Some Physicochemical Aspects of Nanoparticulate Magnetic Iron

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Langmuir 2008, 24, 11489-11496

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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 Fe2+ and Fe3+ 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.

* To whom correspondence should be addressed. E-mail: abakan@ upatras.gr, [email protected]. † University of Patras. ‡ NCSR “Demokritos”.

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 monodispersity10 and in high yields.12 Nevertheless, the aqueous alkaline hydrolytic precipitation route13 is still being widely utilized especially in biomedical applications,14 despite the polydispersity of the particles produced, probably due the simple and cost-effective15 synthetic procedure. Another advantage of the method is that the particles’ surface may be retained free for any subsequent functionalization with the desired molecular14b,f or polymeric14a,c,d entities. It is known

(1) Harben, P. W. Iron Oxides in Industrial Minerals. The Industrial Mineral Handbook II, 2nd ed.; Industrial Minerals: London, 1995; p 85. (2) (a) Jing, Z.; Wu, S. Mater. Lett. 2006, 60, 952. (b) Lu, B.-W.; Chen, W. C. J. Magn. Magn. Mater. 2006, 304, e400. (c) Liu, Z.-M.; Yang, H.-F.; Li, Y.-F.; Liu, Y.-L.; Shen, G.-L.; Yu, R.-Q. Sens. Actuators, B 2006, 113, 956. (3) Odenbach, S. In Handbook of Magnetic Materials; Buschow, K. H. J., Ed.; Elsevier Science Publishers B.V.: Amsterdam, 2006; Vol. 16, p 151. (4) (a) Prinz, G. A. Science 1998, 282, 1660. (b) Redl, F. X.; Black, C. T.; Papaefthymiou, G. C.; Sandstrom, R. L.; Yin, M.; Zeng, H.; Murray, C. B.; O’Brien, S. P. J. Am. Chem. Soc. 2004, 126, 14583. (c) Zhang, D.; Liu, L.; Han, S.; Li, C.; Lei, B.; Stewart, M. P.; Tour, J. M.; Zhou, C. Nano Lett. 2004, 4, 2151. (5) (a) Hashimoto, T.; Yamada, T.; Yoko, T. J. Appl. Phys. 1996, 80, 3184. (b) Bakandritsos, A.; Bouropoulos, N.; Zboril, R.; Iliopoulos, K.; Boukos, N.; Chatzikyriakos, G.; Couris, S. AdV. Funct. Mater. 2008, 18, 1694. (6) (a) Yoon, M.; Kim, Y. M.; Kim, Y.; Volkov, V.; Song, H. J.; Park, Y. J.; Vasilyak, S. L.; Park, I. W. J. Magn. Magn. Mater. 2003, 265, 357. (b) Pankhurst, Q. A.; Pollard, R. J. J. Phys.: Condens. Matter 1993, 5, 8487. (7) (a) Yavuz, C. T.; Mayo, J. T.; Yu, W. W.; Prakash, A.; Falkner, J. C.; Yean, S.; Cong, L.; Shipley, H. J.; Kan, A.; Tomson, M.; Natelson, D.; Colvin, V. L. Science 2006, 314, 964. (b) Ille´s, E.; Tomba´cz, E. J. Colloid Interface Sci. 2006, 295, 115. (c) Wang, C. B.; Zhang, W. X. EnViron. Sci. Technol. 1997, 31, 2154. (8) Pankhurst, Q. A.; Connolly, J.; Jones, S. K.; Dobson, J. J. Phys. D: Appl. Phys. 2003, 36, R167. (9) Gupta, A. K.; Gupta, M. Biomaterials 2005, 26, 3995.

(10) (a) Zhuang, J.; Wu, H.; Yang, Y.; Cao, Y. C. J. Am. Chem. Soc. 2007, ´ ., P. J. Am. Chem. Soc. 129, 14166. (b) Rockenberger, J.; ScherÅ, C.; AlivisatosA 1999, 121, 11595. (c) Hyeon, T.; Lee, S. S.; Park, J.; Chung, Y.; Na, H. B. J. Am. Chem. Soc. 2001, 123, 12798. (d) Sun, S.; Zeng, H.; Robinson, D. B.; Raoux, S.; Rice, P. M.; Wang, S. X.; Li, G. J. Am. Chem. Soc. 2003, 126, 273. (11) Huh, Y. M.; Jun, Y. W.; Song, H. T.; Kim, S.; Choi, J. S.; Lee, J. H.; Yoon, S.; Kim, K. S.; Shin, J. S.; Suh, J. S.; Cheon, J. J. Am. Chem. Soc. 2005, 127, 12387. (12) Park, J.; An, K.; Hwang, Y.; Park, J. G.; Noh, H. J.; Kim, J. Y.; Park, J. H.; Hwang, N. M.; Hyeon, T. Nat. Mater. 2004, 3, 891. (13) Massart, R. IEEE Trans. Magn. 1981, MAG-17, 1247. (14) (a) Gupta, A. K.; Curtis, A. S. G. Biomaterials 2004, 25, 3029. (b) Bra¨hler, M.; Georgieva, R.; Buske, N.; Mu¨ller, A.; Mu¨ller, S.; Pinkernelle, J.; Teichgra¨ber, U.; Voigt, A.; Ba¨umler, H. Nano Lett. 2006, 6, 2505. (c) Steitz, B.; Hofmann, H.; Kamau, S. W.; Hassa, P. O.; Hottiger, M. O.; vonRechenberg, B.; HofmannAmtenbrink, M.; Petri-Fink, A. J. Magn. Magn. Mater. 2007, 311, 300. (d) Kim, D. K.; Mikhaylova, M.; Wang, F. H.; Kehr, J.; Bjelke, B.; Zhang, Y.; Tsakalakos, T.; Muhammed, M. Chem. Mater. 2003, 15, 4343. (e) Dixit, S.; Hering, J. EnViron. Sci. Technol. 2003, 37, 4182. (f) Bucak, S.; Jones, D. A.; Laibinis, P. E.; Hatton, T. A. Biotechnol. Prog. 2003, 19, 477. (g) Son, S. J.; Reichel, J.; He, B.; Schuchman, M.; Lee, S. B. J. Am. Chem. Soc. 2005, 127, 7316. (15) Vayssie`res, L.; Chane´ac, C.; Tronc, E.; Jolivet, J. P. J. Colloid Interface Sci. 1998, 205, 205.

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 technologies1 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

10.1021/la801901j CCC: $40.75  2008 American Chemical Society Published on Web 09/27/2008

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that the iron oxide’s surface may be positively or negatively charged depending on the suspension’s pH (protonations deprotonation of surface OH groups), and subsequently, the capping agents may be electrostatically or even covalently bound, provided that appropriate complexing groups are present (i.e., carboxylic,16 sulfate,5b phosphate,17 and silane groups18). Due to the importance of the hydrolytic route, one of the objectives of the present study was to gain a better insight into the colloidal properties of the particles produced. By appropriate adjustment of the temperature of the reaction, three colloids of different mean nanocrystallite sizes were obtained and studied. It is noted that throughout the text the term nanocrystallite refers to the size of the domain where single crystallinity is retained and not to the size of the colloidal particles/aggregates. The experiments focus particularly on the state and behavior of the particles in H2O or in the presence of the neutral polymer of poly(vinyl alcohol) (PVA). Subsequently, the respective Fe3O4/ PVA nanocomposite polymeric films were fabricated and studied with dielectric spectroscopy, which is considered as a powerful tool for the investigation of molecular mobility, phase changes, conductivity mechanisms, and interfacial effects in polymers and complex systems.19

2. Experimental Section 2.1. Materials and Synthetic Methods. Anhydrous FeCl2 and FeCl3 were purchased from Sigma-Aldrich. NaOH in pellets, NaCl, and PVA of mean molecular weights Mw ) 72 000 and 145 000 were obtained form Merck Chemicals. Hydrochloric acid (37%) for analysis was purchased from Carlo Erba. In a spherical flask containing deionized H2O (40 mL), the following reagents were added in the sequence they appear: FeCl3 (0.380 g), concentrated HCl (37%, 0.75 mL), and FeCl2 (0.140 g). HCl is added because the acidic environment stabilizes the ferrous complexes against oxidation. The flask was placed in a temperature bath, and the reaction was performed at three different temperatures (0, 50, and 90 °C) under a N2 flow. After equilibration of the temperature, an aqueous solution of NaOH (1.65 g, 10 mL) of the same temperature was added with a syringe in one step and in a time interval of seconds under vigorous stirring. After 20 min the flask was removed from the bath, and the contents were stirred for another 10 min to cool. At that point the mixture was centrifuged and the solid product of the reaction precipitated easily (4000 rpm, 2 min). Washing was performed with H2O (30 mL), and isolation of the product was easily performed once more with centrifugation. After the second washing step prolonged centrifugation (4000 rpm, 30 min) was necessary to isolate the product, while the supernatant layer was still light-brown-colored due to nonprecipitated particles of the product. The supernatant was decanted, and the precipitate was washed two more times with an appropriate mixture of water/ ethanol where the product is insoluble (instead of the above washing steps, dialysis may be applied, but it was not tested in the present work). Immediately after the last centrifugation and without drying, the precipitate was suspended in 50 mL of H2O and the suspension sonicated for 5 min and stirred overnight. The mixture was centrifuged (4000 rpm, 20 min), and the very dark brown supernatant layer was collected (pH 8.5, σ ) 17 µS cm-1). Therefore, three colloidal suspensions were produced depending on the temperature of the reaction, hereafter coded as Magn0, Magn50, and Magn100. 2.2. Characterization Methods and Instruments. Powder X-ray diffraction analysis was performed on a Siemens D-500 diffractometer with Cu KR radiation (1.54 Å). Electrophoretic measurements with (16) Lutz, J.-F.; Stiller, S.; Hoth, A.; Kaufner, L.; Pison, U.; Cartier, R. Biomacromolecules 2006, 7, 3132. (17) Mohapatra, S.; Pramanik, N.; Ghosh, S. K.; Pramanik, P. J. Nanosci. Nanotechnol. 2006, 6, 823. (18) Bourlinos, A. B.; Bakandritsos, A.; Georgakilas, V.; Tzitzios, V.; Petridis, D. J. Mater. Sci. 2006, 41, 5250. (19) Kremer, F.; Scho¨nhals, A. In Broadband Dielectric Spectroscopy; Kremer, F., Scho¨nhals, A., Eds.; Springer: Berlin, 2003; pp 35-64.

Bakandritsos et al. laser Doppler velocimetry and dynamic light scattering (DLS) were performed with a Malvern Instruments Nano ZetaSizer equipped with a 4 mW He-Ne laser, operating at a wavelength of 633 nm and having an avalanche photodiode as a detector. In DLS the scattered light is measured at an angle of 173 °. Reported polydispersity index (PDI) values, ranging between 0 for an ideally monodispersed sample and 1 for very large size distributions, derive j H2, where σ is the standard deviation from the formula PdI ) σ2/D j H is the intensity-weighted average of the distribution (nm) and D particle size. The cumulants analysis method was applied. Transmission electron microscopy (TEM) measurements were carried out on a Philips CM20 TEM instrument. Assays of the colloidal stability of the nanoparticulate suspensions, at various ionic strengths and PVA concentrations, were also performed by optical absorption measurements based on the increased scattering of light upon particle growth due to aggregation, which is detected as an increase in the optical absorbance of the sample.20 The measurements were performed on a Hitachi, Digilab U-2800 spectrophotometer. It is amply noted that the aforementioned measurements were performed in colloidal formulations after the addition of the salt in periods of time before the formation of any visible flocculates, where aggregation gives rise only to an increase of turbidity. In this case such measurements can be safely utilized. All measurements were performed using 10 mL of each colloid with a concentration of 0.03% (w/v). Dielectric measurements were conducted by means of broadband dielectric spectroscopy (BDS) in the frequency range between 0.1 Hz and 1 MHz, using an Alpha-N frequency response analyzer, supplied by Novocontrol. The BDS-1200 two-electrode system, supplied also by Novocontol, was used as a test cell. For each of the examined composite Fe3O4/PVA films isothermal frequency scans from ambient to 120 °C, with a temperature step of 10 °C, were performed. To obtain the films (10% (w/w) or 2% (v/v) in inorganic material), the respective colloids containing the appropriate amounts of inorganic nanoparticles and PVA were evaporated to dryness.

3. Results and Discussion 3.1. Observations on the Synthesis of the Colloids. The synthetic procedure toward the preparation of magnetite colloids is the typical alkaline coprecipitation method where the molar ratio Fe2+/Fe3+ is 1/2, as dictated by the magnetite’s molecular formula: (Fe3+)A[Fe2+Fe3+]BO4, where Α and B denote tetrahedral and octahedral coordination sites. After the precipitation of the iron oxide, the product must be washed out or subjected to dialysis to remove the excess cations (i.e., Na+) and therefore reduce the ionic strength of the solution, which is an important prerequisite for increasing the stability of the resulting colloid. It is characteristic that, due to the decrease of the ionic strength, after the two first washing steps, the product would not precipitate during centrifugation, making necessary the change of the washing solvent to a mixture of H2O/ethanol or neat ethanol. It is also indicative that the better the quality of the distilled water used (i.e., smaller ionic conductivity), the higher the concentration of the final colloid. Typical concentration values of the fine dispersions were found in the region of 0.45% (w/v), varying form batch to batch by 0.05%. In this way, colloids are directly attainable without extra steps of basic or acidic peptization, which are often utilized in the coprecipitation method.13 In another case, an interesting acidic treatment with HNO3 in the presence of FeNO3 is followed for the stabilization of the colloids.21 Because of the multitude of preparative methods, a comparison of the various attainable concentrations in magnetic material would be very interesting; (20) Petri-Fink, A.; Steitz, B.; Finka, A.; Salaklang, J.; Hofmann, H. Eur. J. Pharm. Biopharm. 2008, 68, 129. (21) (a) van Ewijk, G. A.; Vroege, G. J.; Philipse, A. P. J. Magn. Magn. Mater. 1999, 201, 31. (b) Sousa, M. H.; Tourinho, F. A.; Depeyrot, J.; da Silva, G. J.; Lara, M. C. F. L. J. Phys. Chem. B 2001, 105, 1168.

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Figure 2. XRD patterns of the products (a) Magn0, (b) Magn50, and (c) Magn100 (bottom lines represent the theoretical diffraction pattern of the iron oxide spinel structure, JCPDS no. 19-0629).

Figure 1. (a) Photographs of magnetite colloids at various concentrations and (b) reference curves of the products (absorbance measured at 550 nm).

therefore, a common and reproducible way of expressing the concentration of a colloid is very important. For this reason, in the present case the concentration of the colloid is considered as that corresponding to the particles retained in suspension after centrifugation at 4000 rpm for 20 min (see the Experimental Section). Turning to the optical properties of the colloids, their suspensions are optically clear, as shown in Figure 1a, while in Figure 1b their optical absorbance is plotted at various concentrations, measured at 550 nm. It is evident that the absorbance of Magn0 is weaker (lower absorption coefficient) than those of Magn50 and Magn100 (not plotted here because it practically overlaps with that of Magn50). This difference is attributed probably to the smaller size of the nanocrystallites in Magn0.22 3.2. Particle Size. The XRD patterns of the three products, corresponding to the inverse spinel structure of magnetite, are available in Figure 2, from which a clear relationship between the temperature of the reaction and the nanocrystallite size can be drawn. At a temperature of 0 °C a mean size of ∼3 nm is deduced from the Scherrer equation, whereas by increasing the temperature of the reaction at 50 and 90 °C the size increased to ∼5 and ∼8 nm, respectively. Representative TEM images of the three products are available in Figure 3. The size distributions for samples Mang50 and Magn100 are in very good agreement with the mean size estimated from Scherrer’s formula. On the other hand, sample Magn0 displays a rather bimodal size distribution. It should be emphasized that the particular size-distribution diagram probably contains significant error due to the fact that the very small particles (such as those indicated by the arrows) cannot be easily distinguished over the larger ones or in areas where many aggregated particles are present. Only in a few cases, as in the image of Figure 3a, are such fine particles discernible. Nevertheless, the significant broadening of the XRD reflection testifies to the dominant (22) Cornell, R. M.; Schwertmann, U. The Iron Oxides, 2nd ed.; Wiley-VCH: Weinheim, Germany, 2003; p 133.

presence of such fine nanocrystallites. The explanation for the presence of particles of higher size should be that, after the end of the reaction at 0 °C, further treatment of the product takes place at ambient temperature, where crystallites may grow to higher sizes. The possibility of centrifugation at 0 °C would probably lift this problem. As one might expect, the three different mean sizes of the primary crystallites obtained would result in an analogous particle size variation in the colloidal state, but this was not the case as DLS measurements revealed. In Figure 4, typical intensityweighted size distribution curves are shown, pointing to the similar aggregate size, irrespective of the size of the primary nanocrystallites. Polydispersity indices were in the region of ca. 0.18. A possible explanation for this behavior could be the similar magnitude of the antagonizing forces participating in charged colloids: the surface tension on one hand (related to the attractive van der Waals interactions) and the electrostatic repulsion of the negatively charged particles on the other. A temporary equilibrium is established when the tendency toward aggregation, driven by the decrease in surface energy, is counterbalanced by the energy barrier imposed by the electrostatic repulsion. Considering the same nature of the particles in all three products, it is quite possible to expect the equilibrium to take place at a similar point. Some aspects of this behavior might be related to the superspheres described by J. Ramsden,23 as the result of the interplay between these forces. At these scales, the crystallite size that defines the magnetic properties of the material is well-known. Specifically, the saturation magnetization drops when the crystallite’s size decreases below a critical value (17-20 nm),24 because the disordered and magnetically inactive shell of the particle, which grows at the expense of the ordered core, becomes dominant. At the same time the onset of superparamagnetic behavior results in a nonhysteritic behavior. On the other hand, above this critical value, the material progressively displays the saturation magnetization, the coercivity, and the remanent magnetization of its bulk counterpart, even at room temperature. The last two properties deteriorate the colloid stability of the magnetic nanoparticles through the dipole/exchange magnetic interac(23) Ramsden, J. J. Proc. R. Soc. London, A 1987, 413, 407. (24) (a) Tuc´ek, J.; Zboril, R.; Petridis, D. J. Nanosci. Nanotechnol. 2006, 6, 926. (b) Zysler, R. D.; De Biasi, E.; Ramos, C. A.; Fiorani, D.; Romero, H. In Surface Effects in Magnetic Nanoparticles; Fiorani, D., Ed.; Springer: New York, 2005; p 239. (c) Jeong, U.; Teng, X.; Wang, Y.; Yang, H.; Xia, Y. AdV. Mater. 2007, 19, 33.

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Figure 3. TEM images of the three magnetic sols (a) Magn0, (b) Magn50, and Magn100. (d-f) Size distribution diagrams obtained thereof and the log-normal fittings, apart form the case of Magn0, where a bimodal distribution is observed.

Figure 4. ζ potential distribution diagrams for (a) Magn0, (b) Magn50, and (c) Magn100 at the pH of the as-prepared dispersions (∼8.5).

tions.25 It is therefore straightforward that, if the size of the primary crystallites in the colloid is close to but below the aforementioned critical value, then they may display the maximum saturation magnetization and colloidal stability. Regarding applications related to physicochemical processes, the smaller colloidal size (resulting in a higher surface area) promotes both the kinetics and yield of the process. In biomedical applications a smaller colloidal size is also advantageous from the blood-circulation-time point of view26 (i.e., avoidance of the reticuloendothelial system and clearance from the small capillaries). On this basis, the three colloids are expected to display similar performances because of the similar size distributions of the aggregates. From the magnetic properties point of view, as (25) Odenbach, S. In Handbook of Magnetic Materials; Buschow, K. H. J., Ed.; Elsevier Science Publishers B.V.: Amsterdam, 2006; Vol. 16, p 130. (26) Arruebo, M.; Fernadez-Pacheco, R.; Ibara, M. R.; Santamaria, J. Nanotoday 2007, 2, 22.

previously described, it is clear that Magn100, with primary crystallites closer to the critical diameter, would perform better than Magn50 and certainly than Magn0. Nevertheless, the mean nanocrystallite size of Magn100, being approximately 8 nm, is still lower than the ideal of 15-20 nm.27 Generally, the alkaline coprecipitation route delivers crystallites below ca. 10 nm.28,29 On the other hand, the alternate method for spinel iron oxide synthesis from a single ferrous precursor is possible to deliver nanocrystallites of a mean size in the region of 17-20 nm after a 30 min reaction at 50 °C.5b Nevertheless, this route requires the copresence of surface capping agents; otherwise, the magnetic precipitate remains insoluble regardless of the ionic strength of the medium. Tzitzios et al. have also recently obtained the same results using similar strategies and a block copolymer as a capping agent.30 3.3. Colloidal Stability. It is generally accepted that such charged colloids are metastable systems; that is, they are only kinetically stabilized, being “trapped” in the secondary minimum of the system’s total potential energy, as predicted by the DLVO theory.31 The secondary minimum is separated from the primary, deep minimum of energy, which takes place at very short particle-particle distances, by an energy barrier imposed by the surface charge of the particles. The difference of the energy barrier from the system’s thermal energy dictates the degree of kinetic stability. (27) Of course, the word “ideal” is relative. The way it is used in the text refers only to the present kind of colloids, where it appears that the hydrodynamic diameter remains the same irrespective of the primary crystallite size. (28) Moeser, G. D.; Green, W. H.; Laibinis, P. E.; Linse, P.; Hatton, T. A. Langmuir 2004, 20, 5223. (29) (a) Li, Z.; Tan, B.; Allix, M.; Cooper, A. I.; Rosseinsky, M. J. Small 2008, 4, 231. (b) Rabias, I.; Fardis, M.; Devlin, E.; Boukos, N.; Tsitrouli, D.; Papavassiliou, G. ACS Nano 2008, 2, 977. (30) Basina, G.; Mountrichas, G.; Pispas, S.; Devlin, E.; Boukos, N.; Niarchos, D.; Tzitzios, V. J. Nanosci. Nanotechnol. 2008, accepted. (31) Verwey, E. J. W.; Overbeek, J. Th. G. The Theory and Stability of Lyophobic Colloids; Dover Publications, Inc.: New York, 1948. (b) Hiemenz, P. C.; Rajagopalan, R. Principles of Colloid and Surface Chemistry, 3rd ed.; Marcel Dekker: New York, 1997; p 465.

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Figure 6. Variation of the ζ potential over the pH for neat iron oxide nanoparticles (a) at 10-3 M, (b) at 10-2 M, and (c) in presence of PVA at 10-3 M NaCl concentration.

Figure 5. Size distribution curves as obtained from DLS measurements for the three colloids based on the scattering intensity-weighted distribution.

Therefore, the stability of the colloids was studied with electrophoretic measurements for the estimation of the ζ potential (a quantity closely related to the surface charge), which was found in the region of -35 to -40 mV (generally, ζp values of g|30| mV denote adequate electrostatic repulsion to provide stability). As shown in Figure 5, Magn0 displays the smaller |ζp| value and a wider distribution, indicating a less stable dispersion. This was also macroscopically manifested since Magn0 precipitated after a few weeks time. Considering that the particles’ composition and origin of the surface charge are the same for all samples, the reason for the faster aggregation of Magn0 should be probably related to the smaller primary crystallite size, which is the only difference between the three products. The smaller particle size gives rise to an increase of the surface to volume ratio, and consequently, the structurally (and magnetically) disordered surface shell of the nanocrystallites also increases at the expense of the ordered core.24 At the same time, it might be possible that limited dissolution of metallic cations from disordered surfaces takes place, which, in turn, may screen the surface charge of the particles, resulting in lower |ζp| and faster aggregation. Subsequently, the behavior of the colloids was studied in aqueous mixtures with the neutral polymer PVA, introducing in the experiments more parameters, such as the ionic strength and pH of the medium, to gain a deeper understanding of the system. Another motive for the study was the fact that PVA-stabilized magnetite particles have been examined as potent candidates for biomedical applications.20,32 In Figure 6a,b the ζ potential of the magnetic particles over a wide pH region is depicted, measured at two different salt (NaCl) concentrations (10-3 and 10-2 M) to locate the point of zero charge (PZC) of the uncoated iron oxide surface. In accordance with other literature sources reporting values of 6.333 and 6.6,34 here it was found at a pH of 6.3. As can be seen in the titration curves of Figure 6a,b, the point of zero ζp occurs (32) (a) Petri-Fink, A.; Chastellain, M.; Juillerat-Jeanneret, L.; Ferrari, A.; Hofmann, H. Biomaterials 2005, 26, 2685. (b) Abu-Much, R.; Meridor, U.; Frydman, A.; Gedanken, A. J. J. Phys. Chem. B 2006, 110, 8194. (33) Marmier, N.; Delise`e, A.; Fromage, F. J. Colloid Interface Sci. 1999, 211, 54. (34) Garcell, L.; Morales, M. P.; Andres-Verge`s, M.; Tartaj, P.; Serna, C. J. J. Colloid Interface Sci. 1998, 205, 470.

slightly earlier than the PZC upon a pH decrease, which is expected since the Na+ cations inside the electrical double layer (at a distance from the solid’s surface shorter than that of the shear plane, where ζp is measured) cancel out the small excess of the negative surface charge. The indifference of PZC to the presence of PVA (see Figure 6c) points to the nonspecific binding of the polymer on the oxide’s surface35 or to the low number of interaction points, leaving the vast majority of the oxide’s surfaceterminating groups free. Therefore, the decrease of ζp is rather attributable to the shift of the plane of shear away from the particle’s surface rather than to a change of the acid-base characteristics of the surface. In a work by B. V. Kavanagh et al.36 quantitative titrations on aluminum oxide colloids in the presence of PVA have also shown that a change neither in the position of the PZC nor in the magnitude of the surface charge takes place, further supporting the findings of this study. The effect of PVA on the stability of the particles against the salt concentration (Ms) was evaluated by monitoring the hydrodynamic diameter and ζ potential change upon NaCl addition. The bare particles are extremely sensitive to the electrolyte presence, and aggregation starts practically at Ms values of 10-3 M, as evidenced by the abruptly increasing particle size in Figure 7a. On the other hand, at a PVA concentration of 0.05% (w/v), the stability has significantly increased and aggregation appears only at Ms values approaching 8 × 10-2 M (Figure 7b). In many studies, the increase of the colloid’s stability in the presence of polymers is discussed in terms of the steric stabilization (reduction of the conformational entropy as two coated particles approach) that the polymeric sheath offers to the particles, and certainly this is the case for specifically adsorbed polymers, i.e., for macromolecules chemically grafted on the particle’s surface through an end-functional group or for macromolecules adsorbed through hydrophobic interactions such as in the case of amphiphilic block copolymers. Nevertheless, PVA is a neutral polymer and rather indifferent for the oxide’s surface, especially when water is the solvent, because both PVA and H2O have affinity for the oxide’s surface through the hydrogen bonds, antagonizing each other. PVA can be weakly absorbed though, due to the large number of -OH groups on the same macromolecule. This is evidenced by the low-affinity Langmuirtype sorption isotherms37 of PVA on various oxides.36,38 As (35) Jolivet, J. P. Metal Oxide Chemistry and Synthesis, 3rd ed., translation; J. Wiley & Sons: Chichester, U.K., 2000; p 218 (36) Kavanagh, B. V.; Posner, A. M.; Quirk, J. P. Faraday Discuss. Chem. Soc. 1975, 59, 242. (37) Lyklema, J. Fundamentals of Interface and Colloid Science, Solid-Liquid Interfaces; Academic Press: New York, 1995; Vol. II.

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Figure 8. ζ potential decrease upon PVA addition for the three magnetic colloids.

Figure 7. Hydrodynamic diameter and ζ potential variation of iron oxide nanoparticles (a) in neat water and (b) in a 0.05% w/v PVA aqueous solution vs the salt concentration.

discussed in the extended review work by B. Vincent,39 in this case “the origin of the repulsion is the work required to desorb molecules as two particles approach” and could be characterized as an enthalpic stabilization. However, irrespective of the origin of the repulsion (entropic, enthalpic, or both), it is expected to be a maximum when the maximum amount of polymer has been absorbed on the particle’s surface (if we consider the Langmuirtype adsorption isotherm, the highest coverage takes place when the plateau of adsorption has been reached). Therefore, no further colloidal stabilization is predicted upon polymer addition beyond the point of maximum coverage. For these reasons other mechanisms of stabilization have been generally suggested, such as those embraced by the term “depletion stabilization” 20,40 (the approaching of two particles produces polymer-depleted regions in the intermediate space, which is thermodynamically unfavorable). Subsequently, another possible stabilization mechanism is discussed that could act either cooperatively or not with the depletion stabilization forces. Colloidal stability assays were performed for PVA concentrations far above the concentration where saturation in the polymer adsorption occurs. This concentration was determined with ζ potential measurements. In Figure 8 the maximum polymer coverage is evidenced by the ζ potential’s leveling off, when the maximum shift of the shear plane takes place. The value estimated from the curves is approximately the same for all three products found at 0.03% (w/v) PVA or 0.9 gPVA/gFe3O4.41 For comparison, (38) (a) Mikkola, P.; Leva¨nen, E.; Rosenholm, J. B.; Ma¨ntyla, T. Ceram. Int. 2003, 29, 393. (b) Wis´niewska, M.; Chibowski, S.; Urban, T. Mater. Chem. Phys. 2007, 103, 216. (39) Vincent, B. AdV. Colloid Interface Sci. 1974, 4, 193. (40) Semenov, A. N. Macromolecules 2008, 41, 2243. (41) This value indicates the total amount of PVA present in the solution which is necessary to obtain the maximum coverage and not the amount of PVA sorbed by the particles.

a value of 1.45 gPVA/gFe3O4 was reported in another work42 using the same technique (the result of the work expressed in a PVA/ Fe mass ratio has been interpreted here to a PVA/Fe3O4 mass ratio). The values are in close proximity to each other, considering that in the referenced work PVA has been added before particle aggregation and/or that the colloid suspension has undergone a size selection process and only the particles with smaller diameters (resulting in a higher surface area per gram) are participating in the absorption experiment. It is reminded that all experiments were performed in a ∼0.03% (w/v) 10 mL colloid of magnetic nanoparticles. In Figure 9 the stability assays for Magn100 are available, based on the indirect estimate of the extent of aggregation through the increment of turbidity, as explained in the Experimental Section. The first finding regards the dependence of the colloid stability on the concentration of PVA, even at values well above the necessary concentration for full particle coverage (deduced from the experiments of Figure 8). More specifically, in Figure 9a the optical absorbance was measured 2 h after the salt addition (0.05 and 0.14 M in NaCl) in test tubes containing increasing PVA concentrations. Depending on the ionic strength, there is always a critical PVA concentration where no turbidity is observed (i.e., the optical absorbance remains at a minimum). The critical PVA concentrations were found at ca. 0.1% (w/v) and 0.2% (w/v) for 0.05 and 0.14 M salt concentrations, respectively. Considering that these values are many times higher than the necessary polymer concentration for full particle coverage, it must be concluded that the dominating stabilization mechanism in the case of PVA is different from those discussed previously (steric or enthalpic). The decrease of the mean velocity and of the mean Brownian path length of the colloidal particles due to the viscosity increase upon polymer addition (as deduced from Einstein’s equation D ) kT/6πηsa), could also contribute to the stabilization mechanism and act cooperatively with the depletion forces. The work by Aksay et al.43 would be of interest, since the particles’ “mobility matter” is also brought up by a theoretical approach. By retreating back to the potential energy diagram of the DLVO theory, the colloid is sufficiently (meta)stable if the kinetic energy of two approaching particles is lower than the electrostatic energy barrier. Of course, due to the Maxwellian distribution of the velocities, the particles eventually overcome the barrier and collide (this is why they are metastable). By increasing the viscosity, the particles’ mean velocity shifts to (42) Chastelain, M.; Petri-Fink, A.; Hoffmann, H. J. Colloid Interface Sci. 2004, 278, 353. (43) Liu, J.; Shih, W. Y.; Kikuchi, R.; Aksay, I. A. J. Colloid Interface Sci. 1991, 142, 369.

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Figure 10. (a) Electric modulus loss index as a function of frequency for all the tested samples at 120 °C and (b) the corresponding Cole-Cole plots. Figure 9. (a) Stability assays of Magn50 colloids at two different salt concentrations and at increasing polymer concentrations of PVA (molecular weight 145 000), recorded 2 h after salt addition. (b) Comparison of the behavior of Magn50 colloids under the same salt and polymer concentrations, the latter differing only in their molecular weight.

lower values (so does their kinetic energy), and therefore, the stability of the colloid is extended to longer periods. Another experiment was performed that does not necessarily prove the truth of the previous notion, but if the outcome were different, it would certainly prove its false state. By inserting the same amount of PVA of lower molecular weight and therefore of lower viscosity, the turbidity of the colloid increases and finally flocculates (see Figure 9b) within 8 h. This is in immediate antithesis to the colloid stabilized with the higher molecular weight polymer, where no evidence of aggregation was observed, even for periods of time many times well beyond those displayed in the figure. Closing this section, it is worth noting that the PVA/iron oxide colloid (for instance, the formulation of 0.2% (w/v) PVA in 0.14 M NaCl) does not destabilize upon dilution with a solution of the same ionic strength. Probably, this is because the decrease in the polymer concentration to 0.1% (w/v) (where otherwise flocculation would have started; see Figure 9a) and its implications on the colloids’ stability as discussed in the text are counterbalanced by the concomitant decrease of the nanoparticles’ volume fraction. The latter is interpreted as a decrease in the number of effective collisions, which otherwise would progressively result in flocculation. 3.4. Dielectric Response: Preliminary Results. Dielectric spectroscopy was performed on the composite films to probe any effects of the different crystallite sizes on the dynamics of the solid-state solution of the iron oxides in PVA. Dielectric data can be analyzed with different formalisms, such as the dielectric

permittivity mode, electric modulus mode, and ac conductivity mode. In the framework of the present study dielectric data are analyzed via the electric modulus formalism; arguments and examples for the resulting benefits have been given elsewhere.44-46 Figure 10a depicts the variation of the electric modulus loss index as a function of the frequency of the applied field at 120 °C for all three composite samples. For comparison reasons the dielectric response of pure PVA is also presented. In all cases a single relaxation process is observed, which becomes evident via the formation of a loss peak. The peak positions display proximity in the case of the composite samples, while the PVA peak is recorded at a significantly lower frequency. The proximity of the peak positions is probably an indication of the similar size of the inorganic aggregates in the films. Figure 10b, which is known as a Cole-Cole plot, presents the imaginary part of electric modulus as a function of the real part. In the Cole-Cole plots each relaxation process becomes apparent through the formation of a semicircle. The variation of the semicircles’ radii, as well as the maximum position and amplitude of the recorded processes, reflects the influence of composition and reinforcement. As can be seen, all traces pass from the origin of the M′′ ) f(M′) diagram, providing a clear indication that no other relaxation process is present in their dielectric spectra at lower frequencies. Under this point of view, the recorded process is attributed to the glass/rubber transition of PVA, since it is present in the spectra of both pure PVA and composite films. Interfacial polarization which occurs in heterogeneous systems44,47 appears to be absent or at least negligible in our case, probably because the volume fraction of the iron oxide (44) Tsangaris, G. M.; Psarras, G. C.; Kouloumbi, N. J. Mater. Sci. 1998, 33, 2027. (45) Psarras, G. C.; Gatos, K. G.; Karahaliou, P. K.; Georga, S. N.; Krontiras, C. A.; Karger-Kocsis, J. eXPRESS Polym. Lett. 2007, 1(12), 837. (46) Kontos, G. A.; Soulintzis, A. L.; Karahaliou, P. K.; Psarras, G. C.; Georga, S. N.; Krontiras, C. A.; Pisanias, M. N. eXPRESS Polym. Lett. 2007, 1(12), 789. (47) Tsangaris, G. M.; Psarras, G. C. J. Mater. Sci. 1999, 34, 2151.

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nanoparticles is kept at a low level (∼2%). Loss peaks for reinforced systems are recorded at higher frequency with respect to the pure polymer matrix, implying a decrease of the glass transition temperature. Recently, it was reported48 that the glass transition temperature could be increased or decreased by adjusting the polymer matrix-nanofiller interactions. Reduced values of Tg are related to the presence of free space between the nanofiller and polymer, due to insufficient wetting of the nanoinclusions by the matrix, while higher values of Tg are attributed to restricted cooperative segmental mobility of the polymer chains48 and strong or moderate interactions between the matrix and filler. Thus, it is reasonable to suggest that the peak shift in the spectra of the nanocomposites is related to the relatively high interfacial volume fraction (between the matrix and nanoparticles) and/or to the perturbation introduced by the nanoparticles’ presence between the polymeric chains. Since the nanofiller content is constant in all composite films, the loss peak position is practically unaffected by the variation of the mean crystallite diameter. Further investigations in the future will certainly shed more light on the processes taking place in such systems.

4. Conclusions Dynamic light scattering and dielectric measurements suggest that the preparation of magnetic iron oxide colloids of different (48) (a) Berriot, J.; Montes, H.; Lequeux, F.; Long, D.; Sotta, P. Macromolecules 2002, 35, 9756. (b) Narayanan, R. A.; Thiyagarajan, P.; Lewis, S.; Bamsal, A.; Schadler, L. S.; Lurio, L. B. Phys. ReV. Lett. 2006, 97, No. 075505.

Bakandritsos et al.

mean crystallite sizes (3, 5, and 8 nm) is not necessarily interpreted in a respective variation of the apparent hydrodynamic size of the colloidal entities. Following this finding and considering the dependence of the magnetic properties on the size of the primary crystallite, an advantage could be envisaged if such colloids were prepared at elevated temperatures, where higher saturation magnetization is realized without a concomitant increase in the colloidal particle size, the latter having many ramifications in various applications as discussed in the text. It was also found that the simple addition of the neutral polymer poly(vinyl alcohol) significantly enhances the stability of the magnetic colloids even up to salinity values similar to that of the body fluids. The experimental results support that the dominating stabilization mechanism is other than those mechanisms related to polymer absorption phenomena (entropic or enthalpic stabilization). Particular emphasis is given on the effect of the particles’ velocity decrease to explain the stabilization mechanism. In addition, it was found that the dilution of a stable magnetite/ PVA colloid of high ionic strength with water of the same ionic strength does not destabilize the formulation, even though the polymer concentration drops to values where, otherwise, flocculation of the particles would have occurred. This behavior is interesting in applications where mixing of the magnetic colloid with another medium takes place (i.e., biomedical applications or preparation of composites through wet mixing). LA801901J