7706 J. Phys. Chem. 1993,97, 7706-7710

Surface Forces between Glass Surfaces in Cetyltrimethylammonium Bromide Solutions

John L. Parker,'*+ Vassily V. Yaminsky,* and Per M. Claesson Department of Physical Chemistry, The Royal Institute of Technology, SI00 44 Stockholm, Sweden, and The Surface Force Group, The Institute for Surface Chemistry, Box 5607, S-I14 86 Stockholm, Sweden

Received: February I I , 1993; In Final Form: April 29, 1993

A new surface apparatus has been used to study the interactions between glass surfaces immersed in CTAB (cetyltrimethylammonium bromide) solutions. At low surfactant concentrations CTAB adsorbs to form a monolayer, and the surfaces become adhesive in contact. At a concentration of 5 X lo-' M the charge on glass is neutralized, and the surfaces experience an attractive force in excess of that expected from van der Waals interactions. At concentrations just below the critical micelle concentration, CTAB adsorbs to the hydrophobic interface to form loosely packed bilayers, but the surfaces always come to an adhesive minimum. These observations are quite different from those made when mica is used as the substrate. A comparison between the results obtained for the two substrates is made, and the reasons for the differences are discussed. Glass has a lower area per charge compared with mica, and this results in less tightly packed monolayers and in turn bilayers on glass when compared with mica.

Introduction A great deal can be learned from studying the surface forces

acting between two macroscopic bodies as a function of the surfactant concentration in the surrounding Ad- sorption of surfactant to a surface changes the surface properties and as a result the surface forces. Analysis of the force as a function of distance provides basic information such as the thickness of the adsorbed layer and the charge of the surface, and often it is possible to derive a model of the adsorption process from these data. The relative strengths and roles of the different interactions involved in the adsorption process can be assessed. The largest body of data exists for surfactants adsorbed to mica surfaces. This is because mica forms an ideal substrate for direct surface force measurement using the Israelachvili measurement technique.

Many different surfactants have been studied including cationic,l-9 anionic,1° and nonionicllJz with single and double alkyl chains. One of the most thoroughly studied surfactants with this technique is CTAB (cetyltrimethylammonium bro- mide).lJ.6*7J3 This surfactant adsorbs to form monolayers on mica at very low concentrations (<1V M). The charge on the mica surface is neutralized at (-3.5 Xlod M). At higher concentrations additional surfactant adsorbs, and the surface begins to recharge. At a concentration of about one-half the critical micelle concentration (cmc = 9 X 1W M), bilayers form with a measured thickness of -3.3 nm.13

A detailed picture of the adsorption of CTAB to mica has been built up, but unfortunately the experimental techniques14 used in these studies are not readily adapted to studying substrates other than mica. As a result, the effects of the surface on adsorption have not been investigated with the surface force technique. There are a number of different surface properties which are likely to drastically affect the adsorption and resulting structure of adsorbed surfactant layers. The degree of surface charge influences the interaction between a charged surfactant head group and, as a result, the adsorption. As an extreme example, surfactant adsorption does not occur when the surface has the same sign of charge as the head group.1° Other important properties may include the area per ionizable site, crystallinity, polarizability, and dielectric constant of the material.

* To whom correspondence should be addressed. t On leave from the Department of Applied Mathematics, Research School

* On leave from The Institute of Physical Chemistry, Leninski Prospect 3 1, of Physical Sciences, G.P.0 Box 4, Canberra A C T 2601, Australia.

Moscow.

0022-365419312097-7706$04.00/0

Glass is an amorphous material with lower surface charge at neutral pH than mica. It is an ideal substrate for surface force measurement because smooth surfaces are conveniently prepared by flame polishing. Glass surfaces have been studied previously with surface force and adhesion force measurements.'5-l8 Glass filaments and spherical glass surfaces have been used since the 1 920s.19J0 Derjaguin pioneered thedirect measurement of surface forces by employing glass.15 More recently, Yaminsky and co- workers developed a surface force apparatus based on capacitance measurement techniques to study the interactions between glass spheres,16 and Horn and co-workers prepared thin glass surfaces suitable for interferometry measurements with the Israelachvili surface force apparatus.17 In this study we have employed glass surfaces prepared by melting the end of a glass rod until a molten droplet is formed at the end. Surface forces were measured with a new type of surface force apparatus which is based on a direct force sensorZ1 and which allows any type of surface to be studied.

Materials and Methods A brief description of the new surface force apparatus follows.

The apparatus is based on the bimorph force sensor described in ref 21. 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 translation stage 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 aluminum enclosure for temperature regulation. A complete description of the apparatus will be given in a forthcoming publication.22

The apparatus is controlled by a computer. A block diagram of the force measuring system is shown in Figure 1. The computer generates a waveform which is used to drive a high-voltage amplifier, which in turn drives the piezoelectric tube. For every point in the waveform which is output the computer measures and stores the signal from the bimorph charge amplifier, and the piezo motion is measured with a displacement sensor. In this way all the parameters from the apparatus are recorded as a function of time. The speed of motion of the upper surface can be varied between - 10 000 and 0.5 nm/s simply by changing the time elapsed between sampled points. The number of points sampledcan also be varied over a wide range from several hundred to tens of thousands.

The motion of the piezoelectric tube is measured during the course of each force run with a linear displacement sensor. The

0 1993 American Chemical Society

Glass Surfaces in CTAB Solutions The Journal of Physical Chemistry, Vol. 97, No. 29, 1993 7707

I Charge Amp n

Reservoir Syringe Pump jll Computer

Figure 1. Block diagram of the surface force apparatus used in this study. One surface is mounted at the end of a piezoelectric bimorph force sensor, and the other is mounted at the end of a piezoelectric tube. A linear displacement sensor is used to calibrate the response of the piezotube to a voltage applied from the high-voltage amplifier. A personal computer is used to control the entire system. It outputs a waveform to a high-voltage amplifier and records the piezo displacement and bimorph charge in response to this. A syringe pump is used to recirculate solution from a reservoir into the force measuring chamber.

sensitivity of the piezotube (i.e., the motion for a given voltage) is obtained from either a linear or a polynomial fit to this data. With a polynomial calibration the nonlinearity and hysteresis are accurately accounted for in the calculation of thedisplacement. 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 follows the procedure given in ref 21. The surfaces are pushed together until they come in contact. When in contact, the motion of the piezotube is transmitted directly to the sensor (an example of the output is shown in Figure 2), and this straight line is used to calibrate the sensitivity of the sensor. Using this procedure, it is not possible to define the separation with respect to the contact between the two surfaces in dry atmosphere. As a result, it is not possible to determine the thickness of an adsorbed layer if it is not pushed away from the contact region during the experiment.

Glass surfaces were prepared by cutting a 3-cm length of 2-mm glass rod. The rod was then cleaned with ethanol, and the end was melted in a gas burner until a molten droplet of glass was 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 close to parallel with the axis of motion of the piezoelectric tube. The radius of curvature of the surfaces was measured with a micrometer at the end of the experiment. The radius of the surfaces is 10 times smaller than the radius normally employed in the Israelachvili apparatus, and as a result the hydrodynamic force which scales with the square of the radius is 100 times ~maller.~3 In other words, the force curve can be recorded 100 times faster without changing the scaled hydrodynamic force for surfaces of radius 2 mm than for surfaces of radius 2 cm. Hence, equilibrium force runs were recorded in about 30 s. Measurements of the hydrodynamic force confirm the fact that equilibrium measurements can be safely made over this time scale (results to be reported elsewhere). Water was purified with a modified Millipore MilliQ water purification system and deaerated; CTAB was obtained from Flub and used without further purification.

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Surface Surface Force Contact

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Figure 2. An example of the raw data collected during a force measurement. The bimorph response is plotted against thevoltageapplied to the piezotube. At large surface separations there is no surface force present, and the bimorph charge remains static. When a force is encountered, the bimorph begins to deflect and a response is measured. When the two surfaces come into contact, the upper surface, Le., the surface mounted at the end of the piezo, pushes the lower surface at a constant rate. The direction of motion is indicated by the arrows. When the direction of motion is reversed, the surfaces remain in contact and adhere until they finally jump out from contact at the point indicated by the arrow.

ResulQ

The force between two glass surfaces in pure water is well described by DLVO theory down to small surface ~eparati0ns.l~ DLVO theory predicts that the surfaces should come into an adhesive contact; however, this is not observed, and instead a exponential repulsion not accounted for by electrical double- layer forces is observed. This additional repulsion has been ascribed to hydration f0r~es.l~ Figures 3 and 4 show the forces between glass surfaces in a range of CTAB concentrations from 10-5 to 8 X 1 V M. The force between glass surfaces in dilute CTAB solutions (1 M) is quite different from the force in pure water. The short-range repulsion present in pure water is no longer observed, and the surfaces experience an attractive force and come toan adhesivecontact. At CTAB concentrations below le5 M the force between the surfaces at large surfaceseparations is repulsive with an a long decay length. At smaller separations the force becomes attractive, and the surfaces come to an adhesive contact. Increasing the surfactant concentration to 5 X l e 5 M results in a force law which is purely attractive. The van der Waals force calculated for two quartz surfaces interacting across water (Hamaker constant = 8 X 10-21 J)24 is also shown for comparison.

Increasing the surfactant concentration to 3.1 X 10-4 M results in a force similar to that observed at 1 X 1 V M. A weak double- layer repulsion with a long decay length is followed by a steeply attractive force observed at shorter distances. At 4.8 X 1 V M CTAB concentration the repulsive force is well fitted by DLVO ($0 = 110 mV and ~ - 1 = 13.8 nm) theory at surface separations down to -8 nm. At this separation the surfaces jump sponta- neously into contact. This is not in agreement with DLVO theory which predicts that the surfaces should jump from a separation of 1-2 nm. This indicates that an additional attraction is present also at this concentration. The calculated Debye length from the CTAB solution concentration assuming that CTAB behaves as

7708 The Journal of Physical Chemistry, Vol. 97, No. 29, 1993 Parker et al.

0 10 20 30 40 50 60

Separation (nm) Figure 3. Surface forces measured between glass surfaces as a function of the CTAB concentration in the surrounding medium (filled squares, 9.22 X 1od M; open triangles and open diamonds, 4.59 X M; filled triangles, 1.36 X 1 o-' M). The upper and lower boundaries of the shaded region indicate the solution of the nonlinear Poisson-Boltzmann equation (surface potential $0 = 22 mV; Debye length r1 = 97.4 nm) with constant charge (upper values) and constant potential (lower values) boundary conditions. The upper solid line is theconstant charge boundary condition fit to the filled triangles, 1.36 X 10-4 M (surface potential $0 = 18 mV; Debye length c1 = 25.8 nm). The calculated van der Waals force is shown by the lower thick line (Hamaker constant A = 8 X J).

a fully dissociated 1:l electrolyte is in close agreement with the value obtained from fitting the experimental results. The double- layer force continues to increase as the surfactant concentration is increased, and the position of the jump into adhesive contact moves inward. Table I gives a summary of the results.

Discussion

Adsorption of CTAB onto glass influences the forces between the surfaces dramatically even at very low surfactant concen- tration, <1od M. The effect of adsorption is to remove the short- range repulsive force which exists between glass surfaces in water. Glass is negatively charged, and adsorption of cationic surfactant is initially electrostatically driven. At a concentration of 5 X 10-5 M there is no detectable double layer force, and the attractive force is of much longer range and stronger than the van der Waals force expected (see Figure 3). Attractive forces in excess of those predicted from van der Waals theory have also been observed between mica surfaces coated with adsorbed surfactant monolayers. CTAB monolayers adsorbed from solution to mica show extra attractive forces.3 Even longer-ranged and stronger attractive forces have been observed between neutral deposited monolayers of double-chain cationic su r fa~ tan t s2~~~ and silanated ~urfaces.2~ The origin of this force is the subject of considerable current interest, and a number of theories have been advanced over recent years.28~29 The results reported here allow some observations to be made which should be considered when possible mechanisms are suggested. The advancing contact angle of a drop of water on a glass surface rendered hydrophobic by immersion in lW5 M CTAB is about 65'. The adhesion between the surfaces in surfactant solutions is also rather low (see Figure 5B), indicating a relatively low surface coverage. The extraor- dinary long-range attractive forces observed between Langmuir-

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I I -0.2 P I I I I I - _- 0 IO 20 30 40 50 60

Separation (nm) Figure 4. Surface forces measured between glass surfaces as a function of the CTAB concentration in the surrounding medium (filled squares, 3.12 X 10-4 M; open squares, 4.83 X 10-4 M; filled circles, 6.48 X 10-4 M; open circles, 8.07 X 1 0 4 M). The solid lines are solutions to the nonlinear Poisson-Boltman equation. Both the constant charge and constant potential solutions are shown for 3.12 X 10-4 M CTAB. Only the constant-charge solutions are shown for the other concentrations.

TABLE I concn, M %(glass), mV d(calc), nm q~(mica),@ mV ,

9.22X 1O-a -22 97.4 -65 4 . 5 9 ~ 10-5 o - 120 1.36 X lo-' 18 25.8 -140 3.12X lo-' 50 17.1 4.83 X lo-' 110 13.8 -150 6.48 X lo-' 145,9ob 11.9 8.07 X lo-' 145,906 10.6

4 From ref 13. b From fitting with the plane of origin of the charge at 3.2 nm.

Blodgett monolayers have normally been associated with surfaces with very high solid/liquid surface tensions. The strength and range of the force are much higher than observed here for CTAB monolayers, but these results nevertheless indicate that the hydrophobic interaction persists even with surfaces with a relatively low contact angle.

The extra attractive interaction persists at higher surfactant concentrations even though the surface is positively charged due to further adsorption of surfactant. The same trend is observed for CTAB adsorbed on mica. This led Parsegian and Podgornik30 to advance a theory to account for those observations. They argued that the apparent extra attraction observed may not be due to the presence of additional attractive forces but may instead be due to a sudden reduction of the repulsive double-layer interaction. The electrical double-layer force is due to the charge produced by the adsorption of surfactant to the hydrophobic interface. Parsegian et al. postulated that when the surfaces interact the system may lower its free energy by expelling the surfactant from between the surfaces, and the loss of surface charge results in a loss of the double-layer interaction. The fact that the surfaces in these experiments finally come to a hydrophobic contact means that the surfactant in the outer layer of the bilayer must be expelled from the gap between the surfaces. However, the results reported here are not entirely consistent with this model. In particular, this is the case for the results

Glass Surfaces in CTAB Solutions The Journal of Physical Chemistry, Vol. 97, No. 29, 1993 7709

-7 -6 -5 -4 -3 -2 Log Surfactant Concentration

-7 -6 -5 -4 -3 -2 Log Surfactant Concentration

Figure 5. Potentials obtained from fitting the measured surface force curves for glass (solid squares) plotted against the logarithm of the surfactant concentration (A). Shown also for comparison are the results from streaming potential measurements (open diamonds) and results for quartz (solid diamonds) from ref 30. The results for the fitted surface potentials for the mica experiments are also shown (open squares), and the solid line is added to guide the eye. Adhesion between glass surfaces plotted against log concentration of CTAB is shown below in (B) (solid circles). For comparison, the resultsobtained for mica from ref 13 scaled by a factor of 2 are shown in the shaded region so that FIR from cross cylinders is compared with the result for spheres.

shown in Figure 3 which show attractive forces much higher than predicted by van der Waals interaction. A double-layer collapse mechanism cannot explain this force simply because this mech- anism cannot produce attractive forces. The presence of this real extra attraction at le5 M CTAB does not rule out a contribution from the double-layer collapse mechanism at the higher surfactant concentrations.

Comparison with Mica. The forces as a function of surfactant concentration between glass surfacesqualitatively follow the same trend as the forces between mica surfaces at low surfactant concentrations.13 Namely, partial neutralization at low concen- tration followed by charging by continued adsorption as the concentration is increased. CTAB adsorbs to both glass and mica surfaces due to favorable electrostatic interactions until the surface is neutralized. The surface charge then changes sign, and adsorption continues driven by the hydrophobic interaction. There are however a number of specific and important differences between the behavior observed when glass is used as the substrate instead of mica. Complete surface neutralization has not been observed for mica, but nearly neutral surfaces are observed at 3.3 X 1W M CTAB13 whereas for glass neutralization occurs at more than 10 times higher surfactant concentration. The reason for this is not entirely obvious. Glass has a lower surface charge density than mica under similar conditions of pH and electrolyte concentration, and so one can expect that the electrostatic driving force for adsorption is reduced somewhat in the case of glass. The surface charge on glass arises through dissociation of hydroxyl groups on the surface. Mica, on the other hand, is charged due to isomorphic substitution of aluminum for silicon in the crystal lattice. This gives rise to one negative charge per 48 At. In

solution complete ionization does not occur because the charge is partially neutralized by cation adsorption. The surface potential obtained from fitting surface force measurements between mica surfaces in pure water is about 120 mV whereas streaming potential measurements on glass give values between 60 and 90 mV.31 The nonlinear DLVO fit to the surface force measurements of Horn et al.1’ give a value for the surface potential of glass in 10-4 M NaCl of 40 mV whereas mica has more than double this value under similar conditions. Surfactant can adsorb to mica via an ion exchange mechanism whereas for glass adsorption occurs at negatively charged S i0 centers, and for further adsorption to occur after neutralization surfactant must exchange with a proton or adsorb via a nonelectrostatic mechanism with a bound counterion. Nevertheless, the difference in surface charging properties alone cannot explain the order of magnitude difference in the concentration at which neutralization occurs for the two surfaces. There is a point during adsorption at which the mica surface has the same charge as the glass surface without any adsorbed surfactant, and at this point the electrostatic driving force for continued adsorption is identical for the two surfaces. The surface potentials obtained by fitting the double-layer forces are plotted against the surfactant concentration in Figure 5A. The values from the mica experiments are shown for comparison. The results for the mica surfaces are remarkably similar to those for glass at very low surfactant concentrations, indicating that the electrostatic driving force for adsorption is very similar. Zorin et al.31 concluded that adsorption of CTAB onto quartz below the isoelectric point is purely electrostatically driven. This was already pointed out by Conner and Ottewill, who studied adsorption of CTAB on latexes.32 Zorin et al. obtained agreement between measured surface charge densities and those calculated for a Langmuir adsorption model assuming that adsorption occurs at negatively charged Si0 centers and that there is no interaction between the hydrophobic tails of the molecules. They calculated an area per molecule for CTAB at the point of zero charge of 20 nm2.

The surface potential as a function of concentration CTAB for mica follows the glass results until just over lW5 M. After this concentration the surface potential jumps from -28 mV at 7.7 X 1V M to 65 mV at 1.1 X 10-5 M. The sudden jump in potential can only be explained by a nonelectrostatic mechanism for further adsorption. For mica the area per adsorbed molecule is lower, and the first layer is moredensely packed than at a similar surface potential on glass. This follows simply from the fact that for mica to reach a similar surface potential more negative sites are neutralized by adsorption because of the higher surface charge, when compared with glass, in pure water. As a result, the density of hydrophobicsites is higher (and also the contact angle). Further adsorption occurs at these hydrophobic sites, and continued adsorption onto mica is hydrophobically driven. The area per ionizable site is also approximately the same as the area per head group, and so both the hydrophobic effect and surface charge promote adsorption.

The lower adsorption density for the first adsorbed layer has profound consequences for the adsorption to form a bilayer. For glass surfaces neutralization occurs well before a tightly packed monolayer can form. The hydrophobicity or interfacial tension against water of this layer is smaller than for a monolayer adsorbed to mica. The pull of force, Le., the force required to separate the surfaces from contact, as a function of thesurfactant concentration is shown in Figure 5B. The adhesion between the glass surfaces is nearly an order of magnitude lower at the point of zero charge than for mica. The pull of force scaled by the radius of curvature of the surfaces (FIR) is the difference in free energy between the solids in contact and the separated solids in the liquid. This is an indication of hydrophobicity; the higher the pull of force, the more hydrophobic thesurface and the higher theinterfacial tension against water. The contact angle reported for mica surfaces is about 90° whereas the contact angle for CTAB adsorbed to glass

7710 The Journal of Physical Chemistry, Vol. 97, No. 29, 1993

ismuchlower (=.aso) at thepointofzerocharge. Oneinteresting

Parker et al.

References and Notes point to note from Figure 5 is that the adhesion continues to increase as the surface charge increases. This reflects the fact that the first surfactant layer is becoming more densely packed with surfactant as the surfactant concentration increases and even as the second layer is forming. This means that the adsorption cannot be described by a simple two-step model, Le., adsorption to form a monolayer followed by further adsorption to form a bilayer.

Onevery obvious difference between the mica and glass results is that with glass it is always possible to push the surfaces into an adhesive contact even after long equilibration times. At a CTAB concentration below half cmc the force between mica surfaces is repulsive at all separations, and the surfaces cannot be pushed to smaller distances than 6 nm. This distance corresponds to the thickness of a full bilayer on each surface with the CTAB molecules fully extended and roughly normal to the surface. The surface charge for glass at a concentration just below the cmc is similar to that for mica when the plane of origin of charge is set to the hydrophobic contact position. This is almost certainly not the position of plane of origin of the charge because of the presence of a surfactant bilayer. The data shown in Figure 4 (8 X l ( r M, open circles) can be equally well fitted with a surface potential $0 = 90 mV and the plane of charge set to 3.2 nm, which is roughly the thickness of a CTAB bilayer. If this is the case, then the density of surfactant in the outer layer of the bilayer adsorbed to glass is much lower than for mica, and one would expect the layers to be easily squeezed away from the contact zone. Another factor which should be considered is that the mechanical deformations of the mica surfaces will make it harder to squeeze a layer from between them. With the present data there is no way of determining the precise location of the plane of charge, and hence it is difficult to estimate the relative densities of surfactant in theouter layers of the bilayers. Neutron scattering experiments33on quartz show that the adsorbed CTAB layer is certainly thicker than a full monolayer but is much thinner than a full bilayer, and these results are entirely consistent with the force measurements.

In conclusion, these results show that even though the driving force for adsorption may have the same origin changes in the relative strengths of the forces from one substrate to another can havedrasticeffects. Substrateeffects havereceivedlittleattention with the surface force measurements. This gap in knowledge can easily be filled with new force measurement techniques such as those described here.

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