Characterization of Adsorbed Ionic Surfactants on a Mica Substrate

The continuity of the network of patches increases until a concentration just below the cmc. At this concentration, the patches become continuous “w...
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Characterization of Adsorbed Ionic Surfactants on a Mica Substrate B. G. Sharma, S. Basu, and M. M. Sharma* Department of Chemical and Petroleum Engineering, The University of Texas at Austin, Austin, Texas 78712 Received June 3, 1996X An AFM study is presented to investigate the morphology of adsorbed cationic surfactants on mica substrates. At low surfactant concentrations and low pH values, discrete aggregates of adsorbed surfactants are found on the surface. As the surfactant concentration is increased, these aggregates become more organized into elongated cylindrical shapes. The continuity of the network of patches increases until a concentration just below the cmc. At this concentration, the patches become continuous “wormlike” admicelles on the surface. Contact angle measurements corroborate the change in the surface properties from hydrophobic at low surfactant concentrations to hydrophilic at concentrations slightly below the cmc. Force versus distance curves also clearly indicate a change in surface morphology. It is clear from our observations that the formation of dense continuous monolayers or bilayers does not occur as postulated earlier. Indeed our observations suggest the formation of discrete surfactant aggregates on the surface which under certain conditions (high pH and high surfactant concentration) become continuous and form wormlike admicelles on the mica surface. This change in surfactant morphology from adsorbed surfactant aggregates to more continuous structures is also responsible for the transition from hydrophilic to hydrophobic surface properties. It is also shown that pH and salt concentration play an important role in this transition.

Introduction Surfactant adsorption on mineral surfaces is a phenomenon of significant importance to many different industrial processes ranging from ore flotation and paint technology to enhanced oil recovery. The process of surfactant aggregation or micelle formation, although well understood in bulk solutions, is not adequately understood on surfaces. The hemimicelle concentration,1-4 critical aggregation concentration, and aggregation number5-7 have been calculated theoretically, but the structure and shape of such aggregates still remain controversial due to limited direct experimental evidence. Although the broad principles which govern surfactant adsorption are well understood, there is very limited data available for accurately describing the structure of surfactant aggregates on substrates. Traditionally adsorption isotherms have been used for describing the adsorption characteristics of surfactants on substrates. Several experimental methods such as NMR,8 calorimetry,9 ellipsometry,10 and surface force measurements11,12 have been employed to elucidate adsorption mechanisms. More recently fluorescence decay13 and neutron reflection14 have been used. Strong indications of the presence of local X Abstract published in Advance ACS Abstracts, December 1, 1996.

(1) Fuerstenau, D. W.; Wakamatsu, T. Faraday Discuss. Chem. Soc. 1975, 59, 157-168. (2) Fuerstenau, D. W.; Healy, T. W.; Somasundaran, P. Trans. AIME 1964, 229, 321-325. (3) Fuerstenau, D. W. In The Chemistry of Biosurfaces 1; Hair, M. L., Ed.; Marcel Dekker: New York, 1971; pp 143-175. (4) Somasundaran, P.; Fuerstenau, D. W. Trans. AIME 1972, 252, 275. (5) Gao, Y.; Du, J.; Gu, T. J. Chem. Soc., Faraday Trans. 1 1987, 83 (8), 2671-2679. (6) Gu, T.; Gao, Y.; He, L. J. Chem. Soc., Faraday Trans. 1 1988, 84 (12), 4471-4473. (7) Gu, T.; Rupprecht, H. Colloid Polym. Sci. 1990, 268, 1148-1150. (8) Soderlind, E.; Stilbs, P. J. Colloid Interface Sci. 1991, 143, 586. (9) Partyka, S. E. Keh; Lindenheimer, M.; Groszek, A. Colloids Surf. 1989, 37, 309. (10) Wangnerud, P.; Olofsson, G. J. Colloid Interface Sci. 1992, 153 (2), 392-398. (11) Kekicheff, P.; Christenson, H. K.; Ninham, B. W. Colloids Surf. 1989, 40, 31. (12) Pashley, R. M.; McGuiggan, P. M.; Horn, R. G.; Ninham, B. W. J. Colloid Interface Sci. 1988, 126, 569-578.

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aggregates have also been obtained using spectroscopic techniques.13 Fluorescence decay of pyrene on spherosil was used15 for predicting that at low coverages the adsorption occurred by micellar adhesion while at high coverages the coalescence of micelles due to steric interactions led to a continuous bilayer structure. Neutron reflection was used16 for proposing a defective bilayer scheme at low coverage which resulted in complete bilayer coverage at high concentration with a hydrocarbon layer thickness of a single alkyl chain length using oxyethylene dodecyl ether on quartz. It was proposed that adsorption proceeded as islands of bilayers which never exceed 75% surface coverage (for oxyethylene dodecyl ether on a silica sol) as determined by small-angle neutron scattering.17 Using modern microcalorimetric techniques,18 a four-step adsorption process for commercial polyoxyethylene alkyl ethers on spherosil silica was proposed. The four stages involved monomer adsorption, followed by hydrophobic association with increased surfactant concentration. Further increases in surfactant concentration corresponding to far less than 50% maximum adsorption resulted in a hydrophilic surface due to the formation of aggregates. The surfactant aggregates become equivalent to micelles in bulk at the maximum surface coverage. Their work was supported by contact angle data which described the hydrophobic-hydrophilic transition with increasing surfactant coverage.19 The proposed adsorbed structure was similar to the four-step adsorption isotherm proposed by other investigators.1-4,31 Using porous silica and TX100, cooperative aggregate formation with an average (13) Chandar, P.; Somasundaran, P.; Turro, N. J. J. Colloid Interface Sci. 1987, 117, 31-46. (14) McDermott, D. C.; McCarney, J.; Thomas, R. K.; Rennie, A. R. J. Colloid Interface Sci. 1994, 162, 304-310. (15) Levitz, P.; Van Damme, H.; Keravis, D. J. Phys. Chem. 1984, 88, 2228. (16) Lee, E. M.; et al. Chem. Phys. Lett. 1989, 162, 196. (17) Cummins, P. G.; Staples, E.; Penfold, J. J. Phys. Chem. 1990, 94, 3740. (18) Lindheimer, M.; Keh, E.; Zaini, S.; Partyka, S. J. Colloid Interface Sci. 1990, 138, 83. (19) Gonzalez, G.; Travalloni-Louvisse, A. M. Langmuir 1989, 5, 26.

© 1996 American Chemical Society

Adsorbed Ionic Surfactants on a Mica Substrate

Figure 1. Typical adsorption isotherm for cationic surfactants on silica substrates.

aggregation number for surfactant adsorption on mineral surfaces was proposed.20 Figure 1 shows a typical adsorption isotherm for an ionic surfactant adsorbing on an oppositely charged hydrophilic surface. As can be seen from the figure, surfactants in the aqueous phase usually follow a twostep adsorption isotherm.21 At concentrations lower than the critical micelle concentration (cmc), the adsorption is thought to be driven by electrostatic attractive forces between the ionic surfactant head group and the oppositely charged surface. Complete monolayer coverage results in the first-stage plateau. With an increase in concentration, surface aggregation around the initial adsorption sites occurs, which results in a reduced surface charge followed by reversal of surface charge at complete surface coverage near the cmc. The increase in adsorption after the first plateau is a result of aggregation of surfactant hydrocarbon tails. The association of surfactant molecules to form surfactant aggregates on surfaces is an entropically favored process due to the removal of hydrocarbons from the aqueous environment. This phenomenon, referred to as the hydrophobic effect, is analogous to micelle formation in solution.22 The hydrophobic effect results in enhanced surfactant adsorption, and an increase in surfactant concentration may result in charge reversal of the surface. At concentrations above the critical aggregate concentration, ionic surfactants are thought to form bilayers on mineral surfaces. Figure 1 shows such a surfactant bilayer structure near the cmc. With a further increase in surfactant concentration, a final adsorption plateau results at or near the cmc. The major driving force for the association of surfactant molecules into aggregates is the reduction in contact area between the hydrocarbon tails of the amphiphiles and the surrounding water molecules. The major factor which opposes the aggregation is the (20) Gu, T.; Zhu, B. Y. Colloids Surf. 1990, 44, 81. (21) Rupprecht, H.; Gu, T. Colloid Polym. Sci. 1991, 269 (5), 506522. (22) Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.; Wiley-Interscience: New York, 1989.

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decrease in the configurational entropy of the polar head group and the accompanying counterion. The electrostatic repulsive force between the charged head groups at the surface of the aggregates is also an important parameter which will ultimately determine the aggregation number. One of the most widely studied systems is the cationic surfactant cetyltrimethylammonium bromide (CTAB), which on mica, quartz, or glass surfaces follows the twoplateau adsorption isotherm.23 On the basis of experimental evidence, it has been postulated that CTAB adsorption on mica surfaces results in bilayer formation at or near the cmc. Bijsterbosch24 postulated the formation of a first adsorbed layer due to electrostatic interactions of positive ions on negative surfaces. The second layer or bilayer is formed with a further increase in surfactant concentration with hydrocarbon tails interacting with each other and the hydrophilic group pointing toward the solution. Other workers have also studied cationic adsorption (alkyltrimethylammonium and pyridinium bromides/chlorides) on silica surfaces.24-28 Adsorption of n-alkyltrimethylammonium ions on quartz29 and polystyrene surfaces30 has also been reported. As an alternative to the concept of monolayer and bilayer formation, some investigators propose the formation of aggregates,5-7,21 hemimicelles,1-4,29,31 and admicelles32 at or near the cmc. The hemimicelle concept was introduced to account for lateral interaction between the hydrocarbon chains. The authors1-4,33 coined the term “hemimicelle” for surfactant aggregates on surfaces. They postulated that at a certain minimum concentration there is an increase in surfactant adsorption due to association of hydrocarbon tails by their removal from the aqueous phase. All subsequent work by these authors34,35 retains the concept of hemimicelle or two-dimensional aggregate formation on surfaces. The admicelle structure of surfactant adsorption was proposed32 assuming surfactant aggregation as bilayers. Both the hemimicelle and admicelle concepts were based on principles of aggregation of micelles in bulk solution. A two-step adsorption model was proposed6-7,21 where the adsorbed structure consisted of small isolated aggregates of surfactant molecules being held to the surface by the electrostatic interaction of one or two surfactant molecules. Other surfactant adsorption models36,37 have addressed the effect of surface heterogeneities on surfactant adsorption. A patchwise adsorption model which incorporates bilayer adsorption, lateral interaction, and two-dimen(23) Pashley, R. M.; Israelachvili, J. N. Colloids Surf. 1981, 2, 169187. (24) Bijsterbosch, B. H. J. Colloid Interface Sci. 1974, 47, 186-198. (25) Tadros, Th. F. J. Colloid Interface Sci. 1974, 46 (3), 528-539. (26) Ralston, J.; Kitchener, J. A. J. Colloid Interface Sci. 1975, 50 (2), 242-249. (27) Wangnerud, P.; Jonsson, B. Langmuir 1994, 10, 3268-3278. (28) Gu, T.; Huang, Z. Colloids Surf. 1989, 40, 71-76. (29) Somasundaran, P.; Healy, T. W.; Fuerstenau, D. W. J. Phys. Chem. 1964, 68, 3562-3566. (30) Connor, P.; Ottewill, R. H. J. Colloid Interface Sci. 1971, 37 (3), 642-651. (31) Somasundaran, P. AIChE Symposium Series No. 150. 1975, 71, 1-15. (32) Harwell, J. H.; Hoskins, J. C.; Schechter, R. S.; Wade, W. H. Langmuir 1985, 1, 251. (33) Gaudin, A. M.; Fuerstenau, D. W. Trans. AIME 1955, 202, 6672. (34) Moudgil, B.; Soto, H.; Somasundaran, P. In Reagents in Mineral Technology; Somasundaran, P., Moudgil, B., Eds.; Surfactant Science Series 27; Marcel Dekker: New York, 1988, pp 79-104. (35) Fuerstenau, D. W.; Herrera-Urbina, R. In Surfactant-Based Separation Processes; Scamehorn, J. F., Harwell, J. H., Eds.; Surfactant Science Series 33; Marcel Dekker: New York, 1989; pp 259-320. (36) Cases, J. M.; Villieras, F. Langmuir 1992, 8, 1251-1264. (37) Cases, J. M.; Goujon, G.; Smani, S. AIChE Symp. Ser. 1975, 71, 100-109.

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sional phase transitions was developed.38 This work included the hydrophobic effect even in the region where the surface coverage was sparse. A pseudophase separation model of surfactant adsorption was presented32 which incorporated hydrophobic contributions, surfactant surface aggregation, surface heterogeneities, and counterion electrostatic effects. They considered the pseudophase separation approach and proposed the admicelle hypothesis for surfactant adsorption. The admicelle hypothesis proposes that surfactant aggregation resulting in a bilayer structure occurs on a given patch of a heterogeneous surface at a critical solution concentration which is specific to that patch. A rather different model for adsorption of weak organic electrolytes was presented39-41 which can be viewed as an extension of the hemimicelle concept. They described surfactant adsorption on the basis of the polymer-segment distribution at the interface. A detailed physicochemical model for ionic surfactant adsorption at an interface was developed42 which accounts for the influence of pH, electrolyte concentration, temperature, lateral interactions, and second-layer adsorption. The site-binding model, which has been used successfully to describe the effect of pH and electrolyte concentration on the electrical double layer, was used. By considering electrostatics using the Poisson-Boltzmann equation and by including the influence of dispersion forces, solvation, and steric interactions between the bilayer and the surface, a surfactant adsorption model has been recently proposed.43 In summary, the phenomenon of surfactant adsorption at solid surfaces has been described through a hemimicelle concept,1-4,29,34,35 an admicelle concept,32 isolated aggregates,5-7,21 and a two-dimensional condensation of the surfactant on heterogeneous surfaces.36,37 All these models postulate the same driving forces for adsorption: the Coulombic attractive forces between the surface and the ionic surfactant head group and the lateral hydrophobic attraction between the hydrocarbon chains. Although considerable advances have been made in understanding semiquantitatively the effect of various system properties, direct experimental evidence of the structure of the adsorbed surfactant aggregates on the surface is not available. The precise morphology of the adsorbed layer, i.e. monolayer, bilayer, hemimicelle, or isolated aggregates, has major implications for the effectiveness of such structures for applications such as lubrication and coatings. Ionic surfactant films44-47 and nonionic surfactant films48 have been studied using atomic force microscopy (AFM). Very recently, the effect of aqueous KBr concentration on the electrical double-layer properties of selfassembled cetyltrimethylammonium bromide (CTAB) (38) Scamehorn, J. F.; Schechter, R. S.; Wade, W. H. J. Colloid Interface Sci. 1982, 85 (2), 463-477. (39) Koopal, L. K.; Ralston, J. J. Colloid Interface Sci. 1986, 112, 362-379. (40) Koopal, L.; Keltjens, L. Colloids Surf. 1986, 17, 371-388. (41) Koopal, L. K. Adsorption. In Colloid Chemistry in Mineral Processing; Laskowski, J. S., Ralston, J., Eds.; Development in Mineral Processing 12; Elsevier Science Publishers: Amsterdam, 1992; pp 3790. (42) Philips, N. D. Ph.D. Dissertation, The University of Texas at Austin, Austin, 1990. (43) Wangnerud, P.; Jonsson, B. Langmuir 1994, 10, 3542-3549. (44) Drummond C. J.; Senden, T. J. Colloids Surf. 1994, 87, 217234. (45) Liu, Y.; Wu, T.; Evans, D. F. Langmuir 1994, 10, 2241-2245. (46) Manne, S.; Cleveland, J. P.; Gaub, H. E.; Stucky, G. D.; Hansma, P. K. Langmuir 1994, 10, 4409-4413. (47) Tsao, Y.-H.; Yang, S. X.; Evans, D. F. Langmuir 1991, 7, 31543159. (48) Rutland, M. W.; Senden, T. J. Langmuir 1993, 9, 412-418.

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surfaces was investigated49 using AFM. The forces between CTAB adsorbed on silica colloids and CTAB adsorbed on flat oxidized silicon wafers were measured. They concluded that a bilayer was present on the two surfaces on the basis of the force distance characteristics of the curve. The morphology of CTAB molecules adsorbed onto a graphite substrate was determined using AFM.46 They found parallel, epitaxially oriented stripes with a spacing of about twice the surfactant molecule length near the cmc. They concluded that the stripe pattern was indicative of cylindrical hemimicelles which are liquidcrystalline aggregates of surfactant molecules at the graphite surface. Noncontact double-layer repulsion between the tip and the graphite surface was used to image the surfactant structure. The molecular structures of monolayers of dihexadecyldimethylammonium (DHDA), dioctadecyldimethylammonium (DODA), and dieicosyldimethylammonium (DEDA) in air and at the micawater interface were obtained using AFM and a surface forces apparatus (SFA).47 They determined the spacing between the individual methyl groups and found the monolayers to be uniform and continuous over length scales of microns. The forces between a spherical silica particle and a smooth silica surface were measured as a function of the nonionic surfactant oxyethylene dodecyl ether using AFM.48 They found that, at a concentration of about one-third the cmc, an attractive force between the tip and the surface resulted in the surface being pulled into adhesive contact. However at concentrations above the cmc, repulsive steric forces were observed. In this paper we investigate the surface morphology of adsorbed surfactant molecules at surfactant concentrations ranging from 10-5 to 10-2 M. These topographic images are also obtained at low, intermediate, and high pH values. In addition, force versus distance curves are measured at different pH values and ionic strengths to investigate the nature of the adsorbed surfactant molecules. The AFM measurements are combined with contact angle measurements on mica surfaces at different surfactant concentrations to provide a more complete picture of the adsorbed surfactant molecules. Experimental Section Cetyltrimethylammonium bromide (CTAB) was obtained from Aldrich Chemical Company. Sodium hydroxide and hydrochloric acid were used to raise and lower the pH of the CTAB surfactant solution. An atomic force microscope (Digital Instruments) with silicon nitride tips was used for imaging. The mica substrates were subjected to thorough cleaning procedures which included rinsing with distilled water followed by a rinse with 2-propanol and methanol. Each mica sample was freshly cleaved just before use. The surfactant film was adsorbed on the surface of the freshly cleaved mica surface by immersing the mica in the surfactant solution for 24 h. For measurements in air, the mica surface was then dried and glued using an epoxy glue-hardener mix onto the circular stainless steel magnetic disk which was mounted on the piezoelectric scanner. The atomic force microscope generally operates by scanning a relatively sharp tip, attached to a cantilevered beam, across the sample in a raster pattern. A constant load is applied while the tip is scanned. In response to the forces between the tip and the sample, the cantilever is deflected. The interatomic force F between the tip and the sample is proportional to k dx, where dx is the variation in height (i.e. the surface topography). The apparatus was operated at a minimum interaction force (DLVO mode) during surface topographic studies, as described below. An AFM fluid cell which houses the cantilever spring is used to form the surfactant films on the substrate in an aqueous (49) Johnson, S. B.; Drummond, C. J.; Scales, P. J.; Nishimura, S. Langmuir 1995, 11, 2367-2375.

Adsorbed Ionic Surfactants on a Mica Substrate

Figure 2. Tip deflection curve at 10-5 M surfactant concentration. medium. The seal for the liquid is provided by a Teflon O-ring which is fitted into a circular groove between the substrate and the cell. The O-ring, connectors, and syringes were thoroughly rinsed with deionized water before use. The aqueous surfactant solutions were injected into the AFM cell via external fluid ports. Before the surfactant solution was introduced, distilled water was first injected into the fluid cell and the mica surface was imaged. This was followed by repeated injection of surfactant solution at a fixed concentration at an interval of 1 h each. The surface force curves and the surface topography were imaged at the lowest surfactant concentration. After imaging, the fluid cell was flushed repeatedly with the next higher concentration of surfactant solution. The surfactant concentration was raised from 10-5 M (below the cmc) to 10-2 M (above the cmc). Surface force curves and surface topography were measured at each concentration. Contact angles were measured using a Rame-Hart goniometer which consisted of a light source, a microscope, and a fluid cell. The goniometer cell was first rinsed thoroughly with acetone and chloroform followed by deionized water. The decane used was analytical grade. Surfactant solutions were prepared in distilled water. Freshly cleaved mica surfaces were used in all the experiments. The cell was filled with the surfactant solution at the lowest concentration (10-5 M). A freshly cleaved mica surface was placed in the cell and allowed to equilibrate with the surfactant solution for 12 h. A sessile drop of decane was then pressed under the equilibrated mica slide in the fluid cell with the surfactant solution. The contact angle was measured at a particular surfactant concentration. The surfactant concentration in the fluid cell was then increased in steps from 10-5 to 10-2 M.

Results and Discussion The surface of the silicon nitride tip is hydrolyzed in aqueous solutions and consists of basic silazane, silylamine (Si2-NH and Si-NH2), and ionizable amphoteric silanol groups (Si-OH).44 At low concentrations, below the cmc, surfactant adsorption on mica is expected as a monolayer and, since the tip has a surface structure similar to that of mica, a monolayer is expected to form on the tip as well. Hence one expects hydrophobic behavior for both the surface and the tip. This should result in an attractive hydrophobic force between the mica sample and the tip. Figure 2 shows the measured force versus distance curve at a surfactant concentration of 10-5 M. When the distance between the tip and the surface is large, there is no deflection of the end of the cantilever (a horizontal line that represents zero force or tip deflection). As the substrate approaches the tip, the cantilever feels an attractive force which is a result of the hydrophobic nature of both the surfaces. When the attractive force becomes larger than the spring constant of the cantilever, the cantilever tip jumps into contact with the substrate. This results in the large attractive minimum seen in Figure 2. Further movement of the sample causes the cantilever

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Figure 3. Surface topography at 10-5 M surfactant concentration.

Figure 4. Contact angle measurements as a function of surfactant concentration.

tip and the substrate to move in unison, giving rise to the constant compliance line. During the retraction mode of the scanner, the cantilever experiences a considerable adhesive force, after which it snaps-off the CTAB-coated mica surface. During retraction of the scanner away from the cantilever, the cantilever tip moves with the substrate, usually beyond the "jump-in" point due to the large attractive hydrophobic forces. Increasing the distance between the cantilever tip and substrate further results in snap-out of the cantilever tip, which returns back to its zero deflection position. Figure 3 shows the corresponding surface topography as measured by AFM at a surfactant concentration of 10-5 M. The surface topography shows patches of different height which may be surfactant molecules or aggregates of molecules. The surfactant patches are widely separated, and the distance between the patches appears to be variable. The average height of the peaks is comparable to the length of the CTAB molecule (1.5 nm). Figure 4 shows that the contact angle measured at this concentration is 140°, which is clearly indicative of the hydrophobicity of the surface. However, the surface topography indicates that molecules are adsorbed not as a complete uniform monolayer but as small isolated aggregates. Figure 5 shows the surface topography at pH 8. It can readily be observed that at higher pH the number of surfactant patches increases and the distance between the aggregates is reduced. It is expected that high pH will result in higher surface charge density on the mica surface, resulting in a larger number of sites available for surfactant adsorption. This is consistently observed in all our experiments with varying pH, reinforcing our hypothesis that the observed patches are electrostatically adsorbed surfactant molecules or aggregates. Figure 6 shows the force versus distance curve at a surfactant concentration of 10-4 M. The corresponding

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Figure 5. Surface topography at 10-5 M surfactant concentration (pH 8). Figure 8. Tip deflection curve at 10-3 M surfactant concentration.

Figure 6. Tip deflection curve at 10-4 M surfactant concentration. Figure 9. Force versus distance curve at bilayer and monolayer concentration.

Figure 7. Surface topography at 10-4 M surfactant concentration.

surface topography is shown in Figure 7. Again at this increased concentration, Figure 6 clearly indicates that as the surface approaches the tip, hydrophobic forces cause a strong attractive minima in the force curve. Another interesting feature is seen on the force curves at this concentration. A second “dip” is seen just before contact. As discussed later, this may correspond to the removal of CTAB monolayers between the two solid surfaces. The surface topography shows more closely packed surfactant patches with roughly the same height as before but with the distance between the patches being markedly smaller. The surfactant aggregates not only are more closely packed but also form a much more well defined structure. With an increase in surfactant concentration, more surfactant molecules are available to be adsorbed either as individual molecules or as aggregates on the surface, increasing surface coverage. The corresponding contact angle (Figure 4) shows a value of 150°, indicative of strongly hydrophobic surfaces.

Figure 8 is the force curve at a surfactant concentration of 10-3 M. At this concentration, we observe that as the mica substrate approaches the tip, the cantilever no longer feels an attractive hydrophobic force. Instead a repulsive force is experienced by the cantilever which increases in magnitude with decreasing separation distance between the sample and the tip. The repulsive force is indicative of the hydrophilic nature of the surfaces. Hydrophilic surfaces at this concentration imply the formation of aggregates (bilayers, hemimicelles). At this concentration two additional steps are visible in the repulsive barrier part of the surface force curve. The dual step-ins in the repulsive barrier part of the curve are reproducible and are a strong indication of layers of CTAB being removed from the film region as the surfaces are brought closer together. Figure 9 shows the corresponding force versus distance curve. At a tip deflection of about 3.3 nm corresponding to a force of 0.4 nN, the first step-in is observed. The second step-in is observed at a tip deflection of 1.5 nm (corresponding to a force of 0.7 nN). We interpret these step-ins to correspond to removal of layers of CTAB molecules. The change in separation distance between the tip and substrate due to the jump corresponds to approximately 1.5 nm (The distance between the dual jump-in corresponds to about 1.8 nm.), which is roughly equal to the length of a CTAB molecule. It is important to note that one step is observed at low concentrations of CTAB whereas two steps are observed at higher concentrations where bilayer type structures are expected. It should also be noted that no steps are observed during the retraction of the tip from the substrate. This is consistent with our explanation of removal of layers of surfactant molecules from the tip and substrate during approach.

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Figure 12. Surface topography at 10-3 M surfactant concentration (pH 3).

Figure 10. Surface topography at 10-3 M surfactant concentration.

Figure 11. Top surface topography at 10-3 M surfactant concentration.

The contact angle as indicated by Figure 4 shows a value of 25°, which is a strong indication of the hydrophilic nature of the surface at this concentration. Figure 10 shows the surface topography of the mica substrate. It is clearly seen that the patches form a more continuous structure with a much reduced separation between the patches as compared to that for 1 × 10-4 M. We also observe that small patches or aggregates seem to have combined to form a continuous “wormlike” adsorbed admicellar structure. The continuous, elongated structure has a distance of approximately 0.1 µm between the stripes. The network seems to be in a zig-zag pattern with about the same distance between stripes. This “wormlike” surfactant aggregate is remarkable in its consistency over the entire surface. The patches are no longer separated but seem to be connected over a domain spanning several clusters in one dimension. Figure 11 shows the AFM top image of the same mica substrate as in Figure 10. We readily notice the continuous zig-zag stripe structure on the substrate at this surfactant concentration. It is evident from the above discussion that the traditional picture of bilayer formation on mica substrates is overly simplistic. Our AFM results indicate that adsorption of CTAB occurs in patches or aggregates that grow around CTAB molecules that are adsorbed due to electrostatic interactions. The driving force for aggregation is the hydrophobic interactions between the surfactant tails. At higher surfactant concentrations, these aggregates become continuous in one-dimension, forming zig-zag adsorbed micellar structures. The structure of these adsorbed micelles should depend on the structure of the underlying substrate and the surface charge distribution on it.

To demonstrate this effect, surface topographic images were collected at different pH values at a surfactant concentration of 10-3 M (same concentration as used above). Figure 12 shows the image at pH 3 for a 1 × 10-3 M surfactant concentration. We no longer observe the continuous striped cylindrical network. Instead more isolated islands of surfactant aggregates can be observed at low pH. It is probable that, at low pH due to reduced surface charge density, surfactant aggregates will tend to be adsorbed as isolated patches and no longer form a continuous cylindrical structure because the patches are too far away from each other. At a pH of about 7, the mica surface is negatively charged and large electrostatic adsorption results in a continuous network of surfactant aggregates with increasing surfactant concentration. At high pH, one readily notices increased surfactant adsorption. Adsorbed zig-zag micellar structures are observed at a surfactant concentration above 10-3 M. The substrate charge distribution must, therefore, play an important role in determining the final shape of the adsorbed surfactant aggregate or bilayer structure. Data on the adsorption density of CTAB on quartz as a function of surfactant concentration and pH have been presented by several researchers. At CTAB concentrations in the range 10-4 to 10-3 and pH values between 6 and 9, the adsorption density varies from 0.2 to 4 µmol/ m2. This corresponds to an area per molecule from approximately 800-40 Å2/mol. Clearly even at surfactant concentrations of 10-3 M and at a pH of 9, the CTAB molecules are not close packed. The increase in surface coverage with pH clearly implies electrostatic interactions with surface sites are important. With a limited number of charged surface sites, additional surfactant molecules will tend to form aggregates through hydrophobic interactions with surface-adsorbed molecules. The force curve at a surfactant concentration of 10-2 M shows trends very similar to those shown in Figure 8 with two step-ins and a strong repulsive force with a reduced tip-sample approach distance. The contact angle as seen in Figure 4 measured at this concentration shows a value of about 20°. Both these observations are a strong indication of a hydrophilic surface and a surfactant bilayer on the surface. Figure 13 shows the surface topography at 1 × 10-2 M surfactant concentration. One readily notices a continuous network of adsorbed surfactant aggregates (a continuous bilayer). The structure is similar in nature to that observed at 10-3 M CTAB concentration. Conclusion It appears from the surface force curves and the contact angle measurements that at low CTAB concentrations (1 × 10-5 M) adsorption takes place (driven by electrostatics) as sparse patches of surfactant molecules on the surface (50) Demond, A. H.; Desai, F. N.; Hayes, K. F. Water Resour. Res. 1994, 30 (2), 333-342.

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Figure 13. Surface topography at 10-2 M surfactant concentration.

forming a hydrophobic surface. An increase in surfactant concentration results in an increased number of patches or surfactant aggregates, which tend to combine to form a more continuous adsorbed micellar structure. The

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surfactant micelles seem to grow in the form of a continuous bilayer that forms a zig-zag pattern across the substrate. At surfactant concentrations higher than 10-3 M, a repulsive force is observed, indicative of a hydrophilic structure. The contact angle data corroborate our AFM observations; i.e., the contact angle decreases from 150° to 25°, which indicates a hydrophilic surface. The surface topography at concentrations higher than the cmc indicates that the continuous surfactant network grows further, resulting in parallel striped surfactant aggregates with a reduced distance between the striped aggregates. These cylindrical surfactant aggregates appear to be curved and wormlike in shape and form only at high pH values (7 or greater). At low pH no such shapes are observed. Isolated islands of surfactant aggregates are observed that do not organize into adsorbed wormlike micelles perhaps because they are too far apart. LA960537J