Stability of Polydimethylsiloxane-Magnetite Nanoparticle Dispersions Against Flocculation: Interparticle Interactions of Polydisperse Materials O. Thompson Mefford, ² Michael L. Vadala, ‡ Jonathan D. Goff, ² Matthew R. J. Carroll, § Raquel Mejia-Ariza, ² Beth L. Caba, ² Timothy G. St. Pierre, § Robert C. Woodward, § Richey M. Davis, ² and J. S. Riffle* ,² Macromolecules and Interfaces Institute, Virginia Tech, Blacksburg, Virginia 24061, NanoMedics, LLC, Erie, PennsylVania 16505, and School of Physics, UniVersity of Western Australia, Crawley, Western Australia 6009, Australia ReceiVed October 10, 2007. In Final Form: February 11, 2008 The colloidal stability of dispersions comprised of magnetite nanoparticles coated with polydimethylsiloxane (PDMS) oligomers was investigated theoretically and experimentally. Particle-particle interaction potentials in a theta solvent and in a good solvent for the PDMS were predicted by calculating van der Waals, electrostatic, steric, and magnetic forces as functions of interparticle separation distances. A variety of nanoparticle sizes and size distributions were considered. Calculations of the interparticle potential in dilute suspensions indicated that flocculation was likely for the largest 1% of the population of particles. Finally, the rheology of these complexes over time in the absence of a solvent was measured to probe their stabilities against flocculation as neat fluids. An increase in viscosity was observed upon aging, suggesting that some agglomeration occurs with time. However, the effects of aging could be removed by exposing the sample to high shear, indicating that the magnetic fluids were not irreversibly flocculated. Introduction In recent years, the use of magnetic nanoparticles in medical applications has grown significantly. Currently, magnetic nano- particles are utilized as contrast agents for MRI to diagnose tumors and cardiovascular disease, as hyperthermia agents for brain cancer therapy, and for magnetic separations of cells and bioagents. 1-5 In addition, researchers in our laboratories have synthesized hydrophobic ferrofluids comprised of polymer-coated magnetite nanoparticles for treating retinal detachments. 6-11 These latter materials are the focus of this paper. Tailoring the surfaces of these materials is critical for the success of these applications. Polymers that form sheaths around the magnetic nanoparticles can function to (i) suspend the nanoparticles in the intended medium, (ii) provide a stabilizing layer that prevents agglomeration, and (iii) reduce immune response. This has been accomplished with a variety of polymers. 12 For example, magnetic iron oxide particles have been coated with homopolymers such as poly(acrylic acid), 13 random copolymers such as poly(oligo(ethylene oxide) meth- acrylate-co-methacrylic acid), 14 or block copolymers such as poly(ethylene oxide-block-methacrylic acid). 15 In addition, iron oxide nanoparticles have been coated with water soluble polymers such as dextran 16 and poly(ethylene oxide) 3,17,18 and with nonpolar materials such as polystyrene and poly(methyl methacrylate). 19 The stabilities of polymer-magnetite complexes in dilute suspensions are related to the net particle-particle interaction potentials. Particles are attracted by van der Waals and magnetic interactions and repelled by steric and electrostatic forces. To maintain stability of a dispersion, the repulsive forces must be substantial enough to prevent agglomeration driven by the attractive forces. This balance has been extensively studied in the realm of colloidal suspensions. The classical approach utilizes Derjaguin-Landau-Verwey-Overbeek (DLVO) theory 20-23 as * Corresponding author. E-mail: JudyRiffle@aol.com. Phone: (540)- 231-8214. ² Virginia Tech. ‡ NanoMedics, LLC. § University of Western Australia. (1) Willard, M. A.; Kurihara, L. K.; Carpenter, E. E.; Calvin, S.; Harris, V. G. Int. Mater. ReV. 2004, 49 (3-4), 125-170. (2) Neuberger, T.; Schopf, B.; Hofmann, H.; Hofmann, M.; von Rechenberg, B. J. Magn. Magn. Mater. 2005, 293 (1), 483-496. (3) Thunemann, A. F.; Schutt, D.; Kaufner, L.; Pison, U.; Mohwald, H. Langmuir 2006, 22, 2351-2357. (4) Pankhurst, Q. A.; Connolly, J.; Jones, S. K.; Dobson, J. J. Phys. D: Appl. Phys. 2003, 36, R167-181. (5) Tartaj, P.; Puerto-Morales, M. D.; Veintemillas-Verdaguer, S.; Gonzalez- Carreno, T.; Serna, C. J. J. Phys. D: Appl. Phys. 2003, 36, R182-R197. (6) Dailey, J. P.; Phillips, J. P.; Li, C.; Riffle, J. S. J. Magn. Magn. Mater. 1999, 194, 140-148. (7) Mefford, O. T.; Woodward, R. C.; Goff, J. D.; Vadala, T. P.; St. Pierre, T. G.; Dailey, J. P.; Riffle, J. S. J. Magn. Magn. Mater. 2007, 311, 347-353. (8) Rutnakornpituk, M.; Baranauskas, V. V.; Riffle, J. S.; Connolly, J.; St. Pierre, T. G.; Dailey, J. P. Eur. Cells Mater. 2002, 3, 102-105. (9) Stevenson, J. P.; Rutnakornpituk, M.; Vadala, M. L.; Esker, A. R.; Charles, S. W.; Wells, S.; Dailey, J. P.; Riffle, J. S. J. Magn. Magn. Mater. 2001, 225, 47-58. (10) Vadala, M. L.; Zalich, M. A.; Fulks, D. B.; St. Pierre, T. G.; Dailey, J. P.; Riffle, J. S. J. Magn. Magn. Mater. 2005, 293, 162-170. (11) Wilson, K. S.; Goff, J. D.; Riffle, J. S.; Harris, L. A.; St. Pierre, T. G. Polym. AdV. Technol. 2005, 16 (2-3), 200-211. (12) Kim, D. K.; Mikhaylova, M.; Zhang, Y.; Muhammed, M. Chem. Mater. 2003, 15 (8), 1617-1627. (13) Si, S.; Kotal, A.; Mandal, T. K.; Giri, S.; Nakamura, H.; Kohara, T. Chem. Mater. 2004, 16, 3489-3496. (14) Lutz, J.-F.; Stiller, S.; Hoth, A.; Kaufner, L.; Pison, U.; Cartier, R. Biomacromolecules 2006, 7, 3132-3138. (15) Wormuth, K. J. Colloid Interface Sci. 2001, 241, 366-377. (16) Molday, R. S. U.S. Patent 4452773, 1984. (17) Suzuki, M.; Shinkai, M.; Kamihira, M.; Kobayashi, T. Biotechnol. Appl. Biochem. 1995, 21, 335-345. (18) Harris, L. A.; Goff, J. D.; Carmichael, A. Y.; Riffle, J. S.; Harburn, J. J.; St. Pierre, T. G.; Saunders, M. Chem. Mater. 2003, 15 (6), 1367-1377. (19) Noguchi, H.; Yanase, N.; Uchida, Y.; Suzuta, T. J. Appl. Polym. Sci. 1993, 48 (9), 1539-1547. (20) Hunter, R. J., Foundations of Colloid Science; Clarendon Press: Oxford, UK, 1992. (21) Verwey, E. J. W.; Overbeek, J. T. G. Theory of the Stability of Lyophobic Colloids; Dover: Mineola, NY, 2000. (22) Deryaguin, B. V.; Landau, L. D. Acta Physicochim. (USSR) 1941, 14, 633-652. (23) Verwey, E. J. W.; Overbeek, J. T. G. Theory of the Stability of Lyophobic Colloids; Elsevier: Amsterdam, 1948. 5060 Langmuir 2008, 24, 5060-5069 10.1021/la703146y CCC: $40.75 © 2008 American Chemical Society Published on Web 03/27/2008