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Effects of Protonation on the Aggregate Structures of Tetradecyldimethylamine Oxide at Solid-Solution Interfaces Hideya Kawasaki,* Masamitsu Syuto, and Hiroshi Maeda Department of Chemistry, Faculty of Science, Kyushu University 33, Hakozaki, Higashi-ku, Fukuoka, 812-8581, Japan Received June 20, 2001. In Final Form: October 9, 2001 We have investigated the effects of ionization on the aggregate structures of tetradecyldimethylamine oxide (C14DAO) at the mica-solution and graphite-solution interfaces by atomic force microscopy. It was found that the C14DAO forms aggregate structures with lower curvature at the mica-solution interface as the degree of ionization of C14DAO increased. Nonionic C14DAO formed short cylindrical aggregates on mica. On the other hand, a flat bilayer on mica was observed for the half-ionized C14DAO1/2HCl and the fully ionized C14DAOHCl. The flat bilayers for C14DAO1/2 HCl and C14DAOHCl suggest the higher packing parameter (or lower mean curvature) character of these surfactants, compared to the cationic quaternary ammonium surfactant with a single tetradecyl tail as well as the nonionic C14DAO which all form the cylindrical aggregates on mica. The reason for this lower curvature of half-ionized C14DAO was suggested to be the hydrogen bonds between the headgroups (i.e., the cationic-nonionic pair for C14DAO1/2 HCl), in addition to the contribution of the electrostatic attractive interaction between the cationic headgroup and the negatively charged mica. Contrary to the aggregates on mica, the dominant structures of C14DAO on graphite were hemicylinders, irrespective of the ionization. It was found that the ionization increased the distances between the hemicylindrical aggregates due to the electrostatic repulsion between the aggregates rather than induced the change in the aggregate structure. The weak dependence of the ionization on the aggregate structures at the graphite-solution interface is suggested to be due to the dominant attractive interaction between the graphite surface and the surfactant tail.
Introduction Adsorption and self-assembly are central characteristic features of surfactant molecules. The adsorption of surfactants from solutions onto solid surfaces has been investigated over many years in relation to numerous practical applications, such as detergency, water purification, oil recovery, and flotation, as well as the scientific interest.1 It has been proposed that surfactants spontaneously aggregate to form microstructures (i.e., hemimicelles) at solid-solution interfaces in a manner analogous to the micellization in bulk solution.2,3 The existence of the hemimicelles has been inferred through the adsorption isotherms.2,3 Recently, atomic force microscopy (AFM) has provided a new insight into the structure of the surfactant aggregate at solid-solution interfaces. On mica, a variety of structures have been found to be formed at the mica-solution interface: a flat bilayer, long ordered cylinders, disordered cylinders, short cylinders, and spheres, depending on the surfactant geometry (the length of the hydrocarbon chains, the size of the polar headgroups, and counterions).4-12 The aggregate structure has been (1) Adamson, A. W.; Gast, A. P. Physical Chemistry of Surfaces, 6th ed.; John Wiley & Sons: New York, 1997. (2) Gaudin, A. M.; Fuerstenau, D. W. Trans. AIME 1955, 1. (3) Gaudin, A. M.; Fuerstenau, D. W. Trans. AIME 1955, 958. (4) Manne, S.; Gaub, H. E. Science 1995, 270, 1480. (5) Ducker, W. A.; Grant, L. M. J. Phys. Chem. B 1996, 100, 11507. (6) Ducker, W. A.; Wanless, E. J. Langmuir 1996, 12, 5915. (7) Manne, S.; Scaffer, T. E.; Huo, Q.; Hansma, P. K.; Morse, D. E.; Stucky, G.; Aksay, I. A. Langmuir 1997, 13, 6382. (8) Reuben, E. L.; Ducker, W. A. J. Am. Chem. Soc. 1998, 120, 7602. (9) Ducker, W. A.; Wanless, E. J. Langmuir 1999, 15, 160. (10) Patrick, H. N.; Warr, G. G.; Manne, S.; Aksay, I. A. Langmuir 1999, 15, 1685. (11) Fielden, M. L.; Claesson, P. M.; Verrall, R. E. Langmuir 1999, 15, 3924. (12) Liu, J.-F.; Ducker, W. A. J. Phys. Chem. B 1999, 103, 8558.
found to depend on the surfactant packing parameter13 as it does in bulk solutions: the higher the packing parameter of a surfactant, the lower the curvature the aggregate favors. But the aggregate structures at the solid-solution interface sometimes differ from the structures formed in the bulk solution because of the additional contribution of the surfactant-surface interactions. Contrary to the surfactant aggregate on mica, the dominant aggregate structures at graphite-solution interfaces are hemicylinders, which are weakly dependent on the chemical structure of the surfactant.5,7,12,14-22 It has been suggested that the graphite surface templates the formation of hemicylindrical aggregates due to the good fit between the surfactant alkyl chain length and the graphite lattice (i.e., epitaxial layering), leading to the hemicylindrical aggregates on graphite.14,21,22 In addition to AFM study, the aggregates at the solid-solution interface have been recently studied by other experimental methods (e.g., neutron reflection,23 ellipsometry,24 fluorescence probe study,25 and calorimetry26) and theoretical modelings.27,28 (13) Israelachvili, J. N.; Mitchel, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525. (14) Manne, S.; Cleveland, J. P.; Gaub, H. E.; Stucky, G. D.; Hansma, P. K. Langmuir 1994, 10, 4409. (15) Wanless, E. J.; Ducker, W. A. J. Phys. Chem. B 1996, 100, 3207. (16) Patrick, H. N.; Warr, G. G.; Manne, S.; Aksay, I. A. Langmuir 1997, 13, 4349. (17) Ducker, W. A.; Lamont, R. J. Colloid Interface Sci. 1997, 191, 303. (18) Wanless, E. J.; Ducker, W. A. Langmuir 1997, 13, 1463. (19) Grant, L. M.; Ducker, W. A. J. Phys. Chem. B 1997, 101, 5337. (20) Wanless, E. J.; Davey, T. W.; Ducker, W. A. Langmuir 1997, 13, 4223. (21) Grant, L. M.; Tiberg, F.; Ducker, W. A. J. Phys. Chem. B 1998, 102, 4288. (22) Holland, N. B.; Ruegsegger, M.; Marchant, R. E. Langmuir 1998, 14, 2790. (23) Schulz, J. C.; Warr, G. G.; Hamilton, W. A.; Butler, P. D. J. Phys. Chem. B 1999, 103, 11057.
10.1021/la010934g CCC: $20.00 © 2001 American Chemical Society Published on Web 11/30/2001
Aggregate Structures of C14DAO
The physicochemical properties of the mixed surfactant systems (e.g., nonionic-ionic and cationic-anionic mixtures) compared to those of the single surfactant system are often changed due to the fact that there is a strong attractive interaction between the headgroups (e.g., decreased critical micellar concentrations (cmc), increased surface activity at air-solution), which is termed “synergism” of the mixed surfactants. The rich structures on the mixed surfactant systems in bulk solution (e.g., nonclassical micelles, equilibrium vesicles, and several lyotropic liquid crystalline phases) have recently attracted interest in experimental study and theoretical modeling. It is interesting to examine how such attractive interaction between the headgroups in the mixed surfactants affects the physicochemical properties and the aggregate structures at the solid-solution interface. There have been extensive studies between the surfactant geometry and the aggregate structures at the solid-solution interface so far. However, only little is known of the aggregate structures of the mixed surfactant systems with the attractive interaction between the headgroups. In this study, we have investigated the effect of protonation on the aggregate structures of tetradecyldimethylamine oxide (C14DAO) surfactants at micasolution and graphite-solution interfaces by AFM. C14DAO solutions are mixtures of the nonionic [C14H25(CH3)2NfO] and the ionized (protonated) cationic species [C14H25(CH3)2N+-OH]. The composition RΜ (the degree of ionization) in the micelle is determined by pH under a given ionic strength. The mixtures of the protonated and unprotonated species show an extreme strong synergism due to the short-range attractive interaction between the headgroups. On dodecyldimethylamine oxide (C12DAO) and C14DAO, several marked effects of the protonation have been found,29-48 and they have been reviewed recently.49 (1) The cmc is minimum at RΜ ) 0.5 and the maximum value of the surface excess Γ is observed at the (24) Tiberg, F.; Landgren, M. Langmuir 1993, 9, 927. (25) Strom, C.; Hansson, P.; Jo¨nsson, B.; So¨dermann, O. Langmuir 2000, 16, 2469. (26) Kiraly, Z.; Findenegg, G. H. J. Phys. Chem. B 1998, 102, 1203. (27) Jonson, R. A.; Nagarajan, R. Colloids Surf., A 2000, 167, 21, 31. (28) Bandyopadhyay, S.; Schelly, J. C.; Tarek, M.; Moore, P. B.; Klein, M. L. J. Phys. Chem. B 1998, 102, 6318. (29) Herrmann, K. W. J. Phys. Chem. 1962, 66, 295. (30) Herrmann, K. W. J. Phys. Chem. 1964, 68, 1540. (31) Ikeda, S.; Tsunoda, M.; Maeda, H. J. Colloid Interface Sci. 1979, 70, 448. (32) Warr, G. G.; Grieser, F.; Evans, D. F. J. Chem. Soc., Faraday Trans. 1 1986, 82, 1829. (33) Rathman, J. F.; Christian, S. D. Langmuir 1990, 6, 391. (34) Maeda, H. Colloids Surf., A 1996, 109, 263. (35) Zhang, H.; Dubin, P. L.; Kaplan, J. I. Langmuir 1991, 7, 2103. (36) Maeda, H.; Muroi, S.; Ishii, M.; Kaimoto, H.; Kakehashi, R.; Nakahara, T.; Motomoura, K. J. Colloid Interface Sci. 1995, 175, 497. (37) Kaimoto, H.; Shoho, K.; Sasaki, S.; Maeda, H. J. Phys. Chem. 1994, 98 (8), 10243. (38) Tokiwa, F.; Ohki, K. J. Phys. Chem. 1966, 70, 3437. (39) Maeda, H.; Tsunoda, M.; Ikeda, S. J. Phys. Chem. 1974, 78, 1086. (40) Maeda, H.; Muroi, S.; Kakehashi, R. J. Phys. Chem. B 1997, 43, 511. (41) Goddard, E. D.; Kung, H. C. J. Colloid Interface Sci. 1973, 175, 497. (42) Maeda, H.; Yamamoto, A.; Souda, M.; Kawasaki, H.; Hossain, K. S.; Nemoto, N.; Almgren, M. J. Phys. Chem. B 2001, 23, 5411. (43) Fukada, K.; Kawasaki, M.; Kato, T.; Maeda, H. Langmuir 2000, 16, 2495. (44) Maeda, H.; Kanakubo, Y.; Miyahara, M.; Kakehashi, R.; Garamus, V.; Pedersen, J. S. J. Phys. Chem. B 2000, 104, 6174. (45) Garamus, V. M.; Pedersen, J. S.; Kawasaki, H.; Maeda, H. Langmuir 2000, 16, 6431. (46) Kawasaki, H.; Fukuda, T.; Yamamoto, A.; Fukada, K.; Maeda, H. Colloids Surf., A 2000, 169, 117. (47) Miyahara, M.; Kawasaki, H.; Fukuda, T.; Ozaki, Y.; Maeda, H. Colloids Surf., A, in press. (48) Kawasaki, H.; Maeda, H. Langmuir 2001, 17, 2279.
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value of RΜ ) 0.5 for C12DAO in 0.1 M NaCl solution.31,33-36 (2) For the intrinsic proton dissociation constants of the micelle KM and the monomer K1, pKM > pK1.38,39 (3) The cmc of C12DAO (RΜ ) 1) is lower than that of the nonionic one at high ion strength (>0.2 M NaCl).40 (4) The micelle size is the largest at RΜ ) 0.5.29,31,35,37,44 (5) Cryotransmission electron micrographs clearly showed a highly entangled network for the C14DAO (RΜ ) 0.5 and RΜ ) 1) solutions while much smaller micelles were observed for the C14DAO (RΜ ) 0) solution, although the viscoelastic properties differed between RΜ ) 0.5 and RΜ ) 1.42 (6) The C12DAO (RΜ ) 0.5) and C12DAO (RΜ ) 1) form infinitely long cylindrical micelles in the liquid crystalline hexagonal phase, while C12DAO (RΜ ) 0) micelles showed short cylinders.43 These protonation effects cannot be understood in terms of the electrostatic interaction alone, and hydrogen bonds have been proposed between the headgroups, the cationic-cationic pair (-N+-OH‚‚OH-N+)40 as well as the cationic-nonionic pair (-N+-OH‚‚ON-).31,32,34 Recently, the spectroscopic evidence of the hydrogen bond between the headgroups on the cationicnonionic pair was found for the half-ionized C12DAO in the lyotropic hexagonal phase as well as that in the solid state by the Fourier transform infrared spectrophotometer (FT-IR).48 A part of the result was found in a preliminary report.53 Experimental Section Sample Preparation. Water was prepared by distillation and then passage through an ultrapure water system consisting of ion exchange, activated carbon cartridge, and a 0.2 µm filter (Branstead Co). The resulting water has a conductivity of 18 MΩ cm-1 at 25 °C. C14DAO (Gerbu Co.) was recrystallized three times from hot acetone. After recrystallization, there was no minimum in a plot of surface tension versus concentration and a single peak in chromatograms of high-performance liquid chromatography (HPLC) (Tosoh Co., Japan) with an ODS-120T column (MeOH/H2O ) 7/3). The degree of ionization (R) for the C14DAO aggregates at the interface is not clear in the present study. Alternatively, we define that values of alpha are the degree of ionization of the micelle in bulk solution (RΜ). We prepared the half-ionized C14DAO solution of 1 mM (cmc × 5) by dissolving the half-ionized solid sample of C14DAO, resulting in the solution pH ∼ 4.5. At this pH, the micelle in bulk solution is considered to be almost halfionized. The intrinsic proton dissociation constant of the amine oxide micelle, pKM, is 5.9 for C12DAO spherical micelles and 6.3 for C14DAO rodlike micelles.44 It is thus considered that there is a small difference of the ionization (at a given pH) between the micelle and the aggregates at the interface due to slightly different curvatures between the two kinds of aggregates. The cmc values for the C14DAO without added salt are 0.15 mM for the C14DAO (RΜ ) 0) at pH 9.0 ( 0.05, 0.2 mM for the C14DAO (RΜ ) 0.5) at pH 4.6 ( 0.1, and 1.5 mM for the C14DAO (RΜ ) 1) at pH 2.8 ( 0.05. The solution pH values were adjusted by HCl or NaOH. Hexadecyltrimethylammonium bromide (C16TAB) (Nacalai Tesque Co.) and tetradecyltrimethylammonium chloride (C14TACl) (Nacalai Tesque Co.) were recrystallized three times from hot acetone. Analytical grade NaCl (99.98%) was used as purchased. Microscopy. AFM images were captured with an SPA 400 (Seiko Instruments Co., Japan), using silicon nitride tips (Olympus Co., Japan) with a nominal spring constant of 0.09 N m-1 and a fluid cell. Muscovite mica and crystalline graphite were glued to microscope slides with low melting temperature (49) Maeda, H.; Kakehashi, R. Adv. Colloid Interface Sci. 2000, 88, 275. (50) Scales, P. J.; Grieser, F.; Healy, T. W. Langmuir 1990, 6, 582. (51) Lin, X.-Y.; Creuzet, F.; Arribart, H. J. Phys. Chem. 1993, 97, 7272. (52) Laughlin, R. G. In The Aqueous Phase Behaviors of Surfactants; Academic: London, 1994; Chapter 9. (53) Kawasaki, H.; Syuto, M.; Maeda, H. Chem. Lett. 2000, 972.
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epoxy glue and then freshly cleaved with adhesive tape in a laminar flow. The surfactant was adsorbed on the surface of the freshly cleaved mica or the graphite by immersing the substrate in the desired surfactant solutions for at least 12 h. AFM imagings were performed for the substrates in the surfactant solutions at 25 ( 2 °C. Both deflection images (showing the error in the feedback signal) and the height images were collected with scan rates of 5-10 Hz. No filtering of images was performed other than that inherent in the feedback loop. The zero of distance in force curves is where a constant high gradient of force was measured, and it is assumed to be where the tip is touching the substrates. Most AFM imagings were performed at a repulsive force with the tip separated from the substrates by about 2-5 nm (so-called “soft contact imaging16). Under this condition, the force on the tip is dominated by the steric repulsion of the surfactant film on the substrates, and thus the imaging gives information about the surfactant film. When there is purely attractive force in the force curve, the imaging force was adjusted to as small a value as possible under an adhesive force. Contact Angle Measurements. Contact angles were measured using CA-DT goniometer with a light source, a microscope, and a fluid cell (Kyowa Interface Science Co., Japan). Surfactant solutions were prepared in distilled water after passage through the ultrapure water system. Freshly cleaved mica was placed in a liquid cell and allowed to equilibrate with the surfactant solution for at least 12 h. A sessile drop of decane was then placed under the equilibrated mica in the fluid cell with the surfactant solution at 25 ( 0.5 °C. The advanced contact angle was measured at a desired surfactant concentration. Decane of analytical grade was used.
Results and Discussion Concentration Dependence of the Morphologies of the C14DAO (rM ) 0.5) Aggregates on Mica. A characteristic feature of surfactant adsorption onto the solid-solution interface is that the adsorbed surfactants form local aggregates at the solid-solution interface at a concentration much lower than the bulk cmc. The AFM image gives information about the local aggregate structure at the solid-solution interface below the bulk cmc. In this study, we examined the concentration dependence of the morphologies and the corresponding force curves for the C14DAO (RΜ ) 0.5) aggregates on mica at pH 4.6 ( 0.2. Figure 1 shows the AFM height images of C14DAO (RΜ ) 0.5) aggregates on the mica in the aqueous surfactant solutions of different concentrations without added salt. The corresponding force curves are shown in Figure 2. The counterion binding constant for the K+ ions on mica and the surface isoelectric point of the Si3N4 AFM tip have been reported to be 2.550 and 6.0 ( 0.4,51 respectively. According to these results, the surface charges of the mica and the AFM tip in water at pH 4.6 have opposite signs (i.e., negative charges for the mica and positive charges for the AFM tip). As might be expected, the attractive interaction was observed between the negatively charged mica and the positively charged tip in water at about pH 5 (Figure 2a). At 10-6 M, the AFM image shows patches of small sizes, which may be the surfactant aggregates (Figure 1a). As the surfactant concentration increases to 10-5 M, the number of patches increases, and they tend to combine to form more continuous adsorbed aggregates (Figure 1b). In this concentration regime (
