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Bilayer Surfactant Stabilized Magnetic Fluids: Synthesis and Interactions at Interfaces Lifen Shen, Paul E. Laibinis,* and T. Alan Hatton* Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received June 26, 1998. In Final Form: November 12, 1998 Aqueous magnetic fluids were synthesized by a sequential process involving the chemical coprecipitation of Fe(II) and Fe(III) salts with ammonium hydroxide (NH4OH) followed by resuspension of the ultrafine particles in water using fatty acids. This procedure produced Fe3O4 nanoparticles stabilized against agglomeration by bilayers of n-alkanoic acids with 9-13 carbons encapsulating the metal particles. The magnetic properties and particle size and size distributions of these magnetic fluids, characterized by transmission electron microscopy and superconducting quantum interference device, indicated the formation of single-domain nanoparticles of mean diameter ∼9.3 and ∼7.5 nm, respectively; the difference in values determined by the two methods implies the presence of a nonmagnetic layer on the particle surface. Thermogravimetric analysis measurements showed the existence of two distinct populations of surfactants on the particle surface, each having surfactant coverage of ∼21-24 Å2/molecule, that was consistent with highly organized surfactant bilayer structures. Differential scanning calorimetry indicated the presence of a phase transition for the bilayer-coated particles that suggests partial interpenetration of the hydrocarbon tails of the primary and secondary surfactants.
Introduction A magnetic fluid is a stable colloidal suspension of magnetic nanoparticles dispersed in a carrier liquid.1,2 Due to their small size and superparamagnetic behavior, these magnetic nanoparticles exhibit different materials properties from their bulk counterparts.3 Applications of magnetic fluids that have already reached commercial maturation (e.g., in dynamic loudspeakers, computer hard drives, and dynamic sealing) are based on their unique superparamagnetic, tribological, thermal, and mechanical properties.4 Potential applications for so-called magnetic mesoscopic materials exist in information storage, color imaging, bioprocessing,5 and magnetic refrigeration.6 Recently, exploration of novel uses of magnetic particles in the separations area has increased significantly. Magnetic particles with appropriately tailored surface characteristics can be used to facilitate the separation of biomolecules, to sort specific cell types from a cell population, or to deliver drugs to a target organ in the body.7 Thus, the preparation of magnetic fluids has continued to attract a great deal of attention since their inception in the late 1960s. A difficulty associated with the preparation of magnetic fluids is that the particles have large surface area-tovolume ratios and thus tend to aggregate to reduce their surface energy. Specifically, magnetic metal oxide surfaces (1) Berkovsky, B. M.; Medvedev, V. F.; Karkov, M. S. Magnetic Fluids: Engineering Application; Oxford University Press: New York, 1993. (2) Rosensweig, R. E. Ferrohydrodynamics; Cambridge University Press: Cambridge, England, 1985. (3) Ziolo, R. F.; Giannelis, E. P.; Weinstein, B. A.; O’Horo, M. P.; Ganguly, B. N.; Mehrotra, V.; Russell, M. W.; Huffman, D. R. Science 1992, 257, 219-223. (4) Raj, K.; Moskowitz, R. J. Magn. Magn. Mater. 1990, 85, 233245. (5) Nixon, L.; Koval, C. A.; Noble, R. D.; Slaff, G. S. Chem. Mater. 1992, 4, 117-121. (6) McMichael, R. D.; Shull, R. D.; Swartzendruber, L. J.; Bennett, L. H.; Watson, R. E. J. Magn. Magn. Mater. 1992, 111, 29-33. (7) Pieters, B. R.; Williams, R. A.; Webb, C. Magnetic Carrier Technology; Butterworth-Heinemann Ltd.: Oxford, England, 1992.
have extremely high surface energies (>100 dyn/cm) that make the production of nanoparticles very challenging. In addition, magnetic dipole-dipole attractions between particles enhance the difficulties experienced in the production of ferrofluids in comparison to nonmagnetic nanoparticles. Prevention of particle agglomeration is one of the critical obstacles to be overcome in producing stable magnetic fluids whether physical grinding or chemical reaction is used in their production. Although well-dispersed magnetic nanoparticles can be obtained by ball-milling,1,8 high-energy requirements and unavoidable contamination of the product have necessitated the development of more economical and reliable ways to produce magnetic particles by chemical coprecipitation. Iron oxide (Fe3O4), the dominant magnetic material in magnetic fluid preparations, can be synthesized through the coprecipitation of Fe(II) and Fe(III) salts by addition of a base. Massart obtained stable aqueous alkaline and acidic magnetic liquids by free precipitation.9,10 The nature of the counterions and the pH of the suspensions played a vital role in stabilizing the charged magnetic particles through interactions between their electrical double layers. The applications of magnetic fluids that are stabilized solely by electrostatic repulsion are somewhat restricted since this system is overly sensitive to conditions such as pH and ionic strength and offers little flexibility for tuning the surface properties of the particles. In several studies, complex microstructures have been used as reactors to obtain ultrafine magnetic iron oxide particles.11-16 These works claim that the constrained (8) Berkowitz, A. E.; Lahut, J. A.; Vanburen, C. E. IEEE Trans. Magn. 1980, MAG-16, 184-190. (9) Massart, R. IEEE Trans. Magn. 1981, MAG-17, 1247-1248. (10) Massart, R.; Dubois, E.; Cabuil, V.; Hasmonay, E. J. Magn. Magn. Mater. 1995, 149, 1-5. (11) Kommareddi, N. S.; Tata, M.; John, V. T.; McPherson, G. L.; Herman, M. F.; Lee, Y.; O’Connor, C. J.; Akkara, J. A.; Kaplan, D. L. Chem. Mater. 1996, 8, 801-809. (12) Lee, J.; Isobe, T.; Senna, M. J. Colloid Interface Sci. 1996, 177, 490-494.
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cavities within microemulsions or unilamellar vesicles provide more effective control over the crystal growth and prevention of particle agglomeration than can be obtained by free precipitation methods. In practice, however, little control can actually be exercised over the size and size distribution of the microstructures, and moreover, only small quantities of iron oxide can be obtained owing to the constraints of low reagent concentrations necessitated by this synthetic procedure. Stabilization of magnetic fluid suspensions can be achieved by coating the particle surfaces with organic surfactants. When two such particles approach each other, the surfactants interpenetrate and the particles are subjected to steric repulsion due to an increase in osmotic pressure.1,2,17 Shimoiizaka et al. proposed the principle of bilayer stabilization in the early 1980s.18 They first precipitated oleic acid-coated particles and then redispersed them in aqueous solutions of sodium dodecyl benzenesulfonate, poly(oxyethylene)nonylphenyl ethers, or di(2-ethylhexyl)adipate, hypothesizing that the second surfactant coated the primary surfactant-coated particles to form a structured surfactant bilayer. Following this work, Khalafalla and Reimers19 and Wooding et al.20 produced stable aqueous magnetic fluids using various saturated and unsaturated fatty acids as primary and secondary surfactants. While some research has focused on the synthesis of magnetic particles stabilized by surfactant bilayers, little is known about the stabilization mechanisms underlying such systems in terms of the interfacial interactions that might exist. Indeed, the concept of surfactant bilayer stabilization has not yet progressed beyond being a reasonable hypothesis. Quantitative experimental characterizations of such systems are still rare and the results are inconsistent with each other. The inconsistency is due to the many assumptions made in the analysis of experimental results, such as monodisperse particles and no free secondary surfactant in solution in equilibrium with adsorbed secondary surfactant, neither of which is true for real systems. The goal of this paper is to demonstrate the existence of surfactant bilayers on the iron oxide nanoparticle surfaces and to quantify the surfactant coverage and interfacial interactions between the primary and secondary surfactants, allowing for the polydispersity of the colloidal particles. We describe the synthesis of these bilayer surfactantstabilized magnetic fluids using saturated fatty acids as the primary and secondary surfactants. These syntheses reliably produced colloidal systems that were stable over periods of more than 6 months as evidenced by the consistency in their magnetic properties over these times and a lack of observable precipitation. We characterized the magnetic fluids in terms of their magnetic properties, their particle morphology, size and size distribution, surfactant coverage, and interfacial interactions by su(13) Lopez-Quintela, M. A.; Rivas, J. J. Colloid Interface Sci. 1993, 158, 446-451. (14) Mann, S.; Hannington, J. P. J. Colloid Interface Sci. 1987, 122, 236-335. (15) Yaacob, I. I.; Nunes, A. C.; Bose, A.; Shah, D. O. J. Colloid Interface Sci. 1994, 168, 289-301. (16) Yaacob, I. I.; Nunes, A. C.; Bose, A. J. Colloid Interface Sci. 1995, 171, 73-84. (17) Hiemenz, P. C. Principles of Colloid and Surface Chemistry; 2nd ed.; Marcel Dekker: New York, 1986. (18) Shimoiizaka, J.; Nakatsuka, K.; Jujita, T.; Kounosu, A. IEEE Trans. Magn. 1980, MAG-16, 368. (19) Khalafalla, S. E.; Reimers, G. W. IEEE Trans. Magn. 1980, MAG16, 178-183. (20) Wooding, A.; Kilner, M.; Lambrick, D. B. J. Colloid Interface Sci. 1991, 144, 236-242.
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Figure 1. Schematic representation for the synthesis of surfactant bilayer stabilized magnetic fluids, using fatty acids as the primary and secondary surfactants to produce stable aqueous magnetic fluids.
perconducting quantum interference device (SQUID) magnetometry, transmission electron microscopy (TEM), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). With the experimental evidence, our data suggest that surfactant bilayer stabilization should be generalizable for preparing aqueous magnetic fluids with tailored surfaces using other surfactants for specific applications. Experimental Section Materials. Iron(II) chloride tetrahydrate (99%), iron(III) chloride hexahydrate (97%), ammonium hydroxide (28% NH3 in water, double distilled), nonanoic acid (96%), decanoic acid (99%), undecanoic acid (97%), dodecanoic acid (99.5%), and tridecanoic acid (98%) were obtained from Aldrich (Milwaukee, WI). Acetone and methanol were purchased from Mallinckrodt (Paris, KY). All chemicals were used as received. Synthesis of Bilayer Surfactant-Stabilized Magnetic Fluids: Step 1. Preparation of Primary Surfactant-Coated Iron Oxide Particles. Our syntheses were based on a procedure reported by Wooding et al.,20 in which coprecipitation of Fe(II) and Fe(III) salts by NH4OH at 60 °C was followed by a two-step addition of the primary and secondary surfactants. Three modifications were made: (i) an initial addition of decanoic acid in acetone into the Fe(II) and Fe(III) chloride aqueous mixture; (ii) the use of a precipitation temperature of 80 °C instead of 60 °C; and (iii) a doubling of the amount of ammonium hydroxide added. The schematic representation of the two-step synthetic procedure is shown in Figure 1. In a typical synthesis to obtain 1 g of Fe3O4 precipitate, 0.86 g of FeCl2‚4H2O and 2.35 g of FeCl3‚ 6H2O were dissolved under Ar in 40 mL of deaerated Milli-Q water with vigorous stirring, such that Fe3+/Fe2+ ) 2. As the solution was heated to 80 °C, a solution of 100 mg of neat decanoic acid in 5 mL of acetone was added, followed by 5 mL of 28% (w/w) NH4OH. Further neat decanoic acid was added to the suspension in five 0.2-g amounts over 5 min. The crystal growth was allowed to proceed for 30 min at 80 °C with constant stirring to produce a stable, water-based suspension, which was then cooled slowly to room temperature. The suspension was precipitated with acetone and MeOH, and the precipitates were isolated from the solvent by magnetic decantation. This washing-decantation procedure was repeated five times to remove the excess decanoic acid used for the primary layer; the removal was confirmed by the lack of decanoic acid in the magnetic decantation supernatant by GC (gas chromatography).
Bilayer Surfactant-Stabilized Magnetic Fluids Step 2. Preparation of Stable Aqueous Magnetic Fluids Using a Series of Secondary Surfactants of Similar Chemical Structure to the Primary Surfactant. n-Alkanoic acids with 9-13 carbons, designated by C9-C13, were utilized as the secondary surfactant. In a typical procedure, 1 g of the fresh precipitate obtained in step 1 was combined with 20 mL of Milli-Q water and the slurry was heated to 60 °C under vigorous stirring. A solution of 10% (w/v) ammonium salt of a fatty acid (pH ) 10) was added to the slurry dropwise using a syringe under constant stirring at 60 °C until the slurry changed into a stable suspension; typical amounts of added solution were 5 mL. To confirm the complete redispersion of the magnetic precipitates, the suspension was exposed to a 3 kG magnetic field (field gradient is 9 kG/cm at the surface of the permanent magnet) for at least 10 min, and no phase separation was observed. These samples were denoted by a combination of their two surfactants, with the first being the primary surfactant. For instance, C10/C9 designates a magnetic fluid coated with a primary surfactant C10 and a secondary surfactant C9. Characterization. The size and morphology of the particles were determined by bright-field TEM (Akashi 002B, 200 kV), with samples prepared by evaporating dilute suspensions on a carbon-coated copper film. The magnetization of the diluted samples of different concentrations was measured by SQUID (Quantum Design, MPMS) at room temperature. In a typical measurement, 10 µL samples were injected into a glass sample holder that was designed specially for liquid samples. The glass tube was sealed with cement to ensure that the concentration of the sample remained constant during the measurement. A two-step treatment was undertaken to prepare dried samples for thermal analysis. The particles were precipitated completely in an ultracentrifuge (Beckman Optima preparative ultracentrifuge, 20 min at 40 000 rpm), following which they were lyophilized (FreezeMobile 25-EL freeze-dryer, 48 h) to remove as much solvent as possible from the samples. To confirm the dryness of the samples, a test run was performed as follows. One sample was heated to 150 °C at a rate of 10 °C/min and maintained at that temperature for 5 h. If no mass loss (80 °C) favor the formation of Fe3O4.3,22 The necessary pH value for the rapid formation of Fe3O4 is attained by addition of excess NH4OH. In our experiments, we observed that the product showed a brownish color, an indication of the presence of Fe2O3, if the precipitation temperature was below 60 °C or insufficient NH4OH was added. On the basis of these observations, the formed crystals were assumed to be Fe3O4. Particle Size, Morphology, and Interaction at the Interface between the Primary Surfactant and the Iron Oxide Surface. The size and size distribution of the colloidal particles in the magnetic fluids were characterized by TEM23 and magnetization measurements.2 The particle size polydispersity can be captured by the log-normal distribution law:23,24
P(x) )
2 2 1 e-(ln x-µ) /2σ (x > 0) x2πσx
(1)
where P(x) is the probability density of the distribution and x ) D/Dp is the reduced diameter, Dp being the median of the particle diameter D. The mean and the standard deviation of ln x are given by µ and σ, respectively, while the mean of x itself is given by eµ+σ2/2, with variance (eσ2 - 1)e2µ+σ2. Generally, this distribution fits the experimental results well and is more physically acceptable than the Gaussian distribution, for instance, since values below zero are not allowed. Size and size distributions were determined from TEM micrographs by measuring the diameters of 500 particles. A fit of the log-normal distribution to our experimental data (sample C10/C9) gave µ ) 1.095 and σ ) 0.102. The significance level of the fit was 95% by an F test. Figure 2 shows that the magnetic oxides formed spherical clusters with a mean diameter of 9.3 nm and a standard deviation of 2.6 nm (sample C10/C9). We observed similar particle morphology, sizes, and size distributions for samples C10/ C9, C10/C11, and C10/C13 irrespective of the secondary surfactants used. These samples appeared well-dispersed in the TEM micrographs. This observation indicates that (21) Glusker, J. P.; Lewis, M.; Rossi, M. Crystal Structure Analysis for Chemists and Biologists; VCH: New York, 1994. (22) Zhang, L.; Papaefthymiou, G. C.; Ying, J. Y. J. Appl. Phys. 1997, 81, 6892-6990. (23) Popplewell, J.; Sakhnini, L. J. Magn. Magn. Mater. 1995, 149, 72-78. (24) Buhrman, R. A.; Granqvist, C. G. J. Appl. Phys. 1976, 47, 22002219.
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Figure 2. TEM micrograph of surfactant bilayer-stabilized magnetic fluids. The primary/secondary surfactants are decanoic/undecanoic acids (C10/C11).
fatty acids of chain length n ) 9-13 can provide an effective stabilization for dispersing the magnetic particles in an aqueous medium and that it is possible to rationalize a two-step approach that uses other secondary surfactants to tune the surface properties of the magnetic particles for particular applications. We also extracted particle size information from magnetization curves, which are sensitive to the particle size and size distribution.25,26 In the absence of an external magnetic field, Brownian motion orients the particles randomly and the magnetic fluid has no net magnetization. When a magnetic field is applied, the dipolar particles align themselves with the field, resulting in a measurable magnetization of the fluid. The measured magnetization is a result of the competition between the random Brownian motion and the oriented magnetic interactions experienced by the particles, both of which depend strongly on the particle size. In contrast to TEM studies, the magnetization measurements can be performed on liquid samples, which make them a complementary tool for obtaining particle size. To preclude the possible effect of chain formation and particle agglomeration,27 all measurements were performed on dilute samples (iron oxide volume
