Synthesis of magnetic nanoparticles in bicontinuous microemulsions. Effect of surfactant concentration
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- Loo, A.L., Pineda, M.G., Saade, H. et al. J Mater Sci (2008) 43: 3649. doi:10.1007/s10853-008-2581-6
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Precipitation was accomplished at 80 °C for magnetic nanoparticles in bicontinuous microemulsions that were stabilized with different concentrations of a surfactants mixture of dodecyltrimethylammonium bromide/didodecyldimethylammonium bromide (3/2, w/w). These nanoparticles were characterized by X-ray Diffraction, Scanning Transmission Electronic Microscopy (STEM), and Vibrating Sample Magnetometry (VSM), which demonstrated that they were composed of magnetite or a mixture of magnetite-maghemite. The particles were found to have average diameters between 6.9 and 7.9 nm with relatively narrow particle size distribution and showed possible superparamagnetic behavior. In addition, we observed an inverse dependence of particle size on surfactant concentration. Yields obtained in these precipitation reactions were found to be up to three times higher than those typically reported in specialized literature about precipitation of magnetic nanoparticles in reverse microemulsions.
Magnetic nanoparticles, such as magnetite (Fe3O4) and maghemite (γ-Fe2O3), show very interesting properties due to their finite size and surface effects that dominate the magnetic behavior of individual particles . Superparamagnetism shown by magnetic particles with diameters <15 nm , are increasingly important to a variety of industrial and biomedical applications ; for example, current biomedical applications are magnetic separation, drug delivery, hyperthermia treatment, and magnetic resonance imaging contrast enhancement [3–6].
Precipitation in reverse microemulsions is a well-established technique for preparation of magnetic particles with diameters smaller than 10 nm and narrow particle size distribution [7–18]. The basis of this technique is the use of aqueous phase nanodroplets, with diameters usually smaller than 10 nm, that contain precursor ions of magnetic nanoparticles, to carry out precipitation reactions. In this way the droplets function as nanoreactors in which nucleation in a very small fraction of those occurs. The further growth of particles occurs by recruiting precursor ions and precipitate molecules from droplets void of particles .
In spite of the desirable characteristics of magnetic nanoparticles, precipitation in reverse microemulsions has the drawback of relatively low yield. Calculations based on the reported recipes for preparing magnetic nanoparticles by this technique, showed yields from 0.1 g  to 0.4 g  of product per 100 g of total mixture; however, by precipitation in bicontinuous microemulsions stabilized with a mixture of the cationic surfactants dodecyltrimethylammonium bromide (DTAB) and didodecyldimethylammonium bromide (DDAB) in 3/2 (w/w) ratio, we were able to obtain particles with diameters near 8 nm with relatively narrow particle size distribution and yields of up to 1.16 g of product per 100 g of total mixture . The increase in yield resulted from the differing capacities of aqueous phase dissolution between reverse and bicontinuous microemulsions. This is because reverse microemulsions usually accept up to 15 wt% while bicontinuous microemulsions are able to accept contents near 45 wt% . Use of bicontinuous microemulsions provides an option to overcome the drawback of low yield while preserving the characteristics of magnetic nanoparticles.
Bicontinuous microemulsions are formed of interconnected aqueous channels, with diameters usually smaller than 10 nm, immersed in an oleic continuous phase , whereas reverse microemulsions are constituted of nanodroplets of aqueous phase dispersed in an oleic continuous phase. Due to the difference in microstructures between both types of microemulsions, the mechanism of the particles precipitation should be different. In bicontinuous microemulsions it would be expected that nucleation inside the channels and further growth of nuclei through recruiting precursor ions and molecules of precipitate would occur.
Our previous report on preparation of magnetic nanoparticles by precipitation in bicontinuous microemulsions was the first on this topic in the specialized literature . To begin to elucidate the mechanism of particle precipitation in bicontinuous microemulsions, we report our findings on the effect of the concentration of the mixture of surfactants DTAB/DDAB (3/2, w/w) on the synthesis of magnetic nanoparticles in bicontinuous microemulsions.
All reagents were high-purity grades from Aldrich used without further purification: ferric chloride (FeCl3 · 4H2O, 99%), ferrous chloride (FeCl2 · 6H2O, 98%), methyl methacrylate (MMA, 99%), aqueous ammonia (NH4OH, 57.6 wt%), dodecyltrimethylammonium bromide (DTAB, 99%), and didodecyldimethylammonium bromide (DDAB, 98%). Water was triple-distilled deionized grade.
Synthesis of magnetic particles
Precipitation reactions were carried out at 80 °C in a 100 mL jacketed glass reactor equipped with a reflux condenser and inlets for argon, and feed of the microemulsion and ammonium hydroxide aqueous solution. To help stabilize the system during the reaction, the reactor was immersed in an ultrasonic bath. Three precipitation reactions in bicontinuous microemulsions were carried out with 30, 33, and 36 wt% surfactant, the same content (30 wt%) of an aqueous solution of a mixture of ferric and ferrous chlorides in a molar ratio of 3/2 with an overall concentration of 0.75 M, and the corresponding MMA contents to complete the total mixture. To preserve the microstructure in bicontinuous microemulsions, agitation was not provided when the reactions were carried out. In all cases, an excess of ammonium hydroxide (twice the stoichiometric requirements) as precipitating agent was used. Precipitation reactions were carried out following the same procedure reported in our previous work .
Electrical conductivities were measured at 80 °C and 1 KHz with an Orion 115 conductivity meter. Magnetic measurements were determined at room temperature in fields up to 12.5 KOe using Lake Shore vibrating sample magnetometer model VSM 735. X-ray determinations were done on a Siemens D-5000 X-ray diffractometer. Particle size was determined from micrographs obtained using a JEOL 1200 EXII Field Emission Scanning Electronic Microscope (FESEM) with a STEM module. Samples for electronic microscopy were prepared by dispersing the powders in acetone using an ultrasonicator and depositing the dispersion on a copper grid.
Results and discussion
All the mixtures of reaction became immediately black after the aggregate of aqueous ammonium hydroxide, typical of precipitation reactions where magnetite has been obtained . With the appearance of black powder, the products of the reactions were recovered and, yield calculations indicated values between 1.4 and 1.6 g of dried solids per 100 g of total mixture. These values are significantly higher than the maximum value of theoretical yield calculated from reported recipes in the specialized literature for preparing magnetic nanoparticles in reverse microemulsions . The maximum value of theoretical yield being 0.4 g of magnetic particles per 100 g of total mixture. These data corroborated the yield advantage of bicontinuous over reverse microemulsions in the synthesis of magnetic nanoparticles .
Number-average diameter (Dn) and polydispersity index (Dw/Dn) as determined from STEM micrographs of particles prepared by precipitation in bicontinuous microemulsions with various surfactant concentrations and 30 wt% 0.75 M aqueous solution of the mixture of iron chlorides at 80 °C
Surfactant concentration (wt%)
Data in this table indicate that the precipitation reactions carried out in bicontinuous microemulsions produce small diameter particles (6.9–7.9 nm) with a relatively narrow particle size distribution. It is noticeably that the diameters of these particles were similar to those characteristic of the channels in bicontinuous microemulsions (<10 nm). This was expected, since it is known that the microstructures of the microemulsions serve like nanoreactors, thus limiting the growth of particles .
Considering that the estimation of response error in our measurements of particle size was ≈ 0.03 nm, data in Table 1 show a diminution in Dn when increasing surfactant concentration. This agrees with the decrease in the frequency of higher particle sizes as surfactant concentration increases as shown in the histograms in Fig. 3. When the content of the dispersed phase is maintained, it is well known that, the characteristic size of the microstructures in a microemulsion, the diameter of the droplets in normal and reverse microemulsions, as well as the channels in the bicontinuous ones, diminish with the increase in surfactant concentration. Thus, in this study, the increase in surfactant concentration caused a diminution in the diameter of the channels (nanoreactors) where the precipitation reactions would have been carried out and therefore would have restricted the particles growth.
In addition, Fig. 5 also shows that the magnetization values of the samples at 12.5 kOe decrease slightly with decreasing surfactant concentration. Because of the previously identified inverse dependency of particle size on surfactant concentration, it is easy to conclude that there is also a direct relationship between magnetization and particle size. This agrees with those reports indicating a decrease in magnetization of small particles (diameters smaller than ca. 15 nm) as particle size decreases [10, 15]. The magnetization values obtained in this study were similar to those previously reported by our group for nanoparticles precipitated in bicontinuous microemulsions  and also for those characteristics of magnetic particles precipitated in reverse microemulsions at relatively high temperatures (90 °C) [17, 32]. In contrast, they were higher than those reported for nanoparticles obtained by precipitation in reverse microemulsions at low temperatures (25–50 °C) [31, 32]. It is a known fact that magnetization of magnetic nanoparticles depends directly on their crystallinity and that an increase in precipitation temperature enhances the crystallinity of the iron oxide nanoparticles . This could be the explanation for this behavior.
Precipitation reactions carried out in bicontinuous microemulsions at different surfactant concentrations allowed to obtain magnetic nanoparticles. X-ray diffraction analysis demonstrated that the particles consisted of magnetite or a mixture of magnetite-maghemite. STEM measurements showed that the particles had average diameters between 6.9 and 7.9 nm and, a relatively narrow particle size distribution. VSM data suggested superparamagnetic behavior. Yields obtained in the precipitation reactions demonstrated the yield advantage of bicontinuous over reverse microemulsions in the synthesis of nanoscale particles. In addition, we found an inverse dependence of particle size on surfactant concentration, that probably was caused by a decrease in the channels diameter of the microemulsions as a response to the change in this variable.
National Council of Science and Technology (CONACyT) supported this research through grant SEP 2003-CO2-45436. We are grateful to Janet Valdés and Patricia Siller for their technical assistance.