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Continuous Flow Synthesis of Superparamagnetic Nanoparticles in Reverse Miniemulsion Systems
Colloid and Interface Science Communications 28 (2019) 1–4
Contents lists available at ScienceDirect
Colloid and Interface Science Communications journal homepage: www.elsevier.com/locate/colcom
Rapid Communication
Continuous Flow Synthesis of Superparamagnetic Nanoparticles in Reverse Miniemulsion Systems
T
Tonghan Gua, Yunfei Zhanga, Saif A. Khanb, T. Alan Hattona,
⁎
a b
Department of Chemical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge 02139, MA, USA Department of Chemical and Bimolecular Engineering, National University of Singapore, 4 Engineering Drive 4 E5-02-28, 117576, Singapore
ARTICLE INFO
ABSTRACT
Keywords: Magnetic nanoparticle Multiphase flow reaction Miniemulsion Reverse micelle
Magnetic nanoparticles were synthesized continuously in a multiphase segmented flow reaction system where one phase was a miniemulsion containing iron precursor solution in nanodroplet confinement and the other phase was diluted ammonium hydroxide solution. Spontaneous formation of reverse micelles at the phase boundary effectively transported ammonium hydroxide into the nanodroplets to trigger the precipitation of magnetic nanoparticles. The nanoparticles possessed superparamagnetism with a mean diameter of about 10 nm. This flow system is potentially generalizable to other emulsion-based continuous nanoparticle synthesis applications.
Magnetic nanoparticles have been applied to a broad range of applications over the past two decades, including magnetic fluids [1], catalysis [2,3], magnetic resonance imaging contrast agents [4], hyperthermia therapy [5], drug delivery [6,7], and protein separations [8,9], to name a few. When magnetic nanoparticles are sufficiently small, such that there is only one magnetic domain in each particle [10], a permanent dipole cannot be formed, thus leading to the phenomenon of superparamagnetism [11,12]. High-quality magnetic nanoparticles can be synthesized by several chemical processes, such as co-precipitation, thermal decomposition, hydrothermal reactions, and microemulsion synthesis [13,14]. Among these methods, microemulsion systhesis uses well-defined nanodroplets as reaction environments, which is easy to carry out and restricts nanoparticle size thus favoring superparamagnetism [15]. The production yield can be improved significantly when miniemulsions, which are orders of magnitude more concentrated, are used instead of microemulsions, though at the expense of creating larger nanoparticles since miniemulsions have droplets of a few hundred nanometers. Recently, continuous flow chemistry has attracted significant research interest; micro- and milli-fluidic systems are advanced tools for continuous production of value-added products in small quantities, such as functional nanoparticles [16,17]. Magnetic nanoparticles have also been successfully synthesized continuously in a coaxially mixed single phase flow [18], gas-liquid flow [19], and liquid-liquid flow [20], thanks to the rapid mixing in microfluidic channels that greatly helps control the size of magnetic nanoparticles. Continuous synthesis
⁎
offers distinct advantages over the more conventional batch processes because they provide a better control over reaction conditions and minimize batch-to-batch variations. We recently introduced a novel droplet-based microfluidic technique in which the continuous phase used for droplet generation consisted of a stable nanoemulsion or miniemulsion phase; we also demonstrated its application in the flow synthesis of micro- and nanoparticles driven by controlled mass transport from nanodroplets in the continuous phase to the dispersed microdroplets [21,22]. In this short communication, we demonstrate how the reversed process, which transports materials from microdroplets to nanodroplets, enables superparamagnetic nanoparticle synthesis. To the best of our knowledge, this is the first report of continuous synthesis of superparamagnetic nanoparticles in reverse miniemulsion systems, and opens the door to facile and scalable continuous production methods for such materials. Fig. 1 shows a schematic illustration of our method. First, a FeCl2 (0.5 M) /FeCl3(1.0 M) /HCl (1.0 M, pH ≈ 0) mixture solution was homogenized into a miniemulsion with dodecane (containing 5 wt% Span 20 as surfactants) as the continuous phase and aqueous droplets of 20% volume fraction as the dispersed phase. The droplet size was reduced to 476 ± 30 nm using a Cole-Parmer Horn-Cup Cell Disruptor (at 40% max. power) to sonicate for 20 min. Then, this emulsion was pumped into a microfluidic T-junction (1.016 mm ID) where it met the NH4OH solution (5 wt%). Both phases flowed at 30 μL/min and a segmented flow with alternating NH4OH solution and miniemulsion segments formed at the junction. Although NH4OH solution was dispersed
Corresponding author. E-mail addresses: [email protected] (S.A. Khan), [email protected] (T.A. Hatton).
https://doi.org/10.1016/j.colcom.2018.10.005 Received 20 September 2018; Received in revised form 20 October 2018; Accepted 27 October 2018 2215-0382/ © 2018 Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Colloid and Interface Science Communications 28 (2019) 1–4
T. Gu et al.
Fig. 1. Illustration of the flow synthesis process. The photo shows the transport of ammonia from the aqueous phase to the organic phase to precipitate Fe2+/Fe3+ solutions in nanodroplets.
as microdroplets in miniemulsion due to tube wetting preferences, we use the term “segment” to denote both phases in the rest of this article to distinguish these macroscopic entities from the nanodroplets dispersed in the miniemulsion segment. Ammonia was quickly transported through the dodecane phase (and across the surfactant layer) into nanodroplets due to the sub-millimeter mass transfer distance, and triggered reactions inside each nanodroplet to form iron (II) hydroxide and iron (III) hydroxide precipitates. The NH4OH segment had almost no color change but the miniemulsion phase turned black. The flow of segments also created strong circulation in the miniemulsion segments for a better mixing to ensure a fast transport of ammonia [23]. Heating of this segmented flow at 50 °C for 5 min caused hydroxides to decompose into oxides to yield Fe3O4 nanoparticles that were suspended in the nanodroplets in the miniemulsion phase. The miniemulsion was collected and demulsified with ethanol and centrifuged to recover the nanoparticles, which were washed by ethanol three times. The particles were stored as a dispersion in deionized water, but can also be dried into a powder.
Fig. 2a and b shows the transmission electron microscope (TEM) characterization of the prepared nanoparticles. Due to aggregation, size measurement cannot rely on dynamic light scattering techniques. We randomly sampled 80 locations in Fig. 2a and measured the size of nearest non-overlapping particle of each location, and plotted a histogram of particle size distribution, as shown in Fig. 2c. It can be inferred directly from the dimensions of the magnetic nanoparticles that they are superparamagnetic [10], and this inference was confirmed with a SQUID (superconducting quantum interference device) test. Fig. 2d showed that the nanoparticles are superparamagnetic at room temperature, which is consistent with the overall small size of nanoparticles. The saturation magnetization (Ms) is about 30 emu/g, which is comparable to those reported previously [24,25] for emulsion-based magnetite nanoparticle preparation, but lower than those in other reports [14]. Future work should focus on tuning the recipe to achieve a higher Ms, which is known to be related to pH, surfactant, etc. [25,26], to increase the superparamagnetic properties. Fig. 2e shows the X-ray
Fig. 2. (a) and (b) TEM characterization of the magnetic nanoparticles, (c) Histogram of primary particle size distribution, (d) SQUID analysis of magnetic hysteresis with saturation magnetization (Ms) ≈ 30 emu/g, remnant magnetization (Mr) ≈ 0.6 emu/g (Ms/Mr = 50), and coercivity (Hc) ≈ 0.1 Oe, (e) XRD characterization of the magnetite nanoparticles. 2
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diffraction (XRD) characterization of the magnetic nanoparticles, which gives an averaged particle size of ~10.4 nm based on the Scherrer equation (d = Kλ/βcosθ, K ≈ 1, λ = 1.79 Å, β = 0.018, and θ = 0.314) [12]. This value is close to the average size (10.1 nm) calculated from the primary particle size histogram (Fig. 2c) obtained from TEM. In this reaction system, two aqueous phases and an inert dodecane oil phase are present at the same time, with one aqueous phase (Fe2+/ Fe3+) dispersed as nanodroplets in dodecane segments and the other aqueous phase (NH4OH) itself forms segments. As mentioned above, only the dodecane phase that contains aqueous nanodroplets turns black, while the NH4OH segments remain colorless. While our previous research has shown that materials can be transferred from the nanodroplets into the NH4OH segments under thermal, chemical, or electrical stimuli [21,22], it is still unclear how the reverse process, where material is transferred from the large NH4OH segments into nanodroplets, occurs without any stimuli. The highly polar molecule ammonia has almost no molecular solubility in dodecane (0.0021 g/g) [27] and thus a different mechanism other than molecular dissolutiondiffusion/convection must be responsible for the mass transport. We conducted a few extra experiments to gain insight into the underlying mechanism. The experimental setup is shown in Fig. 3 with four different conditions. Briefly, two vials containing full (Fig. 3a, b, and c) or half (Fig. 3d) level of aqueous NH4OH and FeCl2/FeCl3 solutions were submerged into an oil phase, which contains no surfactants (Fig. 3a) or 5 wt% Span 20 (Fig. 3b, c, and d). Heating time was controlled at 10 min (Fig. 3a, b, d) or 2 h (Fig. 3c). First, we discovered that adding Span 20 is essential for enabling the mass transport. To understand the role of surfactants in this process, we did a control experiment with dodecane free of Span 20. As shown in Fig. 3a, when the dodecane phase contains no surfactant, heating at 50 °C for 10 min did not change the color of the separated NH4OH and FeCl2/ FeCl3 solutions and the dodecane phase remained colorless, indicating negligible mass transport between these two solutions across the dodecane phase. However, when 5 wt% Span 20 was loaded into dodecane, the dodecane phase turned yellow in < 10 min, which indicates that FeCl2/FeCl3 had entered the dodecane phase (Fig. 3b). NH4OH, due to its colorless nature, cannot be easily detected. However, extended heating for 2 h generally turned the dodecane phase from yellow to grey, showing that NH4OH must have entered the dodecane phase and reacted with FeCl2/FeCl3 (Fig. 3c). A closer inspection of Fig. 3b and c showed large black particles formed at the interface between the FeCl2/FeCl3 solution and dodecane (with Span 20), which meant that NH4OH not only entered dodecane, but also diffused into the FeCl2/FeCl3 solution. There were far fewer particles formed in the NH4OH solution, showing that NH4OH is more active in being transported than FeCl2/FeCl3, leading to a net transport direction from the NH4OH solution to the FeCl2/FeCl3 solution that is consistent with the observation in the flow experiment (Fig. 1) that NH4OH remained almost colorless.
Second, it is unlikely that the improvement in polarity of dodecane by Span 20 significantly increases the molecular solubility of FeCl2/ FeCl3. Molecular solubility can be estimated based on the Hansen solubility parameters of the solutes. In general, two molecules of similar solubility parameters have similar solubilities in a single solvent, provided there are no additional molecular interactions besides dispersion force (δD), dipolar force (δP), and hydrogen bonding (δH). The solubility parameters of Span 20 are 17.1, 8.4, and 13.6 (δD, δP and δH respectively) [28]. Thus, a 5 wt% solution of Span 20 in dodecane (δD = 16.1) yields a solubility parameter of 16.15, 0.42, 0.68 through linear combination. Such a slight increase in polarity and hydrogen bonding forces is less likely to be enough to create a detectable yellow color from the dissolved FeCl2/FeCl3. As a comparison, no yellow color appears when we try to dissolve FeCl2/FeCl3 in a 9-to-1 mixture of dodecane and ethyl acetate, which gives an even slightly more polar environment with solubility parameters of 16.07, 0.53, 0.72 (δD, δP and δH respectively). Third, it is more likely that the apparent increase in ammonium hydroxide solubility is achieved via the formation of reverse micelles. As shown in Fig. 3d, when we created a static dodecane layer on top of NH4OH and FeCl2/FeCl3 solutions (negligible convection inside the vial), we can clearly observe the spontaneous formation of emulsions after 10 min of heating, which is probably caused by the coalescence of reverse micelles. NH4OH formed a thicker and more opaque emulsion layer than FeCl2/FeCl3, indicating that NH4OH is more active in forming reverse micelles, which as a result contributed to a stronger mass transport activity. With the evidence above, it is clear that reverse micelles formed by Span 20 are more likely to be the mass transfer carriers, and a schematic illustration of the mass transport process is shown in Fig. 4. Berg et al. thoroughly studied the reverse micelle formation capability of Span series in hydrocarbons [29] and concluded that Span 20 formed the largest reverse micelles of 4.2 ± 0.1 nm with deionized water based on small angle neutron scattering studies. This aligns well with our experimental observations that using Span 80 results in a much slower reaction process. The larger reverse micelles formed by NH4OH could be explained in terms of two factors - cation valency and pH. Reverse micelle size is inversely proportional to cation valency, which contributes to a larger micelle size for NH4OH than FeCl2/FeCl3 [30]. A higher pH has also been shown to reduce the hydrocarbon-water interfacial tension [31,32], which facilitates the spontaneous generation of more reverse micelles. Compared to batch synthesis, flow synthesis provides a high control over the reaction conditions, such as mixing and heat treatment. The high reproducibility of segments, such as the size and flow speed of each segment, ensures that mixing between different segments is consistent. Heat treatment is very efficient compared to batch processes due to a much smaller heat transfer distance. A thermal entrance length of only < 5 mm (Lentrance/Dtube ≈ 0.05ReDPr, ReD ≈ 10, Pr < 10) was
Fig. 3. Pictures of vials of FeCl2/FeCl3/HCl and NH4OH solution submerged in dodecane. Left vials contain FeCl2/FeCl3/HCl solution and right vials contain NH4OH solution. Process conditions: (a) 10-min heat treatment without surfactant, (b)10-min heat treatment with Span 20, (c) 2-h heat treatment with Span 20, (d) 10-min heat treatment with Span with a static dodecane layer on top of each solution. 3
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[5] [6] [7] [8] [9] [10] [11]
Fig. 4. A schematic illustration of micelle-assisted mass transport.
[12] [13]
achieved, which means that the emulsion reached thermal equilibrium with the surrounding oil bath in < 5 s, and ensured that the fluid was in a constant temperature environment for most of the heating duration. Another benefit is that the oxidation of the unstable Fe2+ salt and hydroxide is significantly mitigated, due to the intrinsic oxygen-free environment inside the FEP plastic tubing filled with liquids. The common clogging issue of solid-generating reactions in small channels was not observed in our experiment due to the small size of magnetic nanoparticles. In summary, we reported a novel multiphase segmented flow system for the continuous synthesis of superparamagnetic nanoparticles with a mean size of about 10 nm. The flow reactor offers good temperature and mixing control, as well as an oxygen-free environment. A mechanistic study shows that the ammonia transport from the large aqueous segments to nanodroplets containing iron precursors is likely achieved through reverse micelles as material carriers, and the mass transfer direction is controlled by the difference in the rates of micelle formation and the sizes of micelles generated. This technique could potentially be a continuous synthesis solution to other nanoparticles prepared in batch emulsion systems.
[14] [15] [16] [17] [18] [19] [20]
[21] [22] [23]
Declaration of Interest
[24]
None.
[25]
Acknowledgement
[26]
The authors thank Center for Material Science and Engineering (CMSE) of Massachusetts Institute of Technology for providing transmission electron microscope access and Dr. Paul Brown for conducting magnetic property and Mr. Kai-Jher Tan for conducting X-ray diffraction measurements.
[27] [28] [29]
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Okuyama, Correlation between particle size/domain structure and magnetic properties of highly crystalline Fe3O4 nanoparticles, Sci. Rep. 7 (2017) 9894. M. Mikhaylova, D.K. Kim, N. Bobrysheva, M. Osmolowsky, V. Semenov, T. Tsakalakos, M. Muhammed, Superparamagnetism of magnetite nanoparticles: dependence on surface modification, Langmuir 20 (2004) 2472–2477. S. Sun, H. Zeng, Size-controlled synthesis of magnetite nanoparticles, J. Am. Chem. Soc. 124 (2002) 8204–8205. A.H. Lu, E.L. Salabas, F. Schüth, Magnetic nanoparticles: synthesis, protection, functionalization, and application, Angew. Chem. Int. Ed. 46 (2007) 1222–1244. J. Zhi, Y. Wang, Y. Lu, J. Ma, G. Luo, In situ preparation of magnetic chitosan/ Fe3O4 composite nanoparticles in tiny pools of water-in-oil microemulsion, React. Funct. Polym. 66 (2006) 1552–1558. J.A. Lopez Perez, M.A. Lopez Quintela, J. Mira, J. Rivas, S.W. Charles, Advances in the preparation of magnetic nanoparticles by the microemulsion method, J. Phys. Chem. 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Demello, Direct synthesis of dextran-coated superparamagnetic iron oxide nanoparticles in a capillary-based droplet reactor, J. Mater. Chem. 22 (2012) 4704–4708. T. Gu, C. Zheng, F. He, Y. Zhang, S.A. Khan, T.A. Hatton, Electrically controlled mass transport into microfluidic droplets from nanodroplet carriers with application in controlled nanoparticle flow synthesis, Lab Chip 18 (2018) 1330–1340. T. Gu, E.W.Q. Yeap, A. Somasundar, R. Chen, T.A. Hatton, S.A. Khan, Droplet microfluidics with a nanoemulsion continuous phase, Lab Chip 16 (2016) 2694–2700. A. Günther, S.A. Khan, M. Thalmann, F. Trachsel, K.F. Jensen, Transport and reaction in microscale segmented gas–liquid flow, Lab Chip 4 (2004) 278–286. G. Liu, H. Yang, J. Zhou, S.J. Law, Q. Jiang, G. Yang, Preparation of magnetic microspheres from water-in-oil emulsion stabilized by block copolymer dispersant, Biomacromolecules 6 (2005) 1280–1288. Z.H. Zhou, J. Wang, X. Liu, H.S.O. 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Contents lists available at ScienceDirect
Colloid and Interface Science Communications journal homepage: www.elsevier.com/locate/colcom
Rapid Communication
Continuous Flow Synthesis of Superparamagnetic Nanoparticles in Reverse Miniemulsion Systems
T
Tonghan Gua, Yunfei Zhanga, Saif A. Khanb, T. Alan Hattona,
⁎
a b
Department of Chemical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge 02139, MA, USA Department of Chemical and Bimolecular Engineering, National University of Singapore, 4 Engineering Drive 4 E5-02-28, 117576, Singapore
ARTICLE INFO
ABSTRACT
Keywords: Magnetic nanoparticle Multiphase flow reaction Miniemulsion Reverse micelle
Magnetic nanoparticles were synthesized continuously in a multiphase segmented flow reaction system where one phase was a miniemulsion containing iron precursor solution in nanodroplet confinement and the other phase was diluted ammonium hydroxide solution. Spontaneous formation of reverse micelles at the phase boundary effectively transported ammonium hydroxide into the nanodroplets to trigger the precipitation of magnetic nanoparticles. The nanoparticles possessed superparamagnetism with a mean diameter of about 10 nm. This flow system is potentially generalizable to other emulsion-based continuous nanoparticle synthesis applications.
Magnetic nanoparticles have been applied to a broad range of applications over the past two decades, including magnetic fluids [1], catalysis [2,3], magnetic resonance imaging contrast agents [4], hyperthermia therapy [5], drug delivery [6,7], and protein separations [8,9], to name a few. When magnetic nanoparticles are sufficiently small, such that there is only one magnetic domain in each particle [10], a permanent dipole cannot be formed, thus leading to the phenomenon of superparamagnetism [11,12]. High-quality magnetic nanoparticles can be synthesized by several chemical processes, such as co-precipitation, thermal decomposition, hydrothermal reactions, and microemulsion synthesis [13,14]. Among these methods, microemulsion systhesis uses well-defined nanodroplets as reaction environments, which is easy to carry out and restricts nanoparticle size thus favoring superparamagnetism [15]. The production yield can be improved significantly when miniemulsions, which are orders of magnitude more concentrated, are used instead of microemulsions, though at the expense of creating larger nanoparticles since miniemulsions have droplets of a few hundred nanometers. Recently, continuous flow chemistry has attracted significant research interest; micro- and milli-fluidic systems are advanced tools for continuous production of value-added products in small quantities, such as functional nanoparticles [16,17]. Magnetic nanoparticles have also been successfully synthesized continuously in a coaxially mixed single phase flow [18], gas-liquid flow [19], and liquid-liquid flow [20], thanks to the rapid mixing in microfluidic channels that greatly helps control the size of magnetic nanoparticles. Continuous synthesis
⁎
offers distinct advantages over the more conventional batch processes because they provide a better control over reaction conditions and minimize batch-to-batch variations. We recently introduced a novel droplet-based microfluidic technique in which the continuous phase used for droplet generation consisted of a stable nanoemulsion or miniemulsion phase; we also demonstrated its application in the flow synthesis of micro- and nanoparticles driven by controlled mass transport from nanodroplets in the continuous phase to the dispersed microdroplets [21,22]. In this short communication, we demonstrate how the reversed process, which transports materials from microdroplets to nanodroplets, enables superparamagnetic nanoparticle synthesis. To the best of our knowledge, this is the first report of continuous synthesis of superparamagnetic nanoparticles in reverse miniemulsion systems, and opens the door to facile and scalable continuous production methods for such materials. Fig. 1 shows a schematic illustration of our method. First, a FeCl2 (0.5 M) /FeCl3(1.0 M) /HCl (1.0 M, pH ≈ 0) mixture solution was homogenized into a miniemulsion with dodecane (containing 5 wt% Span 20 as surfactants) as the continuous phase and aqueous droplets of 20% volume fraction as the dispersed phase. The droplet size was reduced to 476 ± 30 nm using a Cole-Parmer Horn-Cup Cell Disruptor (at 40% max. power) to sonicate for 20 min. Then, this emulsion was pumped into a microfluidic T-junction (1.016 mm ID) where it met the NH4OH solution (5 wt%). Both phases flowed at 30 μL/min and a segmented flow with alternating NH4OH solution and miniemulsion segments formed at the junction. Although NH4OH solution was dispersed
Corresponding author. E-mail addresses: [email protected] (S.A. Khan), [email protected] (T.A. Hatton).
https://doi.org/10.1016/j.colcom.2018.10.005 Received 20 September 2018; Received in revised form 20 October 2018; Accepted 27 October 2018 2215-0382/ © 2018 Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Colloid and Interface Science Communications 28 (2019) 1–4
T. Gu et al.
Fig. 1. Illustration of the flow synthesis process. The photo shows the transport of ammonia from the aqueous phase to the organic phase to precipitate Fe2+/Fe3+ solutions in nanodroplets.
as microdroplets in miniemulsion due to tube wetting preferences, we use the term “segment” to denote both phases in the rest of this article to distinguish these macroscopic entities from the nanodroplets dispersed in the miniemulsion segment. Ammonia was quickly transported through the dodecane phase (and across the surfactant layer) into nanodroplets due to the sub-millimeter mass transfer distance, and triggered reactions inside each nanodroplet to form iron (II) hydroxide and iron (III) hydroxide precipitates. The NH4OH segment had almost no color change but the miniemulsion phase turned black. The flow of segments also created strong circulation in the miniemulsion segments for a better mixing to ensure a fast transport of ammonia [23]. Heating of this segmented flow at 50 °C for 5 min caused hydroxides to decompose into oxides to yield Fe3O4 nanoparticles that were suspended in the nanodroplets in the miniemulsion phase. The miniemulsion was collected and demulsified with ethanol and centrifuged to recover the nanoparticles, which were washed by ethanol three times. The particles were stored as a dispersion in deionized water, but can also be dried into a powder.
Fig. 2a and b shows the transmission electron microscope (TEM) characterization of the prepared nanoparticles. Due to aggregation, size measurement cannot rely on dynamic light scattering techniques. We randomly sampled 80 locations in Fig. 2a and measured the size of nearest non-overlapping particle of each location, and plotted a histogram of particle size distribution, as shown in Fig. 2c. It can be inferred directly from the dimensions of the magnetic nanoparticles that they are superparamagnetic [10], and this inference was confirmed with a SQUID (superconducting quantum interference device) test. Fig. 2d showed that the nanoparticles are superparamagnetic at room temperature, which is consistent with the overall small size of nanoparticles. The saturation magnetization (Ms) is about 30 emu/g, which is comparable to those reported previously [24,25] for emulsion-based magnetite nanoparticle preparation, but lower than those in other reports [14]. Future work should focus on tuning the recipe to achieve a higher Ms, which is known to be related to pH, surfactant, etc. [25,26], to increase the superparamagnetic properties. Fig. 2e shows the X-ray
Fig. 2. (a) and (b) TEM characterization of the magnetic nanoparticles, (c) Histogram of primary particle size distribution, (d) SQUID analysis of magnetic hysteresis with saturation magnetization (Ms) ≈ 30 emu/g, remnant magnetization (Mr) ≈ 0.6 emu/g (Ms/Mr = 50), and coercivity (Hc) ≈ 0.1 Oe, (e) XRD characterization of the magnetite nanoparticles. 2
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diffraction (XRD) characterization of the magnetic nanoparticles, which gives an averaged particle size of ~10.4 nm based on the Scherrer equation (d = Kλ/βcosθ, K ≈ 1, λ = 1.79 Å, β = 0.018, and θ = 0.314) [12]. This value is close to the average size (10.1 nm) calculated from the primary particle size histogram (Fig. 2c) obtained from TEM. In this reaction system, two aqueous phases and an inert dodecane oil phase are present at the same time, with one aqueous phase (Fe2+/ Fe3+) dispersed as nanodroplets in dodecane segments and the other aqueous phase (NH4OH) itself forms segments. As mentioned above, only the dodecane phase that contains aqueous nanodroplets turns black, while the NH4OH segments remain colorless. While our previous research has shown that materials can be transferred from the nanodroplets into the NH4OH segments under thermal, chemical, or electrical stimuli [21,22], it is still unclear how the reverse process, where material is transferred from the large NH4OH segments into nanodroplets, occurs without any stimuli. The highly polar molecule ammonia has almost no molecular solubility in dodecane (0.0021 g/g) [27] and thus a different mechanism other than molecular dissolutiondiffusion/convection must be responsible for the mass transport. We conducted a few extra experiments to gain insight into the underlying mechanism. The experimental setup is shown in Fig. 3 with four different conditions. Briefly, two vials containing full (Fig. 3a, b, and c) or half (Fig. 3d) level of aqueous NH4OH and FeCl2/FeCl3 solutions were submerged into an oil phase, which contains no surfactants (Fig. 3a) or 5 wt% Span 20 (Fig. 3b, c, and d). Heating time was controlled at 10 min (Fig. 3a, b, d) or 2 h (Fig. 3c). First, we discovered that adding Span 20 is essential for enabling the mass transport. To understand the role of surfactants in this process, we did a control experiment with dodecane free of Span 20. As shown in Fig. 3a, when the dodecane phase contains no surfactant, heating at 50 °C for 10 min did not change the color of the separated NH4OH and FeCl2/ FeCl3 solutions and the dodecane phase remained colorless, indicating negligible mass transport between these two solutions across the dodecane phase. However, when 5 wt% Span 20 was loaded into dodecane, the dodecane phase turned yellow in < 10 min, which indicates that FeCl2/FeCl3 had entered the dodecane phase (Fig. 3b). NH4OH, due to its colorless nature, cannot be easily detected. However, extended heating for 2 h generally turned the dodecane phase from yellow to grey, showing that NH4OH must have entered the dodecane phase and reacted with FeCl2/FeCl3 (Fig. 3c). A closer inspection of Fig. 3b and c showed large black particles formed at the interface between the FeCl2/FeCl3 solution and dodecane (with Span 20), which meant that NH4OH not only entered dodecane, but also diffused into the FeCl2/FeCl3 solution. There were far fewer particles formed in the NH4OH solution, showing that NH4OH is more active in being transported than FeCl2/FeCl3, leading to a net transport direction from the NH4OH solution to the FeCl2/FeCl3 solution that is consistent with the observation in the flow experiment (Fig. 1) that NH4OH remained almost colorless.
Second, it is unlikely that the improvement in polarity of dodecane by Span 20 significantly increases the molecular solubility of FeCl2/ FeCl3. Molecular solubility can be estimated based on the Hansen solubility parameters of the solutes. In general, two molecules of similar solubility parameters have similar solubilities in a single solvent, provided there are no additional molecular interactions besides dispersion force (δD), dipolar force (δP), and hydrogen bonding (δH). The solubility parameters of Span 20 are 17.1, 8.4, and 13.6 (δD, δP and δH respectively) [28]. Thus, a 5 wt% solution of Span 20 in dodecane (δD = 16.1) yields a solubility parameter of 16.15, 0.42, 0.68 through linear combination. Such a slight increase in polarity and hydrogen bonding forces is less likely to be enough to create a detectable yellow color from the dissolved FeCl2/FeCl3. As a comparison, no yellow color appears when we try to dissolve FeCl2/FeCl3 in a 9-to-1 mixture of dodecane and ethyl acetate, which gives an even slightly more polar environment with solubility parameters of 16.07, 0.53, 0.72 (δD, δP and δH respectively). Third, it is more likely that the apparent increase in ammonium hydroxide solubility is achieved via the formation of reverse micelles. As shown in Fig. 3d, when we created a static dodecane layer on top of NH4OH and FeCl2/FeCl3 solutions (negligible convection inside the vial), we can clearly observe the spontaneous formation of emulsions after 10 min of heating, which is probably caused by the coalescence of reverse micelles. NH4OH formed a thicker and more opaque emulsion layer than FeCl2/FeCl3, indicating that NH4OH is more active in forming reverse micelles, which as a result contributed to a stronger mass transport activity. With the evidence above, it is clear that reverse micelles formed by Span 20 are more likely to be the mass transfer carriers, and a schematic illustration of the mass transport process is shown in Fig. 4. Berg et al. thoroughly studied the reverse micelle formation capability of Span series in hydrocarbons [29] and concluded that Span 20 formed the largest reverse micelles of 4.2 ± 0.1 nm with deionized water based on small angle neutron scattering studies. This aligns well with our experimental observations that using Span 80 results in a much slower reaction process. The larger reverse micelles formed by NH4OH could be explained in terms of two factors - cation valency and pH. Reverse micelle size is inversely proportional to cation valency, which contributes to a larger micelle size for NH4OH than FeCl2/FeCl3 [30]. A higher pH has also been shown to reduce the hydrocarbon-water interfacial tension [31,32], which facilitates the spontaneous generation of more reverse micelles. Compared to batch synthesis, flow synthesis provides a high control over the reaction conditions, such as mixing and heat treatment. The high reproducibility of segments, such as the size and flow speed of each segment, ensures that mixing between different segments is consistent. Heat treatment is very efficient compared to batch processes due to a much smaller heat transfer distance. A thermal entrance length of only < 5 mm (Lentrance/Dtube ≈ 0.05ReDPr, ReD ≈ 10, Pr < 10) was
Fig. 3. Pictures of vials of FeCl2/FeCl3/HCl and NH4OH solution submerged in dodecane. Left vials contain FeCl2/FeCl3/HCl solution and right vials contain NH4OH solution. Process conditions: (a) 10-min heat treatment without surfactant, (b)10-min heat treatment with Span 20, (c) 2-h heat treatment with Span 20, (d) 10-min heat treatment with Span with a static dodecane layer on top of each solution. 3
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[5] [6] [7] [8] [9] [10] [11]
Fig. 4. A schematic illustration of micelle-assisted mass transport.
[12] [13]
achieved, which means that the emulsion reached thermal equilibrium with the surrounding oil bath in < 5 s, and ensured that the fluid was in a constant temperature environment for most of the heating duration. Another benefit is that the oxidation of the unstable Fe2+ salt and hydroxide is significantly mitigated, due to the intrinsic oxygen-free environment inside the FEP plastic tubing filled with liquids. The common clogging issue of solid-generating reactions in small channels was not observed in our experiment due to the small size of magnetic nanoparticles. In summary, we reported a novel multiphase segmented flow system for the continuous synthesis of superparamagnetic nanoparticles with a mean size of about 10 nm. The flow reactor offers good temperature and mixing control, as well as an oxygen-free environment. A mechanistic study shows that the ammonia transport from the large aqueous segments to nanodroplets containing iron precursors is likely achieved through reverse micelles as material carriers, and the mass transfer direction is controlled by the difference in the rates of micelle formation and the sizes of micelles generated. This technique could potentially be a continuous synthesis solution to other nanoparticles prepared in batch emulsion systems.
[14] [15] [16] [17] [18] [19] [20]
[21] [22] [23]
Declaration of Interest
[24]
None.
[25]
Acknowledgement
[26]
The authors thank Center for Material Science and Engineering (CMSE) of Massachusetts Institute of Technology for providing transmission electron microscope access and Dr. Paul Brown for conducting magnetic property and Mr. Kai-Jher Tan for conducting X-ray diffraction measurements.
[27] [28] [29]
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