Polyelectrolyte-Modified Inverse Microemulsions and Their Use as

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Polyelectrolyte-Modified Inverse Microemulsions and Their Use as Templates for the Formation of Magnetite Nanoparticles† Jennifa Baier,‡ Joachim Koetz,*,‡ Sabine Kosmella,‡ Brigitte Tiersch,‡ and Heinz Rehage§ Institut fu¨r Chemie, UniVersita¨t Potsdam, Karl-Liebknecht-Strasse 24-25, Haus 25, 14476 Potsdam, Germany, and Institut fu¨r Physikalische Chemie II, UniVersita¨t Dortmund, 44221 Dortmund, Germany ReceiVed: December 28, 2006; In Final Form: March 26, 2007

This paper is focused on the characterization of polyelectrolyte-modified inverse microemulsions and their use as templates for the synthesis of magnetite nanoparticles. It is shown that the cationic polyelectrolyte poly(diallyldimethylammonium chloride) (PDADMAC) of low molar mass can be incorporated into the individual inverse microemulsion droplets (L2 phase) consisting of heptanol, water, and an amphoteric surfactant with a sulfobetaine head group. Up to a polymer concentration of 20% by weight in the aqueous phase and for different molecular weights of the polymer, an isotropic phase still exists. At a PDADMAC concentration of 10% the area of the isotropic L2 phase is shifted in direction to the water corner. In the percolated area of the L2 phase, i.e., at higher water content, a temperature-dependent change in the conductivity can by observed, and bulk water can be detected by means of differential scanning calorimetry measurements. The unusual temperature-dependent behavior of the polymer-modified system, i.e., the conductivity decrease with increasing temperature, can be explained by temperature-sensitive polyelectrolyte-surfactant interactions, influencing the droplet-droplet interactions. These PDADMAC-modified microemulsions can be successfully used as a template for the formation of ultrafine magnetite particles, in contrast to the nonmodified microemulsion, where the process is misdirected due to the “disturbing” effect of the surfactants. However, in the presence of PDADMAC the surfactant head groups were masked, and therefore magnetite can be synthesized. During the process of magnetite formation the PDADMAC controls the particle growing and stabilizes spherical magnetite particles with a diameter of 17 nm, which can be redispersed without a change in size.

Introduction Microemulsions are thermodynamically stable surfactantwater-oil mixtures, which can be subdivided into different classes, i.e., water-in-oil (w/o), bicontinuous, and oil-in-water (o/w) microemulsions.1,2 Recently, it has been shown that microemulsions can be modified in their properties by adding polymers.1-3 For example, we were able to show that watersoluble polymers, i.e., charged polyelectrolytes, can be incorporated into inverse w/o microemulsions without a macroscopic phase separation.4-6 However, the molar mass and type of polymer can strongly influence the properties of the resulting polymer-modified microemulsion. When the molar mass of the polymer is low in comparison to the droplet size, polymers can be incorporated into individual microemulsion droplets. Polymers of significantly higher molar mass induce a cluster formation. Additional effects have to be taken into account when the surfactant head groups can interact with the functional groups of the polymer. It is already well-known that inverse microemulsions are spontaneously formed by mixing well-defined amounts of surfactant, water, and a long-chain alcohol. Especially, Friberg et al. were able to show that the ternary system consisting of a long-chain alcohol, sodium dodecyl sulfate (SDS), and water † Part of the special issue “International Symposium on Polyelectrolytes (2006)”. * To whom correspondence should be addressed. Telephone: +49 331 977 5220. Fax: +49 331 977 5054..E-mail: [email protected]. ‡ Universita ¨ t Potsdam. § Universita ¨ t Dortmund.

exhibits a large region of inverse micelles.7,8 In these systems the chain length of the alcohol strongly influences the formation of inverse micelles. However, investigations about the influence of polymers on structure formation in such water-alcoholsurfactant systems are rather scarce.5 One very interesting applicational feature of microemulsions is their use as templates for the formation of well-defined nanoparticles. The concept is to control the size of the formed nanoparticles due to the size of the microemulsion droplets depending on the water-surfactant ratio of an inverse microemulsion.9-11 The formation of well-defined nanoparticles has been intensively pursued not only due to the fundamental scientific interest but also because of their big potential in technological applications.12-15 For many of these applications, the synthesis of monodisperse, nanoscalic particles is of key importance, because the special electrical, optical, optoelectronical, and magnetic properties of these nanoparticles strongly depend on their dimensions. For example, uniformly sized magnetic nanoparticles have attracted a lot of attention due to their use as magnetic storage media, ferrofluids, magnetic resonance imaging (MRI), magnetically guided drug delivery, medical diagnostics, and alternatingcurrent (AC) magnetic-field-assisted cancer therapy.16 Various synthetic procedures have been used to synthesize magnetic nanoparticles. For example, magnetite (Fe3O4) of colloidal dimensions dispersed in solvents are frequently used as ferrofluids.17 In general, magnetite can be produced by the coprecipitation of ferrous (Fe2+) and ferric (Fe3+) ions under basic

10.1021/jp068995g CCC: $37.00 © 2007 American Chemical Society Published on Web 05/25/2007

Formation of Magnetite Nanoparticles conditions or by strong glowing (1130 °C) of R-Fe2O3.18 However, it is still an advantage to produce ultrafine magnetite particles of much smaller dimensions on the nanometer scale. Taking this knowledge into account, we have tried to use a microemulsion system with an amphotereric surfactant, i.e., 3-(N,N-dimethyldodecylammonio)propanesulfonate, as a template phase for the magnetite nanoparticle formation. However, first experiments failed due to strong interactions between the surfactant head groups and the Fe ions inhibiting the magnetite formation. Therefore the aim of these investigations was to use polymer-modified microemulsions as a template for the formation of well-defined magnetite particles. In the first part of the paper the polymer-modified microemulsions are characterized, and in the second part the magnetite particle formation inside the droplets is studied. Experimental Section A. Materials. Low molecular weight poly(diallyldimethylammonium chloride) (PDADMAC), purchased from Aldrich, was purified by ultrafiltration and freeze-drying. The average molecular weights of Mn ) 5400 g/mol and Mw ) 13 700 g/mol were determined by gel permeation chromatrography (GPC). The z-average of the square of the radius of gyration can be related according to Dautzenberg et al.19 to Rg < 8 nm. The following commercially available chemicals were used without further purification: heptanol (99+%, Fluka), 3-(N,Ndimethyldodecylammonio)propanesulfonate (SB, >97%, Fluka), iron(II) chloride tetrahydrate (FeCl2‚4H2O, p.a., Fluka), iron(III) chloride hexahydrate (FeCl3‚6H2O, p.a., Fluka), sodium hydroxide pellet (p.a., Merck), and ammonia solution (32%, p.a., Merck). Water was purified by the water purification system MODULAB PureOne (Continental). B. Characterization of the Microemulsion Template Phase. The phase diagrams were determined optically by titrating the alcohol-surfactant mixture with water or the corresponding aqueous PDADMAC solution at room temperature (22 °C), 35 °C, and 45 °C. At first a mixture of surfactant, alcohol, and the aqueous solution was shaken or treated in an ultrasonic bath until the system becomes optically clear. The area of the isotropic L2 phase was determined then by adding more of the aqueous solution drop by drop to the system. One phase diagram was constructed by using at least 20 measuring points. The droplet structure of the inverse microemulsion was investigated by means of ultrahigh-resolution cryoscanning electron microscopy (cryo-SEM; S-4800, Hitachi). Samples were frozen into nitrogen slush at atmospheric pressure, freezefractured at -180 °C, etched for 60 s at -98 °C, sputtered with platinum in the cryopreparation chamber (Alto 2500, GATAN), and finally transferred into the cryo-SEM. For 1H NMR self-diffusion measurements a Bruker spectrometer operating at a proton resonance frequency of 300.13 MHz using the LED pulse sequence with bipolar pulse pair (bppLED) at 25 °C was used. The summation runs over all diffusing species contributing to the corresponding signal intensity according to the procedure already described in more detail in ref 20. By using frequency-dependent ultrasound relaxation measurements (DT 1200, Dispersion Technology), the droplet size of the inverse microemulsion can be calculated according to sound weakening of a sample within the range of 3-100 MHz. Conductometric measurements were realized by means of a microprocessor conductivity meter (LF 2000, WTW) at different temperatures.

J. Phys. Chem. B, Vol. 111, No. 29, 2007 8613 The rheological properties of the mixtures were checked with the Rheometer LS100 (Paar Physica) with a double slit cell. Flow curves were measured by variation of the rotation momentum from 0.1 to 100 mN m. The samples were investigated in a temperature range between 20 and 50 °C. Calorimetric measurements were carried out with a MicroDSC III (Setaram) in a temperature region between -20 and +80 °C. The heating and cooling rate was fixed to 0.3 K/min. After cooling, the sample was kept frozen at -20 °C for 3 h, before the heating curve starts again. The differential scanning calorimetry (DSC) curves were repeated several times. C. Synthesis of Magnetite Particles. In general, we used two routes to produce magnetite. Two sulfobetaine/heptanolbased microemulsions in the absence or presence of the cationic polyelectrolyte (PDADMAC) were prepared separately. The first one, i.e., microemulsion I, was made from a iron(II)-iron(III) chloride solution (molar ratio of 1:2), and the other one, i.e., microemulsion II, from a ammonia solution (route 1) or a sodium hydroxide solution (route 2). Microemulsions I and II were mixed together (procedure A). In addition, the sulfobetaine/heptanol-based microemulsion containing the cationic polyelectrolyte PDADMAC and the iron chloride salts (i.e., Fe(II):Fe(III) in a molar ratio 1:2) was dropwise treated by a 2 mol/L aqueous ammonia solution (procedure B). D. Characterization of the Magnetite Particles. Dynamic light scattering measurements were employed at 25 °C at a fixed angle of 173° (backscattering) by using a Nano Zetasizer (Malvern) equipped with a He-Ne laser (4 mW) and a digital autocorrelator. For determining the average size of the main particle fraction, an automatic peak analysis by number was used. Electrophoretic light scattering was used as an optical detection system to check the movement of the charged nanoparticles in an electric field. The collected signal of the particle movement shifted to higher or lower frequencies depending on their charge. The frequencies were than converted to electrophoretic mobility and finally to the ζ potential. Measurements were carried out with a Nano Zetasizer (Malvern) at a fixed angle of 17°. The shape and size of the magnetite nanoparticles was determined by transmission electron microscopy (EM 902, Zeiss). Samples were prepared by dropping a small amount of the dispersion on the copper grids, dried, and examined in the electron microscope at an acceleration voltage of 90 kV. Results A. Microemulsion Template Phase. The partial phase diagram in Figure 1 shows an optically clear area in the heptanol corner (L2 phase) and in the water corner (L1 phase) of the ternary system water/heptanol/sulfobetaine (SB). The region of the L1 phase is not significantly changed by replacing water with an aqueous 10% by weight PDADMAC solution. However, the isotropic area of the L2 phase is shifted in the direction of the water corner after incorporating the polycation. In addition, the area of the L2 phase can be enlarged by increasing the temperature, as shown in Figure 2. To understand this behavior, some general statements have to be made here. The addition of the PDADMAC leads first of all to a small increase of the ionic strength. Noteworthy sulfobetaine head groups are not so sensitive to an increase of the ionic strength than anionic or cationic surfactants. Taking into account that the molar concentration of our PDADMAC-modified microemulsions is varied between

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Figure 3. Conductivity of the PDADMAC-modified microemulsions with increasing water content (from point P1 to P6) at 30 °C.

Figure 1. Influence of the PDADMAC concentration on the area of the isotropic phase in the water-heptanol-SB system: (;) water and (--) 10% PDADMAC at room temperature.

Figure 4. Conductivity measurements with dependence on the temperature at points P1-P6.

TABLE 1: Composition of the Microemulsions at Different Points of Investigation composition (wt %)

Figure 2. Temperature dependence of the isotropic L2 phase in the water-heptanol-SB system at a PDADMAC concentration of 10% by weight ((;) room temperature (22 °C); (--) 35 °C; (;) 45 °C) including the marked points P1-P6.

0.235 and 0.285 mmol and the solubilization capacity for low molecular salts is two magnitudes higher (e.g., for CaCl2, 100 mmol; for Na2SO4, 50 mmol; and for ZnSO4, 10 mMol), one can conclude that the change of the ionic strength is only of minor priority. Additional investigations have shown that a moderate change of the pH does not change the phase diagram of the SB-based microemulsion. Only under very strong basic conditions, i.e., by using a 4 M NaOH, the L2 phase is decreased. That means, when PDADMAC is solubilized in the microemulsion droplets, intermolecular interactions between the sulfonate group of the surfactant and the quaternary N-function of the polymer are of pronounced meaning in concurrence with the intramolecular sulfobetain interactions. One can assume that these moderate surfactant-polymer interactions are responsible for changes in the phase behavior and have to be checked by some additional measurements along an imaginary line with increasing water to surfactant ratio (Table 1) from point P1 to P6. However, in dependence on the method used not all points were characterized in much more detail. ConductiVity. The conductivity at 30 °C increases along the line P1 to P6 (Figure 3). Points P1-P3 show a low conductivity

point

aqueous solution

heptanol

SB

rw(water/surfactant)a

A B C P1 P2 P3 P4 P5 P6

5 5 10 25 29 34 38 42 46

80 75 70 50 46 42 38 35 31

15 20 20 25 25 24 24 23 23

0.33 0.25 0.5 1 1.2 1.4 1.6 1.8 2

a

rw ) weight ratio.

value (below 200 µS/cm), and beginning with point P4 one can observe a steep conductivity increase, which can be explained by a percolation of the microemulsion droplets.3-5 In Figure 4, the conductivity was plotted against the temperature.21-23 Points P1-P3 show a low conductivity value and only a weak increase of the condutivity with the temperature. Beginning with point P4, one can observe a jump. However, the percolated system shows quite unusual behavior, which means the conductivity decreases with increasing temperature, remains constant, and finally increases again. The unusual decrease of the conductivity in the temperature region between 20 and 50 °C can by related to temperaturedependent polyelectrolyte-surfactant interactions. That means, at higher temperatures, the polymer-surfactant interactions were increased, resulting in a more stable polyelectrolyte-surfactant film, and, as a consequence, the droplet-droplet interaction is weakened. Rheology. In general, rheology is a useful method to detect structural changes in microemulsions. Sample P1, i.e., a nonpercolated microemulsion, and P4, i.e. a percolated one, show a typical low shear viscosity and Newtonian flow behavior, as expected for a reverse microemulsion. The higher viscosity

Formation of Magnetite Nanoparticles

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Figure 5. DSC heating curves of the PDADMAC-modified microemulsions at points P1-P6.

Figure 6. Reduced diffusion coefficients for water, sulfobetaine, and heptanol of the PDADMAC-modified microemulsions at point P1 determined by 1H NMR spectroscopy.

at point P4, in comparison to P1, can be related to the higher polymer concentration. Moreover, the shear viscosity decreases by increasing the temperature and is in full agreement to pure solvent heptanol. Calorimetry. In the micro-DSC experiments characteristic endothermic peaks in the heating curve were observed (Figure 5). At point P1 no freezable water can be detected. In mixture P2 a peak maximum at -1 °C indicates a polyelectrolyteinduced shift of the melting point to lower temperatures.3,4 At higher water concentrations (P4-P6) the bulk water peak at 0 °C is accompanied by the polyelectrolyte-influenced water peak, given by a shoulder at -1 °C. Moreover, an additional peak around -17 °C can be related to “bound” water nearby the surfactant head groups. Similar results were observed by Xu et al.24 in a poly(vinyl pyrrolidone)-modified microemulsion with a betaine-based surfactant. 1H NMR self-diffusion measurements can be successfully used to detect the diffusion coefficients D and the reduced diffusion coefficients (D/Do) of the different components of the microemulsion. The diffusion coefficients can give information about the type of microemulsion as well as structural changes in the system. However, the very small values for the reduced diffusion coefficient of water and the surfactant molecule below 0.05, in contrast to the high values for the heptanol of about 0.7 (compare Figure 6), clearly indicate the typical behavior of small water spheres surrounded by heptanol, which means the existence of PDADMAC filled w/o microemulsion droplets. Ultrasound relaxation measurements can be employed for detecting the particle dimensions of water droplets in a reverse microemulsion by measuring the sound relaxation in a frequency range between 3 and 100 MHz. Therefore, the mean droplet diameter of 11.6 nm was observed for the PDADMAC-modified microemulsion at point P1. Figure 7 shows the unimodal particle size distribution. It has to be mentioned here that P1 is the point with the lowest water/surfactant ratio (rw value). One can assume that the droplet size is increased in the order P1-P6 with increasing rw value. However, we dispense with details due to the large amounts of substances needed for each ultrasound experiment. Ultrahigh-Resolution Cryoscanning Electron Microscopy. Recently, we were able to show that cryo-SEM can be successfully applied for visualizing individual microemulsion droplets.4,25 On the basis of this knowledge, cryo-SEM was used for the characterization of the polymer-modified microemulsions in dependence on the water content. Figures 8 and 9 show the

micrographs for the mixtures P1 and P4. In the case of the nonpercolated sample P1 individual water droplets of at least 12 nm can be observed. This observation is in good agreement to the droplet size determined by means of our sound relaxation measurements, described above. Turning to sample P4, by using adequate freezing conditions, a coalescence to significant larger droplet associates can be observed, which is in full agreement with the percolation observed by conductometry. Concluding Remarks. First of all our results show that the polycation of low molar mass can be solubilized in the inverse microemulsion droplets. Taking into account that the smallest droplet size of about 12 nm (determined by means of ultrasound relaxation measurements) is significantly larger than the radius of gyration of the polymer coil, one can conclude that the polymer is incorporated into individual w/o microemulsion droplets. The existence of polymer-filled w/o droplets was determined independently by means of NMR spectroscopic diffusion measurements. Conductometric investigations show a percolation boundary at about 35% water content, and cryo-SEM micrographs visualize the droplet associates. One characteristic feature of these percolated microemulsions (samples P4-P6) is the presence of an additional bulk water peak, and the unusual conductivity decrease with increasing temperature, which can be related to temperature-sensitive polyelectrolyte-surfactant interactions. That means the polyelectrolyte-surfactant interactions influence the droplet-droplet interactions but not yet the viscosity of the system. B. Magnetite Particle Formation. Absence of PDADMAC. When the process is realized in a microemulsion template phase at point A, B, or C (compare Table 1), orange colored, small nanoparticles in the size range between 4 and 15 nm were formed in the template phase. However, during the precipitation, washing, and re-dispersion process the particles aggregate to larger dimensions, and finally the mean diameter of the re-dispersed particles varies between 50 and 120 nm (compare Table 2). Nevertheless, the particles formed are nonmagnetic, which means Fe3O4 was not synthesized under these conditions. Additional measurements made in an aqueous surfactant solution show that in the presence of the sulfobetaine detergent the formation of magnetite particles failed. One can assume that the surfactant molecules inhibit the process of magnetite formation, and Fe2O3, and/or FeOOH particles were formed. However, the “disturbing” effect of the surfactant can be related

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Figure 7. Particle size distribution determined by ultrasound relaxation of the PDADMAC-modified microemulsions at point P1.

TABLE 2: Characteristic Data of the Redispersed Particles in Absence of PDADMAC route

point

color

ζ potential (mV)

particle sizea (nm)

1b

A B C A B

orange orange orange orange orange

+35.0 ( 1.8 +23.9 ( 0.6 +28.3 ( 0.8 -33.3 ( 3.5 -28.2 ( 1.8

54 ( 3 80 ( 2 69 ( 3 116 ( 27 85 ( 15

2b

a Mean value obtained by Zav. b Procedure A by mixing two microemulsions.

TABLE 3: Characteristic Data of the Redispersed Particles in Presence of 1% PDADMAC route

point

color

ζ potential (mV)

particle sizea (nm)

1b

A B C A B

orange orange orange orange orange

+39.0 ( 5.5 +36.0 ( 3.5 +37.7 ( 2.2 +18.8 ( 1.7 +21.5 ( 2.5

27 ( 11 10 ( 2 25 ( 4 248 ( 40 312 ( 62

2b

Figure 8. Cryo-SEM micrograph of the PDADMAC-modified microemulsions at point P1.

a Mean value obtained by Zav. b Procedure A by mixing two microemulsions.

TABLE 4: Characteristic Data of the Redispersed Particles in Presence of 10% PDADMAC route

point

color

ζ potential (mV)

particle sizea (nm)

1b

P4 P6 P4 P6

black/brown black/brown black/brown black/brown

+46.6 ( 4.8 +35.3 ( 3.2 +54.1 ( 3.5 +42.2 ( 1.3

25 ( 8 23 ( 8 17 ( 6 28 ( 10

1c

a Mean value obtained by automatic peak analysis by number. Procedure A by mixing two microemulsions. c Procedure B by adding the ammonia solution.

b

Figure 9. Cryo-SEM micrograph of the PDADMAC-modified microemulsions at point P4.

(a) to the adsorption of hydroxid ions at the N-function of the sulfobetain and/or (b) to the complexation of the sulfonate group of the sulfobetain by iron ions. Presence of PDADMAC. When similar experiments were made in a microemulsion template phase at a PDADMAC

concentration of 1%, no significant changes were observed, and again orange-colored Fe particles were formed. The particle dimensions of the re-dispersed particles in presence of 1% PDADMAC are significantly smaller, but the magnetite formation is still inhibited (compare Table 3). However, things were changed drastically, when the polymer concentration was increased to 10%, and black/brown magnetic particles were observed. In general, the experiments can be realized in the percolated microemulsion at point P4 or P6 according to procedure A or B by adding ammonia (route 1). However, independent of the adding procedure (A or B) wellstabilized nanoparticles can be formed with ammonia, but the

Formation of Magnetite Nanoparticles

Figure 10. TEM micrograph of re-dispersed magnetite nanoparticles synthesized in the PDADMAC-modified microemulsion at point P4 (route 1, with two microemulsions).

smallest best stabilized particles were formed according to procedure B at point P4. A transmission electron microscopy (TEM) micrograph (Figure 10) shows uniform, spherically shaped magnetite particles of about 17 nm, which are in full agreement with the results obtained by dynamic light scattering (Table 4). The high positive ζ potential indicates that these nanoscale particles are well-stabilized by a polyelectrolyte adsorption layer. Conclusions First of all our experiments show that an optically clear region in the water-heptanol-sulfobetaine system still exists after incorporation of the cationic polyelectrolyte PDADMAC at a polymer concentration of 10% by weight. At lower water content the sample P1 shows the typical behavior of a non-percolated inverse microemulsion with a low shear viscosity, a Newtonian flow behavior, and a low conductivity. The disappearance of freezable water and the very small value for the reduced diffusion coefficients for water (determined by NMR diffusion measurements) are in full agreement with this statement. The non-percolated inverse microemulsions P2 and P3 show also a low conductivity and a small “bulk” water peak, which is shifted to a lower temperature in the presence of the polyelectrolyte. The points (P4-P6) in the percolated inverse microemulsion area indicate a higher conductivity with a sharp rise and a growing “bulk” water peak with increasing water content. Furthermore, a low shear viscosity and a Newtonian flow behavior were detected for point P4. The unusual decrease of the conductivity with increasing temperature can be explained by temperature-sensitive polyelectrolyte-surfactant interactions, leading to repulsive droplet-droplet interactions by increasing the temperature from 20 to 50 °C. These polymer-filled inverse microemulsions can be used as a template for the nanoparticle formation. Especially the formation of magnetite nanoparticles of small dimensions is still a challenge in such template phases due to the fact that in the nonmodified microemulsion the process failed, and orangecolored, nonmagnetic nanoparticles were formed. Taking this knowledge into account, we tried to use our wellcharacterized PDADMAC-modified microemulsions as a template phase for the nanoparticle formation.

J. Phys. Chem. B, Vol. 111, No. 29, 2007 8617 At point P1 in a non-percolated microemulsion of lower water concentration brown, nonmagnetic aggregates were formed. This might be caused by the nonexistence of bulk water, which is needed for the formation of magnetite particles in the nonpercolated microemulsion. When the same experiments were realized at point P4 or P6 according to route 1, that means in microemulsions having much more water incorporated, including a large amount of bulk water, it becomes possible to produce well-defined nanoscalic, spherically magnetite particles. The nanoparticles with particle diameter smaller than 20 nm are stabilized by a PDADMAC adsorption sheet (indicated by the high positive ζ potential) and can be re-dispersed without problems of particle aggregation. That means only in a percolated microemulsion, under “similar” conditions such as in pure water, i.e., in the presence of enough bulk water, under suppression of the “disturbing” effect of the surfactant head groups by means of polyelectrolyte-surfactant interactions, it becomes possible to produce nanoscalic magnetite particles. In such a complex system the polyelectrolytes have quite different tasks: stabilizing the microemulsion droplets; “masking” the surfactant head groups; controlling the particle growth; stabilizing of the nanoparticles. In general, it has to be stated here that the polyelectrolytes used have to be checked therefore on the one hand with regard to polyelectrolyte-surfactant head group interactions and on the other hand to polyelectrolyte-nanoparticle interactions. Acknowledgment. The authors thank Erich Kleinpeter and Gunter Wolf for the 1H NMR self-diffusion experiments. Supporting Information Available: The shear viscosities of samples P1 and P4 in dependence on the temperature are given in an additional figure. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Shah, O. Micelles, Microemulsions and Monolayers, Science and Technology; Dekker: New York, Basel, Hong Kong, 1998. (2) Beitz, T.; Koetz, J.; Wolf, G.; Kleinpeter, E.; Friberg, S. E J. Colloid Interface Sci 2001, 240, 581-589. (3) Koetz, J.; Andres, S.; Kosmella, S.; Tiersch, B. Compos. Interface 2006, 13 (4-6), 461-475. (4) Note, C.; Kosmella, S.; Koetz, J. J. Colloid Interface Sci. 2006, 302, 662-668. (5) Koetz, J.; Gu¨nther, C.; Kosmella, S.; Kleinpeter, E.; Wolf, G. Progr. Colloid Polym. Sci. 2003, 122, 27-36. (6) Koetz, J.; Bahnemann, J.; Kosmella, S. J. Polym. Sci., Part A: Polym. Chem. 2004, 42 (3), 742-751. (7) Rong, G.; Friberg, S. E. J. Dispersion Sci. Technol. 1988, 9, 401. (8) Koetz, J.; Beitz, T.; Tiersch, B. J. Dispersion Sci. Technol. 1999, 20, 139-163. (9) Lee, Y.; Lee, J.; Bae, C. J.; Park, J.-G.; Noh, H.-J.; Park, J.-H.; Hyeon, T. AdV. Funct. Mater. 2005, 15, (3), 503. (10) Lui, Z. L.; Wang, X.; Yao, K. L.; Du, G. H.; Lu, Q. H.; Ding, Z. H.; Tao, J.; Ning, Q.; Luo, X. P.; Tian, D. Y.; Xi, D. J. Mater. Sci. 2004, 39, 2633. (11) Lee, H. S.; Lee, W. C.; Furubayashi, T. J. Appl. Phys. 1999, 85 (8), 5231. (12) Schmid, G. Nanoparticles: From Theory to Application; WileyVCH: Weinheim, Germany, 2004. (13) Klabunde, K. J. Nanoscale Materials in Chemistry; Wiley-Interscience: New York, 2001. (14) Sugimoto, T. Monodispersed Particles; Elsevier Science: Amsterdam, 2001. (15) Fendler, J. H. Nanoparticles and Nanostructured Films; WileyVCH: Weinheim, Germany, 1998. (16) Hafeli, U.; Schutt, W.; Teller, J.; Zborowski, M. Scientific and Clinical Applications of Magnetic Carriers; Plenum: New York, 1997. (17) Bashtovoy, V. G.; Berkovsky, B. M.; Vislovich, A. N. Introduction to Thermomechanics of Magnetic Fluids; Hemisphere: Washington, DC, 1988.

8618 J. Phys. Chem. B, Vol. 111, No. 29, 2007 (18) Brabers, V. A. M. The preparation of tetragonal single crystals in the MnxFe3-xO4 system. J. Cryst. Growth 1971, 8 (1), 26. (19) Dautzenberg, H.; Go¨rnitz, E.; Jaeger, W. Macromol. Chem. Phys. 1998, 199, 1561. (20) Wolf, G.; Kleinpeter, E. Langmuir 2005, 21, 6742. (21) Nazario, L. M. M.; Hatton, T. A.; Crespo, J. P. S. G. Langmuir 1996, 12, 6326.

Baier et al. (22) Paul, B. K.; Mitra, R. K. J. Colloid Interface Sci. 2006, 295, 230. (23) Mehta, S. K.; Sharma, S. J. Colloid Interface Sci. 2006, 296, 690. (24) Xu, G.; Zhang, L.; Yuan, S.; Huang, X.; Li, G. J. Dispersion Sci. Technol. 2001, 22, 563. (25) Note, C.; Ruffin, J.; Kosmella, S. J. Dispersion Sci. Technol. 2007, 28, 155-164.