Langmuir 2002, 18, 6259-6265
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Scanning Probe Microscopy Studies of Mesostructured Nonstoichiometric Polyelectrolyte-Surfactant Complexes Xiangmin Liao and Daniel A. Higgins* Department of Chemistry, Kansas State University, Manhattan, Kansas 66506 Received March 20, 2002. In Final Form: June 6, 2002 High-resolution tapping-mode atomic force microscopy (AFM) and near-field scanning optical microscopy (NSOM) are used to study nonstoichiometric polyelectrolyte-surfactant complexes (PSCs) formed between poly(vinyl sulfate) and alkyltrimethylammonium bromide (CnTAB). The morphological evolution of PSC films prepared with different surfactant loadings (i.e., different surfactant/anionic-site stoichiometries) is explored as a function of surfactant chain length (n ) 12, 14, 16, 18). PSC films were prepared with 1%, 10%, 25%, and 50% surfactant loading. At the lowest loading, relatively uniform films are obtained, despite the fact that the surfactant concentrations in the solutions employed were all above the critical aggregation concentration. At 10% loading, for the longer chain surfactants (n ) 16 and 18), small protrusions (10-40 nm in height) appear in the films. These are attributed to the formation of spheroidal and/or discoidal polyelectrolyte-surfactant micelles. For films of 25-50% surfactant loading, lamellar PSC bilayer domains are clearly observed for all but those prepared with C12TAB. In many samples, both micellar structures and lamellar phases are found, indicative of the complicated morphological attributes of these materials. Evidence for surfactant-dependent dewetting of the glass substrate surface is also obtained. Fluorescence NSOM images of dye-doped versions of the PSCs provide additional proof for the film structural assignments. AFM images of the lamellar structures at 50% surfactant loading show the presence of interesting defected bilayer regions. The possible origins of these defected regions and their associated bilayer height variations are discussed.
Introduction The development of new thin-film surface coatings continues to be an important motivation behind many materials research programs. For example, new materials are being sought for applications in chemical separations,1,2 optical and electrochemical sensors,3,4 fuel-cellbased power systems,5,6 and the fabrication of biocompatible materials.7-9 All such applications require surface coatings with tailored chemical and physical properties. Often, materials exhibiting spatial variations in their properties on mesoscopic length scales are desired. A particularly important class of materials now under development are amphiphilic systems, which incorporate both hydrophilic regions (i.e., polar and possibly ionic regions) and hydrophobic regions. The complexation chemistry of polyelectrolytes and charged surfactants represents one substantial research area that will likely lead to the development of valuable new surface coatings with the aforementioned attributes.10,11 A notable advantage of these materials is that they are easily synthesized by simply mixing (usually) * Corresponding author. E-mail:
[email protected]. (1) Svec, F.; Peters, E. C.; Sykora, D.; Frechet, J. M. J. J. Chromatogr., A 2000, 887, 3. (2) Dorsey, J. G.; Cooper, W. T.; Siles, B. A.; Foley, J. P.; Barth, H. G. Anal. Chem. 1998, 70, 591R. (3) Albert, K. J.; Lewis, N. S.; Schauer, C. L.; Sotzing, G. A.; Stitzel, S. E.; Vaid, T. P.; Walt, D. R. Chem. Rev. 2000, 100, 2595. (4) Demas, J. N.; DeGraff, B. A.; Coleman, P. B. Anal. Chem. 1999, 71, 793A. (5) Mamak, M.; Coombs, N.; Ozin, G. Adv. Mater. 2000, 12, 198. (6) Steele, B. C. H.; Heinzel, A. Nature 2001, 414, 345. (7) Nguyen, S. T.; Gin, D. L.; Hupp, J. T.; Zhang, X. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 11849. (8) Satulovsky, J.; Carignano, M. A.; Szleifer, I. Proc. Natl. Acad. Sci. U.S.A. 2000, 97, 9037. (9) Stewart, M. P.; Buriak, J. M. Adv. Mater. 2000, 12, 859. (10) Antonietti, M.; Conrad, J.; Thu¨nemann, A. Macromolecules 1994, 27, 6007. (11) Ober, C. K.; Wegner, G. Adv. Mater. 1997, 9, 17.
aqueous polyelectrolyte and surfactant solutions. The polyelectrolyte-surfactant complexes (PSCs) that are formed have widely tunable properties that can be controlled by simply altering the chemical nature of either the polymer or the surfactant. The chemical properties of these materials can also be altered by varying the surfactant/ionic-site loading ratio. Stoichiometric PSCs are formed when the amount of surfactant employed is equivalent to the number of oppositely charged ionic sites on the polyelectrolyte. Such materials are insoluble in water yet provide highly charged regions that may adsorb water and other polar substances. These films have good mechanical and chemical stability. The surfactant layers are stabilized by their electrostatic interactions with the oppositely charged polymer. Hydrophobic interactions between the surfactant hydrocarbon chains are also important to material stability. As a result of their tremendous potential as new materials, such stoichiometric complexes have been studied extensively by a number of physical and optical methods,12-15 both in solution15-18 and in the solid state.11,14,19 It is now well-known that well-ordered lamellar structures exist in stoichiometric PSCs.13 Indeed, Antonietti et al. have clearly shown the existence of such structures in PSCs formed between poly(styrenesulfonate) (PSS) and alkyltrimethylammonium bromide (CnTAB),10 a system very similar to the one studied here. The lamellar phases in (12) Ren, B.; Tong, Z.; Gao, F.; Liu, X.; Zeng, F. Polymer 2001, 42, 7291. (13) Thu¨nemann, A. F.; Ruppelt, D. Langmuir 2000, 16, 3221. (14) Thu¨nemann, A. F.; Ruppelt, D. Langmuir 2001, 17, 5098. (15) Chen, L.; Xu, S.; McBranch, D.; Whitten, D. J. Am. Chem. Soc. 2000, 122, 9302. (16) Dembo, A. T.; Starodoubtsev, S. G. Macromolecules 2001, 34, 2635. (17) Hayakawa, K.; Tanaka, R.; Kurawaki, J.; Kusumoto, Y.; Satake, I. Langmuir 1999, 15, 4213. (18) Kogej, K.; Skerjanc, J. Langmuir 1999, 15, 4251. (19) MacKnight, W. J.; Ponomarenko, E. A.; Tirrell, D. A. Acc. Chem. Res. 1998, 31, 781.
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these materials exhibit a periodicity in small-angle X-ray scattering (SAXS) data of 3-4 nm. Harada et al. have shown that the stoichiometric complex of poly(vinyl sulfate) (PVS) and cetyltrimethylammonium bromide (C16TAB) also possesses a lamellar structure, with a longer period of about 5-6 nm.20 When cast as thin films, because of their lamellar morphologies, such stoichiometric materials most likely form relatively chemically uniform coatings, at least in the film plane. Depending on the preparation methods employed, their surfaces may be terminated by either the hydrophobic alkane chains of the surfactant or the hydrophilic ionic regions of the complex. Thin-film materials possessing lateral nanometer to micrometer scale (i.e., mesoscale) variations in their chemical properties may provide several advantages over these stoichiometric materials. For example, PSCs incorporating hydrophilic and hydrophobic domains may yield films more readily accessible to certain solvents and solutes. The preparation of nonstoichiometric PSCs21 represents one potential route to such materials. The solubility of nonstoichiometric PSCs in aqueous solution provides clear evidence that their properties differ substantially from those of insoluble stoichiometric materials and also indicates that they much more readily incorporate highly polar solvents. The water solubility of nonstoichiometric materials is due to the presence of uncomplexed ionic sites along the polyelectrolyte backbone. These ionic sites will similarly make films prepared from nonstoichiometric PSCs much more accessible to polar and/or ionic adsorbants, an attribute that may be of particular importance in applications such as DNA immobilization22-24 and/or ion-exchange separations.25,26 While nonstoichiometric PSCs possess potentially useful chemical properties, thin films prepared from these materials have not been extensively studied. Specifically, the ramifications of nonstoichiometric surfactant loading on the local chemical and physical properties of their films remain unclear. However, it is believed that highly inhomogeneous thin films may be formed from nonstoichiometric PSCs. Significant spatial variability in the chemical properties of these films is expected from the “string-ofpearls” model originally proposed for PSCs at low surfactant loadings.10 PSC micelles form in local regions along individual polymer chains, leaving the remaining ionic sites uncomplexed. Materials incorporating highly polar regions (uncomplexed polymer) and nonpolar regions (micelles) result. At relatively high surfactant loadings, a transition to lamellar structures is also expected, providing additional material complexity. Indeed, the results of previous studies from our lab27 clearly show the presence of both PSC micelles and lamellar phases in model PSC films, proving that nonstoichiometric PSC thin films are chemically inhomogeneous on mesoscopic length scales. In the present work, new insights into the mesoscale chemical variability of nonstoichiometric PSC thin films are sought for common PSC systems. Such information is best obtained from high-resolution optical and physical (20) Harada, A.; Nozakura, S. Polym. Bull. 1984, 11, 175. (21) Bakeev, K. N.; Shu, Y. M.; MacKnight, W. J.; Zezin, A. B.; Kabanov, V. A. Macromolecules 1994, 27, 300. (22) Wang, J.; Bard, A. J. Anal. Chem. 2001, 73, 2207. (23) Kumar, A.; Larsson, O.; Parodi, D.; Liang, Z. Nucleic Acids Res. 2000, 28, e71. (24) Caruso, F.; Rodda, E.; Furlong, D. N.; Niikura, K.; Okahata, Y. Anal. Chem. 1997, 69, 2043. (25) Suzuki, Y.; Quina, F. H.; Berthod, A.; Williams, R. W., Jr.; Culha, M.; Mohammadzai, I. U.; Hinze, W. L. Anal. Chem. 2001, 73, 1754. (26) Cheetham, A. K.; Ferey, G.; Loiseau, T. Angew. Chem., Int. Ed. 1999, 38, 3268. (27) Liao, X.; Higgins, D. A. Langmuir 2001, 17, 6051.
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microscopic methods. Atomic force microscopy (AFM) and near-field scanning optical microscopy (NSOM) are two methods that provide the high spatial resolution necessary to investigate mesoscale variations in PSC film properties. AFM has been successfully used to study self-assembled monolayers,28-30 phase-separated Langmuir-Blodgett (LB) films,31-34 and recently stoichiometric and nonstoichiometric PSCs.13,14,27 In the present study, high-resolution tapping-mode AFM images are used to investigate the morphological evolution of PSCs as a function of surfactant loading and surfactant chain length. These systems are shown to evolve from spheroidal and/or discoidal micellar structures to lamellar phases. The latter structures appear as terraces across much of the sample surface. However, interesting new features are also observed within these terraces. Their observation clearly points to local variability in the polymer-surfactant bilayer structure. While the AFM results provide detailed physical information on the PSC films, NSOM images provide more chemically relevant data.35 Transmission and fluorescence NSOM methods have been employed in the characterization of a wide variety of thin-film materials.27,34-41 Here, the hydrophobic dye nile red, which preferentially partitions into regions of high surfactant content, is used as a probe of local film composition. The fluorescence NSOM images clearly show the incorporation of the dye into micellar structures and lamellar phases. Experimental Section Dodecyltrimethylammonium bromide (C12TAB), tetradecyltrimethylammonium bromide (C14TAB), hexadecyltrimethylammonium bromide (C16TAB), and octadecyltrimethylammonium bromide (C18TAB) were purchased from Aldrich and were used as received. Stock solutions of 2 × 10-3 M surfactant were prepared using high-purity water (18 MΩ cm). The C18TAB stock solution was found to be turbid at this concentration, because of its low solubility. Potassium poly(vinyl sulfate) (Aldrich, Mw ) 140 000) was purified prior to use by reprecipitation after neutralization to pH ) 7.0 with aqueous KOH. For PSC preparation, a stock PVS solution with an approximate monomer concentration of 4 × 10-3 M (in high-purity water) was used. Thin PSC films were prepared from these solutions to yield samples with surfactant loadings of 1%, 10%, 25%, and 50%. Surfactant loading is defined here as the percentage of anionic polymer sites that are complexed with surfactant, assuming all available surfactant interacts with the polymer. To form the PSCs, appropriate amounts of surfactant solution and high-purity water were added to the PVS stock solution to yield a final PVS monomer concentration of 2 × 10-3 M in each case. All PSC solutions were found to be clear. A small drop of PSC solution was then transferred to a 200 µm thick glass substrate (Fisher Premium). (28) Auletta, T.; Van Veggel, F. C. J. M.; Reinhoudt, D. N. Langmuir 2002, 18, 1288. (29) Doudevski, I.; Schwartz, D. K. J. Am. Chem. Soc. 2001, 123, 6867. (30) Tamada, K.; Ishida, T.; Knoll, W.; Fukushima, H.; Colorado, R., Jr.; Graupe, M.; Shmakova, O. E.; Lee, T. R. Langmuir 2001, 17, 1913. (31) Imae, T.; Aoki, K. Langmuir 1998, 14, 1196. (32) Luo, G.; Liu, T.; Ying, L.; Zhao, X. S.; Huang, Y.; Wu, D.; Huang, C. Langmuir 2000, 16, 3651. (33) Imae, T.; Takeshita, T.; Kato, M. Langmuir 2000, 16, 612. (34) Vickery, S. A.; Dunn, R. C. Langmuir 2001, 17, 8204. (35) Dunn, R. C. Chem. Rev. 1999, 99, 2891. (36) Higgins, D. A.; Liao, X.; Hall, J. E.; Mei, E. J. Phys. Chem. B 2001, 105, 5874. (37) Teetsov, J.; Vanden Bout, D. A. J. Phys. Chem. B 2000, 104, 9378. (38) Vanden Bout, D. A.; Kerimo, J.; Higgins, D. A.; Barbara, P. F. Acc. Chem. Res. 1997, 30, 204. (39) Hollars, C. W.; Dunn, R. C. J. Phys. Chem. B 1997, 101, 6313. (40) Cordero, S. R.; Weston, K. D.; Buratto, S. K. Thin Solid Films 2000, 360, 139. (41) Hwang, J.; Tamm, L. K.; Bo¨hm, C.; Ramalingam, T. S.; Betzig, E.; Edidin, M. Science 1995, 270, 610.
SPM Study of Polyelectrolyte-Surfactant Complexes Table 1. Critical Micelle Concentrations and Critical Aggregation Concentrations for the CnTAB and CnTAB-PVS Systemsa surfactant
cmc (mM)
cac (µM)
C12TAB C14TAB C16TAB C18TAB
17.2 3.7 0.9 0.2
12.0 2.1 1.5 1.3
a These values were determined using fluorescence spectroscopy, as described in the text.
The substrate was subsequently spun at 3000 rpm for 30 s on a spin-coating apparatus. The resulting films were dried at 60 °C for at least 12 h prior to use. For NSOM fluorescence imaging, PSC films were doped to 5 × 10-7 M with nile red, using a methanolic stock solution. Nile red was also obtained from Aldrich and was used as received. A Digital Instruments Multimode AFM was used to obtain high-resolution topographic images of the PSC film surfaces. All such images were recorded in tapping mode, as films with high surfactant content were damaged by strong probe-sample interactions in contact-mode imaging experiments. A modified, commercially available TM-Microscopes Aurora NSOM was used to simultaneously record fluorescence and topographic images of the PSC samples. This system has been described previously.27,36 Briefly, the tapered, aluminum-coated near-field fiber probes used were fabricated in house by conventional means.27 Surface topography was sensed by optically detected shear-force methods.42,43 Light from an argon ion laser (514 nm, 1-2 mW) was coupled into the probe fiber and was used to excite the nile red doped into the PSC films. Fluorescence from the sample was collected in transmission by a microscope objective (Nikon, 0.8 numerical aperture). The fluorescence was isolated from residual excitation light by use of appropriate notch, long-pass, and shortpass filters and was detected with a single-photon-counting avalanche diode (Perkin-Elmer). Conventional solution-phase fluorescence spectroscopy was used for the determination of the critical micelle concentration (cmc) and critical aggregation concentration (cac) for each surfactant and each polyelectrolyte-surfactant system, respectively. Nile red was employed for the cmc determination, and pyrene for the cac determination. Fluorescence spectra of these solutions were recorded on a commercial spectrofluorometer (Spex Fluoromax-2).
Results and Discussion To better understand the structures formed in the PSC films, the cmc and cac were measured for each surfactant and PSC system. The cmc was measured by recording the peak fluorescence intensity for 1 × 10-5 M nile red solutions as a function of surfactant concentration. Nile red is only weakly fluorescent in pure water, having a fluorescence quantum yield of
