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Direct Visualization of Surfactant Hemimicelles by Force Microscopy of the Electrical Double Layer S. Manne,*’t J. P. Cleveland,$ H. E. Gaub,? G. D. Stucky,@and P. K. Hansma$ Lehrstuhl fur Biophysik E22, Technische Universitat Miinchen, 85748 Garching, Germany, Department of Physics, University of California, Santa Barbara, California 93106,and Department of Chemistry, University of California, Santa Barbara, California 93106 Received July 1, 1994. I n Final Form: September 28, 1994@ The morphology ofionic surfactant molecules adsorbed from aqueous solutiononto hydrophobicsubstrates has been determined by atomic force microscopy. Near the critical micelle concentration(cmc),noncontact imaging using double-layer repulsion between the tip and sample shows parallel, epitaxially oriented stripes spaced apart by about twice the surfactant length. This represents the first direct imaging of “hemimicelles”,liquid-crystallineaggregates of amphiphilic molecules (analogousto bulk micelles)which form at interfaces. The striped pattern is indicative of hemicylindrical hemimicelles, which is further corroborated by images of the monolayer adsorbate (in contact mode) below the cmc. Our results suggest that the hemimicelle structure is templated by the epitaxially bound monolayer, in contrast with previous interpretations of the adsorption mechanism.
Introduction Understanding the adsorption mechanism of surfactant molecules a t the solid-liquid interface is a n important step toward modeling industrial processes which use surfactants on a large scale, such as detergency, water purification, oil recovery, and ore refinement by fl0tation.l In addition, adsorption behavior can give useful clues to the chemistry of the surfactant molecules themselves. Their strong intermolecular interactions in bulk solution leads to a variety of self-assembled liquid-crystalline structures (micelles) which have been well-studied.2 At a n interface, however, the normal self-assembly process is perturbed by competing surfactant-surface and solvent-surface interactions, which can in principle lead to novel structures termed “hemimi~elles”.~ Less is known about these structures than about their bulk-solution counterparts; although properties such as the critical c ~ n c e n t r a t i o nand ~ ~ aggregation number3f of such hemimicelles have been calculated theoretically, a lack of direct evidence has hindered efforts to determine the structure and shape of these proposed aggregates. Over the last few decades, the adsorption characteristics of a wide variety of surfactant-solvent-substrate systems have been investigated, traditionally by adsorption isotherms: and more recently by fluorescencedecay: neutron reflection,6 and the surface force apparatus (SFA).’ Ionic surfactants in aqueous solutions usually follow a twostep adsorption mechanism as a function of surfactant
* Author t o whom correspondence may be addressed. f
Lehrstuhl fiir BiophysikE22, Technische Universitat Miinchen.
+ Department of Physics, University of California. Department of Chemistry, University of California. Abstract published in Advance ACS Abstracts, November 1, 1994. (1)Adamson, A. W. Physical Chemistry of Surfaces, 5th ed.; John Wiley & Sons: New York, 1990;Chapter MII. (2) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed., Academic Press: London, 1992. (3)(a) Gaudin, A. M.; Fuerstenau, D. W. Trans. AIME 1966,202, 958. (b)Wakamatsu, T.;Fuerstenau, D. W. Adu. Chem. Ser. 1968,No. 79,161-172. (c) Fuerstenau, D.W.; Wakamatsu, T. Faraday Discuss. Gu, T. J.Chem. SOC., Faraday Chem.Soc. 1976,59,157.(d)Zhu, B.-Y.; Trans. 1989,85,3813-3817.(e) Gu, T.;Huang, Z. Colloids Surf. 1989, 40, 71-76. (0Gu, T.;Rupprecht, H. Colloid Polym. Sci. 1990,268, 1148-1150. (4)Kipling, J. J. Adsorption from Solution of Non-Electrolytes; Academic Press: London, 1965. (5)Chandar, P.; Somasundaran, P.; Turro, N. J. J. Colloid Interface Sci. 1987,117,31-46. (6)McDermott, D.C.;McCamey, J.;Thomas, R. K.; Rennie, A. R. J. Colloid Interface Sci. 1994,162,304-310. 5
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concentration. At low concentration ( 10%of the critical micelle concentration or cmc) the adsorption is driven primarily by surfactant-surface interactions, resulting in a low surface density of adsorbed molecules. As the solution concentration is increased, the adsorbate density also increases until it levels off past the cmc; this increase often occurs rapidly near the c ~ c although , ~ ~ gradual increases have also been observed starting well below the c ~ c In. this ~ ~regime, the adsorption is driven primarily by interactions between surfactant molecules themselves, until a saturation coverage is reached. This two-step behavior is true for both charged and hydrophobic surfaces, although different physical mechanisms of adsorption have been proposed for the two cases. The adsorption of ionic surfactants onto oppositely charged hydrophilic substrates is thought to be driven initially by electrostatic attraction between headgroups and surface charges, followed by surfactant aggregation around these initial adsorbates as the concentration is increased. Thus as adsorption proceeds, the surface electric field is first reduced a s a result of charge neutralization, then reversed as a result of surfactant aggregation. For the specific case of cetyltrimethylammonium bromide (CTAB) on silica a t neutral pH, the adsorbate structure is thought to be a bilayer above the cmc8(although hemispherical aggregates have also been proposed3e). For ionic surfactant adsorption on hydrophobic surfaces (primarily graphite), there is general agreement that the lower-density adsorbed layer (below the cmc) is oriented with surfactant tails parallel to the substrate plane (Figure lA),driven by a strong hydrophobic interaction between the tails and the s ~ b s t r a t ethe ; ~ measured occupation area per molecule agrees well with the cross sectional area along the molecular axis.l0 However, there have been two schools of thought regarding the higher-density adsorbate structure. Some authorsl0J propose a gradual (7)(a) Pashley, R. M.; Israelachvili, J. N. Colloids Surf. 1981,2, 169-187. (b) Pashley, R. M.; McGuiggan, P. M.; Hom, R. G.;Ninham, B. W. J. Colloid Interface Sci. 1988,126,569-578. (c) Kekicheff, P.; Christenson, H. K.; Ninham, B. W. Colloids Surf 1989,40,31-41. (8)Iler, R. K. The Chemistry of Silica; John Wiley & Sons: New York, 1979;pp 680-687. (9)Electrostatic interactions with the headgroups are considered relatively less important since the pH of our surfactant solution (5.56.0)is very close to the isoelectric point of the graphite surface. (pH -5.8;see Lau, A. C.; Furlong, D. N.; Healy, T. W.; Grieser, F. Colloids Surf. 1986,18, 93-104.) (10)Zettlemoyer, A. C. J Colloid Interface Sci. 1968,28,343-369.
0743-746319412410-4409$04.50/0 0 1994 American Chemical Society
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4410 Langmuir, Vol. 10, No. 12, 1994
force curves and images were obtained in surfactant solution by a commercial AFM16 using dc mode beam deflection. Two types of tip-sample interactions were used to image the adsorbate structure: standard contact interaction (“constantcompliance” region of the force curve17), and a precontact double-layer repulsion (discussed below). CTAB (99% purity) was used as supplied, without further purification; water was deionized, treated with an active charcoal filter, and deionized again to a resistivity 18 MQ cm. All experiments were performed at room temperature, 22-25 “C.
n n n n n
C
Results and Discussion Figure 1. A previously proposed adsorption model (which is not consistent with the results presented in this work) for ionic surfactant molecules at the graphite-aqueous solution interface (after Zettlemoyer12). This model attempts t o explain an observed increase in adsorbate density with increasing surfactant concentration in solution. (A) At low concentration (-10% of the cmc), molecules adsorb with their alkane chains extended on the substrate plane. (B)As the concentration is increased, the chains gradually desorb so that a portion of the adsorbate molecule is oriented perpendicular to the substrate. (C)At concentrationsnear the cmc the adsorbates are oriented perpendicular t o the substrate plane, with the hydrophilic headgroups completely shielding the hydrophobic substrate from solution. The surfactantmolecule is drawn approximately t o scale for CTAB. change from a parallel to a perpendicular adsorption scheme (Figure 1)as the concentration approaches the cmc, citing evidence that the occupation area per molecule approaches the headgroup area. However, others12propose hemimicelle formation (due to micelle collapse near the surface) as being responsible for the increased adsorption near the cmc. Since these conflicting models for hydrophobic adsorbents predict similar overall adsorbate density and degree of surface ionization, it is difficult to resolve this issue using surface techniques which average over ‘more than a micellar area. Consequently we used an atomic force microscope (AFM)13to image the adsorbate structure of a n ionic surfactant, both above and below the cmc, on a hydrophobic substrate. We present results which indicate the formation of hemimicelles above the cmc, although not in the configurations envisioned previously. The adsorbed layer below the cmc is also shown to differ slightly from previous expectations. Finally we discuss the possible role played by the low-density adsorbate in determining the structure of the hemimicelles.
Experimental Section We investigated the adsorption and self-assembly of the cationic surfactant CTAB (cetyltrimethylammoniumbromide, C1&33N+(CH&Br-, cmc = 0.9 mM) at the interface between an aqueous solution and a hydrophobic substrate, namely the cleavage plane of highly oriented pyrolitic graphite.14 Silicon cantilevers16 exposed to ultraviolet light for 0.5-2 h gave the best imaging results, although unmodified silicon nitride cantilevers16also worked. Surfactantadsorptionwas accomplished merely by introducing an aqueous solution of CTAB into the fluid cell and allowing the tip and freshly-cleaved substrate to stand in this solution for a few minutes before operation; adsorption occurred within 2 min of introducingthe solution.All ~
(11) Greenwood, F. G.;Parfltt, G. D.; Picton, N. H.; Wharton, D. G. InAdsorptwnfromAqueous Solutions;Weber, W. J., Matijevic,E., Eds.; American Chemical Society: Washington, DC, 1968; pp 135-144. (12) Koganovskii, A.M. Colloid J . USSR 1962,24, 702-708. (13) (a) Binnig, G.; Quate, C. F.; Gerber, Ch. Phys. Rev. Lett. 1986, 56,930-933. (b)Rugar, D.;Hansma, P. K. Phys. Today 1990,43,23Oxford University Press: 30. (c) Sarid, D. Scanning Force M~CFOSCOP~; New York, 1990. (14) Courtesy of Union Carbide Corp.(Cleveland, OH); graphite is 0.5%boron-doped. (15) Park Scientific Inc., Palo Alto, CA. (16) Digital Instruments, Santa Barbara, CA.
Below the cmc, adsorption of CTA+is expected to charge the graphite surface but neutralize the tip surface, which is chemically similar to silica; consistent with this expectation, force vs distance curve@ showed no double layer forces at these concentrations (nor in pure water). As the CTAB concentration in solution was increased, however, force curves began to show long-range repulsion around the cmc (Figure 2). Since substantial surfactant adsorption is expected on both the tip and sample surfaces at this concentration, the long-range repulsion is analogous to a stabilization force between solubilized colloidal particles. This force was exponential a t moderate separations but grew more steeply a t small separations, similar to previous AFM17 and SFA7 force data on surfactantadsorbed surfaces. The exponential decay length became smaller in electrolyte (Figure 21, indicating that the force arises from electrical double layer repulsion,lg which is expected since surfactant aggregation should charge both the tip and sample.20 Under conditions of low thermal drift, it proved possible to use this long-range repulsion as the imaging force, resulting in a “double-layer force map” (i.e., charge map) of the adsorbate layer without contacting the sample. (Quite recently, Senden et aZ.21used a similar technique to image silicon nitride surfaces in electrolyte.) Parts A and B of Figure 3 show such double-layer images taken at a CTAB concentration of about 0.8 mM; similar images were obtained up to concentrations of 5 mM. The adsorbate images showed approximately parallel stripes spaced 4.2 f 0.4 nm apart, i.e., about twice the length of the CTA+ ion.22 The adsorbed stripes were generally observed to occur in three orientations (all evident in Figure 3A); these orientations were perpendicular to the three symmetry directions of the graphite lattice, as determined by lattice images (not shown). Langmuir isotherm data, as mentioned above, have indicated a high adsorbate density starting around the cmc, which is consistent in our case with the presence of double-layer repulsion. However the adsorbate images showing stripes and grain boundaries cast doubt on the model (Figure 1) that the adsorbed molecules switch from a parallel to a perpendicular adsorbtion scheme, since no adsorbate (17) Rutland, M. W.; Senden, T.J. Langmuir 1993,9, 412-418. (18)Weisenhorn,A. L.; Hansma, P. K.; Albrecht,T. R.; Quate, C. F. Appl. Phys. Lett. 1989,54, 2651-2653. (19) For other AFM measurements of electrical double layer forces, see (a) Ducker, W. A,;Senden, T.J.; Pashley, R. M. Nature 1991,353, 239-241. (b) Butt, H.-J. Biophys. J . 1991, 60, 1438-1444. (20) Steric and hydrationforces have also been documented between surfactant layers at very small separations(pp 304-307 and 399-408 of ref 2), with measured decay lengths typically 1.0 nm (beforejump-in)is more consistent with double layer forces; however additional steric and hydration contributions are also possible. (21) Senden, T. J.;Drummond, C. J.;Kekicheff,P. Langmuir 1994, 10,358-362. (22) The length of a heptadecane chain adsorbed on graphite has been measured to be 1.9 nm (see McGonigal, G. C.; Bernhardt, R. H.; Thomson, D. J. Appl. Phys. Lett. 1990, 67, 28-30). Scaling this to a 16-carbonchain and adding the ionic radius of a trimethylammonium headgroup (0.35 nm; pp 109-112 of ref 2) gives an expected length of 2.1 nm for CTA+ adsorbed on graphite.
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Figure 2. (A, top) AFM force curve on graphite in -0.8 mM CTAE! (-90% of the cmc). As the sample approaches the tip (right to left on the force curve), the cantilever feels a longrange repulsive force starting at a separation of -40 nm. This repulsive force grows in magnitude until the cantilever irreversibly jumps to the sample surface, after which the cantilever and sample move together (constant compliance regime). (During retraction the cantilever experiences considerable adhesion before a sudden snap-off.) The long-range repulsion can be used for imaging in the same way as contact repulsion, by using feedback mode and selecting an imaging force within the noncontact portion of the curve as indicated. (B, bottom)Semilogplots offorce vs true tip-sample separation on graphite for 0.8 mM CTAE!solution with and without added electrolyte (50 mM NaCl). These are AFM force curves converted by mapping the constant-compliance region into a vertical line at zero tip-sample separation.lgaThe linearity of the data points, at separations >5 nm, indicates an exponential double layer force. (MeasuredDebye lengths ranged from 7.0 to 9.0 nm for plain CTAl3, and from 1.5 t o 2.0 nm for CTAB NaC1; the expected theoretical values for 1:lelectrolytes are 11 and 1.3 nm, respectively.)
+
structure should then be visible (at > 1 nm scale) above the cmc. Therefore we propose that Figure 3 shows direct images of CTAB hemimicelles on the graphite surface.23 These hemimicelles apparently have a linear structure (stripes),24 and the hemimicellar stripes are separated from each other by twice the molecular length. One simple way to account for these observations is to propose that CTAB forms roughly hemicylindrical hemimicelles on graphite (Figure 3 0 . Such a model is also consistent with three key earlier observations. First, the model agrees with Langmuir isotherm data showing that the occupation area per molecule (in the high-density adsorbate structure) is close to the headgroup cross section.1° (23) Theterm'%emimicelle" was originallyused to describe interfacial . ~ ~ the aggregation which occurred well below the bulk c ~ c However term has since been generalized to interfacial aggregates at any concentration that form specifically as a result of attraction between amphiphilic molecules.3e We use the term in this latter sense.
Second, it makes the graphite surface hydrophilic by exposing only ionic headgroups to the aqueous solutionconsistent with data showing a steep reduction in contact angle with increasing surfactant concentration." Third, it maximizes the area of interaction between surfactant tails and the substrate plane, a n interaction which is known to be quite strong. Published thermodynamic data have in fact indicated that adsorption of a n ionic surfactant molecule a t the graphite surface is energetically favored over incorporation into a micellar e n v i r ~ n m e n tcreat,~~ ing difficulties with the chain desorption model of Figure 1. It is natural to expect that hemimicelles at the cmc and above build on the monolayer structure established at low concentrations (i.e., in the first adsorption step). The proposed hemimicelle model (Figure 3C) thus implies that the adsorbate molecules at low concentration arrange themselves parallel to the substrate plane (as suggested by adsorption isotherm data), but with heads neighboring heads and tails neighboring tails; the implied periodicity is twice the molecular length. While such a head-to-head arrangementhas been proposed26and observedz7for other adsorbates (e.g., fatty acids) which can form hydrogen bonds between headgroups, a simpler head-to-tail arrangement (as in Figure 1A)has often been assumed1° for ionic surfactants since the headgroups are expected to repel each other electrostatically. Nevertheless, when we imaged the adsorbate structure under low surfactant concentration, we observed a periodicity of twice the molecular length (Figure 4), corresponding to a head-tohead arrangement and further supporting the proposed model of hemicylindrical hemimicelles. In view of these results, we suggest that the two-step adsorption isotherm of CTAB (and probably other ionic surfactants) on graphite can be interpreted in the following way. At low concentrations, a monolayer of surfactant binds strongly to the surface (driven by the hydrophobic interaction), forming a solid film which does not exchange much with solution. These molecules self-assemble horizontally on the graphite plane, parallel to each other (within a single two-dimensional domain) and along a graphite symmetry axis. (STM data have shown similar epitaxy for a variety of alkanes and alkyl derivatives on graphite.27) The monolayer self-assembles with a headto-head, tail-to-tail arrangement. This serves as a template for the growth of (roughly) hemicylindrical hemimicelles near the cmc, which is driven by hydrophobic interactions between the exposed alkane chains of the initial monolayer and those of molecules in solution.28 The hemicylindrical shape is unexpected, since earlier m ~ d e l s have ~ ~ Joften ~ assumed that hemimicelle shape is related to bulk micelle shape, which is spherical for CTAB (24) Sometimes the boundary between two stripe domains shows a narrow (-5-15 nm) region where both orientations are visible (e.g., bottom right corner of Figure 3A), creating a cross-hatched pattern. This can in principlebe a real feature ofthe adsorbatestructure-either a static spatial transition or a dynamic region where stripe orientations change rapidly on the time scale of imaging. However, tip resolution limits seem a more likely explanation. First, even a perfectly sharp tip which follows an exponential force contour will interact with an adsorbate radius of -1 Debye length before the force decays laterally. Second, the tip itself probably has a radius of tens of nm, further increasing the lateral area of interaction with the adsorbate. (25) For the specific example of sodiumdodecyl sulfate,the free energy change of micellizationin water is roughly -8kT per molecule (from p 355 of ref 2), whereas the enthalpy of monolayeradsorptionon graphite has been reported as -15kT per molecule;1°the free energy change for adsorption should be even more negative, since entropy undergoes a net increase as a result of hydrophobic aggregation (k = Boltzmann's constant; T = room temperature). (26) Kipling, J. J.; Wright, E. H. M. J . Chem. SOC.1962, 855-860. (27) (a)Yeo, Y. H.; Yackoboski, K.; McGonigal, G. C.; Thompson, D. J. J . Vac.Sci. Technol. 1992,AI0,600-602. (b) Rabe, J. P.; Buchholz, S. Science 1991,253, 424-427.
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4412 Langmuir, Vol. 10,No. 12, 1994
- Symmetry Axis
Figure 3. Images and proposed model of the adsorbate structure of CTAB on graphite at a solution concentration of -0.8 mM (similarimages have been obtained at concentrationsup to 5 mM). Images were obtained in noncontact mode using double layer forcesbetween tip and sample (Figure2). (A, top left)(Imagesize 240 x 240 nm, z-range 1.2 nm.) The adsorbate structure is imaged as stripes which are spaced 4.2 f 0.4 nm apart (about twice the length of the adsorbed surfactant),organized into two-dimensional domainsin which all the stripes are parallel. The three domain orientations evidentin this figure are generallythe only orientations that are observed, and they are related t o the symmetry directions of the underlying substrate (see text). (B, top right) (Image size 300 x 300 nm.) Thermal drift of the cantilever occasionally caused an irreversiblejump to the surface during imaging. In this image (sample scanned from bottom to top) the jump-in occurred a little over halfway up, showing a contact-mode image at the top in which the adsorbate stripes are still slightlyvisible. (C,bottom)We propose that the stripes are images of hemicylindrical hemimicelles on the graphite surface. This schematic shows a perpendicular cross section through two neighboring hemimicelles (i.e., the cylindrical axis is into the page). A graphite symmetry axis is shown running left t o right, consistentwith the observation that stripes are oriented perpendicularto symmetry axes. The bottom molecules (shaded)are probably bound expitaxiallyby the graphite surface, while the rest of the hemimicelle is more dynamic, with the schematic representing a typical snapshot. Some bound counterionsare shown to suggest that the hemimicelle is not entirely dissociated (CTABmicelles and bilayers are evidently only -20% dis~ociated);~~ however the actual degree of counterion binding in the hemimicelle is not known, since the surfacecharge was not measured. The number of molecules shown in each hemimicelle is somewhat arbitrary (althoughthe molecules are drawn roughly to scale) and is not meant to suggest an actual aggregation number.
at concentrations not far above the cmc. Thus hemispherical hemimicelles (with diameters lying on the substrate plane) have been proposed for CTAB on graphite,12requiring somehow a surface rearrangement of the monolayer into circular patches prior to (or simultaneous with) hemimicelle formation. Our results, on the other hand, suggest that the presence of the head-to-head monolayer changes the thermodynamics of aggregation so that cylindrical curvature is favored at the surface. Since the initially adsorbed monolayer thus seems to play a crucial templating role in determininghemimicelle structure, a relevant question is why the monolayer selfassembles with headgroups neighboring each other. We (28) The hemicylinder model is roughly consistent with the length of the irreversible jump to contact measuredon the force curve (Figure 2). This distance is 3.4 nm, which is close to a CTAB monolayer thickness (1.6 nmn) a micelle radius (2.1 nm; calculated from the measured radius of the 12-carbonanalog in Chatenay, D.; Urbach, W.; Messager, R.; Langevin, D. J . Chem. Phys. 1987,86,2343-2351). This suggests that as the tip and sample jump into contact, the hemimicelle on the sample and one monolayer on the tip are expelled, and the alkane chains of the remaining tip monolayer come into strong adhesive contact with the hydrophobic sample (analogusto irreversible flocculationof colloidal particles). SFAresults have shown simlar“flocculation”of mica surfaces with adsorbed CTAl3 bilayer^.^"
+
suggest three possibilities. First, the structure of bulk crystalline CTAB also implies a head-to-head packing, since it has a long-axis periodicity of 5.2nm.29 The CTA+ ions in bulk are presumably stabilized by an equal number of Br- ions in an arrangement similar to that of an ionic crystal. There is good reason to suspect that a similar association with Br- ions occurs in the monolayer structure, thus stabilizing the headgroups against electrostatic repulsion; namely, previous studies have shown that CTAB micelles and bilayers are both only -20% dissociated, with the rest of the surfactant charge neutralized by bound counterions.7bA second possible reason for stable head-to-head packing is the moderate electrical conductivity of graphite, which leads to attractive image forces between the headgroups and the graphite surface. This effectively means that an adsorbed headgroup emanates not a monopole electric field but a weaker dipole field (due to headgroup image), reducing the repulsive interaction with a neighboring adsorbate. Lastly, an attractive hydrophobic interaction between the outer
+
(29) Donnay, J. D. H., Ondik, H. M., Eds. Crystal Datu: Organic Compounds, 3rd ed.; National Bureau of Standards: Washington, DC, 1983; Vol. 3.
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Langmuir, Vol. 10,No.12,1994 4413
groups can achieve arbitrarily close separations whereas the charges (centered a t the nitrogen atoms) are always separated by a t least the headgroup diameter (0.69 nm).
Figure4. Contact mode image (200 x 200 nm)of the adsorbate structure of CTAB on graphite below the cmc (concentration 0.3 mM). The spacing between stripes is 4.5 nm, again about twice the length of the CTAB molecule. The bright stripes here presumably indicate rows of headgroups, since the headgroup diameter is almost twice that of the alkane chain. methyl groups of neighboring heads may overcome the electrostatic repulsion, particularly since the methyl
Conclusion Using force microscopy, we have investigated the adsorbate structure formed by an ionic surfactant (CTAl3) on a hydrophobic surface (graphite) in contact with the aqueous surfactant solution. The noncontact double layer repulsion between tip and sample was used to image the hemimicelle structure on the sample,which was consistent with a hemicylindrical morphology. This was further corroborated by imaging the monolayer structure at low concentrations, which showed a periodicity of twice the molecular length, consistent with a head-to-headand tailto-tail self-assembly. We postulate that this monolayer structure serves as a template for hemimicelle formation as the concentrationis increased. Thus hemimicelle shape need not be related to bulk micelle shape; rather it can be determined primarily by the monolayer structure. In light of this new intepretation, our results agree well with previous adsorption measurements of ionic surfactants on hydrophobic substrates. Acknowledgment. We thank V. T. Moy, T. Schwinn, W. Ducker, J. N. Israelachvili, and an anonymous referee for helpful suggestions. This work was supported by fellowshipsfrom AT&T and the Alexander von Humboldt Stiftung (S.M.) and by grants from the Office of Naval Research (J.P.C.),the National Science Foundation (MCB 92-02775to G.D.S. and P.K.H.), and Digital Instruments.
