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Atomic Force Microscopy Observation of the Nanostructure of Tetradecyltrimethylammonium Bromide Films Adsorbed at the Mica/Solution Interface Hideki Sakai,*,†,‡ Hidemoto Nakamura,† Kozo Kozawa,† and Masahiko Abe†,‡ Faculty of Science and Technology, Science University of Tokyo, 2641 Yamazaki, Noda-shi, Chiba 278-8510, Japan, and Institute of Colloid and Interface Science, Science University of Tokyo, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan Received June 24, 1999. In Final Form: October 2, 2000 An investigation was performed by atomic force microscopy (AFM) using two measuring modes on the nanostructure of adsorbed films of tetradecyltrimethylammonium bromide (C14TAB) formed at the solution/ mica interface. Cylindrical micelle-like aggregates, quite similar to those reported by Manne and Gaub (Science 1995, 270, 1480-1482), were found in noncontact AFM observations based on the electrostatic repulsive force between mica substrate with adsorbed C14TAB and AFM probe. The cylindrical micelle-like structure was observed only above the critical micelle concentration (cmc) of C14TAB (3.5 mM), and the diameter of the cylinder was ca. 5-6 nm. In contrast, a hexagonal lattice-like structure with a 1.0-1.2 nm lattice spacing, which differs from that for mica substrate, was found in a wide range of surfactant concentrations covering those above and below the cmc when contact mode AFM observations were carried out in the adhesive force region. The adsorbed state of C14TAB on the mica substrate was discussed based on these experimental findings.
1. Introduction Surfactant molecules having both hydrophobic and hydrophilic groups in one molecule form molecular aggregates such as micelles in aqueous solutions and adsorb mono- or bimolecularly at solid/liquid interfaces. No detailed examination was conducted before at the nanometer level on the nanostructure of such adsorbed surfactant films because no suitable measuring method was available. In recent years, however, interesting reports have appeared showing that ionic surfactants form cylindrical micelle-like aggregates due to their interaction with a solid surface through in situ atomic force microscopy (AFM) observations1-8 that allow observation of the surface at a molecular scale even in solutions. When the surface of a solid substrate is hydrophilic, surfactant molecules adsorb on the substrate surface with their hydrophilic groups directed toward the surface to form the first layer. Although surfactant molecules were formerly thought to orient vertcally to the surface in the second layer taking a similar structure to that in the first layer, they have been reported to form cylindrical films on the substrate because adjacent molecules in the film repel each other and are recurved due to a stronger electrostatic repulsion between the adsorbed surfactant molecules than their adsorption forces on the substrate. * To whom all the correspondence should be addressed: phone and fax, +81-471-24-8650; e-mail,
[email protected]. † Faculty of Science and Technology. ‡ Institute of Colloid and Interface Science. (1) Manne, S.; Gaub, H. E. Science 1995, 270, 1480-1482. (2) Ducker, W. A.; Wanless, E. J. Langmuir 1996, 12, 5915-5920. (3) Wanless, E. J.; Ducker, W. A. J. Phys. Chem. 1996, 100, 32073214. (4) Wanless, E. J.; Ducker, W. A. Langmuir 1997, 13, 1463-1474. (5) Grant, L. M.; Tiberg, F.; Ducker, W. A. J. Phys. Chem. 1998, B102, 4288-4294. (6) Manne, S.; Cleveland, J. P.; Gaub, H. E.; Stucky, G. D.; Hansma, P. K. Langmuir 1994, 10, 4409-4413. (7) Jaschke, M.; Butt, H.-J.; Gaub, H. E.; Manne, S. Langmuir 1997, 13, 1381-1384. (8) Manne, S.; Schaffer, T. E., Huo, Q.; Hansma, P. K.; Morse, D. E.; Stucky, G. D.; Aksay, I. A. Langmuir 1997, 13, 6382-6387.
In contrast, if the substrate surface is hydrophobic, semicylindrical films are formed since surfactant molecules adsorb with their hydrophobic groups orienting to the substrate.1-5 We can expect to have fine lithographs that are prepared with an accuracy of less than the wavelength of light if such cylindrical micelle-like adsorption films formed by surfactant molecules become usable as a micro reaction field of polymerization and other chemical reactions. Meanwhile, the substrates used thus far for examining the nanostructure of cylindrical micelle-like adsorption films have been limited to highly oriented pyrolytic graphite (HOPG), mica, MoS2 silica, and gold single cystals, which have a smooth surface at the atomic level owing to the requirements of high-resolution AFM observation. Thus, Gaub and others examined the adsorption of cationic surfactant molecules on the hydrophobic HOPG surface and reported that cationic surfactant molecules adsorb on the substrate 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.6-8 When mica was used as the substrate, cationic surfactant molecules were considered to be chemically adsorbed firmly on the surface forming the first layer through ionic exchange with potassium ions of the substrate.9 Since the adsorption forces of cylindrical or semicylindrical micelles on the substrate surface would be relatively weak, the forces between the probe and the surface should be controlled precisely to rule out the possibility that the cantilever scrapes off the adsorbed films when AFM observations are made of their surface structure. In this investigation, the structure of adsorbed films of cationic tetradecyltrimethylammonium bromide (C14TAB) was examined by in situ AFM taking note of the kinds and magnitudes of the forces observed between the AFM cantilever and the sample surface. Moreover, the adsorbed (9) Fujii, M.; Kikukawa, T.; Sugano, T. Prepr. Symp. Colloid Surf. Chem., 51st 1998, 277.
10.1021/la990825q CCC: $20.00 © 2001 American Chemical Society Published on Web 02/16/2001
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state of C14TAB on the mica substrate was discussed based on these experimental findings. 2. Experimental Section 2.1. Materials. Cationic tetradecyltrimethylammonium bromide (C14TAB, Tokyo Kasei Kogyo Co., Ltd.) was used as the surfactant without further purification. Mica whose surface is smooth at the atomic level (Niraco Co., Ltd.) was used as the solid substrate. Clean cleavage surfaces obtained immediately before observations were employed in the AFM observation. Distilled water for injection (Ohtsuka Pharmaceutical Co., Ltd.) was used as the solvent as supplied. A microcantilever SN-AF01-A made of Si3N4 (Olympus Optical Co., Ltd) was employed as the AFM probe. It was coated with gold and had a spring constant of 0.11 N/m, a length of 100 µm, and a thickness of 4000 Å. 2.2. AFM Observations. AFM observations were performed using a model SPI3800 (Seiko Instruments Co., Ltd.). After a mica substrate was fixed in the bottom of a solution cell, an aqueous C14TAB solution was poured into the cell. The surfactant concentration was varied in a wide range. AFM observations were made when an adsorption equilibrium was attained after allowing the cell to stand for 2 h. To prevent adsorbed surfactant films from being damaged by the AFM probe, observations were made on the surface of adsorbed films through (a) noncontact observation based on the electrostatic repulsive force between substrate and probe and (b) contact mode observation in the attractive force region. The details of each method are described later.
Figure 1. Force curve on the mica surface in 10 mM C14TAB aqueous solution.
3. Results and Discussion 3.1. Noncontact Observations Using Electrostatic Substrate-Probe Repulsion. In ordinary contact mode AFM observations, forces between the probe and the substrate surface are sensed and the probe is allowed to scan in two dimensions so that the forces remain constant to detect surface roughness. However, the damage caused to the sample surface by the probe is not inappreciable in this method. When a solid surface is observed in surfactant aqueous solutions using AFM, surfactant molecules adsorb on the cantilever (the gold-coated tip in this study) and they form adsorbed films on the substrate (mica in this study). In the case of ionic surfactants, an electrostatic repulsive force begins to act between the substrate and the probe (both with adsorbed surfactant films) when they approach each other. The electrostatic repulsion between the probe and the sample surface is a long range force, and hence, we would be able to observe the surface without contact if the probe is permitted to scan two-dimensionally so that this force remains constant. The effect of added C14TAB was then examined on the force curve in aqueous solutions. The force curve in pure water is similar to that obtained in the air, except that a smaller adhesive force was observed when the cantilever was withdrawan from the sample. Meanwhile, measurement in an aqueous 10 mM C14TAB solution, an about three times higher concentration than the critical micelle concentration (cmc) gave the force curve shown in Figure 1, indicating a region of repulsive force (not observed in either water or the air) appeared when the probe-sample gap was shortened (region A in the figure). This region results from the electrostatic repulsion between C14TAB molecules adsorbed on the substrate and those on the AFM probe. Noncontact AFM observations were then conducted keeping the force constant between the probe and the sample in the repulsion region shown in Figure 1. Thus, the probe was allowed to approach the substrate surface with a piezoelectric device and was brought to a stop when it detected a repulsive force due to electrostatic repulsion at a certain position. The observations were made possible by permitting the probe to scan in the x-y directions while the repulsive force was kept constant.
Figure 2. Noncontact AFM image using electrostatic repulsive force of the mica surface in 10 mM C14TAB aqueous solution (scan area 150 nm × 150 nm).
The use of a Si3N4 probe without gold coating, instead of the one with the coating, did not permit us to make noncontact AFM observation because it failed to detect repulsive force clearly. Figure 2 is a typical noncontact AFM image of the mica surface in aqueous 10 mM C14TAB solution using electrostatic repulsive force. Aggregates are seen to form stripes running from top to bottom in the figure. Such a structure was observed with good reproducibility and independent of scan angle and scan speed (1-2.16 Hz). Further observation of the cross section revealed that the diameter of the rodlike aggregates is 5.0-6.0 nm. Since this value is about twice the length of a C14TAB molecule (2.6 nm), the surfactant aggregates would have a cylindrical micelle-like structure. Cylindrical micelle-like aggregates were not observed in aqueous C14TAB solutions at concentrations below 3.5 mM, the cmc of the surfactant. The repulsice force between the probe and the sample was very weak at this concentration range, suggesting that the surface coverage with the surfactant is not enough. On the other hand, a formation of cylindrical micelle-like aggregates was confirmed at concentrations above the cmc (30 and 70 mM) with noncontact AFM observations using the electrostatic force. 3.2. Contact Mode Observation in the Adhesive Force Region. When contact AFM observations are made, two types of forces act between the probe and the sample surface, that is, repulsion (region B in Figure 1)
Nanostructure of C14TAB Films
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Figure 3. Contact-mode AFM image at the adhesive force region of the mica surface in C14TAB aqueous solution (scan area 20 nm × 20 nm): (a) in 1.0 mM C14TAB aqueous solution; (b) in 10 mM C14TAB aqueous solution; (c) in 70 mM C14TAB aqueous solution.
and adhesion (region C in Figure 1). Since contact observations in the adehesive region would cause less damage to the sample surface than those in the repulsion region, observations were conducted that allow the probe to stop at a certain position in the adhesive region and scan in two dimensions on the sample surface making the adhesive force unchanged. A contact observation of the mica surface at a C14TAB concentration of 10 mM (the same concentration as that in Figure 2) failed to show cylindrical micelle-like aggregates in a scanning area of 150 nm × 150 nm, suggesting that the scanning broke the cylindrical micellelike structure even under the relatively weak adhesive force. A hexagonal lattice-like surface structure shown in Figure 3b, however, was found when a high-resolution observation in a scanning area of 20 nm × 20 nm was made. The spacing of this hexagonal lattice-like roughness was about 1.2 nm, and this spacing is independent of scan angle and scan speed (3.5-8.5 Hz). Similar contact observations at surfactant concentrations lower than 10 mM showed that a hexagonal lattice structure was also formed on the mica surface at concentrations below the cmc (1.0, 1.75, and 3.5 mM) with a lattice spacing of about 1.0-1.2 nm (the AFM image at 1.0 mM is shown in Figure 3a). Moreover, a similar hexagonal lattice-like roughness was observed on the mica surface in aqueous C14TAB solutions at 30 and 70 mM, showing again a lattice spacing of about 1.0-1.2 nm (Figure 3c). Similar contact mode (adhesive force) observation in pure distilled water gave the hexagonal lattice structure with a spacing of 0.5-0.6 nm, corresponding to the lattice spacing of mica. In addition, a clear AFM image was not obtained when contact mode observation at the repulsive force region (B in Figure 1) was carried out. The relation shown in Figure 4 was obtained when the spacing of the hexagonal roughness was plotted against C14TAB concentration based on the contact observations
Figure 4. Relationship between concentration of C14TAB and lattice spacing of adsorbed structure observed at contact mode.
conducted so far. In the observation in pure water, a hexagonal lattice structure was observed with a lattice spacing of 0.52 nm that corresponds to the mica hexagonal lattice, while the hexagonal structure with a lattice spacing of 1.0-1.2 nm, which differs from the spacing for mica, was observed in the presence of C14TAB even at concentrations below the cmc (3.5 mM). Adsorption of surfactant molecules on the solid substate is reported to take place at concentrations well below the cmc,10 and the results obtained in the present work are consistent with this report. At concentrations below the cmc, multilayer adsorption would not have occurred since rodlike aggregates were not observed in noncontact observations. Although the exact reason is still unknown for the AFM images with such a large lattice spacing (1.0-1.2 nm), these images may correspond to a dimer-like structure (10) Bitting, D.; Harwell, J. H. Langmuir 1987, 3, 501-511.
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reported in recent papers11,12 arguing that the process of ionic surfactant adsorption proceeds in such a way that surfactant molecules form small aggregates such as dimers after being trapped in a narrow space within the electrical double layer and these aggregates adsorb on the solid surface. In contact observations, the lattice images with a spacing of 1.0-1.2 nm were obtained even at concentrations above the cmc and these results are quite different from those (cylindrical micelle-like structure) obtained in the previous section in noncontact observations. On the basis of these different AFM images resulting from the different types of forces acting between the substrate and the probe, the mechanism of the adsorption of C14TAB molecules on the mica surface would be given as follows. C14TAB molecules adsorb on the substrate surface to form the first layer which is perpendicular to the surface. The structure of this layer is rather rigid because the adsorption process involves ion exchange with potassium ions of mica, and hence, the layer can be observed without being destroyed in contact observations in the attraction region. Cylindrical micelles are formed on top of the first layer. Since the adsorption forces of these cylindrical micelles are weak, the micelles are scraped off by the probe to make them nonobservable in contact observation in the attraction region. Cylindrical micelles can be observed without their disruption, however, in noncontact obervations using the electrostatic repulsion. We are now conducting a quan(11) Fujii, M. J. Jpn. Oil Chem. Soc. 1996, 45, 1099-1196. (12) Wangnerud, P.; Jonsson, B. Langmuir 1994, 10, 3542-3549.
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titative study on the multilayer adsorption of C14TAB molecules through measurements of AFM image change with time and the amount adsorbed on the mica surface. 4. Conclusions Cylindrical micelle-like aggregates with a diameter of 5.0-6.0 nm were observed when the nanostructure of surfactant films adsorbed on the mica substrate was examined by noncontact AFM measurements making use of the electrostatic repulsive force in aqueous C14TAB solutions. These cylindrical micelle-like aggregates could be observed only at C14TAB concentrations above the cmc. In contrast, a hexagonal lattice-like structure of a 1.01.2 nm spacing, which differs from that of mica, was observed in contact AFM observations in the adhesive force region at both below and above the cmc. In these observations, only the first layer of surfactant adsorption, which is rather strongly bound to the substrate, is suggested to be observable because cylindrical micelles observed in noncontact observations are scraped off through the interaction between the substrate and the probe. Acknowledgment. This work was partially supported by a Grant-in-Aid for Scientific Research on the Priority Area of CARBON ALLOYS from the Ministry of Education, Science, Sports and Culture, Japan. LA990825Q
