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Self-Assembly of Cationic Surfactants on a Graphite Surface Studied by STM Sai-Long Xu, Chen Wang,* Qing-Dao Zeng, Peng Wu, Zhi-Gang Wang, Hai-Ke Yan, and Chun-Li Bai* Center for Molecular Science, Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100080, China Received July 23, 2001. In Final Form: October 30, 2001 The self-assembly of hexadecyltrimethylammonium bromide (C16TAB) and dodecyltrimethylammonium bromide (C12TAB) from ethanol solution that was allowed to evaporate was imaged on graphite with a scanning tunneling microscope (STM). The STM images showed that these cationic surfactants could form ordered parallel stripes with approximately twice the length of a surfactant molecule, displaying that the cationic surfactants were arranged in a head-to-head configuration. The high-resolution image indicated that three bright spots amid a stripe could correspond to three methyl groups of a couple of head-to-head molecules. This observation provided direct support for interpretation of the model proposed from atomic force microscope studies concerning the first adsorbed layer at solid-aqueous interfaces.
Introduction Surfactants can self-assemble into a wide range of ordered structures including spheres, hemispheres, cylinders, hemicylinders, bilayers, flat sheets, and so forth. The well-ordered structures have attracted intensive scientific interest, providing a useful understanding of the adsorption or self-assembly mechanism of surfactants and the self-assembled structures, largely because of the importance of surfactant self-assembly in modeling industrial processes such as detergency, froth flotation, lubrication, crude oil refinement, purification, and so forth. Over the past decades, many techniques such as adsorption isotherms,1 fluorescence probe analyses,2 X-ray photoelectron spectroscopy (XPS),3 neutron reflection,4 and surface force apparatus (SFA)5 have been used to investigate the assembly or adsorption of surfactants. But little information about the topology of the adsorbed or selfassembled surfactants can be obtained. Recently, Manne et al.6 first used the atomic force microscope (AFM) to obtain the microscopic evidence of the organization of hexadecyltrimethylammonium bromide (C16TAB) at the interface between graphite and aqueous solution. The images showed straight parallel stripes spaced apart by approximately twice the surfactant length. Accordingly, the initially arranged monolayer with head-to-head structure was proposed to serve as the template for the further formation of hemicylinders. In subsequent studies, structures of a variety of different surfactants (such as cationic,7-12 anionic,13-15 zwitterionic,16-18 gemini,9,19 and * To whom correspondence should be addressed. E-mail: clbai@ infoc3.icas.ac.cn (C.L.B.);
[email protected] (C.W.). Fax: (86)-10-62557908. (1) Kipling, J. J. Adsorption from Solutions of Non-Electrolytes; Academic Press: London, 1965. (2) Chandar, P.; Somasundaran, P.; Turro, N. J. J. Colloid Interface Sci. 1987, 117, 31. (3) Chen, Y.-L.; Chen, S.; Frank, C.; Israelachvili, J. J. Colloid Interface Sci. 1992, 153, 22. (4) McDermott, D. C.; McCarney, J.; Tjomas, R. K.; Rennie, A. R. J. Colloid Interface Sci. 1994, 162, 304. (5) Pashley, R. M.; McGuiggan, P. M.; Horn, R. G.; Ninham, B. W. J. Colloid Interface Sci. 1988, 126, 569. (6) Manne, S.; Cleveland, J. P.; Gaub, H. E.; Sturcky, G. D.; Hansma, P. K. Langmuir 1994, 10, 4409. (7) Manne, S.; Gaub, H. E. Science 1995, 270, 1480. (8) Jaschke, M.; Butt, H. J.; Gaub, H. E.; Manne, S. Langmuir 1997, 13, 1381.
nonionic20-23) adsorbed from aqueous solution on many different types of substrates (such as graphite,11,15,20 mica,10,11,19 gold,8 and silica11,20) were studied by this technique. For example, AFM images indicated that tetradecyltrimethylammonium bromide (C14TAB) formed full cylinders on mica and spherical micelles on amorphous silica.7 The zwitterionic surfactant (dodecyldimethylammonio)propanesulfonate (DDAPS) formed spherical micelles on mica, whereas the cationic surfactant dodecyltrimethylammonium bromide (C12TAB) formed cylinder micelles.17 Hexadecyltrimethylammonium hydroxide (C16TAOH) and sodium dodecyl sulfate (SDS) formed halfcylinders on gold, and C14TAB formed full cylinders.8 On the other hand, Gaub and co-workers reported that cationic surfactant molecules adsorbed on the hydrophobic graphite forming a layer parallel to the surface through relatively strong adsorption forces to give a solid film as the first layer, on top of which semicylindrical micelles are formed.8,9 Sakai et al. proposed that C14TAB molecules adsorbed on mica formed the first layer perpendicular to the surface and templated the formation of cylindrical micelles.12 Marchant and others suggested that the N-alkylmaltonnamide nonionic diblock surfactants assembled into an ordered monolayer on graphite as a template for subsequent hemicylinder formation and the periodic banding structures.24 In addition, molecular (9) Manne, S.; Scha¨ffer, T. E.; Huo, Q.; Hansma, P. K.; Morse, D. E.; Stucky, G. D.; Aksay, I. A. Langmuir 1997, 13, 6382. (10) Lamont, R. E.; Ducker, W. J. J. Am. Chem. Soc. 1998, 120, 7602. (11) Liu, J.-F.; Ducker, W. A. J. Phys. Chem. B 1999, 103, 8558. (12) Sakai, H.; Nakamura, H.; Kozawa, K.; Abe, M. Langmuir 2001, 17, 1817. (13) Wanless, E. J.; Ducker, W. A. J. Phys. Chem. 1996, 100, 3207. (14) Wanless, E. J.; Ducker, W. A. Langmuir 1997, 13, 1463. (15) Wanless, E. J.; Davey, T. W.; Ducker, W. A. Langmuir 1997, 13, 4233. (16) Ducker, W. A.; Wanless, E. J. Langmuir 1996, 12, 5915. (17) Ducker, W. A.; Grant, L. M. J. Phys. Chem. 1996, 100, 11507. (18) Grant, L. M.; Ducker, W. A. J. Phys. Chem. B 1997, 101, 5337. (19) Fielden, M. L.; Claesson, P. M.; Verrall, R. E. Langmuir 1999, 15, 3924. (20) Grant, L. M.; Tiberg, F.; Ducker, W. A. J. Phys. Chem. B 1998, 102, 4288. (21) Patrick, H. N.; Warr, G. G.; Manne, S.; Aksay, I. A. Langmuir 1997, 13, 4339. (22) Grant, L. M.; Ederth, T.; Tiberg, F. Langmuir 2000, 16, 2285. (23) Dong, J.; Mao, G. Langmuir 2000, 16, 6641.
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dynamics (MD) simulations showed that the monolayer arrangement of C16TAB on a hydrophobic surface was unstable and evolved into hemicylindrical type aggregates,25 which supported the proposed model.6,7 However, there was limited direct evidence of these initially adsorbed monolayers at the solvent/solid interface to determine the proposed arrangement and shape. Meanwhile, the scanning tunneling microscope (STM) has already been successful in imaging with atomic resolution important structural and dynamical properties for a number of self-assembled layers, molecular adsorbates, and thin polymer films. Using the STM, Wang et al. first observed a uniform layer of SDS with tailgroups fixed on the graphite surface and headgroups shielding the surface after the evaporation of aqueous solution.26 So far, there have been few reports on the fine structures of the surfactant self-assembled layers. To acquire more direct and detailed information about the surfactant selfassembled structure on graphite, herein we used the STM to detect the assembled layer structure of C16TAB and C12TAB from evaporable ethanol solution on graphite. The result supported the assumed models of the initial absorbed monolayer with a head-to-head arrangement from AFM studies and could be potentially used in the enhanced oil recovery. Material and Methods Hexadecyltrimethylammioum bromide (C16TAB) and dodecyltrimethylammioum bromide (C12TAB) (purity >99%) were purchased from Aldrich and used without further purification. The solutions of C16TAB and C12TAB were made in ethanol (analytical grade, purity g 99.7%, Beijing Chem. Co.) solution with a concentration of less than 1 mM. The samples were prepared by depositing a drop of the solution on a freshly cleaned highly oriented pyrolytic graphite (HOPG) surface and allowing ethanol to evaporate under ambient conditions. The STM experiments were performed with a Nanoscope IIIa SPM (Digital Instruments, Santa Barbara, CA) at room temperature. The Pt/Ir (90/10) tips were mechanically formed.
Results and Discussion We imaged, using the STM, the periodic structure of surfactant kept for 1 month under ambient conditions to allow ultimate equilibrium. The self-assembly of surfactant on HOPG after ethanol evaporation is very low to reach equilibrium, similar to the slow process of the formation of surfactant ordered structures adsorbed at the interface between substrate and aqueous solution.3,10,15 Figure 1 shows an image of a C16TAB ordered structure on graphite. In this image, C16TAB self-assembled into a two-dimensional (2-D) lamella with four domains, occurring in three different orientations (indicated as 1, 2, and 3). The size of these domains varies from about 60 nm × 70 nm to 100 nm × 100 nm. Within each domain, parallel and meandering stripes can be resolved. This result showed that these surfactants could form 2-D ordered structures on graphite, although surfactant molecules have both nonpolar and polar groups in one molecule. Actually, self-assembly is largely determined by the alkyl chain of the surfactant, including its composition, length, flexibility, and geometry. It is probable that surfactant molecules with a hydrocarbon alkyl chain containing more than 10 carbon atoms formed a 2-D ordered layer in registry with the graphite surface in a tail-to-tail con(24) Holland, N. B.; Ruegsegger, M.; Marchant, R. E. Langmuir 1998, 14, 2790. (25) Bandyopadhyay, S.; Shelley, J. C.; Tarek, M.; Moore, P. B.; Klein, M. L. J. Phys. Chem. B 1998, 102, 6318. (26) Zhang, J.; Chi, Q.; Dong, S.; Wang, E. Surf. Sci. 1994, 321, L195.
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Figure 1. STM image of the C16TAB self-assembled layer on graphite with three orientations. Tunneling conditions: 932 pA, 848 mV; scan area, 265 nm × 265 nm.
figuration of alkyl groups,14,24 as observed for a rich variety of STM studies showing that long-chain alkyl derivatives with desired functional groups (such as -COOH and -OH) could form ordered monolayers in polar or nonpolar solvents.27 In parts a and b of Figure 2, the stripes of C16TAB (corresponding to orientation 2 in Figure 1) and C12TAB are shown, respectively. Each stripe comprises three parts. The alkyl chains of surfactant molecules on both sides of the stripe are closely packed parallel to each other and along the HOPG surface. In contrast to the parallel chains on both sides of the stripes, the middle portion of each stripe is a bright strand. Neighboring stripes are aligned in a tail-to-tail arrangement of alkyl groups. In Figure 2a, the mean periodicity of a C16TAB stripe is 5.3 ( 0.2 nm, which is about twice the length of a C16TAB molecule (estimated as approximately 2.6 nm by adding the radius of the headgroup to the length of the alkyl chain28a). The C12TAB (about 2.1 nm28b) stripes in Figure 2b have a periodicity of 4.1 ( 0.2 nm. Apparently, these observations reveal that the stripes of C16TAB and C12TAB are arranged in a head-to-head structure. The arrangement could provide a microscopic level support to the 2-D structure of the first adsorbed monolayer of surfactant molecules at solid-aqueous interfaces studied by AFM.6,8,9 Figure 3a is a high-resolution image of the stripes of C16TAB in Figure 2a. On the stripe, the darker regions correspond to the alkyl chains, which are closely packed with their long axis parallel to the graphite substrate and also parallel to each other. This orientation of alkyl chains with respect to the graphite substrate is typical, similar to that of simple molecules such as alkanes, alcohols, fatty acids, and alkylbenzenes observed by STM.27 The even (27) (a) Foster, J. S.; Frommer, J. E. Nature 1988, 333, 542. (b) Spong, J. K.; Mizes, H. A.; LaComb, L. J., Jr.; Dovek, M. M.; Frommer, J. E.; Foster, J. S. Nature 1989, 338, 137. (c) Rabe, J. P.; Buchholz, S. Science 1991, 253, 424. (d) Cyr, D. M.; Venkataraman, B.; Flynn, G. W. Chem. Mater. 1996, 8, 1600. (e) Grim, P. C. M.; De Feyter, S.; Gesquie`re, A.; Vanoppen, P.; Rucker, M.; Valiyaveetil, S.; Moessner, G.; Mu¨llen, K.; De Schryver, F. C. Angew. Chem., Int. Engl. 1997, 36, 2601. (f) Padowitz, D. F.; Messore, B. W. J. Phys. Chem. B 2000, 104, 9943. (28) (a)Velegol, S. B.; Fleming, B. D.; Biggs, S.; Wanless, E. J.; Tilton, R. D. Langmuir 2000, 16, 2548. (b) Scaling (a) to a dodecyl chain, the length of C12TAB is 2.1 nm.
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Figure 3. (a) The high-resolution image of the head-to-head arrangement of C16TAB on graphite. Tunneling conditions: 478 pA, 775 mV; scan area, 13 nm × 13 nm. (b) A random model of the head-to-head arrangement of C16TAB was proposed. Molecules A′ correspond to the methyls’ arrangement of molecules A indicated in Figure 3a, and molecules B′ correspond to molecules B.
Figure 2. STM images of the stripes of C16TAB and C12TAB on graphite. (a) Image of the stripes of C16TAB (corresponding to orientation 2 in Figure 1). Tunneling conditions: 839 pA, 848 mV; scan area, 49 nm × 49 nm. (b) Image of the stripes of C12TAB. Tunneling conditions: 804 pA, 720 mV; scan area, 21 nm × 21 nm.
darker regions correspond to the gaps between the terminal methyls of the tail-to-tail alkyl chains, and the lighter strand corresponds to the headgroups. The angle R between two head-to-head C16TAB molecules is about 176.9 ( 0.4°, revealing that a couple of head-to-head alkyl chains were not in the same line. The angle β of the headgroups’ strand relative to the alkyl chain is varied. On the headgroups’ strand, three bright spots (indicated by an arrow in Figure 3a) indicating enhanced tunneling current can be roughly discerned, corresponding to the three methyls extending outside of the alkyl backbone planes. On the basis of these STM observations, a molecular model (Figure 3b) using the Hyperchem soft-
ware package (version 5.02 for Win95/NT) was proposed to account for the arrangement imaged by STM. Figure 3b shows that the three bright spots in Figure 3a could result from the common contributions of three extending methyl groups of a couple of head-to-head molecules. Two methyl groups on the left side of molecules A′ (a couple of head-to-head molecules are labeled as A′) and one counterpart on the right side could correspond to the three bright spots of molecules A (labeled in Figure 3a). These three methyl groups are packed out of both alkyl backbone planes. Similarly, molecules B′ corresponding to molecules B could be arranged in one methyl group adjacent to two other ones. The extending methyl groups’ arrangements could not alternate between those similar to molecules A′ and those similar to molecules B′, indicative of the disordered alignments of the overall head-to-head molecules within the stripe. Although this head-to-head molecular pattern is similar to those of simple molecules such as alkanes, alcohols, fatty acids, and alkylbenzenes, the formation of the surfactant self-assembled lamella in our result could occur in two distinct stages. A fast initial nonequilibrium stage during the wetting of ethanol solution on graphite is very rapid. As ethanol evaporation proceeds, the tailgroups of surfactants adsorb strongly to the graphite surface in tailto-tail packing due to the interactions caused by the alkyl
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tail in registry with the substrate lattice. These alkyl chains horizontally adsorbed onto the graphite plane along the graphite symmetry axis and parallel to each other. Then, this stage could be followed by a much slower process involving rearrangement of CTA+ ions and their counterions (i.e., Br-) toward the final equilibrium. CTA+ ions continue to assemble on graphite carrying negatively charged Br-. Br- ions diffuse within the regions of polar headgroups to create the electrostatic stabilization at the confluence of the headgroups via dipole bonds. As a result, the 2-D lamella was favored on the surface. To explain the stable head-to-head structure, there are briefly three possibilities considered. First, the equilibrium structures result from the compromise of many forces including interactions between surfactant tails and graphite, repulsive headgroup interactions, and geometric interactions.16,22,29 During the wetting of ethanol solution, the interfacial interactions between alkyl chains and the graphite surface dominate; the alkyl chains preferentially assemble onto the graphite lattice. A sterically favored packing pattern for alkyl chains having their carboncarbon skeleton oriented parallel on the graphite is obtained in their tail-to-tail configuration. Next, the equal number of Br- ions dissociating with CTA+ ions stabilized the head-to-head headgroups against electrostatic repulsion, as pointed out by Manne et al.6 In addition, the (29) Pamer, B. J.; Liu, J.; Virden, J. Langmuir 1999, 15, 7426.
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electrostatic attraction between the positive surfactant headgroups and the mobile electrons in the conducting graphite surface may also help to stabilize the headgroups.12 Finally, the attractive interactions between the outer methyl groups of neighboring or head-to-head headgroups may overcome the electrostatic repulsion.6 Conclusion Using the STM, we imaged the 2-D ordered lamella formed by a cationic surfactant (C16TAB and C12TAB) on a graphite surface from an evaporating ethanol solution. The acquired images showed that the well-ordered lamella was arranged in a head-to-head structure, which confirmed the first monolayer structure at the solid-aqueous interfaces proposed by Manne at al.6,7 The headgroups’ arrangement shown in the high-resolution image indicated that three bright spots extended outside of the alkyl backbone planes, resulting from the common contributions of three methyl groups of a couple of head-to-head molecules. Acknowledgment. We acknowledge the National Natural Science Foundation and the foundation of the Chinese Academy of Science for financial support. Support from the National Key Project on Basic Research (Grant G2000077501) is also acknowledged. LA0111506
