Langmuir 1996, 12, 4345-4349
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Brewster Angle Microscopy Study of a Magnetic Nanoparticle/Polymer Complex at the Air/Water Interface Young Soo Kang,†,‡ Subhash Risbud,‡ John Rabolt,§ and Pieter Stroeve*,‡ Center on Polymer Interfaces and Macromolecular Assemblies, Department of Chemical Engineering and Materials Science, University of California at Davis, Davis, California 95616, and Center on Polymer Interfaces and Macromolecular Assemblies, IBM Almaden Research Center, San Jose, California 95120 Received February 20, 1996X The Langmuir layer behavior of a polymer/magnetite nanoparticle complex at the air/water interface was studied beyond the liquid-expanded region with Brewster angle microscopy (BAM). The copolymer surfactant was poly(octadecene-co-maleic anhydride) (POMA) with an average molecular weight of 30 000. The subphase was an aqueous colloidal solution of Fe3O4 nanoparticles with an average diameter of 8.5 ( 1.3 nm. In comparison to the case of a pure water subphase, the surface pressure versus specific area isotherm of the POMA was significantly expanded in the presence of the nanoparticles in the subphase. At 24 mN/m a phase transition was observed between the liquid-expanded and the liquid-condensed states. In the absence of the magnetite nanoparticles the POMA monolayer exhibited no domains as observed with BAM. In the presence of nanoparticles, formation of domains, was observed at surface pressures of 24 mN/m or higher. The domain size increased with surface pressure, suggesting the growth of a twodimensional solid complex in the liquid-compressed monolayer. Transmission electron microscopy (TEM) of the Langmuir layer deposited on a TEM grid showed the presence of magnetite particles.
Introduction Surfactant-induced aggregation or crystallization of organic or inorganic materials has been studied to obtain information on crystal growth and electron transport processes occurring within limited distances between nuclei.1-8 Precious metal particulates have been extensively studied in terms of their internal structure, their growth mechanisms, and their physical properties in the areas of conducting materials9 and catalysis.10 Controlled precipitation of quantum-confined semiconductor clusters has been achieved in a wide range of media including surfactant micelles,11-13 vesicles,14,15 zeolites,16 random ionomers,17-19 and ion-exchange complexing block co* E-mail address for correspondence:
[email protected]. † Permanent address: Department of Chemistry, National Fisheries University of Pusan, Pusan 608-737, Korea. ‡ University of California at Davis. § IBM Almaden Research Center. X Abstract published in Advance ACS Abstracts, August 15, 1996. (1) Martin, C. R. Science 1994, 266, 1961. (2) Zhao, X. K.; Herve, P. J.; Fendler, J. H. J. Phys. Chem. 1989, 93, 908. (3) Meldrum, F. C.; Kotov, N. A.; Fendler, J. H. Chem. Mater. 1995, 7, 1112. (4) Moffit, M.; Eisenberg, A. Chem. Mater. 1995, 7, 1178. (5) Moffit, M.; McMahon, L.; Pessel, V.; Eisenberg, A. Chem. Mater. 1995, 7, 1185. (6) Gorer, S.; Albu-Yaron, A.; Hodes, G. Chem. Mater. 1995, 7, 1243. (7) Charych, D. H.; Majda, M. Thin Solid Films 1992, 210/211, 348. (8) Du, Z.; Zhang, Z.; Zhao, W.; Zhu, Z.; Zhang, J.; Jin, Z.; Li, T. Thin Solid Films 1992, 210/211, 404. (9) Cheung, J. H.; Punkka, E.; Rikukawa, M.; Rosner, R. B.; Royappa, A. T.; Rubner, M. F. Thin Solid Films 1992, 210/211, 246. (10) Henglein, A. J. Phys. Chem. 1993, 97, 5457. (11) Meyer, M.; Wallberg, C.; Kurihara, K.; Fendler, J. H. J. Chem. Soc., Chem. Commun. 1984, 90. (12) Lianos, P.; Thoas, J. K. Chem. Phys. Lett. 1986, 125, 299. (13) Petit, C.; Lixon, P.; Pileni, M. P. J. Phys. Chem. 1990, 94, 1598. (14) Tricot, Y.-M.; Fendler, J. H. J. Phys. Chem. 1986, 90, 3369. (15) Watzke, H. J.; Fendler, J. H. J. Phys. Chem. 1987, 91, 854. (16) Wang, Y.; Herron, N. J. Phys. Chem. 1987, 91, 257. (17) Krishnan, M.; White, J. R.; Fox, M. A.; Bard, A. J. J. Am. Chem. Soc. 1983, 105, 7002. (18) Mau, A.-J.; Huang, C. B.; Kakuta, N.; Bard, A.-J.; Campion, A.; Fox, M. A.; White, J. M.; Webber, S. E. J. Am. Chem. Soc. 1984, 106, 6537. (19) Kuczynski, J. P.; Milosavljevic, B. H.; Thomas, J. K. J. Phys. Chem. 1984, 88, 980.
S0743-7463(96)00148-5 CCC: $12.00
Figure 1. Structure of poly(octadecene-co-maleic anhydride) (POMA).
polymers.20-22 It is possible to control the size of particles in materials such as reverse micelles, random ionomers, and polymer resins by varying the structure of their constituents.23-25 Due to steric hindrances the growth of nanoparticles can be confined and consequently very uniform size distributions of the nanoparticles can be obtained. Magnetic fluids, which are composed of colloidal suspensions of magnetic nanoparticles, are of interest in applications for ferromagnetic materials30,31 and magnetic (20) Cummins, C. C.; Schrock, R. R.; Cohen, R. E. Chem. Mater. 1992, 4, 27. (21) Yue, J.; Sankaran, V.; Cohen, R. E.; Schrock, R. R. J. Am. Chem. Soc. 1993, 115, 4409. (22) Ng Cheong Chan, Y.; Schrock, R. R.; Cohen, R. E. J. Am. Chem. Soc. 1992, 114, 7295. (23) Moore, R. B.; Bittercourt, D.; Gauthier, M.; Williams, C. E.; Eisenberg, A. Macromolecules 1991, 24, 1376. (24) Ziolo, R. F.; Giannel, E. P.; Weinstein, B. A.; O’Horo, M. P.; Ganguly, B. N.; Mehrotra, V.; Russell, M. W.; Huffman, D. R. Science 1992, 257, 219. (25) Nguyen, M. T.; Diaz, A. F. Adv. Mater. 1994, 6, 858. (26) Gunther, L. Phys. World 1990, 3, 28. (27) Audran, R. G. L.; Huguenard, A. P. U.S. Patent 4, 302, 523, 1981. (28) Okata, H. Chem. Mater. 1990, 2, 89. (29) Mann, S.; Hannington, J. P. J. Colloid Interface Sci. 1989, 122, 326. (30) Papell, S. S. U.S. Patent 3215572, 1965. (31) Zahn, M.; Sherton, K. E. Magnetic Fluids Bibliography. IEEE Trans. Magn. 1980, MAG-16, 387.
© 1996 American Chemical Society
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Figure 2. Pressure-area isotherm of POMA monolayer in the subphase of pure water (A), water with pH ) 3.5 (B), and the hydrosol of Fe3O4 nanoparticles with pH ) 3.5-4.0 (C).
refrigeration.32 Papell et al.30 reported the synthesis of ferro- or ferrimagnetic nanoparticles to produce stable magnetic fluids. The method consisted of ball-milling magnetite in a liquid in the presence of a stabilizing surfactant until the magnetite was in the colloidal state. Resler and Rosensweig reported the use of a ferromagnetic responsive liquid in order to develop an efficient magnetocaloric energy conversion cycle for refrigerating materials.33 From this initial work a new field called ferrohydrodynamics has evolved, and magnetic fluids have been the subject of numerous studies. Nanocomposites composed of ultrathin polymeric films containing nanoparticles have been prepared for study as functional materials in optics, electronics, and magnetics.26-29 Fendler et al. studied surfactant-stabilized Fe3O4 nanoparticles at the air/water interface by both transmission electron microscopy (TEM) and Brewster angle microscopy (BAM).34 They prepared an iron oxide spreading solution, containing lauric acid as a surfactant to stabilize the nanoparticles, and applied the solution directly to the air/water interface. The average particle size of their Fe3O4 nanoparticles was 13 nm. They observed two-dimensional domains of packed nanoparticles in a lauric acid matrix. The focus of this study is to investigate the behavior of nanocomposite monolayers composed of a polymer surfactant and Fe3O4 nanoparticles. Polymeric monolayers can lead to more robust systems, in particular if ultrathin films are fabricated by the Langmuir-Blodgett technique.35 The nanoparticle size used here is smaller than those reported previously.34 Our procedure is to spread (32) McMichael, R. D.; Shull, R. D.; Swartzendruber, L. J.; Bennett, L. H.; Watson, R.E. J. Magn. Magn. Mater. 1992, 111, 29. (33) Resler, E. L., Jr.; Rosensweig, R. E. AIAA J. 1964, 2, 1418. (34) Meldrum, F. C.; Kotov, N. A.; Fendler, J. H. J. Phys. Chem. 1994, 98, 4506. (35) Kowel, S. T.; Selfridge, R.; Eldering, C.; Matloff, N.; Stroeve, P.; Higgins, B. G.; Srinivasan, M. P.; Coleman, L. B. Thin Solid Films 1987, 152, 377.
the surface active polymer on a subphase of a colloidal aqueous suspension (hydrosol) of magnetic Fe3O4 nanoparticles.36 The hydrosol does not contain the added complication of an additional surfactant. To achieve a stable nanocomposite Langmuir layer composed of polymer and magnetic nanoparticles, it is necessary to study the morphology and phase behavior of the monolayer complex at the air/liquid interface. The state of the complex Langmuir layer affects the magnetic dipole interactions between the particles. The negative surface charge of the polymeric Langmuir layer plays an important role with regard to maximizing the electrostatic interaction with the positively charged Fe3O4 nanoparticles. The surface charge of the Langmuir layer can be increased by compressing the monolayer and increasing the surface pressure. Experimental Section The preparation of Fe3O4 nanoparticles has been described in a previous paper.36 The clear, blue-black, aqueous sol (hydrosol) was prepared by dispersing the nanoparticles into an aqueous solution with a pH equal to 3.5. The aqueous solution was a mixture of distilled water and concentrated hydrochloric acid. The magnetite particle concentration in the hydrosol was 0.38 × 10-4 mol of the iron oxide per liter of solution. The Fe3O4 nanoparticles had a diameter of 8.5 ( 1.3 nm, as analyzed by TEM.36 The surfactant was the copolymer poly(octadecane-co-maleic anhydride) (POMA), as shown in Figure 1, which was obtained from Johnson Wax and Sons (Racine, WI). The molecular weight of the polymer was 30 000. The POMA spreading solution was prepared by dissolving 1 mg of POMA in 1.0 mL of chloroform. Langmuir monolayers were spread in a circular NIMA Langmuir-Blodgett trough. The subphases in the trough were either distilled water (pH ∼ 6), distilled water at a pH of 3.5, made by addition of HCl, or the hydrosol of magnetite nanoparticles. The subphase temperature in the trough was controlled at 25 °C. (36) Kang, Y. S.; Risbud, S.; Rabolt, J. F.; Stroeve, P. Chem. Mater., in press.
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Figure 3. Brewster angle microscopic image (240 × 450 µm) of poly(octadecane-co-maleic anhydride) subphase of pure water, at a surface pressure of 10 mN/m and a mean molecular area of 27 Å2/mol for the repeat unit of POMA (A); subphase of the hydrosol of Fe3O4 nanoparticles, at a surface pressure of 10 mN/m and a mean molecular area of 33 Å2/mol for the repeat unit of POMA (B); subphase of pure water, at a surface pressure of 48 mN/m, which is the collapse point (C); subphase of the hydrosol of Fe3O4 nanoparticles, at a surface pressure of 24 mN/m and a mean molecular area of 28 Å2/mol for the repeat unit of POMA (D); subphase of the hydrosol of Fe3O4 nanoparticles, at a surface pressure of 34 mN/m and a mean molecular area of 26 Å2/mol for the repeat unit of POMA (E); subphase of the hydrosol of Fe3O4 nanoparticles, at a surface pressure of 45 mN/m and a mean molecular area of 22 Å2/mol for the repeat unit of POMA (F). Brewster angle microscopy was carried out with a setup similar to that described by Honig and Mobius.37 Depositions of the Langmuir monolayer were made by the Langmuir-Schaefer or the horizontal lifting method on a 400 mesh copper TEM grid coated with formvar film. TEM measurements on the Langmuir-Schaefer film were carried out on a Hitachi-600 transmission electron microscope.
Results and Discussions Pressure-area isotherms of POMA on the subphase of purified water, water with pH ) 3.5, and the Fe3O4 nanoparticle colloidal subphase at pH ) 3.5 are shown in Figure 2. The phase behavior of the POMA monolayer film does not show any significant difference between the purified water (A) and the water subphase with a pH ) 3.5 (B). Previous results, using a pure water subphase, have shown that the POMA monolayer at the air/water interface is stabilized as hydrolyzed dicarboxylates formed from the anhydride ring in the polymeric backbone.39 Since (37) Honig, D.; Mobius, D. J. Phys. Chem. 1991, 95, 4590. (38) Hallmark, V. M.; Shih, L.-B.; Stroeve, P.; Rabolt, J. F. Proc. SPIEsInt. Soc. Opt. Eng. 1989, 1145, 557.
the isotherms for the pure water subphase are nearly identical to those for the subphase at pH ) 3.5, the results indicate that the conversion of some of the carboxylates to the acid form does not significantly influence the monolayer behavior. The isotherm of the POMA monolayer on the Fe3O4 nanoparticle colloidal subphase is approximately 30% expanded at lower and 20% expanded at higher surface pressures (C). It is obvious that there is a significant interaction between the positively charged Fe3O4 nanoparticles and the negatively charged surface of the POMA monolayer and that this interaction increases with increasing surface pressure. Related studies by Peng et al. reported results on the isotherm of stearate on a subphase containing diamagnetic R-Fe2O3 nanoparticles, and the isotherm showed considerable expansion compared to that of a stearate monolayer on pure water.39 In a later study Peng et al.40 found that the isotherms of a polymaleic acid monoester on a R-Fe2O3 (39) Peng, X.; Zhang, Y.; Yang, J.; Zou, B.; Xiao, L.; Li, T. J. Phys. Chem. 1992, 96, 3412. (40) Peng, X.; Gao, M.; Zhao, Y.; Kang, S.; Zhang, Y. H.; Zhang, Y. Z.; Wang, D.; Xiao, L.; Li, T. Chem. Phys. Lett. 1993, 209, 233.
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nanoparticle colloidal subphase were almost the same as that obtained with purified water. The longer hydrophobic polymer backbone between the carboxylate groups in the polymaleic acid monoester (compared to the POMA backbone) could lead to less or no expansion if the properties of the backbone determine the isotherm character. On the other hand, the monolayer of POMA is critically affected by the hydrophilic portion of the monolayer, especially when the POMA monolayer is attached to iron oxide nanoparticles. In Figure 2 the limiting mean molecular area of the repeat unit of POMA is 29 Å2 in the iron oxide hydrosol subphase compared to 24 Å2 in distilled water. The difference in the mean molecular area at the higher surface pressures could be due to nanoparticles attached to the Langmuir layer being compacted under the monolayer. The phase behavior of the POMA monolayer observed with BAM is shown in Figure 3. The POMA monolayer was completely homogeneous both in the pure water and in the Fe3O4 nanoparticle hydrosol in the liquid-expanded region (below the phase transition), as shown in Figure 3A and B. Further compression beyond the liquidexpanded region of the monolayer on pure water causes the collapse of the monolayer, as shown in Figure 3C. In contrast to those results, further compression of the POMA monolayer in the Fe3O4 nanoparticle hydrosol subphase resulted in the formation and growth of domains with surface pressures as shown in Figure 3D, E, and F. These results can be interpreted as a growth of a polymeric monolayer complex containing attached iron oxide nanoparticles. Compression of the monolayer decreases the distance between the particles and increases the particle concentration. As the particles come into closer proximity, there is an increase in the van der Waals attraction and in the electrostatic repulsion between the particles.41 The growth of the size of the domain with surface pressure is reversible; upon reduction of the surface pressure the domains decrease in size. It is not clear how the particle interactions with the monolayer induce the domain formation in the monolayer. In comparison, the POMA monolayer without Fe3O4 nanoparticles shows no domain formation except when the monolayer is collapsed. These domains are smaller in size than those observed with the hydrosol subphase. TEM photographs obtained from Langmuir-Schaefer films are shown in Figure 4 and 5 at two different surface pressures: 11 and 40 mN/m. The bars on the figures are 300 Å long. Figure 4 indicates a low concentration of the Fe3O4 nanoparticles, randomly distributed on the monolayer. The nanoparticles are roundlike in shape and on the order of 80 Å in size. Aggregates of particles are apparent on the film, leading to larger irregular shapes. In Figure 5, the particle concentration of the Fe3O4 nanoparticles at 40 mN/m is greater than that seen in Figure 4 at the lower surface pressure. Again the distribution of the nanoparticles on the film appears to be random. Also more nanoparticles have aggregated on the film at the higher surface pressure, leading to a crowded appearance. As the surface pressure increases, the surface concentration of the POMA repeat unit increases, which leads to a higher negative surface charge. With an increase in surface charge, the electrostatic interactions between the Fe3O4 nanoparticles and the monolayer increase, causing more of the positively charged nanoparticles to adhere to the monolayer at higher surface pressures. The attachment of the nanoparticles at the surface of the monolayer (41) Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press: Orlando, FL, 1985.
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Figure 4. TEM photograph of a Langmuir-Schaefer film of a POMA monolayer at a surface pressure of 11 mN/m on the subphase of the hydrosol of Fe3O4 nanoparticles. The bar indicates a length of 300 Å.
Figure 5. TEM photograph of a Langmuir-Schaefer film of a POMA monolayer at a surface pressure of 40 mN/m on the subphase of the hydrosol of Fe3O4 nanoparticles. The bar indicates a length of 300 Å.
should make the collapse of the monolayer more difficult, particularly if the collapse is caused by the monolayer folding over. Our results in Figure 2 indeed show a substantially increased collapse pressure in the presence of the nanoparticles. Further, the presence of many nanoparticles at the interface of the polymer monolayer would prevent the polymer at high surface pressures from achieving the same minimum area per repeat unit that the monolyer would achieve without the presence of the particles. The 20% expansion in curve C of Figure 2 near the collapse pressure is significant. Conclusion POMA monolayers show nearly the same pressurearea isotherms for subphases of pure water and water with a pH of 3.5. In comparison the POMA monolayer on
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the subphase of the hydrosol of Fe3O4 nanoparticles was expanded. The POMA monolayer on a subphase of colloidal Fe3O4 nanoparticles induces domain formation beyond the liquid-expanded region, as observed by BAM. This is in distinct contrast with the pure POMA monolayer behavior at the air/water interface, which shows no domains. TEM photographs show a random distribution of the Fe3O4 nanoparticles on the POMA film. The results indicate that the anionic carboxylate groups of POMA
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interact electrostatically with the cationic surface charges of the Fe3O4 nanoparticles. Acknowledgment. This research is supported by the MRSEC Program of the National Science Foundation under Award Number DMR-9400354. We thank Dr. Marcin Majda for use of the Brewster angle microscope at UC Berkeley. LA960148P
