Some Physicochemical Aspects of Nanoparticulate Magnetic Iron Oxide Colloids in Neat Water and in the Presence of Poly(vinyl alcohol) Aristides Bakandritsos,* ,† Georgios C. Psarras, † and Nikos Boukos ‡ Materials Science Department, School of Natural Sciences, UniVersity of Patras, Rio 26504, Patras, and Institute of Materials Science, NCSR “Demokritos”, Agia ParaskeVi 15310, Athens, Greece ReceiVed June 17, 2008. ReVised Manuscript ReceiVed July 21, 2008 The preparation of magnetic iron oxide colloids directly from the coprecipitation of Fe 2+ and Fe 3+ species at different temperatures may lead to crystallites of higher size as the temperature of the reaction increases. On the other hand, dynamic light scattering investigations and dielectric measurements rather point to the similar colloidal size of the entities existing in their aqueous or solid-state dispersions, irrespective of the size of the primary nanocrystallites. Significant enhancement of the stability of the colloids, even in the presence of high electrolyte concentrations, is furnished after the addition of relatively small amounts of poly(vinyl alcohol), and the stabilization mechanism is discussed in terms of the various forces participating in the system. The experimental results suggest that the increased colloidal stability is triggered from the particles’ decrease of velocity rather than from steric (entropic) effects originating from polymer absorption. 1. Introduction Magnetic nanoparticles of iron oxide, magnetite and maghemite, are increasingly receiving the interest of contemporary research due to their important physicochemical properties with extensions in many technological and scientific fields. From the physics standpoint such particles may exhibit superparamagnetism and single magnetic domain particle behavior, while from the chemistry point of view their large surface area and rich surface chemistry are critical factors dominating in phenomena related to (bio)chemical reactivity. Owing to the above properties, numerous applications have emerged in the field of pigment and ink technologies 1 as gas and biosensors, 2 in sealing, positioning, heat transfer, and separation processes, 3 in magnetoelectronics, 4 in nonlinear optics, 5 in magnetic storage media, 6 in environmental remediation, 7 and in numerous biomedical technologies. 8,9 Specifically, the preparation of nanoparticles in the colloidal state (ferrofluids) is of paramount importance in many of the aforementioned fields of application, since the fine dispersion of the particles facilitates their effective incorporation in polymers or other matrixes (through mixing of the constituent materials in a nanoscopic scale), the fabrication of films over various substrates, and, if adequate monodispersity is provided, the preparation of self-organized 1-D, 2-D, or 3-D arrays. 10 In addition, being in the colloidal state, their chemistry can be considerably expanded by attaching molecular or polymeric capping agents on the surface of the particles through wet chemical routes, 5b,11 imparting to them properties and functionalities beyond the intrinsic properties of the oxide core. Considering the synthetic routes toward iron oxide colloids, the thermolytic decomposition of ferric or ferrous complexes in high boiling point organic solvents has been reported to deliver particles with a high degree of monodispersity 10 and in high yields. 12 Nevertheless, the aqueous alkaline hydrolytic precipita- tion route 13 is still being widely utilized especially in biomedical applications, 14 despite the polydispersity of the particles produced, probably due the simple and cost-effective 15 synthetic procedure. Another advantage of the method is that the particles’ surface may be retained free for any subsequent functionalization with the desired molecular 14b,f or polymeric 14a,c,d entities. It is known * To whom correspondence should be addressed. E-mail: abakan@ upatras.gr, arisbakan@gmail.com. † University of Patras. ‡ NCSR “Demokritos”. (1) Harben, P. W. Iron Oxides in Industrial Minerals. 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