Langmuir 1993,9, 1965-1967
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Time-Dependent Adhesion between Glass Surfaces in Dilute Surfactant Solutions John L. Parkerf and Mark W.Rutland’ The Laboratory for Chemical Surface Science, Department of Physical Chemistry, The Royal Institute of Technology, SlOO 44 Stockholm, Sweden, and The Institute for Surface Chemistry, Box 5607,S-11486 Stockholm, Sweden Received March 31,1993.I n Final Form: June 18,1993 Measurements of surface forces between glass surfaces in very dilute cationic surfactant solutions at pH 10 are presented. As the surfaces approach the forces are purely repulsive and correspond exactly to the interaction of glass surfaces in aqueous solution at the same pH. However,a small adhesion is observed on separation, the magnitude of which is dependent on the time the surfaces are left in contact. The adhesion arises due to adsorption of surfactant in a narrow gap around the contact area and is induced by the favorable interaction of hydrophobic tails across the gap. The time dependence of the adhesion suggests that the adsorption is diffusion limited.
Introduction The measurement of surface forces reveals a great deal about the interaction of surfaces with one another and with the medium in which they reside’ as well as information on how a corresponding colloidal suspension might be expected to behave. For example, a repulsive electrostatic force is observed between glass surfaces in aqueous solution. This arises due to charging of the glass surfaces by the dissociation of silanol groups. The forces are well described by DLVO theoryuntil small separations, where, instead of the predicted van der Waals attraction, an extra repulsion This extra “exponential” repulsion cannot be explained by electric double layer theory and is thought to be due to hydration of the surface. In general, the addition of cationic surfactant changes this situation dramatically. Rather than an increasing repulsion as the surfaces are pushed into contact, the surfaces attract each other and finally come to an adhesive contact.BJ The reason for this adhesion is due to adsorption of surfactant on the silica surface. The presence of adsorbed surfactant renders the surface partially hydrophobic, and the resulting high interfacial tension against water leads to adhesion. Here we report measurements of the forces between two glass surfaces in 6 X 1O-lM cetyltrimethylammonium bromide (CTAB) and the adhesion between the surfaces as a function of time. In order to increase the number of sites available for adsorption, the experiments were performed at pH 10 since at this pH the surface charge of glass is much higher than at normal pH. Experimental Section Surface forces were measured with a new type of surface force apparatus which is based on a bimorph force sensor (described
* To whom correspondenceshould be addressed at The Institute for Surface Chemistry. + On leavefrom the Department of Applied Mathematics,Research School of Physical Sciences, G.P.O.Box 4, Canberra ACT 2601, Australia. (1)Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press: London, 1986. (2) Horn, R. G.: Smith,D. T.: Haller, W. Chem. Phrs. Lett. 1989,162, 404-408.
(3)Rabinovich,Y. I.; Derjaguin, B. V.; Churaev, N. Adu. Colloid Interface Sci. 1982, 16,63-78. (4) Peschel, G.; Belouschek,P.; Muller, M. M.; Muller, M. R.; Konig, R. Colloid Polym. Sci. 1982, 260,444-451. (5) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Nature (London) 1991, 353,239. (6) Parker, 3. L.; Yaminsky, V.; Claesson, P. M. Langmuir, in press. (7) Shchukin, E. D.; Yaminsky, V. V. Colloids Surf. 1988,32,33-35. 0743-7463/93/2409-1965$04.00/0
in ref 8). One surface is mounted at the end of the sensor, and the other is mounted at the end of a piezoelectric tube. The bimorph is enclosed in a Teflon sheath, and this is mounted inside a small chamber (volume -10 mL). The chamber is clamped to a translationstage which is used to control the coarse position of the piezoelectric tube and the upper surface. The apparatus has a footprint of 6.5 by 5 cm and is mounted inside an aluminium casing for temperature regulation. A complete description of the apparatus will be given in a forthcoming publication. The apparatus is controlled by a computer which generates the waveform used to drive a high-voltage amplifier which in turn drives the piezoelectric tube. The signal from the bimorph chargeamplifierand the piezoelectricmotion from a displacement sensor are recorded in response to the motion of the piezoelectric tube. In order to hold the surfacesin contact for variable amounts of time, the waveform applied to the piezo consisted of a positivegoing ramp followed by a rest period and then a negative-going ramp. In this way the surfaces could be pushed to the same force value and the time in contact varied simply by changing the length of the rest time in the middle of the force run. The motion of the piezoelectric tube is measured during the course of each force run with a linear displacement sensor. The sensitivityof the piezoelectric tube (ie.,the movement for agiven voltage) is obtained from either a linear or polynomial fit to this data. In this way the nonlinearity and hysteresis associated with all piezoelectric devicesis accuratelyaccounted for. The chamber of the apparatus is designed for use as a flow cell, and a syringe pump is used to circulate the solution between an external reservoir and the chamber. The analysis of the raw experimental data followsthe procedure given in ref 8. The surfacesare pushed together until they come to contact, at which point the motion of the piezoelectrictube is transmitted directly to the sensor and the resulting straight line is used to calibrate the deflection of the bimorph signal. The force is then obtained using the measured spring constant and Hooke’s law. Glass surfaces were prepared by cutting a 3-cm length of a 2-mm glass rod. The rod was then cleaned with ethanol and one end melted in a gas burner until a molten dropletof glass formed with a radius of 2 mm. Two such surfaces were mounted in the apparatus and aligned so that the centers of both spheres were as close as possible to parallel with the axis of motion of the piezoelectric tube. Water was purified with a Millipore MilliQ water purification system and deaerated; CTAB was obtained from Fluka and used without further purification. The pH of the solution was adjusted by addition of a solution of 2 X le2 M NaOH (BDH) until the desired pH was reached. Results and Discussion The forces between two glass surfaces in dilute CTAB solution at pH 10 are shown in Figures 1(linear plot) and 2 (log plot). The repulsive forces agree extremely well (8) Parker, J. L. Langmuir 1992,8,551. 0 1993 American Chemical Society
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2A Figure 3. A schematicillustrationof the contact between surfaces and the presence of surfactant in the gap around the contact region. The negative midplane potential c a w s surfactant to accumulate in the gap. The aggregation of surfactantin the gap is favored when the gap is lese than twice the length of the hydrocarbon tail as the hydrocarbon tails can overlap. The time required to attain the equilibrium surfactant concentration is limited by diffusion of surfactantinto the gap, and this gives rise to the time dependence of the adhesion as shown in Figure 4.
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Separation (nm) Figure 1. Force between two glass surfaces immersed in 6 X 10-7 M CTAB solution (pH 10)as a function of the separation between the surfaces. Three separate measurements are shown. In each run the surfaceswere left in contact (defied as an applied load of 10 mN/m) for a different length of time: In order from top to bottom, 9.3, 18.2,and 28.7 e. The increase in adhesion as a function of time can be seen clearly.
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Separation (nm) Figure2. Same data as shown in Figure 1 plotted on a logarithmic scale. Only the repulsive parts of the force laws are shown. The solid lines represent the best fit to the nonlinear PoissonBoltzman equation with boundary conditions of constant charge (upper smooth line) and constant potential (lower smooth line) with the parameters 90= -90 mV and .rl= 19.5 nm. with the nonlinear Poisson-Boltzmann (P-B) equation at large surface separation. The measured interaction lies between the constant charge and constant potential boundary condition solutions,indicating that some charge regulation is occurring at the surfaces. The parameters for the fit are 90= -90 mV and ~ - 1= 19.5 nm (surface potential and Debye length, respectively), corresponding to a surface charge density of 5.2 mC m-2 or an area per surface charge of 30.8 nm2. (Note that the surface charge obtained in this way is somewhat of an underestimate.9) The measured Debye length corresponds to a bulk concentration of 2.5 X lo-" M electrolyte. The forces between two glass surfaces at pH 10 are almost identical (results not shown), with the only difference being that (9) Gulbrand, L.; Jiineeon, B.; Wenneretriim, H.; Linee, P. J. Chem. Phys. 1984,80, 2221.
there is no adhesion present between the surfaces regardless of how long the surfaces are left in contact. At very short separations the force begins to deviate from DLVO theory and becomes progressively more repulsive (seeinset in Figure 2). This occursat a separation of about 2 nm. It is not possible to say exactly at what separation the forces deviate, however, a lower bound is given by the intersection of the measured curve and the constant charge solution to the P-B equation at 1.5 nm. At a similar electrolyte concentration (1.1X lo-" M NaC1) Horn et observed the onset of an additional repulsive force at about the same surface separation (-1.5-2 nm). Similar observations have been made using other experimental techniquesSgb Horn and co-workersascribed this extra repulsion to a "hydration force" similar to that which occurs between mica in some electrolyte solutions10 and that postulated to occur between some surfactant bilayers."-13 The measured electrical double layer interaction and the presence of hydration forces are entirely consistent with previous work. When surfactant is added to the system at a concentration of 6 X lo-' M,ita only effect is to promote an adhesion. As the surfaces approach, the short-range forces are repulsive and there is no indication of the presence of an attractive force; however, as the surfaces are separated, they stick and an adhesive minimum is observed. The degree of binding of surfactant to the surface at infinite separations must be extremely small. One must keep in mind the fact that the Na+ ion concentration outweighs the surfactant concentration by a factor of almost a thousand. The predominant species in the double layer is Na+, and it is almost certainly this species which neutralizes the surfaces when they finally contact, fulfilling the requirements of electroneutrality. It is thus not surprising that purely repulsive forces are observed on approach. In order to explain the adhesion and ita time dependence, we must consider adsorption in the gap around the contact region. The onlyreasonable explanation for the adhesion is that adsorption of surfactant is occurringin a truncated annulus of small separation around the contact between the surfaces (see Figure 3). If surfactant were to adsorb, the result would be partially hydrophobic surfaces and the concomitant increase in the interfacial tension against water would result in adhesion. The reason why surfactant should adsorb in the gap is not obvious. In order to maintain charge neutrality across the gap, there must already be a sufficientcomplement of Na+ ions and a small amount of CTA+ ions. The reason that CTA+ displaces (10) Paehley, R M.J. Colloid Interface Sci. 1981,83,531-646. (11) Lyklema, J.; Mysele, K. J. J. Am. Chem. SOC.1966,87, 2639.
(12)Clunie,J.S.;Corkhill,J.M.;Goodman,J.R.;Symons,P.C.;Tate, J. R. J. Chem. SOC.Faraday, Trans. 1 1967,63,2839. (13) Le Neveu, D.M.;Rand, R. P.; Paraegian, P. A. Nature (London) 1976, 259, 601.
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Langmuir, Vol. 9, No. 8,1993 1967
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Figure 4. A plot of the pull-off force as measured by the depth of the minima as a function of the time the two glass surfaces were left in contact in 6 X lo-' M CTAB solution at pH 10. The solid line is intended as a guide for the eye. Inset: The adhesion plotted as a function of t-llz. The solid line is a linear estimate to the data at larger t where diffusion theory predicts that the pointa should fall on a straight line.l6
Na+ must be due to a lowering in free energy due to the removal of hydrocarbon from water. If the maximum diameter of the annulus is of the same order as the length of the CTAB molecules, then the exchange of Na+ for CTA+on the surfacewill be favored when the other surface is less than a molecular extension away. This is because the hydrocarbon chains of molecules adsorbed on opposing surfaces are able to interact via the hydrophobic effect. Hence, the free energy of adsorption of surfactant is lowered for separations of less than 3.0 nm. When two spherical surfaces are pushed into contact, a flattened region forms where the separation between the surfaces is zero. The size of the flattened region can be estimated from the Hertz formula:
WRD = as (1) If the load (W) is set to the maximum measured force value of 10mN/m and D,the elastic constant, to 1X then using the measured radius, R,of 2 mm, the radius of the flattened regiona = 0.6 pm. Around the contact region the separation increases with distance away from the central axis between the surfaces. In comparison there is quite a considerable area where the surface separation is below 3 nm (seeFigure 3). The area of the midplane where the separation between the surfaces is less than 3.0nm is about 19 pm2 for R = 2 mm. This area is more than 10 times larger than the area where the surface is in contact. It follows then that even if the force in the contact region is repulsive, an attractive interaction in the area surrounding the contact region can, and in this case does, give rise to a net adhesion. The measurements of the depth of the adhesive minima as a function of time are plotted in Figure 4. The adhesion increases rapidly as a function of time and reaches an (14) Parker,J. L.;Attard, P. J. Phya. Chem. 1992, W,10398.
equilibrium value after a contact time of about 1min. At first sight this time scale for adsorption may seem rather long. It has been shown that the adsorption of CTAB from submicromolar solutions to a droplet of CCl, is diffusion controlled and the surface tension of the oilwater interface reaches an equilibrium after only 2 s.15J6 This is a considerably shorter time scale than that observed in these experiments. However, the geometry of the region to which adsorption takes place in our experiments must considerably slow the diffusion process. A plot of the adhesion against time, t , should go as t112 except at short t , if in fact the process is diffusion controlled. The inset in Figure 4 shows a plot of the adhesion against t-l12, and it can be seen that the adhesion does indeed follow this trend, which suggests that the effect observed here is diffusion limited. However, it should also be observed that there is an equally good linear correlation of the adhesion with t at short t . It follows that on separating the surfaces to "infinity" the bound CTA+ ions are displaced by Na+ ions by simple ion exchange as the driving force for adsorption is removed. Hence, the force runs on approach are reproducible and repulsive with no evidence of surfactant adsorbed to the surfaces.
Conclusions Glasssurfacesin very dilute surfactant solutions display a small adhesion which increases as a function of the time the surfaces are in contact. The adhesion is due to adsorption of surfactant in a narrow region around the flattened contact. The adsorption occurs as a result of the free energy of adsorption being reduced by favorable interactions of hydrophobictails across the gap. The time dependence of the adhesion correlates well to diffusion limitation, and it is concluded that the relatively long equilibration times are due to the narrowness of the slit through which surfactant molecules must diffuse. In some senses the adsorption could be considered to be a surfaceinduced micellization since it is driven primarily by the reduction of exposed hydrophobictail area across the gap rather than within a layer, or by a favorable interaction with the glass surface. We believe that this is the first study where surface force measurementshave been used to probe the dynamics of adsorption. This has only recently become possible since the developmentof new instrumentation for surface force measurements which will also enable this work to be greatly extended. For instance with the appropriate preprogrammed control of the upper surface it should be possible to determine precisely the separation at which "surface-induced adsorption" of surfactant begins. Such experimentsare planned, and we hope to be able to report the results soon. (15)Daviea, J. T.; C o h Smith, J. A.; Humphreye, D. G. Proceedings
2nd International Conference on Surface Activity; Butterwortbe: Lon-
don, 1957; pp 281. (16) van Huneel, J.; Bleye, G.; Joos,P. J. Colloid Interface Sci. 1986, 114,432-41.
