavities formed only after the process of subsequent separation from contact was started. As the remote end of the force-measuring spring was moved to effectuate separation, large, annular cavities formed around the contact zone. On dilution of the concentrated solution, cavity formation once more occurred as soon as the surfaces came into contact. The adhesion at contact between the fluorocarbon surfaces decreased slightly with electrolyte concentration, from about 250 mNJm in dilute solutions to 180 mNJm at IF2M. The measured adhesion is consistent with previous results with similar surfamlo but lower than expected from a comparison with the interfacial energy of fluorocarbon-water interfaces,18 which predicts values (18) Handa, T.: Mukerjee. P. J. Phys. Chem. 1981.85, 3916
Fluorocarbon Surfactant Monolayers
The Journal of Physical Chemistry, Val. 93. No. 4, 1989 1475
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Figure 5. Comparison of the attractive force measured between differently prepared, uncharged surfaces immersed in water. The main figure illustrates the slope of the force, plotted on a logarithmic scale, as a function of surface separation. D = 0 is at bare mica contact. The inset shows the force at large distances on a linear scale. The solid line reprcscnts the upper limit of the van der Waals farce. and e represents the attraction between surfaces coated with a 0.4-nmthin layer adsorbed from solution, 0 a 2-nm-thick layer adsorbed from solution, and a 2-nm-thick layer deposited by the Langmuir-Blodgett technique. The nonretarded van der Waals force for the case of a Hamaker constant bctween 0.35 X IFmand 2.2 X IFmJ is illustrated by the shaded region. These values represent fluorocarbonx and mica, respectively, interacting across water.
in the range 5o(t600 mN/m. By wntrast, the adhesion measured between hydrocarbon surfaces has been shown to he close to the theoretically expected values.',8 We have no satisfactory explanation for the low adhesion values other than that the formation of often numerous vapor cavities around the contact zone makes a precise analysis of the adhesion problem difficult. The fluorocarbon monolayers were not stable in sodium chloride or potassium bromide solutions. After addition of these salts, irreproducible, overall repulsive forces were invariably measured. These showed one or two "steps" or "oscillations" of 4-5 nm a t short range. We believe that this reflects the formation of bilayers or multilayers on the surfaces. Such multilayer formation is made possible by exchange of the adsorbed fluorocarbon surfactant ions for sodium or potassium and assisted by the cavity formation, which provides an avenue for transport and piling up of surfactant layers as cavities first form and then dissipate on separation of the surfaces. Surface Forces. Adsarptionfrom Solution. The adsorption behavior of surfactant from the sonicated solutions was found to be rather involved. The adsorbed amount and the measured interaction depend not only on the hulk wncentration of surfactant, but also on the adsorption rate as regulated by the mixing procedure and the surface separation during the adsorption process. In these experiments the zero of separation was taken as micamica contact and the actual fluorocarbon-water interface was consequently found at varying distances. When the solution was thoroughly mixed and the surfaces kept far apart ( > I mm) a 1.9-nm-thick monolayer formed on each surface within 2 h after injection to IO4 M. No repulsion was measured, and the force became attractive a t distances below 50-60 nm. This is illustrated in Figure 5 (open squares), which shows the gradient of the force on a logarithmic scale. As with the LB surfaces the force becomes more attractive a t small separations and, if exponential at short range (below 15 nm), would give a decay length of 2-3 nm. The measured adhesion was 280-320 mN/m. One day after injection a weak repulsion had appeared with a force maximum at about 30 nm and an attraction a t smaller separations (not shown). The surfaces did not jump directly into contact from the position of the instability close to the force
Figure 6. Normalized force as a function of surface separation in fluorocarban surfactant solutions (prepared by sonication). The slow adsorption situation is illustrated. 0 represents farces measured after 2 h of equilibration m a IO4 M solution. The double-layer force and the van der Waals force (solid line) were well described by assuming interaction at constant potential (120 mV), a Debye length of 75 nm, and a nonrelarded Hamaker constant of 2.2 X J. No hydrophobic attraction was observed at this stage. After 24 h the total wncentration was increased to 2 X IO4 M. The forces measured after a further equilibration for 24 and IW h are represented by A and e, respectively. The solid line represent DLVO forces calculated by assuming constant charge canditions. The Debye length was 30 nm in bath cases, whereas the surface potential on isolated surfaces was 16 mV after 24 hand I00 mV after 100 h. To account for the strong attraction observed after 100 h, an exponential attraction (FIR(mN/m) = -8 exp[-D (nm)/7]) had to be added to the DLVO force CUNC (dashed line). J indicates jump positions. The final eontact was located 0.8 nm out from bare mica contact at D = 0, except after 100 h equilibration when the final wntact was at D = 4 nm
maximum hut were instead observed to stop briefly, for a second or two, at a separation of 4 nm from final monolayer contact. We interpret this as evidence for the partial formation of a second layer of surfactant molecules on the hydrophobic monolayers. No further changes in the nature of the interaction were observed over an additional 2-day period. When the surfaces were kept clme together (=1-2 r m ) during the adsorption process, the measured forces were initially quite different. The thickness of the adsorbed layer 2 h after injection to lo4 M was only 0.4 nm per surface. The repulsive doublelayer interaction measured in pure water before addition of surfactant still dominated the force curve, but the magnitude of the repulsion had decreased, indicating partial neutralization of the surface. The force maximum at 6 nm from bare mica-mica contact was 1.1 mN/m (Figure 6, open circles). With time the force barrier decreased in magnitude and shifted to larger separations, and 1 day after injection the repulsion had disappeared completely and an attraction was measurable below 25 nm (Figure 5, inset). The thickness of the adsorbed layer remained at 0.4 nm throughout, but the adhesion force increased slightly from 60 to 80 mN/m. Similar results were obtained when the surfaces were kept far apart with a bulk surfactant concentration of IO-! M, except that even after a day the double-layer repulsion had not vanished completely. After an adsorption time of 1 day a t lo4 M the surfactant concentration was doubled by injection to 2 X IO4 M. The forces measured after subsequent adsorption times of a further 24 and 100 h are illustrated in Figure 6. To begin with the force remained purely attractive with a measurable strength below separations of 25 nm, but 1 day later a weak repulsion had developed. The force maximum had shifted to 11-12 nm but the thickness of the adsorbed layer did not change (Figure 6, triangles). Three days later the monolayer thickness had increased to 1.9 nm, as found on rapid adsorption after only 2 h. Both the repulsion at long range and the attraction further in had increased (Figure 6, filled circles). The adhesion was now 260-290 mN/m, close to what was measured after rapid adsorption. The surfaces jumped together from about 30 nm, but they were now also seen to stop
1476 The Journal of Physical Chemistry, Vol. 93, No. 4, 1989
Christenson et al. A
briefly a few nanometers from final contact before coming into molecular contact. No cavitation was ever observed with a thin layer of adsorbed surfactant. As soon as a thick layer had formed cavitation occurred, but only during the process of separation from contact. In this regard the behavior was similar to that observed for the M Pe4NBr (see above). LB monolayer surfaces in
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Discussion Comparison of the Hydrophobic Forces between Differently Prepared Surfaces. We have obtained virtually uncharged fluorocarbon surfaces in three different ways using the same surfactant. Slow adsorption from solution gave an initially very thin surfactant layer but also led to (almost) complete surface neutralization. A layer thickness of only 0.4 nm indicates that the surfactants must be lying rather flat on the mica surface and that large areas of the more hydrophilic headgroups are exposed to the solution (for this surfactant the area of the headgroup when viewed side-on is comparable to the fluorocarbon part of the molecule). The area per headgroup must be substantially larger than the lattice site area of the mica (0.48 n d ) . If we estimate the density of the surfactant to be 1.5, the volume per molecule is about 1.3 nm3, which gives an area per molecule of 3.2 nm2. As with Langmuir-Blcdgett deposition of neutral lipidsIgthe lattice charge on the mica must be largely neutralized by other ions, presumably hydrogen ions. These surfaces thus consist of a mixture of fluorocarbon groups, hydrocarbon groups, and polar groups, and the attraction measured is stronger than a continuum van der Waals force but considerably weaker than what is observed between more homogeneous hydrophobic surfaces (see Figure 5). Once the negatively charged surface has been neutralized, the electrostatic driving force for adsorption is gone and further adsorption occurs very slowly. In spite of the surfactant concentration being increased, it took 2-5 days before a reasonably well-packed monolayer had formed. We note that the thickness of 1.9 nm eventually reached indicates a headgroup area of about 0.7 nm2. Such a layer thickness is achieved within a few hours when the adsorption takes place with the surfaces far apart. The difference, however, is that the surface is neutral at this stage, whereas after the slow adsorption a layer of the same thickness is charged. The attraction between these surfaces is considerably stronger than between the thin layers obtained initially by slow adsorption. It is clear that the uncharged “monolayer” is in both cases only a transient state. With time a (presumably positive) surface charge builds up, and this appears to be due to adsorption of additional surfactant outside the hydrophobic surface. In both cases the surfaces were observed to halt at a separation corresponding to a bilayer thickness before moving into final contact. Whether the reason is patchy adsorption of a second layer or some loose association of surfactant molecules with the surface, it throws considerable doubt on the idea that surfactants adsorb in strictly stepwise fashion to surfaces. A similar adsorption mechanism has been observed with dihexadecyldimethylammonium and we have evidence that the same occurs with hexadecyltrimethylammonium bromide (CTAB, Christenson and Claesson, unpublished results). It is possible that the slow adsorption takes place via monomers only because aggregates might have difficulty in diffusing in between the surfaces when these are kept only a few micrometers apart. If the surfaces are separated, sufficiently rapid adsorption from positively charged aggregates colliding with the negatively charged mica surface can take place. The result is a surface that rapidly becomes packed with surfactant molecules. When most of the surfactant is contained in aggregates, which is especially true of the injected solution, it is very difficult to be certain when and if equilibrium adsorption is attained. By contrast, Langmuir-Blodgett films, though nonequilibrium structures, give deposited monolayers that do not change meas~~~
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