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Membrane dispersion precipitation method to prepare nanopartials
Powder Technology 139 (2004) 180 – 185 www.elsevier.com/locate/powtec
Membrane dispersion precipitation method to prepare nanopartials G.G. Chen, G.S. Luo *, J.H. Xu, J.D. Wang The State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, P.R. China Received 4 June 2003; received in revised form 21 November 2003; accepted 1 December 2003
Abstract In this paper, a new method for preparing nanoparticles with membrane dispersion technology was developed by integrating direct chemical precipitation and membrane emulsification. Barium sulfate nanoparticles were prepared with the new method in a membrane dispersion module with a plate microfiltration membrane as dispersion medium. Barium chloride solution and sodium sulfate solution with pure water or 20% ethyl alcohol in water as solvents were selected as the reactants. The influences of the reactant concentrations, two-phase flow rate and membrane pore size were investigated. The morphology of the nanoparticles was characterized by TEM images, and the particle-size distribution was measured. The results showed that the spheric nanoparticles of barium sulfate could be prepared with the new method. The average size was in the range of 20 – 200 nm. The particles prepared by the new method were much more uniform, compared with those by direct precipitation method. The average size of barium sulfate nanoparticles was decreased with an increase of the concentration and the flow rate of sodium sulfate solution quickly. However, those of barium chloride solution had little influence on barium sulfate nanoparticles. The decreasing of the membrane pore size resulted in the decrease of the average size of barium sulfate nanoparticles. And by changing 20% ethyl alcohol in water as solvent instead of pure water, the nanoparticle size was decreased from 70 to 20 nm. D 2004 Elsevier B.V. All rights reserved. Keywords: Membrane dispersion; Precipitation; Barium sulfate; Nanoparticles
1. Introduction Over the past decades, nanoparticles have triggered an explosion of scientific and industrial interest. Distinguished from conventional materials, nanoparticles mean that their sizes lie between that of molecules, as well as atoms, and that of bulk materials; in other words, nanoparticles generally vary in size from 10 to 1000 nm. Because of their outstanding characteristics, such as surface effect and quantum effect, nanoparticles have great potential for use in applications in the electronic, chemical or mechanical industries. And they have been used in many technologies, including superconductors, catalyst, drug carriers, sensors, magnetic materials, and in structural and electronic materials. For example, barium sulfate particles with the diameter in the range of sub-200 nm have wonderful optical characteristics and flow behavior, and have been used in pigment, printing ink and medicine widely. * Corresponding author. Tel.: +86-106-2783-870; fax: +86-106-2770304. E-mail address: [email protected] (G.S. Luo). 0032-5910/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2003.12.003
For nanoparticles, it is well known that an unagglomerated spherical particle with a narrow size distribution is the preferred state for applications. Numerous techniques for the preparations of nanoparticles that satisfy this requirement have been developed, which include gas-phase synthesis [1,2], solution-phase synthesis [3,4] and template synthesis [5,6]. The method of solution-phase synthesis is the most popular since it is easy to control characteristics including size, morphology and composition. Most of the methods use metal salts as a starting precursor, such as hydrothermal processes, homogeneous precipitation methods, microemulsions technology or reversed micelles. For liquid phase chemical techniques, it can be classified that there are two kinds methods to control the size of nanoparticles: one of them is to put the synthesis process in a ‘‘micro-reaction-environment’’ such as microemulsions technology and reversed micelles; the other is to offer a homogeneous supersaturation environment and control the nucleation and growth process, and in which micromixing is an very important factor which determines the particle morphology. However, most of these approaches require complex process control, high-reaction temperature or long synthesis time. And somehow they are batch processes.
G.G. Chen et al. / Powder Technology 139 (2004) 180–185
Fig. 1. The experimental set-up. (1) Continuous phase inlet, (2) Dispersed phase inlet, (3) slurry outlet, (4) dispersed phase store tank, (5) mixing tank, (6) microfiltration membrane.
181
dispersion medium. It has been proved the technology with the characteristics of monodisperse droplets, controlled size and mild conditions [10 – 12]. In addition, some wonderful characteristics of the technology have been shown when it is applied to extraction process, such as wider operating range (phase ratio from 1:20 to 20:1), high extraction efficiency (larger than 95%) and very short residence time (between 0.1 and 0.4s) [13,14]. It has great potential for mixing the reactants on molecular scale and giving a homogeneous environment for nanoparticles synthesis. Sodium sulfate and barium chloride were selected as the working system to prepare barium sulfate nanoparticles with membrane dispersion technology in this paper. The different factors such as initial concentrations, flow rates, membrane pore size and so on have been investigated. In this work, direct chemical precipitation method was also applied to prepare barium sulfate nanoparticles, and the advantages of the new method have been disclosed by comparing the new method with other methods.
2. Experimental equipment and analytical method
Fig. 2. SEM image of particles prepared by direct chemical precipitation.
Nowadays, it is an important challenge to obtain the nanoparticles by simple, powerful and continuous method. In this paper, a new low-energy consumption method, membrane dispersion precipitation technology, has been developed. It is one of solution-phase synthesis methods, and developed from direct precipitation method and membrane dispersion technology. Membrane dispersion technology, especially membrane emulsification, has a lot of good performances [7 –9]. Membrane dispersion technology is a new low-energy consumption dispersion technology or emulsification technology with a macroporous membrane as
The mainly used raw materials were sodium sulfate, barium chloride and absolute ethyl alcohol. All these chemicals were supplied by Beijing Chemicals Company (Beijing, P.R. China) and used directly without any further purification. And the particles were synthesis by two methods, common direct chemical precipitation and membrane dispersion precipitation method. A membrane dispersion module used in this paper is showed in Fig. 1. Barium chloride solution and sodium sulfate solution with different concentrations were pumped into the module with two metering pumps, and barium chloride solution in the forms of droplets with the diameter of micrometers passed through the microfiltration membrane into sodium sulfate solution. The two phases were micro-mixed in the module. Since the system was supersaturation, barium sulfate nanoparticles were
Fig. 3. SEM images of BaSO4 nanoparticles prepared by membrane dispersion precipitation technology.
182
G.G. Chen et al. / Powder Technology 139 (2004) 180–185
Fig. 4. TEM images of BaSO4 nanoparticles prepared by membrane dispersion precipitation technology.
synthesis. Three kinds of microfiltration membranes (purchased from the Central Iron and Steel Research Institute, Beijing, P.R. China), namely 5-Am stainless steel membrane, 0.9-Am Ni membrane and 0.2-Am Ni membrane
were applied in this work. The active membrane area was 12.5 mm2. As to the preparation of the nanoparticles by direct chemical precipitation method, barium chloride solution
Fig. 5. Size distribution of BaSO4 nanoparticles prepared by membrane dispersion precipitation technology.
G.G. Chen et al. / Powder Technology 139 (2004) 180–185
183
There are three steps in a chemical precipitation process, reaction in the liquid phase, nucleation and crystal growth. As all the steps can be finished in a short time, therefore, the process is influenced by mixing condition; in other words, the process is a ‘‘mixing sensitive’’ process. Although the morphology can be improved by means of intense mixing, common direct precipitation method cannot mix the reactants on a molecular scale before reaction, so it is difficult to prepare nanoparticles with this method, and also the shape and size of the particles by the method are hardly controlled. 3.2. Membrane dispersion precipitation method Fig. 6. Effect of reactant concentrations on d32.
was added into sodium sulfate solution dropwise under high-speed magnetic stirring condition. The particles with the new method or with direct chemical precipitation method were separated with a centrifugal separator (LD5-2A, Beijing Medical Centrifugal Separator Factory, P.R. China). After being washed for five times with distilled water and once more with absolute ethyl alcohol, and then dried in drying cabinet in 80 jC for 12 h, barium sulfate nanoparticles were obtained. The morphology of the nanoparticles was characterized by SEM (JEM-6301F Japan) and TEM (JEM200CX Japan) images, and the particle-size distributions were measured through the micrographs.
3. Results and discussion 3.1. Direct chemical precipitation method Fig. 2 is the SEM micrographs of the particles prepared with direct chemical precipitation method. It is clear that the shapes of the particles are platelets and sandroses, which are the typical shapes of barium sulfate [15], and the size of the particles is in the range of 0.3 –1 Am.
3.2.1. Morphology and particle-size distribution The SEM and TEM images of barium sulfate nanoparticles prepared by membrane dispersion precipitation technology with 0.2-Am Ni membrane as dispersion medium are showed in Figs. 3 and 4. Fig. 5 is the result of particle-size distributions. The flow rate of dispersed phase was 22 ml/min and that of continuous phase was 24 ml/min. The particles prepared by the new method are spherical and the particle size is in nanometer range, and their distribution is very narrow. Compare particles in Fig. 3 with those in Fig. 2, the morphology of the nanoparticles prepared with the new method is much better than that prepared by direct precipitation method. It also can be seen that the particle size is influenced by the reactant concentrations greatly. 3.2.2. Influences of reactant concentrations The average particle size was measured from TEM images, and the influence of the reactant concentrations on the nanoparticles are shown in Fig. 6. In the experiments, BaCl2 solution was used as the dispersed phase, Na2SO4 as the continuous phase and the phase ratio of the dispersed phase to the continuous phase was 11:12. A total of 0.2-Am Ni membrane was applied. From Fig. 6, it can be seen that the average size of the particles is decreased quickly with an increase of sodium sulfate
Fig. 7. Effect of the operational conditions on d32.
184
G.G. Chen et al. / Powder Technology 139 (2004) 180–185
kinetic equations of nucleation and growth, in which the weight coefficients of the concentrations are equivalent. This phenomenon must be studied further.
Fig. 8. Influence of membrane pore size on BaSO4 nanoparticles.
3.2.3. Influences of operation conditions The influence of the continuous and the dispersed phase flow rates on the average size was investigated, and the results are shown in Fig. 7. The average sizes were mainly affected by the continuous phase flow rate of sodium sulfate, and the flow rate of barium chloride as the dispersed phase had little influence on the size, similar with the influences of the concentrations on the particle size. That may be caused by the difference of the diffusion coefficients of the reactants in the solutions. As discussed above, we will study the phenomenon further.
concentrations. From the traditional theories, the size of particles prepared by precipitation is affected by the saturation degree of the solutions greatly. With the increasing of sodium sulfate concentrations, the saturation degree was increased, so the nucleation formation was enhanced greatly. But the mass production of BaSO4 was kept the same as before, so the average size of the particles dropped quickly. But it can be found that barium chloride concentration does not play the same role as that of sodium sulfate concentration. It did not agree very well with the traditional theories about the
3.2.4. Influence of membrane pore size The average sizes of barium sulfate prepared by three different membranes are shown in Fig. 8. It can be seen that the average particle sizes decrease with the membrane pore size decreasing. According to the results of membrane emulsification process, the size of dispersed phase droplets is mainly determined by the pore diameter of microfiltration membranes. So that, with a decrease of the membrane pore size, the micromixing between barium chloride and sodium sulfate solution is improved greatly, and results in improving the morphology and the size distribution of the nanoparticles.
Fig. 9. TEM images of BaSO4 nanoparticles prepared in different solvent. (solvent 2: 20% ethyl alcohol in water).
G.G. Chen et al. / Powder Technology 139 (2004) 180–185 Table 1 BaSO4 nanoparticles prepared in different solvents
Acknowledgements
BaCl2 0.1 M, Na2SO4 0.1 M
BaCl2 0.3 M, Na2SO4 0.1 M
Solvent 1
Solvent 2
Solvent 1
Solvent 2
22.6 F 2.0
60.9 F 3.0
15.1 F 2.0
d32 (nm) 69.3 F 3.0
185
We wish to acknowledge the support of the National Science Foundation of China on this work gratefully.
References 3.2.5. Influence of solvents It is well known that the solubility of a material in different solvents is changed. And it will greatly influence the saturation degree, one of the most important factors that influence the particle size in chemical precipitation method. So in this work, 20% ethyl alcohol in water was used as solvent 2 instead of solvent 1, pure water. The TEM images of the nanoparticles prepared in different solvents with 0.2Am Ni membrane as dispersion medium and BaCl2 solution as dispersed phase are shown in Fig. 9. And the flow rate of dispersed phase was 22 ml/min and that of continuous phase was 24 ml/min. The comparison results are listed in Table 1. It is clearly shown that the average sizes of the nanoparticles prepared in ethyl alcohol solvent are much smaller than that in pure water, and the average sizes decreased from 70 to 20 nm. The most likely reason is not only the change of solubility but also the decrease of the interfacial force.
4. Conclusions In the paper, a new membrane dispersion precipitation method for preparing nanoparticles was introduced. Barium sulfate nanoparticles with different size have been prepared with the new method. The morphology of the nanoparticles has been characterized by TEM images. It has been found that the average size of barium sulfate nanoparticles was decreased with an increase of the concentration and the flow rate of sodium sulfate solution quickly. However, those of barium chloride solution had little influence on barium sulfate nanoparticles. The decrease in membrane pore size resulted in the decrease of the average size. And by changing 20% ethyl alcohol in water as solvent instead of pure water, the nanoparticle size was decreased from 70 to 20 nm. The results prove that the new technology has the characteristics of lowenergy consumption, easy operation and well-controlled size. The new technology can also be operated in a continuous mode.
[1] A.S. Edelstein, R.C. Cammarata (Eds.), Nanomaterials: synthesis, properties and applications, Institute of Physics Pub., Bristol, Philadelphia, UK, 1996, Chap. 2. [2] W.Z. Wang, Y.K. Liu, Y.J. Zhan, C.L. Zheng, G.H. Wang, A novel and simple one-step solid-state reaction for the synthesis of PbS nanoparticles in the presence of a suitable surfactant, Materials Research Bulletin 36 (2001) 1977 – 1984. [3] P. Dutta, J.H. Fendler, Preparation of cadmium sulfide nanoparticles in self-reproducing reversed micelles, Journal of Colloid and Interface Science 247 (1) (2002) 47 – 53. [4] J.C. Leroux, E. Allemann, E. Doelker, R. Gurnay, New approach for the preparation of nanoparticles by an emulsification – diffusion method, European Journal of Pharmaceutics and Biopharmaceutics 41 (1995) 14 – 18. [5] Y. Xie, Y.T. Qian, W.Z. Wang, A benzene-thermal synthetic route to nanocrystalline GaN, Science 272 (1996) 1926 – 1927. [6] W. Zhou, J.M. Thomas, D.S. Shephard, et al., Ordering of ruthenium cluster carbonyls in mesoporous silica, Science 280 (1998) 705 – 708. [7] M.C. Yang, E.L. Cussler, Designing hollow-fiber contactors, AIChE Journal 32 (11) (1986) 1910 – 1916. [8] R. Prasad, S. Khare, A. Sengupta, K.K. Sirkar, Noval liquid-in-pore configurations in membrane solvent extraction, AIChE Journal 36 (10) (1990) 1592 – 1596. [9] Y. Sun, G.S. Luo, Y. Pu, J.D. Wang, A novel extraction process membrane dispersion extraction, Journal of Tsinghua University 40 (10) (2000) 40 – 42 (in Chinese). [10] T. Nakashima, M. Shimizu, M. Kukizaki, Membrane emulsification by microporous glass, Key Engineering Materials 61-62 (1991) 513 – 516. [11] R. Latoh, Y. Asano, A. Furuya, K. Sotoyama, M. Tomita, Preparation of food emulsions using a membrane emulsification system, Journal of Membrane Science 113 (1996) 131 – 135. [12] V. Schroder, O. Behrend, H. Schubert, Effect of dynamic interfacial tension on the emulsification process using microporous ceramic membranes, Journal of Colloid and Interface Science 202 (1998) 334 – 340. [13] G.G. Chen, G.S. Luo, Y. Sun, Y. Pu, Mass transfer characteristics with ceramic macroporous membrane as dispersion medium, Journal of Chemical Industry and Engineering 53 (6) (2002) 644 – 647 (in Chinese). [14] Y. Sun, Membrane dispersion and mass transfer performance of liquidliquid system, Ph.D. dissertation, Tsinghua University, Beijing. 2002. [15] B. Benrnard-Michel, M.N. Pons, H. Vivier, Quantification, by image analysis, of effect of operational conditions on size an shape of precipitated barium sulphate, Chemical Engineering Journal 87 (2002) 135 – 147.
Membrane dispersion precipitation method to prepare nanopartials G.G. Chen, G.S. Luo *, J.H. Xu, J.D. Wang The State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, P.R. China Received 4 June 2003; received in revised form 21 November 2003; accepted 1 December 2003
Abstract In this paper, a new method for preparing nanoparticles with membrane dispersion technology was developed by integrating direct chemical precipitation and membrane emulsification. Barium sulfate nanoparticles were prepared with the new method in a membrane dispersion module with a plate microfiltration membrane as dispersion medium. Barium chloride solution and sodium sulfate solution with pure water or 20% ethyl alcohol in water as solvents were selected as the reactants. The influences of the reactant concentrations, two-phase flow rate and membrane pore size were investigated. The morphology of the nanoparticles was characterized by TEM images, and the particle-size distribution was measured. The results showed that the spheric nanoparticles of barium sulfate could be prepared with the new method. The average size was in the range of 20 – 200 nm. The particles prepared by the new method were much more uniform, compared with those by direct precipitation method. The average size of barium sulfate nanoparticles was decreased with an increase of the concentration and the flow rate of sodium sulfate solution quickly. However, those of barium chloride solution had little influence on barium sulfate nanoparticles. The decreasing of the membrane pore size resulted in the decrease of the average size of barium sulfate nanoparticles. And by changing 20% ethyl alcohol in water as solvent instead of pure water, the nanoparticle size was decreased from 70 to 20 nm. D 2004 Elsevier B.V. All rights reserved. Keywords: Membrane dispersion; Precipitation; Barium sulfate; Nanoparticles
1. Introduction Over the past decades, nanoparticles have triggered an explosion of scientific and industrial interest. Distinguished from conventional materials, nanoparticles mean that their sizes lie between that of molecules, as well as atoms, and that of bulk materials; in other words, nanoparticles generally vary in size from 10 to 1000 nm. Because of their outstanding characteristics, such as surface effect and quantum effect, nanoparticles have great potential for use in applications in the electronic, chemical or mechanical industries. And they have been used in many technologies, including superconductors, catalyst, drug carriers, sensors, magnetic materials, and in structural and electronic materials. For example, barium sulfate particles with the diameter in the range of sub-200 nm have wonderful optical characteristics and flow behavior, and have been used in pigment, printing ink and medicine widely. * Corresponding author. Tel.: +86-106-2783-870; fax: +86-106-2770304. E-mail address: [email protected] (G.S. Luo). 0032-5910/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2003.12.003
For nanoparticles, it is well known that an unagglomerated spherical particle with a narrow size distribution is the preferred state for applications. Numerous techniques for the preparations of nanoparticles that satisfy this requirement have been developed, which include gas-phase synthesis [1,2], solution-phase synthesis [3,4] and template synthesis [5,6]. The method of solution-phase synthesis is the most popular since it is easy to control characteristics including size, morphology and composition. Most of the methods use metal salts as a starting precursor, such as hydrothermal processes, homogeneous precipitation methods, microemulsions technology or reversed micelles. For liquid phase chemical techniques, it can be classified that there are two kinds methods to control the size of nanoparticles: one of them is to put the synthesis process in a ‘‘micro-reaction-environment’’ such as microemulsions technology and reversed micelles; the other is to offer a homogeneous supersaturation environment and control the nucleation and growth process, and in which micromixing is an very important factor which determines the particle morphology. However, most of these approaches require complex process control, high-reaction temperature or long synthesis time. And somehow they are batch processes.
G.G. Chen et al. / Powder Technology 139 (2004) 180–185
Fig. 1. The experimental set-up. (1) Continuous phase inlet, (2) Dispersed phase inlet, (3) slurry outlet, (4) dispersed phase store tank, (5) mixing tank, (6) microfiltration membrane.
181
dispersion medium. It has been proved the technology with the characteristics of monodisperse droplets, controlled size and mild conditions [10 – 12]. In addition, some wonderful characteristics of the technology have been shown when it is applied to extraction process, such as wider operating range (phase ratio from 1:20 to 20:1), high extraction efficiency (larger than 95%) and very short residence time (between 0.1 and 0.4s) [13,14]. It has great potential for mixing the reactants on molecular scale and giving a homogeneous environment for nanoparticles synthesis. Sodium sulfate and barium chloride were selected as the working system to prepare barium sulfate nanoparticles with membrane dispersion technology in this paper. The different factors such as initial concentrations, flow rates, membrane pore size and so on have been investigated. In this work, direct chemical precipitation method was also applied to prepare barium sulfate nanoparticles, and the advantages of the new method have been disclosed by comparing the new method with other methods.
2. Experimental equipment and analytical method
Fig. 2. SEM image of particles prepared by direct chemical precipitation.
Nowadays, it is an important challenge to obtain the nanoparticles by simple, powerful and continuous method. In this paper, a new low-energy consumption method, membrane dispersion precipitation technology, has been developed. It is one of solution-phase synthesis methods, and developed from direct precipitation method and membrane dispersion technology. Membrane dispersion technology, especially membrane emulsification, has a lot of good performances [7 –9]. Membrane dispersion technology is a new low-energy consumption dispersion technology or emulsification technology with a macroporous membrane as
The mainly used raw materials were sodium sulfate, barium chloride and absolute ethyl alcohol. All these chemicals were supplied by Beijing Chemicals Company (Beijing, P.R. China) and used directly without any further purification. And the particles were synthesis by two methods, common direct chemical precipitation and membrane dispersion precipitation method. A membrane dispersion module used in this paper is showed in Fig. 1. Barium chloride solution and sodium sulfate solution with different concentrations were pumped into the module with two metering pumps, and barium chloride solution in the forms of droplets with the diameter of micrometers passed through the microfiltration membrane into sodium sulfate solution. The two phases were micro-mixed in the module. Since the system was supersaturation, barium sulfate nanoparticles were
Fig. 3. SEM images of BaSO4 nanoparticles prepared by membrane dispersion precipitation technology.
182
G.G. Chen et al. / Powder Technology 139 (2004) 180–185
Fig. 4. TEM images of BaSO4 nanoparticles prepared by membrane dispersion precipitation technology.
synthesis. Three kinds of microfiltration membranes (purchased from the Central Iron and Steel Research Institute, Beijing, P.R. China), namely 5-Am stainless steel membrane, 0.9-Am Ni membrane and 0.2-Am Ni membrane
were applied in this work. The active membrane area was 12.5 mm2. As to the preparation of the nanoparticles by direct chemical precipitation method, barium chloride solution
Fig. 5. Size distribution of BaSO4 nanoparticles prepared by membrane dispersion precipitation technology.
G.G. Chen et al. / Powder Technology 139 (2004) 180–185
183
There are three steps in a chemical precipitation process, reaction in the liquid phase, nucleation and crystal growth. As all the steps can be finished in a short time, therefore, the process is influenced by mixing condition; in other words, the process is a ‘‘mixing sensitive’’ process. Although the morphology can be improved by means of intense mixing, common direct precipitation method cannot mix the reactants on a molecular scale before reaction, so it is difficult to prepare nanoparticles with this method, and also the shape and size of the particles by the method are hardly controlled. 3.2. Membrane dispersion precipitation method Fig. 6. Effect of reactant concentrations on d32.
was added into sodium sulfate solution dropwise under high-speed magnetic stirring condition. The particles with the new method or with direct chemical precipitation method were separated with a centrifugal separator (LD5-2A, Beijing Medical Centrifugal Separator Factory, P.R. China). After being washed for five times with distilled water and once more with absolute ethyl alcohol, and then dried in drying cabinet in 80 jC for 12 h, barium sulfate nanoparticles were obtained. The morphology of the nanoparticles was characterized by SEM (JEM-6301F Japan) and TEM (JEM200CX Japan) images, and the particle-size distributions were measured through the micrographs.
3. Results and discussion 3.1. Direct chemical precipitation method Fig. 2 is the SEM micrographs of the particles prepared with direct chemical precipitation method. It is clear that the shapes of the particles are platelets and sandroses, which are the typical shapes of barium sulfate [15], and the size of the particles is in the range of 0.3 –1 Am.
3.2.1. Morphology and particle-size distribution The SEM and TEM images of barium sulfate nanoparticles prepared by membrane dispersion precipitation technology with 0.2-Am Ni membrane as dispersion medium are showed in Figs. 3 and 4. Fig. 5 is the result of particle-size distributions. The flow rate of dispersed phase was 22 ml/min and that of continuous phase was 24 ml/min. The particles prepared by the new method are spherical and the particle size is in nanometer range, and their distribution is very narrow. Compare particles in Fig. 3 with those in Fig. 2, the morphology of the nanoparticles prepared with the new method is much better than that prepared by direct precipitation method. It also can be seen that the particle size is influenced by the reactant concentrations greatly. 3.2.2. Influences of reactant concentrations The average particle size was measured from TEM images, and the influence of the reactant concentrations on the nanoparticles are shown in Fig. 6. In the experiments, BaCl2 solution was used as the dispersed phase, Na2SO4 as the continuous phase and the phase ratio of the dispersed phase to the continuous phase was 11:12. A total of 0.2-Am Ni membrane was applied. From Fig. 6, it can be seen that the average size of the particles is decreased quickly with an increase of sodium sulfate
Fig. 7. Effect of the operational conditions on d32.
184
G.G. Chen et al. / Powder Technology 139 (2004) 180–185
kinetic equations of nucleation and growth, in which the weight coefficients of the concentrations are equivalent. This phenomenon must be studied further.
Fig. 8. Influence of membrane pore size on BaSO4 nanoparticles.
3.2.3. Influences of operation conditions The influence of the continuous and the dispersed phase flow rates on the average size was investigated, and the results are shown in Fig. 7. The average sizes were mainly affected by the continuous phase flow rate of sodium sulfate, and the flow rate of barium chloride as the dispersed phase had little influence on the size, similar with the influences of the concentrations on the particle size. That may be caused by the difference of the diffusion coefficients of the reactants in the solutions. As discussed above, we will study the phenomenon further.
concentrations. From the traditional theories, the size of particles prepared by precipitation is affected by the saturation degree of the solutions greatly. With the increasing of sodium sulfate concentrations, the saturation degree was increased, so the nucleation formation was enhanced greatly. But the mass production of BaSO4 was kept the same as before, so the average size of the particles dropped quickly. But it can be found that barium chloride concentration does not play the same role as that of sodium sulfate concentration. It did not agree very well with the traditional theories about the
3.2.4. Influence of membrane pore size The average sizes of barium sulfate prepared by three different membranes are shown in Fig. 8. It can be seen that the average particle sizes decrease with the membrane pore size decreasing. According to the results of membrane emulsification process, the size of dispersed phase droplets is mainly determined by the pore diameter of microfiltration membranes. So that, with a decrease of the membrane pore size, the micromixing between barium chloride and sodium sulfate solution is improved greatly, and results in improving the morphology and the size distribution of the nanoparticles.
Fig. 9. TEM images of BaSO4 nanoparticles prepared in different solvent. (solvent 2: 20% ethyl alcohol in water).
G.G. Chen et al. / Powder Technology 139 (2004) 180–185 Table 1 BaSO4 nanoparticles prepared in different solvents
Acknowledgements
BaCl2 0.1 M, Na2SO4 0.1 M
BaCl2 0.3 M, Na2SO4 0.1 M
Solvent 1
Solvent 2
Solvent 1
Solvent 2
22.6 F 2.0
60.9 F 3.0
15.1 F 2.0
d32 (nm) 69.3 F 3.0
185
We wish to acknowledge the support of the National Science Foundation of China on this work gratefully.
References 3.2.5. Influence of solvents It is well known that the solubility of a material in different solvents is changed. And it will greatly influence the saturation degree, one of the most important factors that influence the particle size in chemical precipitation method. So in this work, 20% ethyl alcohol in water was used as solvent 2 instead of solvent 1, pure water. The TEM images of the nanoparticles prepared in different solvents with 0.2Am Ni membrane as dispersion medium and BaCl2 solution as dispersed phase are shown in Fig. 9. And the flow rate of dispersed phase was 22 ml/min and that of continuous phase was 24 ml/min. The comparison results are listed in Table 1. It is clearly shown that the average sizes of the nanoparticles prepared in ethyl alcohol solvent are much smaller than that in pure water, and the average sizes decreased from 70 to 20 nm. The most likely reason is not only the change of solubility but also the decrease of the interfacial force.
4. Conclusions In the paper, a new membrane dispersion precipitation method for preparing nanoparticles was introduced. Barium sulfate nanoparticles with different size have been prepared with the new method. The morphology of the nanoparticles has been characterized by TEM images. It has been found that the average size of barium sulfate nanoparticles was decreased with an increase of the concentration and the flow rate of sodium sulfate solution quickly. However, those of barium chloride solution had little influence on barium sulfate nanoparticles. The decrease in membrane pore size resulted in the decrease of the average size. And by changing 20% ethyl alcohol in water as solvent instead of pure water, the nanoparticle size was decreased from 70 to 20 nm. The results prove that the new technology has the characteristics of lowenergy consumption, easy operation and well-controlled size. The new technology can also be operated in a continuous mode.
[1] A.S. Edelstein, R.C. Cammarata (Eds.), Nanomaterials: synthesis, properties and applications, Institute of Physics Pub., Bristol, Philadelphia, UK, 1996, Chap. 2. [2] W.Z. Wang, Y.K. Liu, Y.J. Zhan, C.L. Zheng, G.H. Wang, A novel and simple one-step solid-state reaction for the synthesis of PbS nanoparticles in the presence of a suitable surfactant, Materials Research Bulletin 36 (2001) 1977 – 1984. [3] P. Dutta, J.H. Fendler, Preparation of cadmium sulfide nanoparticles in self-reproducing reversed micelles, Journal of Colloid and Interface Science 247 (1) (2002) 47 – 53. [4] J.C. Leroux, E. Allemann, E. Doelker, R. Gurnay, New approach for the preparation of nanoparticles by an emulsification – diffusion method, European Journal of Pharmaceutics and Biopharmaceutics 41 (1995) 14 – 18. [5] Y. Xie, Y.T. Qian, W.Z. Wang, A benzene-thermal synthetic route to nanocrystalline GaN, Science 272 (1996) 1926 – 1927. [6] W. Zhou, J.M. Thomas, D.S. Shephard, et al., Ordering of ruthenium cluster carbonyls in mesoporous silica, Science 280 (1998) 705 – 708. [7] M.C. Yang, E.L. Cussler, Designing hollow-fiber contactors, AIChE Journal 32 (11) (1986) 1910 – 1916. [8] R. Prasad, S. Khare, A. Sengupta, K.K. Sirkar, Noval liquid-in-pore configurations in membrane solvent extraction, AIChE Journal 36 (10) (1990) 1592 – 1596. [9] Y. Sun, G.S. Luo, Y. Pu, J.D. Wang, A novel extraction process membrane dispersion extraction, Journal of Tsinghua University 40 (10) (2000) 40 – 42 (in Chinese). [10] T. Nakashima, M. Shimizu, M. Kukizaki, Membrane emulsification by microporous glass, Key Engineering Materials 61-62 (1991) 513 – 516. [11] R. Latoh, Y. Asano, A. Furuya, K. Sotoyama, M. Tomita, Preparation of food emulsions using a membrane emulsification system, Journal of Membrane Science 113 (1996) 131 – 135. [12] V. Schroder, O. Behrend, H. Schubert, Effect of dynamic interfacial tension on the emulsification process using microporous ceramic membranes, Journal of Colloid and Interface Science 202 (1998) 334 – 340. [13] G.G. Chen, G.S. Luo, Y. Sun, Y. Pu, Mass transfer characteristics with ceramic macroporous membrane as dispersion medium, Journal of Chemical Industry and Engineering 53 (6) (2002) 644 – 647 (in Chinese). [14] Y. Sun, Membrane dispersion and mass transfer performance of liquidliquid system, Ph.D. dissertation, Tsinghua University, Beijing. 2002. [15] B. Benrnard-Michel, M.N. Pons, H. Vivier, Quantification, by image analysis, of effect of operational conditions on size an shape of precipitated barium sulphate, Chemical Engineering Journal 87 (2002) 135 – 147.