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Nanoparticle Precipitation in Reverse Microemulsions: Particle Formation Dynamics and Tailoring of Particle Size Distributions Bjo¨rn Niemann,† Peter Veit,‡ and Kai Sundmacher*,†,§ Max Planck Institute for Dynamics of Complex Technical Systems, Sandtorstr. 1, D-39106 Magdeburg, Germany, and Institute for Experimental Physics, and Process Systems Engineering, Otto-Von-Guericke UniVersity Magdeburg, UniVersita¨tsplatz 2, D-39106 Magdeburg, Germany ReceiVed NoVember 17, 2007. In Final Form: January 10, 2008 In this work, a detailed experimental analysis of the nanoparticle formation dynamics and the formation mechanism in a reverse microemulsion system is given. The precipitation of barium sulfate nanoparticles inside microemulsion droplets is investigated at the molecular scale with respect to the evolution of the particle size distribution and the particle morphology by an extensive transmission electron microscope (TEM) analysis. Different mixing procedures (feeding strategies) of two reactants, barium chloride and potassium sulfate, are evaluated concerning their ability for a tailored particle design under consideration of the complete particle size distribution (modality and polydispersity). It is shown that improved knowledge about the particle formation mechanisms, the dynamics, and the influence of the colloidal microemulsion structure could be used for a tailored design of particles,for example, controlled synthesis of nanoparticles with a bimodal particle size distribution by the application of a sophisticated feeding strategy.
1. Introduction The controlled and reproducible production of nanoparticles with tailored properties such as defined size, shape, morphology, or size distribution is one of the major issues in current research activities in nanotechnology. Exact control of these properties is an important task in the design of advanced materials due to the size dependent physical and chemical properties of nanoparticles which can be significantly different from the corresponding bulk materials. Numerous examples of outstanding nanoparticle properties are known and can be,for example, found in refs 1-3. Many different processes, such as chemical vapor deposition, etching, milling, biomineralization, and so forth, are currently under investigation for the production of nanoparticles.4 * To whom correspondence should be addressed. Telephone: +49 391 6110 350. Fax: +49 391 6110 353. E-mail:
[email protected]. † Max Planck Institute for Dynamics of Complex Technical Systems. ‡ Institute for Experimental Physics, Otto-von-Guericke University Magdeburg. § Process Systems Engineering, Otto-von-Guericke University Magdeburg. (1) Hosokawa, M.; Nogi, K.; Naito, M. Nanoparticle Technology Handbook; Elsevier Science: Oxford, 2007. (2) Rotello, V. M. Nanoparticles: Building blocks for nanotechnology; Springer: New York, 2004. (3) Schmid, G. Nanoparticles. From Theory to Application; Wiley-VCH: Weinheim, 2003. (4) Luther, W. Industrial application of nanomaterials - chances and risks. Technological analysis; Future Technologies Division of VDI Technologiezentrum GmbH: Du¨sseldorf, 2004; Vol. 54. (5) Zhang, W. Z.; Qiao, X. L.; Chen, J. G. Colloids Surf., A 2007, 299, 22-28. (6) Sugimoto, T. J. Colloid Interface Sci. 2007, 309, 106-118. (7) Pileni, M. P.; Motte, L.; Billoudet, F.; Petit, C. Surf. ReV. Lett. 1996, 3, 1215-1218. (8) Chen, W. P.; Zhu, Q. Mater. Lett. 2007, 61, 3378-3380. (9) Xu, P.; Han, X. J.; Wang, M. J. J. Phys. Chem. C 2007, 111, 5866-5870. (10) Rauscher, F.; Veit, P.; Sundmacher, K. Colloids Surf., A 2005, 254, 183191. (11) Nagy, J. B. Colloids Surf. 1989, 35, 201-220. (12) Boutonnet, M.; Kizling, J.; Stenius, P. Colloids Surf. 1982, 5, 209-225. (13) Debuigne, F.; Cuisenaire, J.; Jeunieau, L.; Masereel, B.; Nagy, J. B. J. Colloid Interface Sci. 2001, 243, 90-101. (14) Lade, M.; Mays, H.; Schmidt, J.; Willumeit, R.; Schomacker, R. Colloids Surf., A 2000, 163, 3-15. (15) Norakankorn, C.; Pan, Q. M.; Rempel, G. L.; Kiatkamjornwong, S. Macromol. Rapid Commun. 2007, 28, 1029-1033. (16) Hirai, T.; Sato, H.; Komasawa, I. Ind. Eng. Chem. Res. 1993, 32, 30143019.
The microemulsion precipitation process used in this work is a laboratory scale and well-established production method, which is known for its ability to synthesize narrowly distributed nanoparticles with a defined size. Examples of nanoparticles produced by this method can be found in Table 1 (refs 5-16) together with the obtained mean particle diameters. The main benefit of this technique results from the nanosized droplets (reverse micelles, disperse water droplets in the continuous oil phase), which limit the possible amount of reactants to be stored inside one droplet. Thus, always well-defined conditions for nucleation and growth are provided in the system. Additionally, adsorbed molecules from the surfactant shells of the droplets can act as growth directors to form complex particle morphologies when the particle dimension reaches the boundaries of the nanoscale cavity.17-19 Further, the surfactant shell also protects the nanoparticles against agglomeration to larger particles or breakage into smaller particles. Different process modes can be used for the mixing of the reactants and thus the initialization of the reaction. These are sufficiently discussed in detail by several authors.20-22 In this work, we focus on a semi-batch, exchange-based mode, where the two reactants (dissolved salts) are stored inside two separated, equally composed microemulsions. One microemulsion is placed inside the reactor, and the other is used as a feed solution which is added to the reactor by a certain feeding procedure (see cases A-C, left side of Figure 1 and section 3.1). As a model reaction, we used the precipitation of barium sulfate according to
K2SO4 + BaCl2 f 2KCl + BaSO4V
(1)
Particle formation in the applied exchange-based mode is initialized by fusion and fission of droplets (droplet exchange) (17) Heywood, B. R.; Mann, S. AdV. Mater. 1994, 6, 9-20. (18) Mann, S.; Ozin, G. A. Nature 1996, 382, 313-318. (19) Mann, S. Nature 1993, 365, 499-505. (20) Bandyopadhyaya, R.; Kumar, R.; Gandhi, K. S. Langmuir 2001, 17, 10151029. (21) Bandyopadhyaya, R.; Kumar, R.; Gandhi, K. S.; Ramkrishna, D. Langmuir 1997, 13, 3610-3620. (22) Ethayaraja, M.; Dutta, K.; Bandyopadhyaya, R. J. Phys. Chem. B 2006, 110, 16471-16481.
10.1021/la703566v CCC: $40.75 © 2008 American Chemical Society Published on Web 02/29/2008
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Table 1. Examples of Nanoparticles Synthesized in Microemulsions material Ag AgCl Ag2S Ba0.7Sr0.3TiO3 BaFe12O19 CaCO3 Co2B, Ni2B, NiCoB Ir, Pd, Pt, Rh Nimesulide Pd PMMA TiO2
particle size [nm] 1-10 10 1.5-10 60-100 (diameter) 450-1200 (length) 30 1-7 (small particles) 2000-8000 (filaments) 2-7 3-5 4.6-6 4-7 10-80 ∼3
ref Zhang et al.5 Sugimoto6 Pileni et al.7 Chen and Zhu8 Xu et al.9 Rauscher et al.10 11
Nagy Boutonnet et al.12 Debuigne et al.13 Lade et al.14 Norakankorn et al.15 Hirai et al.16
initially present in the reactor and the feed. A scheme of the particle formation mechanism is given in Figure 1. The two dissolved salts inside the droplets are mixed by droplet exchange. If a critical amount for the formation of a stable nucleus is present inside one droplet, nucleation of primary particles occurs with a certain rate and these particles grow by further exchanges of ions from other droplets to bigger particles. The product barium sulfate has an industrial relevance due to its whiteness, inertness, and opaqueness to X-rays. It is mainly used as a radiocontrast agent, filler in plastics, extender in paints, and additive in pharmaceutical products and in printing ink. In Table 2, an overview of important publications about barium sulfate precipitation inside microemulsion systems is given (refs 28-34). The second column contains the microemulsion system in the order surfactant/oil/water and the two reactants. The third column contains information about the size range of the precipitated particles and the observed morphology. Investigated control parameters for the size and shape of the particles are listed in the fourth column. Droplet sizes are shown in the fifth column, and the last column contains a short description of the assumed precipitation mechanism which is in all cases related to the droplet exchange. This overview shows that the microemulsion technique offers a lot of possibilities for the controlled synthesis of tailored nanoparticles. The obtained particle sizes are in the range of 2 nm to 100 µm. Different particle shapes, such as spherical, cubic, filaments, and platelike particles with a rectangular, rhombic, or hexagonal main crystal face, were observed. Possible control variables such as the choice of microemulsion system, the choice of stabilizing substances, the different applied concentrations, and the temperature were identified. Additionally, interesting progress has been achieved by the group of Koetz23-25 concerning the stabilization of nanoparticles for redispersion purposes, which appeared to be a main disadvantage of this technique besides the low yield of nanoparticles in relation to the reactor volume. They showed that different polymers can be used for stabilization and that the particle size does not change when being redispersed in a new medium. Nevertheless, all works shown in Table 2 do not investigate the engineering aspects of this technique. In all cases, the two microemulsions were rapidly mixed without any sophisticated feeding strategy, the dynamics of particle formation were not analyzed, and the used components were too expensive for a technical realization of this process. The work presented here is (23) Koetz, J.; Bahnemann, J.; Lucas, G.; Tiersch, B.; Kosmella, S. Colloids Surf., A 2004, 250, 423-430. (24) Note, C.; Kosmella, S.; Koetz, J. Colloids Surf., A 2006, 290, 150-156. (25) Note, C.; Ruffin, J.; Tiersch, B.; Koetz, J. J. Dispersion Sci. Technol. 2007, 28, 155-164.
an extension of the study by Adityawarman et al.26 These authors were the first who investigated engineering aspects of this technique. They used a cheap technical surfactant and investigated different control parameters concerning their effect on the final particle size distribution. The most important result was the identification of the initial concentration difference
∆c0 ) |creactor (t ) 0) - cfeed (t ) 0)|
(2)
as an important control parameter for the final particle size and particle morphology. Very often, the ion concentration ratio
R)
cfeed (t ) 0) creactor (t ) 0)
(3)
is also used instead of ∆c0, but both values are easily transferable into each other. Figure 2 shows the final mean particle diameter calculated as the equivalent sphere diameter from the particle volume obtained by TEM analysis (recalculation of the experimental data of Adityawarman et al.26). The error bars symbolize the standard deviation of all counted particles sizes from the mean particle size. In set 1, the concentration of K2SO4 in the reactor was kept constant at 0.1 mol/L, and in set 2 the concentration of BaCl2 in the feed droplets was kept constant at 0.1 mol/L. It can be seen that the resulting final particle size distributions are very narrow and that particle size control is possible by ∆c0 above a value of 0.075 mol/L. Below this value, no significant influence on the final mean particle diameter (∼6 nm) was observed. Comparable results for this control parameter were obtained by Tojo et al.27 for the precipitation of CdS. In the present work, several open questions are addressed, which were not investigated in the work of Adityawarman et al.26 These are the experimental investigation of the particle formation dynamics at the molecular scale with respect to the evolution of the particle size distribution and the particle morphology as well as the utilization of the feed for controlling the final particle size distributions. These results will lead to a better understanding of the particle formation mechanism at the nanoscale and an improved knowledge about the possibilities to design particle size distributions by the application of sophisticated feeding strategies. 2. Experimental Setup and Materials The experiments were performed in a semi-batch operated standard stirred tank reactor. The important dimensions of the reactor and the constant operation parameters are shown in Figure 1 and Table 3. These dimensions are almost similar to the conditions applied by Adityawarman et al.26 Only the stirrer speed is higher in our experiments, because a higher liquid volume has been used and the increased stirrer speed guarantees ideally mixed conditions inside the reactor (proved by computational fluid dynamics studies; see O ¨ ncu¨l et al.35). The stirrer was a six-blade Rushton turbine positioned (26) Adityawarman, D.; Voigt, A.; Veit, P.; Sundmacher, K. Chem. Eng. Sci. 2005, 60, 3373-3381. (27) Tojo, C.; Blanco, M. C.; Lopez-Quintela, M. A. Langmuir 1998, 14, 6835-6839. (28) Qi, L. M.; Ma, J. M.; Cheng, H. M.; Zhao, Z. G. Colloids Surf., A 1996, 108, 117-126. (29) Ivanova, N. I.; Rudelev, D. S.; Summ, B. D.; Chalykh, A. A. Colloid J. 2001, 63, 714-717. (30) Hopwood, J. D.; Mann, S. Chem. Mater. 1997, 9, 1819-1828. (31) Li, M.; Mann, S. Langmuir 2000, 16, 7088-7094. (32) Koetz, J.; Andres, S.; Kosmella, S.; Tiersch, B. Compos. Interfaces 2006, 13, 461-475. (33) Iida, S.; Shoji, T.; Obatake, N.; Sato, H.; Ohgaki, K. J. Chem. Eng. Jpn. 2005, 38, 357-359. (34) Summers, M.; Eastoe, J.; Davis, S. Langmuir 2002, 18, 5023-5026. (35) O ¨ ncu¨l, A.; Niemann, B.; Sundmacher, K.; The´venin, D. Chem. Eng. J., published online July 25, http://dx.doi.org/10.1016/j.cej.2007.07.073.
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Figure 1. Scheme of the investigated microemulsion precipitation process, the involved population dynamic mechanisms of this system, and the different applied feeding strategies. The shown parameters are given and explained in Table 3 and in the Notation section. The reactants barium chloride (supplier, Merck; purity, g99%) and potassium sulfate (supplier, AppliChem; purity, g99.5%) were added to water before mixing of the components of the microemulsion. The influence of the ions on the phase behavior of the microemulsion was already investigated by Adityawarman et al.,26 and a stable one phase region has been chosen in the experiments for all possible salt concentrations. The phase behavior of this microemulsion system was investigated in detail by Rauscher et al.10 and Adityawarman et al.26
3. Experimental Results Figure 2. Final mean particle diameter (equivalent sphere diameter) with corresponding standard deviation as a function of the initial concentration ∆c0 defined by eq 2. in the middle of the initial height of the reactor content. Four symmetrically adjusted baffles were used to improve the mixing. The feed position was directly below the surface of the initial reactor content and in the middle between the stirrer axis and the reactor wall. The inner diameter of the feed pipe was 3.45 mm, and the outer diameter was 6 mm. The temperature inside the reactor was adjusted with thermostated water inside the double jacket. The same microemulsion system as that of Adityawarman et al.26 has been used in this work. The dispersed water phase was deionized water, the continuous oil phase was cyclohexane (supplier, ROTH; purity, g99.5%), and the non-ionic, technical surfactant was Marlipal O13/40 (an alkyl polyethyleneglycol ether; supplier, Sasol; purity, 98%). The applied composition is represented by the oil mass fraction36 R)
moil ) 0.96 moil + mw
(4)
and the surfactant mass fraction36 γ)
msf ) 0.15 msf + moil + mw
(5)
With this composition and a temperature of 25 °C, a mean droplet size of approximately 5 nm was determined by dynamic light scattering (DLS) measurements (Horiba LB-500). (36) Handbook of microemulsion science and technology; Kumar, P., Mittal, K. L., Eds.; Marcel Dekker Inc.: New York, 1999.
3.1. Performed Experiments. The performed experiments can be divided into two categories: the standard experiments where the particle formation dynamics have been investigated in detail with respect to the ∆c0 value (section 3.2; see case A in Figure 1) and the experiments where the influence of the feeding strategy on the final product has been analyzed (section 3.3). Two different feeding strategies have been investigated: the application of a smaller feed rate at different ∆c0-values (see case B in Figure 1) and the application of a pulsed feed with different concentrations of the pulses (see case C in Figure 1). Table 4 provides an overview of the performed experiments and all important operating parameters. (See the Supporting Infromation for information concerning the particle analysis procedure via TEM and statistical values.) 3.2. Particle Formation Dynamics. Figure 3 illustrates the particle formation dynamics of experiments E1-E3. The figures on the left-hand side depict the time evolution of the mean particle diameter along with its standard deviation. The gray rectangles indicate the applied feeding policy. The figures on the righthand side show the corresponding final particle size distributions. Exemplarily, one TEM picture is shown for each final particle size distribution in Figure 4. Different particle formation dynamics can be identified for each experiment. In E1, particles reach their final mean diameter already during the feeding period, thus indicating that this value is a limiting size for these conditions which cannot be increased. Further addition of reactant via the feed leads to the formation of new nuclei which grow very fast to the size of the limiting particle size of approximately 6 nm. The same behavior was also observed for an experiment at ∆c0 ) 0.05 mol/L (the final particle size distribution is shown in Figure 6 E4).
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Table 2. Literature Survey on Barium Sulfate Precipitation in Microemulsion Systems ref
reaction system
Size/ shape control
particle size
droplet size (nm)
Qi et al., 199628
Triton X-100 & n-hexanol/ cyclohexane/water, Ba(OAc)2/ (NH4)2SO4
5-9 nm spheres, 8-16 nm cubic
w) [water]/[surf], ∆c0
10-20
Ivanova et al., 200129 Hopwood and Mann, 199730
like Qi et al. 1996 with BaCl2 instead of Ba(OAc)2 AOT/isooctane/water, Ba(AOT)2/Na2SO4
Main fraction: 5-12 nm, polydisperse: dp,max ≈ 100 nm 2-5 nm spheres, 1-100 mm filaments (20-50 nm width) 5-7 nm oval, 50-400 nm tabular, 10-60 nm hexagonal, rectangle, rhombic ∼200 × 150 × 60 nm rhombic up to 20 µm filaments (50-200 nm width), cones, 60-300 nm spindle-shaped aggregates avg size: 11.8-58.2 nm, main fraction: 4.4-6.9 nm (73 - 88%) avg size: 11.7-933 nm
not investigated csurf, w, S
7.5-11
C12EO4/isooctane/water, BaCl2/ Na2SO4
Li and Mann, 200031 Koetz et al., 200423 Koetz et al., 200632 Note et al., 200725 Iida et al., 200533
Summers et al., 200234
DDAB/isooctane/water, BaCl2/ Na2SO4 AOT/isooctane/water, Ba(AOT)2/ Na2SO4 polyelectrolyte (PDADMAC) modified SB/heptanol/water, BaCl2/ Na2SO4 polymer (PEG) modified SDS/pentanol & xylene/water, BaCl2/ Na2SO4 polyampholyte modified SDS/pentanol & toluene/water, BaCl2/ Na2SO4 AOT/isooctane/water, Ba(AOT)2/Na2SO4
partially polymerized surfactants/ethyl nonanoate (cyclohexane)/water, BaCl2/ Na2SO4
avg size: 2.0-3.9 nm nanofilaments µm-sized, short rags < 200 nm, thick spirals < 1 mm, fine particles < 50 nm, whiskerlike rods > 1.5 µm 2.7-35 nm spheres, cylinders: 52.8-66.7 nm (length) and 28.8-36.1 nm (width), partially cubic, hexagonal
Table 3. Reactor Dimensions and Constant Operating Parameters d1 d2 D baffles stirrer height stirrer blades dd h (t ) 0) T Vfeed Vreactor R γ ω
reactor dimensions 30 mm 15 mm 100 mm 4, width: 10 mm h (t ) 0)/2 6
constant operating parameters 5 nm ∼55 mm 25 °C 350 mL 350 mL 0.96 0.15 800 rpm
A different situation appears in E2 and E3, where the steadystates were reached 20 min and almost 1.5 h after the end of the semi-batch reactor operation. In these cases, a different particle formation mechanism led to an increase of the particle size and a change of the particle morphology. The resulting mean particle diameter in E2 is approximately 17 nm, and the particles are plates with a quadratic or rectangular main crystal face. In E3, the mean diameter is approximately 37 nm, and the particles are also plates with a hexagonal main crystal face. Both final particle size distributions have a low polydispersity, but during processing also very broad particle size distributions were observed similar
exchange-based with surfactant adsorption for directional growth exchange-based
W, S
not investigated T, ∆c0
PDADMAC concentration
exchange-based exchange-based with surfactant adsorption for directional growth 20-60
exchange-based with polyelectrolyte adsorption for stabilization exchange-based with polymer adsorption for stabilization exchange-based with polyampholyte adsorption for stabilization exchange-based with surfactant adsorption on specific crystal faces w anisotropic growth
1.4-2.8
exchange-based with templating effect of the partially polymerized surfactant shells
microemulsion composition choice and amount of polyampholyte ∆c0
influence of cyclohexane on morphology
precipitation mechanism exchange-based, assumption: cubic shape due to surfactant and/or free/bound water influence exchange-based
to the bimodal distribution shown in Figure 5 for ∆c0 ) 0.095 mol/L 30 min after starting the experiment. 3.3. Application of Different Feeding Strategies. In experiments E4 and E5, a lower feed rate is applied (see case B in Figure 1). The feeding period is extended from ∼4 min to ∼6 h, and thus, the feed rate is reduced from 80 to 1 mL/min. In Figure 6, the resulting particle size distributions (gray) are compared with the particle size distribution with the standard feed rate applied in the experiments shown in section 3.2 (dark gray). No influence of the feed rate is observed for ∆c0 ) 0.05 mol/L (E4), but a remarkable influence is observed for ∆c0 ) 0.09 mol/L (E5). In the latter case, the particle size and polydispersity are increased significantly. While the particles obtained with the standard feed rate are almost monodisperse with a quadratic or rectangular main crystal face (see Figure 4 E2), the particles synthesized with the lower feed rate show different shapes such as rhombic, quadratic, rectangular, and hexagonal main faces of the plates (see Figure 7 E5). In experiments E6 and E7, two feed solutions with different reactant concentrations are added separately in a certain sequence (see scheme for case C in Figure 1). Half of the feed volume with the concentration cfeed,1 is added with the standard feed rate (80 mL/min) at the beginning, and the second half of the feed volume with the concentration cfeed,2 at the same rate is added after a break which is long enough to ensure complete consumption of the reactant added by the first pulse (see Table 4 for the applied times). For E6, the concentration of the first feed solution cfeed,1 was higher than that for the second feed solution cfeed,2. In E7,
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Figure 3. Dynamics of the mean particle diameter with standard deviation (left) and final particle size distributions (right) for (from top to bottom) ∆c0 ) 0 mol/L (E1), ∆c0 ) 0.09 mol/L (E2), and ∆c0 ) 0.095 mol/L (E3). Table 4. Performed Experiments (cK2SO4 ) creactor, cBaCl2 ) cfeed) exp
case
E1 E2 E3 E4 E5 E6 E7
A A A B B C C
cfeed [mol/L]
V˙ feed [mL/min]
creactor [mol/L]
cfeed,1
80 80 80 1 1 80 80
0.1 0.1 0.1 0.1 0.1 0.1 0.1
0.1 0.01 0.005 0.05 0.01 0.1 0.01
the order of the feed solutions has been changed to cfeed,1 < cfeed,2. Figure 8 shows the final particle size distributions and a representative TEM picture of the particles for each experiment. The particle size distribution of E6 has a low polydispersity, and the mean particle size is approximately 6 nm. No effect of the feeding strategy can be observed for this experiment. A possible explanation can be given by the ∆c0-values at the beginning of both feeding periods if complete consumption of the feed reactant after the first pulse is assumed. For the first pulse, ∆c0,1 equals 0.0 mol/L, and for the second pulse ∆c0,2 equals 0.023 mol/L. The results shown in Figure 2 indicate that these ∆c0-values generally lead to the observed particle size of
cfeed,2
∆c0 [mol/L] 0.0 0.09 0.095 0.05 0.01
0.01 0.1
tfeed [s] t1
t2
t3
262.5 262.5 262.5 21 000 21 000 131 131
1331 3731
1462.5 3862.5
approximately 6 nm and thus an influence of this feeding strategy is improbable. In contrast to E6, a significant influence of the feeding strategy can be seen for E7. The resulting particle size distribution is bimodal, and two different particle shapes can be identified on the TEM picture. The two peaks of the particle size distribution have a low polydispersity when being regarded separately. Particles with a size between 9 and 15 nm were not observed at all in the steady-state. The first peak covers a particle subpopulation with a mean particle size of approximately 6 nm and a spherical particle shape, while the second peak covers a particle subpopulation with a mean particle size of approximately
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Figure 5. Particle size distribution obtained after 1780 s for ∆c0 ) 0.095 mol/L (E3).
Figure 6. Comparison of the final particle size distributions obtained with two different feed rates for ∆c0 ) 0.05 mol/L (E4, top) and ∆c0 ) 0.09 mol/L (E5, bottom).
Figure 4. Selected TEM pictures of BaSO4 nanoparticles at final conditions for (from top to bottom) ∆c0 ) 0 mol/L (E1), ∆c0 ) 0.09 mol/L (E2), and ∆c0 ) 0.095 mol/L (E3).
comparison with the results shown in Figure 2 leads to the conclusion that the larger particles of the second subpopulation result from the big concentration difference at the beginning and the small particles are synthesized by the second pulse with the low ∆c0 value.
21 nm and a quadratic/ rectangular main crystal face of the platelike particles. Note that both subpopulations have been normalized individually with the total number of particles for better visualization of the results. The total number of particles of the first subpopulation is much higher compared to the larger particles of the second subpopulation. It is a surprising result that only a change of the feed orders results in such a significant influence on the particle size distribution. A possible explanation for this can be again given by the two ∆c0-values for the two pulses. The concentration difference at the beginning is ∆c0,1 ) 0.09 mol/L, and at the beginning of the second feeding period it is ∆c0,2 ) 0.037 mol/L, if complete consumption of the feed reactant is assumed. The
The presented results for the dynamics of barium sulfate nanoparticle synthesis make an analysis of the particle formation mechanism at the nanoscale possible. Depending on the applied conditions, generally three different particle shapes have been observed: spherical particles, platelike particles with a main quadratic/rectangular face (rectangular rhombohedra), and platelike particles with a main hexagonal crystal face (polyhedra). Figure 9 illustrates schematically the formation and the transition of the observed particle shapes as a function of the mean particle diameter and the mean particle thickness. In experiments with ∆c0 e 0.075 mol/L (e.g., E1), only spherical particles were observed with a size of approximately 6 nm, which is close to
4. Discussion
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Figure 7. TEM picture with BaSO4 nanoparticles obtained with the low feed rate at ∆c0 ) 0.09 mol/L (E5).
the size of an empty droplet (5 nm). Presumably, droplets are slightly expanded with the appearance of a particle inside while the water content remains constant. Experimental conditions with higher ∆c0-values (E2 and E3) led to the formation of platelike crystals with quadratic/rectangular or hexagonal main faces which are much larger compared to the spherical particles. An interesting linear correlation can be identified for the particle thickness as a function of the mean particle size in Figure 9 as well (see the trend line). It is given by
hhP ) 0.34‚dhP + 2.9 nm
(6)
Different mechanisms can be responsible for such a particle formation behavior. The first stage of particle formation (indicated by I in Figure 10) seems to be strongly influenced by the droplet size and the surfactant monolayer. In this stage, the final particle size of approximately 6 nm is independent of the application of a feeding strategy and changes of ∆c0. The particle formation proceeds very fast like it is expected for ionic precipitation reactions (see Figure 3 E1 (top)), but growth of the particles is almost stopped when the particles reach the limiting mean diameter of approximately 6 nm. It can be assumed that particle growth in this stage is comparable to the fast bulk-phase mechanism, only governed and slowed down by the droplet exchange. However, when the particle size approaches the dimension of the slightly expanded droplet, growth is drastically slowed down. This stabilization of the nanoparticles can happen by the adsorption of surfactant molecules on the particle surface or by the complete loss of free water domains. In the latter case, all water molecules inside the droplet are bound to surfactant molecules or to the particle surface, consequently losing the typical properties of free water.28,37 In both cases, the exchange behavior of the droplets is drastically changed and dissolved reactants for further growth will not be supplied anymore. Although definite identification of the responsible mechanism for this growth behavior is difficult, it is unquestionable that a barrier for particle growth exists and that nucleation of new particles is more probable than further growth of particles being close to the barrier size. Supporting facts for the here discussed stabilization effect of the nanostructured microemulsion on particles having a size similar to the droplet size are as follows: (a) The observed dynamics in Figure 3 E1 (top) clearly show that feeding of reactants results in nucleation and not in particle growth. (b) Almost similar final particle size distributions were obtained (39) Osseoasare, K.; Arriagada, F. J. Colloids Surf. 1990, 50, 321-339.
at significantly different supersaturation levels during feeding (see Figure 6 (E4, left)). (c) Two coexisting particle populations of different mean sizes were observed in experiment E7. The formation of larger particles with different shapes is only possible by crossing the above identified barrier and by subsequent destabilization of the particle/surfactant aggregates. This happens if high reactant concentration differences between the droplets in the reactor and in the feed are applied (∆c0 > 0.075 mol/L, indicated by III in Figure 10). In such a case, one reactant is in excess. Destabilization at such conditions could be caused by the known effect that an increase of electrolyte concentration decreases the solubility of surfactants in water.38 This means in our case that the excess electrolyte destabilizes the oil-water interface (surfactant shell), whereby enabling the formation of larger particles. A realistic mechanism of particle formation has to be in accordance with the appearance of a bimodal distribution during the time evolution (see Figure 5). Ostwald ripening can be excluded due to the coexistence of two particle populations with a different mean size in the final state of experiment E7. Coagulation seems to be the most probable mechanism for the formation of larger particles if the surfactant shells are destabilized, although no agglomerates were observed in the TEM pictures. The time evolutions of the particle size distributions in our experiments show a typical behavior of populations where coagulation occurs. The first peak of the bimodal distribution remains at its particle size while its height is continuously decreased, thus indicating that the small particles with a size of approximately 6 nm are coagulating among each other to form larger particles and that they are integrated in already existing larger particles. The amount of small particles is depleted (height of the peak decreases) by a direct consumption of the complete particle. Dissolution of the small particles which would be necessary for Ostwald ripening is not observed. During this second nanoparticle formation stage, the morphology of the particles changes drastically. The formation of the platelike shaped particles can be a consequence of, for example, a surfactant directed growth mechanism as proposed by Mann and Ozin,18 crystal face specific adsorption effects (see Kubota et al.39,40), or the existence of different growth velocities for each crystal face.41 In the first case, the colloidal surfactant structure acts as a template for the particle, and in the second case preferential surfactant molecule adsorption on specific crystal faces leads to an individual influence of the growth velocities of the crystal faces. Consequently, all three mechanisms lead to the same effect of morphology changes due to face-specific growth laws, although the proposed microkinetic mechanisms are in all cases significantly different. A small concentration range around ∆c0 ) 0.075 mol/L represents a transient stage (indicated by II in Figure 10). It is introduced because an exact identification of the barrier cannot be presented and also the results by Adityawarman et al.26 showed slightly bigger particles with a quadratic/rectangular main face already for ∆c0 ) 0.075 mol/L. Besides the analysis of the particle formation dynamics, the ability of the microemulsion process for a tailored particle design was studied with regard to the feeding strategy. The final results of experiments E4 and E5 (see set 3 depicted in Figure 10) show that the feed rate has a significant influence on the product if the applied conditions are chosen to be above the identified ∆c0 (40) Kahlweit, M.; Lessner, E.; Strey, R. J. Phys. Chem. 1984, 88, 19371944. (41) Kubota, N. Cryst. Res. Technol. 2001, 36, 749-769. (42) Kubota, N.; Mullin, J. W. J. Cryst. Growth 1995, 152, 203-208. (43) Zhang, Y. C.; Doherty, M. F. AIChE J. 2004, 50, 2101-2112.
Nanoparticle Formation in ReVerse Microemulsions
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Figure 8. Final particle size distribution for two pulsed feeding experiments and corresponding TEM pictures. Top (E6): creactor ) 0.1 mol/L, cfeed,1 ) 0.1 mol/L, and cfeed,2 ) 0.01 mol/L. Bottom (E7): creactor ) 0.1 mol/L, cfeed,1 ) 0.01 mol/L, and cfeed,2 ) 0.1 mol/L.
Figure 9. Particle shape dynamics represented by the mean particle thickness as a function of the mean particle diameter.
barrier. For low ∆c0-values below 0.075 mol/L, it is not possible to transcend a mean particle diameter of 6 nm with a smaller feed rate due to the strong protection of the particles by the surfactant monolayer. However, for high ∆c0-values above 0.075 mol/L, small feed rates drastically change the final particle size distribution by an increase of the polydispersity and mean particle size. Such a behavior can be explained by different growth histories of the particles. Particles which were formed early during the feeding period passed the particle size barrier earlier and grew to large platelike crystals with a main hexagonal face, while particles which were formed later during the long feeding period could only attain the quadratic/rectangular platelike shape due to the consumption of all reactants at the end of the reactor processing. The second investigated possibility to tailor particle size distributions in microemulsions was the application of a pulsed feed with different reactant concentrations (E6 and E7). By the
Figure 10. Identification of the different possible particle size distribution control regimes by the initial concentration difference.
use of Figure 10, together with the individual calculations of the ∆c0-values for each pulse and the knowledge about the influence of the nanostructured microemulsion system (i.e., stabilization by surfactant adsorption), it is possible to synthesize particles with a defined particle size distribution within the limits of the observed morphology effects. The results of E6 and E7 reveal that it is easily possible to obtain bimodal distributions if the desired larger particles are synthesized with the first pulse and the small particles with the second pulse. Generalization of this observation is not possible, because an experiment with appropriate conditions (∆c0-values) where the smaller particles are primarily formed has not been performed. Nevertheless, primary synthesis of the large particles is reasonable due to the surfactant protection. Consequently, the influence of the second pulse on this already existing particle population is very low and the small particles can be obtained almost unaffected by the large particles. The large particles of the first subpopulation in E7 increase their
4328 Langmuir, Vol. 24, No. 8, 2008
mean diameter only by 2 nm after the second pulse of reactant molecules was added to the reactor.
Niemann et al. tipping stage examinations showing the platelike morphology of BaSO4 nanoparticles. This material is available free of charge via the Internet at http://pubs.acs.org.
5. Conclusions In this work, an extensive experimental study concerning the particle formation mechanism in reverse microemulsion systems and the tailoring of the particle size distribution is presented. It is shown that particle formation is governed by fast nucleation and a growth/coagulation mechanism strongly coupled with the microemulsion structure at the nanoscale. If both reactants are present in considerable amounts (small ∆c0-values), fast nucleation and subsequent fast particle growth are the dominant particle formation mechanisms until the particle diameter reaches a barrier at approximately 6 nm, which corresponds to the size of a slightly expanded microemulsion droplet (stabilization of the particles). Under these conditions, particle growth is stopped and nucleation of new particles is preferred. If one of the two reactants only appears in small amounts (high ∆c0-values), the growth barrier can be passed due to the destabilization of the surfactant shell by the excess ions from the other reactant. The time evolution of the particle size distribution clearly indicates that the larger particles can only be formed by coagulation. These large particles change their morphology from spherical particles to platelike shaped particles with a main quadratic, rectangular, or hexagonal main crystal face due to different growth laws for the individual faces. Experiments with different feeding strategies showed that the microemulsion technique is a powerful tool for the synthesis of tailored nanoparticles with very precise specifications concerning the particle size distribution, because the nanostructure of the microemulsion significantly influences the particle formation. Small feed rates at conditions above the growth barrier can be applied for the production of large nanoparticles with a high polydispersity, and a pulsed feed operation appeared to be reasonable if a multimodal particle size distribution with a low polydispersity of the individual subpopulations should be obtained. Supporting Information Available: Description of particle synthesis and statistics; cryo-TEM images of particles produced at ∆c0 ) 0 and 0.095 mol/L; HR-TEM image of a BaSO4 nanoparticle; table showing statistical parameters for each experiment; and images from
Notation cfeed cfeed,1 cfeed,2 creactor csurf d1 d2 dhP dD D hhP moil msf mw Nfeed R S t t1 t2 t3 tfeed T Vfeed V˙ feed Vreactor w ∆c0 ∆c0,1 ∆c0,2 R γ ω
Feed concentration of BaCl2 [mol/L] Feed concentration of BaCl2 in the first pulse [mol/L] Feed concentration of BaCl2 in the second pulse [mol/L] Reactor concentration of K2SO4 [mol/L] Surfactant concentration [mol/L] Inner stirrer diameter [mm] Outer stirrer diameter [mm] Mean particle diameter [nm] Droplet diameter [nm] Reactor diameter [mm] Mean particle thickness [nm] Mass of the oil phase [kg] Mass of the surfactant [kg] Mass of the water phase [kg] Droplet feed rate [s-1] Initial ion concentration ratio [-] Supersaturation ratio [-] Time [s] End of feed pulse 1 [s] Begin of feed pulse 2 [s] End of feed pulse 2 [s] Feed time [s] Temperature [°C] Feed volume [mL] Volume feed rate [mL/s] Reactor volume [mL] Water/surfactant molar ratio [-] Initial concentration difference [mol/L] Concentration difference for pulse 1 [mol/L] Concentration difference for pulse 2 [mol/L] Oil mass fraction [-] Surfactant mass fraction [-] Stirring rate [1/s]
LA703566V
