In situ preparation of hydrophobic CaCO3 nanoparticles in a gas–liquid microdispersion process

G Model

ARTICLE IN PRESS

PARTIC-503; No. of Pages 7

Particuology xxx (2013) xxx–xxx

Contents lists available at SciVerse ScienceDirect

Particuology journal homepage: www.elsevier.com/locate/partic

In situ preparation of hydrophobic CaCO3 nanoparticles in a gas–liquid microdispersion process Le Du, Yujun Wang, Guangsheng Luo ∗ The State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China

a r t i c l e

i n f o

Article history: Received 30 April 2012 Received in revised form 13 June 2012 Accepted 19 July 2012 Keywords: CaCO3 nanoparticles In situ surface modification Microreactor Mass transfer

a b s t r a c t This study presents a novel process of in situ surface modification of CaCO3 nanoparticles using a multipleorifice dispersion microreactor. CO2 /Ca(OH)2 precipitation reaction was employed to prepare CaCO3 nanoparticles with sodium stearate surfactant. Synthesized CaCO3 products were characterized by thermogravimetric analysis (TGA), infra-red (IR), X-ray diffraction (XRD), transmission electron microscopy (TEM) and Brunauer–Emmet–Teller analysis (BET). The effect of various operation parameters on nanoparticles and the dosage of sodium stearate were determined. The results showed that the preparation process could be precisely controlled with efficient mass transfer process. The particles were highly hydrophobic with a contact angle of 117◦ and monodisperse with an average size of 30 nm. The adsorptions of sodium stearate and calcium ion on solid particles during the in situ surface modification process were investigated. © 2013 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.

1. Introduction Nanoparticles have been widely studied in recent decades for their special characteristics such as the surface effect and quantum effect (Katz & Willner, 2004; Raschke et al., 2003). As an example, calcium carbonate (CaCO3 ) nanoparticle is an important multifunctional additive used primarily for paints, rubber, pigment, paper, plastics, etc. (Sahebian, Zebarjad, Khaki, & Sajjadi, 2009; Wang, Piao, Zhai, Hickman, & Li, 2010; Wang, Tang, Wu, Dai, & Qiu, 2007). Several techniques have been developed to prepare hydrophilic CaCO3 nanoparticles, including emulsion liquid membrane method (Sun & Deng, 2004) and gas–liquid carbonation (Cao, Wang, & Zhang, 2003). Considering the economy and practicality, the method of gas–liquid carbonation seems to be one of the best industrial processes. Carbon dioxide (CO2 ) gas and calcium hydroxide (Ca(OH)2 ) suspension are employed as the reactants. But the hydrophilic surfaces of common CaCO3 particles are incompatible with the hydrophobic polymers, which cause agglomeration in the polymer matrix (Chen et al., 2006; Konopacka-Lyskawa & Lackowski, 2011). Rubber, plastics and other materials with such kind of filler particles tend to fracture and age fast. Thus the surface modification of CaCO3 nanoparticles is required to reduce the

∗ Corresponding author. E-mail address: [email protected] (G. Luo).

surface energy and improve the dispersion stability in the matrix. The surfaces of CaCO3 particles are often modified by a variety of surfactants, such as lauric acid, stearic acid, silane coupling agent, and polyethylene glycol (Ma et al., 2008; Novokshonova et al., 2003; Wang, Lu, & Wang, 1997). Among these surfactants, fatty acids and their salts are commonly used. The modification process ends up with hydrophobic alkyl chains chemisorbed to the particle surface, which can significantly improve the wettability and binding force between the filler and polymer matrix (Kong et al., 2008). However, the traditional techniques for industrial production, such as fluidization, batch bubbling precipitation, batch stirred reaction and emulsification, cause several problems (Li, 2009). With these techniques people cannot easily control the particle quality, especially in the large-scale production process. Normally the surface modification is carried out in the wet process after the particles are synthesized. The process usually requires excess addition of surfactants for sufficient modification (Ding, Lu, Deng, & Du, 2007; Shui, 2003), which causes high-energy consumption. Thus strategies of in situ modification, which are effective to solve the problems as mentioned above, have been extensively studied (Wang et al., 2010; Wang, Sheng, et al., 2007; Wang, Zhao, et al., 2007; Ye & Zhang, 2004). A variety of surfactants have been added with the reactants at one step, which simplifies the procedures and utilizes the surfactants effectively. However, for Ca(OH)2 slurry system, most of the surfactants tend to coat on the solid particles and prevent Ca2+ ions from diffusing into liquid phase.

1674-2001/$ – see front matter © 2013 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.

http://dx.doi.org/10.1016/j.partic.2012.07.009

Please cite this article in press as: Du, L., et al. In situ preparation of hydrophobic CaCO3 nanoparticles in a gas–liquid microdispersion process. Particuology (2013), http://dx.doi.org/10.1016/j.partic.2012.07.009

G Model PARTIC-503; No. of Pages 7

ARTICLE IN PRESS L. Du et al. / Particuology xxx (2013) xxx–xxx

2

Nomenclature Notation A d32 d32G Fc Fd NCO2 MCO2 SBET T 

interfacial area in the reactor, m2 average diameter of CaCO3 nanoparticles, nm average diameter of microbubbles, ␮m flow rate of continuous phase, mL/min flow rate of disperse phase, mL/min mass transfer flux density of CO2 , mol/(m2 s) mass transfer flux of CO2 , mol/s specific surface area of CaCO3 particles, m2 /g reaction temperature, ◦ C concentration of Ca(OH)2 solution/suspension, g/L

Song, Kim, and Kim (2003) studied the typical two-step reactions as follows: [Ca2+ ]surface + (stearate)− → [Ca(stearate)]+ surface ,

(1)

− [Ca(stearate)]+ surface + (stearate) → [Ca(stearate)2 ]surface ,

(2)

Fig. 1. The experimental set-up for preparing CaCO3 nanoparticles.

which caused most adsorption processes on solid particles. Furthermore, the addition of surfactants enhances mass transfer resistance in the liquid membrane. Therefore, a process with both in situ modification method and precise controllable operation is in demand. In the last two decades, microfluidics has exhibited excellent performance in mixing, transfer rate and controllable operating conditions (Duraiswamy & Khan, 2009; Guenther, Gross, Wagner, Jahn, & Koehler, 2008; Koehler, Held, Huebner, & Wagner, 2007; Lee et al., 2009). Reactants with various properties in the form of water/oil/gas have been effectively utilized and the processes can be precisely controlled. In our previous study, several types of microfluidic devices have been developed and successfully used to synthesize nanoparticles in homogeneous and heterogeneous reaction systems (Chen, Luo, Li, Xu, & Wang, 2005; Chen, Luo, Yang, Sun, & Wang, 2004; Li, Xu, Wang, & Luo, 2008; Luo, Du, Wang, Lu, & Xu, 2011). Surface treatment and functionalization of various particle materials have also been developed (Guo, Luo, & Wang, 2003; Yang, Wang, Luo, & Dai, 2008; Shen, Wang, Xu, Lu, & Luo, 2012). Especially the large-scale preparation of hydrophilic CaCO3 nanoparticles has been industrialized by using the membrane microdispersion reactors, which make the annual output up to 50,000 tons (Wang, Wang, Chen, Luo, & Wang, 2007). In this study, an in situ preparation strategy to prepare hydrophobic CaCO3 nanoparticles with a gas–liquid microdispersion process has been developed. A multiple-orifice dispersion microreactor was designed and applied to generate gas–liquid microdispersed systems. Calcium hydroxide suspension and carbon dioxide/nitrogen mixed gas (29.8% volume fraction CO2 ) were selected as the reactants while sodium stearate (RCOONa) was selected as the modifying agent. The operation parameters were varied and their influences on the properties of CaCO3 nanoparticles were investigated. The dosage of sodium stearate was optimized. CaCO3 nanoparticles with high-class quality were successfully prepared.

The chemicals include sodium stearate (RCOONa), calcium hydroxide and mixed gas (29.8% volume fraction CO2 , 0.3 MPa). RCOONa was first mixed with Ca(OH)2 suspension and stirred for 3 h at 80 ◦ C before being pumped into the microreactor. Then the Ca(OH)2 suspension as the continuous phase and the mixed gas as the dispersed phase were mixed in the microreactor. Pressure difference between the two sides of the dispersion media was employed as the driving force to disperse the gas phase into the continuous phase in the form of microbubbles. CaCO3 precipitates were synthesized when the two phases contacted each other in the mixing chamber. To realize complete consumption of Ca(OH)2 in the continuous phase, the suspension was circulated at a certain feed rate. At the beginning, the pH of the system was 12.2. The reaction process was stopped when the pH was 7. After an aging treatment for 1 h, CaCO3 precipitates were separated from the suspension using a centrifugal separator (LD5-2A, Beijing Medical Centrifugal Separator Factory). The CaCO3 precipitates were washed 3 times with distilled water, twice with ethanol and dried in a drying cabinet at 100 ◦ C for 24 h. Finally, the product of CaCO3 was obtained. In addition, experiments to observe and record the status of gas–liquid microdispersion were carried out. A microscope at magnifications from 20× to 200× with a high-speed CCD video camera was used to record the microbubble and measure the diameter. The morphology of nanoparticles was recorded by transmission electron microscopy (TEM, JEOL-2010, 120 kV). The crystal form of the nanoparticles was characterized by X-ray diffraction analysis (XRD, Rigaku Corporation D/max-RB). Specific surface area of the particles was determined by BET (Quantachrome autosorb1). The weight loss was measured by thermogravimetric analysis (TGA, STA 409 PC). The photos of the contact angle were taken using a high-speed CCD camera attached to an Olympus U-TV05xC-3 microscope. Additionally, the contact angles were measured using DataPhysics SCA20 contact angle measurement software Ver. 3.12.11. Fourier transform infrared spectroscopic measurements (FTIR, BRUKER Corporation TENSOR 27) were taken to record FTIR spectra.

2. Experimental

3. Results and discussion

Fig. 1 shows the experimental set-up. The major component was a six-orifice dispersion microreactor, in which the diameter of the micro-orifice was 0.2 mm with orifice spacing of 1.5 mm. The geometric size of the main channel was 20 mm × 2 mm × 0.5 mm (length × width × height).

3.1. Influence of two phase flow rates on the process In order to test the possibility of the synthesis and select suitable operation conditions, the preparation of hydrophilic CaCO3 nanoparticles with no addition of surfactant was conducted.

Please cite this article in press as: Du, L., et al. In situ preparation of hydrophobic CaCO3 nanoparticles in a gas–liquid microdispersion process. Particuology (2013), http://dx.doi.org/10.1016/j.partic.2012.07.009

G Model

ARTICLE IN PRESS

PARTIC-503; No. of Pages 7

L. Du et al. / Particuology xxx (2013) xxx–xxx

3

Fig. 2. TEM images of the hydrophilic CaCO3 nanoparticles. Images (a)–(d) correspond to sample (Ca-1)–(Ca-4) in Table 1.

Table 1 lists concentrations of the reactants and the preparation conditions, such as the continuous phase flow rate, Fc , and the dispersed phase flow rate, Fd . TEM images of CaCO3 particles are shown in Fig. 2, indicating that the average particle size is about 36 nm with the surface area of 53 m2 /g. The size is relatively uniform and its distribution is narrow. Compared to the products of high-class standard (d ≤ 80 nm, SBET ≥ 18 m2 /g), the nanoparticles prepared by the multiple-orifice microdispersion reactor are completely qualified. In addition, the particles became relatively smaller and showed better dispersibility with the increase of the continuous phase flow rate. Thus higher flow rate of the continuous phase was applied in the following experiments. 3.2. Influence of surfactant on the process A certain amount of RCOONa was added into Ca(OH)2 suspension. The experiment was carried out at 20 ◦ C, with the continuous phase flow rate of 240 mL/min and the dispersed phase flow rate of 160 mL/min. The concentration of Ca(OH)2 suspension was 50 g/L as above. The adding ratio of RCOONa was based on the weight of CaCO3 product, which was 0, 1, 1.5, 2, 2.5 and 3 wt%, respectively. Fig. 3 exhibits the relation between RCOONa adding ratio and contact angles of the hydrophobic particles. The powders were compacted into flakes and held a drop of deionized water on the surface. The contact angle increases with RCOONa adding ratio, from 8.8◦ to 117.7◦ . When the RCOONa adding ratio exceeds 2.5 wt%, the contact angle changes unnoticeably. The process of hydrophobicity changing is considered to take place in this way: when the modification is carried out, the first layer (monolayer) of stearate ions is chemically adsorbed onto the particle surface, leaving a hydrophobic alkyl chain on the outermost surface (Tran, Tran, Vu, & Thai, 2010).

Fig. 3. Contact angles of hydrophobicity of CaCO3 nanoparticles with different RCOONa contents.

Crystal structures of the particles are shown in Fig. 4, given by the X-ray diffraction analysis. Main characteristic planes of (0 1 2), (1 0 4), (0 0 6), (1 1 0), (1 1 3), (2 0 2), (0 2 4), (0 1 8), (1 1 6), (2 1 1) and (1 0 1 0), corresponding to 2 value of 23.2◦ , 29.5◦ , 31.8◦ , 36.0◦ , 39.3◦ , 43.0◦ , 47.0◦ , 47.5◦ , 48.4◦ , 57.3◦ and 58.5◦ , respectively, appear in all patterns and indicate a calcite structure. The sharp crystalline

Table 1 Preparation conditions of hydrophilic CaCO3 particles. Sample

Ca-1

Ca-2

Ca-3

Ca-4

Fc (mL/min) Fd (mL/min) Temperature (◦ C) Concentration of Ca(OH)2 (g/L)

60 40

120 80

180 120

240 160

20 50

Fig. 4. XRD patterns of CaCO3 powders.

Please cite this article in press as: Du, L., et al. In situ preparation of hydrophobic CaCO3 nanoparticles in a gas–liquid microdispersion process. Particuology (2013), http://dx.doi.org/10.1016/j.partic.2012.07.009

G Model PARTIC-503; No. of Pages 7

ARTICLE IN PRESS L. Du et al. / Particuology xxx (2013) xxx–xxx

4

Fig. 5. FTIR spectra of CaCO3 nanoparticles.

Fig. 8. Change of pH values with reaction time with/without RCOONa added in (MR: microreactor; BR: batch reactor).

the same as the RCOO− dosage. However, when the RCOONa adding ratio exceeds 2.5 wt%, the weight loss changes unnoticeably, which also implies the optimal adding ratio is around 2.5 wt%. Fig. 7 exhibits some TEM images of hydrophobic CaCO3 particles. With the surfactant added, more particles of large size are generated and agglomeration occurs simultaneously. Fig. 8 shows the change of pH value when RCOONa is added. Although the mass transfer could be enhanced by introducing the microreactor compared with traditional batch reactor, the whole process still required nearly 35 min to be completely finished. At the same time, the reaction time was 10 min shorter for the surfactant-free process. The results were far from the initial anticipation which might be caused by inefficient mixing and mass transfer process. Fig. 6. The thermogravimetric curves of CaCO3 nanoparticles.

3.3. Mechanism analysis and enhancement of mass transfer peaks demonstrate highly crystallized calcite phase even with the surfactant added. To verify the existence of the surfactant layer, FTIR spectra of CaCO3 nanoparticles are recorded, as shown in Fig. 5. Three strong peaks at 1454.9, 873.3 and 706 cm−1 , corresponding to 2 , 3 and 4 vibrations respectively, are considered to arise from CO3 2− . The broad and weak peaks at 3500 cm−1 are assigned to the stretching vibration of OH. The peak at 1557 cm−1 is attributed to the antisymmetric and symmetric stretching of COO . FTIR spectra of RCOONa show strong peaks at 2920.2 and 2843.5 cm−1 corresponding to C H and C C vibrations respectively, which are not observed in the unmodified sample Ca-4. In addition, it is observed that the intensity of C H and C C increases with RCOONa adding ration, indicating the degree of the modification. TGA curves in Fig. 6 show the differences between the modified and unmodified particles. The unmodified hydrophilic particles remain stable until the temperature raised beyond 700 ◦ C and the decomposition occurs. For the modified hydrophobic particles, a significant weight loss occurs when temperature ranges from 420 ◦ C to 500 ◦ C. Furthermore, the value of weight loss is exactly

Temperature is one of the most important factors for the gas–liquid system. Generally, the reaction temperature influences the solubility of reactants, the diffusion coefficient and supersaturation. With the increase of temperature, the dissolution of solid Ca(OH)2 and the reaction rate were accelerated for the molecular thermal motion. However, the dissolution of CO2 gas was blocked, and the reduction of bubble diameter, namely the mixing scale, was hardly to achieve. Thus there is an optimal value for the efficient diffusion and reaction. The experiment results exhibited that temperature control was of great importance and the particle size was decreased significantly with the temperature decreasing, as shown in Fig. 9. The specific surface area of CaCO3 particles increases apparently with the decrease of temperature, as a result of the particle size reduction. It is necessary to analyze the mechanism of the processes of surface modification and carbonation. Fig. 10(a) shows the adsorption of RCOO− and Ca2+ ions on solid Ca(OH)2 in aqueous medium. First, RCOO− and Ca2+ ions are adsorbed on the solid surface by

Fig. 7. TEM images of the modified hydrophobic CaCO3 nanoparticles.

Please cite this article in press as: Du, L., et al. In situ preparation of hydrophobic CaCO3 nanoparticles in a gas–liquid microdispersion process. Particuology (2013), http://dx.doi.org/10.1016/j.partic.2012.07.009

G Model PARTIC-503; No. of Pages 7

ARTICLE IN PRESS L. Du et al. / Particuology xxx (2013) xxx–xxx

5

The related experiments also exhibited similar results. Fig. 11 shows the microscope images of the microbubbles at different Ca(OH)2 concentrations and TEM images of nanoparticles in the corresponding conditions. The diameter of the smallest bubble could be reduced to 20–30 ␮m. In these conditions, the particle size was decreased to 25–35 nm and the monodispersity of particles was significantly improved. However, with the Ca(OH)2 concentration decreasing, the particles became larger again, which might be caused by the decrease of the supersaturation and inefficient nucleation process. Based on the mass transfer film theory, the mass transfer flux density can be obtained. Fig. 9. Effect of temperature on particle size and specific surface area.

electrostatic force (Song et al., 2003). Ca2+ ions are saturated on the Stern layer, and the interaction between RCOO− and Ca2+ persists until sufficient number of RCOO− is adsorbed. The adsorption ends up with the monolayer saturation of stearate. After the monolayer saturation of stearate, surfactants are associated in the form of two-dimensional aggregate between hydrophobic groups of surfactants. Finally, the adsorption of surfactant reaches a plateau corresponding to complete surface coverage and the excess positive ions such as Ca2+ and Na+ ions are attached strongly. In this case, Ca2+ ions generated from the dissolution of solid Ca(OH)2 are hard to diffuse into the aqueous solution owing to the blocking by the coated stearate layers. Thus the mass transfer is inefficient and the mixing of reactants is uneven, which causes the generation of large particles. To avoid the coating effect on the solid Ca(OH)2 , it is feasible to decrease the concentration of Ca(OH)2 . For example, the saturated solution can be utilized instead of suspension. The concentration of RCOONa is 8 × 10−4 mol/L while the concentration of Ca(OH)2 saturated solution is 0.002 mol/L, quantitatively about 120 times larger than RCOONa. In this case, most of Ca2+ and OH− ions exist in the solution. RCOO− combined with Ca2+ exists in the form of micelles, as shown in Fig. 10(b). Thus mass transfer processes of Ca2+ , HCO3 − and CO3 2− are significantly improved, which are available to synthesize smaller particles with narrow size distribution.

NCO2 =

MCO2 A

,

(3)

where NCO2 is the mass transfer flux density of CO2 (mol/(m2 s)), MCO2 represents the mass transfer flux of CO2 (mol/s), and A is the interfacial area in the reactor (m2 ). A could be calculated by measuring average diameters (d32G ) of the bubbles from microscope images. Fig. 12 shows the relationship between particle size and mass transfer flux with the concentration of Ca(OH)2 . Obviously, the mass transfer rate of CO2 was enhanced by decreasing Ca(OH)2 concentration, which might be caused by reduction of the coating effect mentioned before. However, with the Ca(OH)2 concentration decreasing, the particle size increased and NCO2 decreased sharply. The reason might be the decrease of the supersaturation and inefficiently nucleating with relatively low concentration. Therefore, saturated Ca(OH)2 solution with 2.5 wt% RCOONa added was considered to be the optimal condition for in situ surface modifying CaCO3 nanoparticles in the microreactor. 3.4. Comparison with other preparation methods The preparation of hydrophobic CaCO3 nanoparticles with a microreactor was compared with some typical methods, including in situ surface modification. The results are shown in Table 2. It is obvious that the microdispersion method is excellent in both synthesis and product qualities. The process can be carried out continuously and is controllable with relatively simple operation. The

Fig. 10. Schematic representation for adding RCOONa into the system. (a) Adsorption process of RCOO− and Ca2+ ions on solid Ca(OH)2 in aqueous medium and (b) micelles exist in Ca(OH)2 saturated solution.

Please cite this article in press as: Du, L., et al. In situ preparation of hydrophobic CaCO3 nanoparticles in a gas–liquid microdispersion process. Particuology (2013), http://dx.doi.org/10.1016/j.partic.2012.07.009

G Model PARTIC-503; No. of Pages 7

ARTICLE IN PRESS L. Du et al. / Particuology xxx (2013) xxx–xxx

6

Fig. 11. Microscope images of the microbubbles and TEM images of modified CaCO3 nanoparticles at different Ca(OH)2 concentrations. (a) (Ca(OH)2 ) = 1.5 g/L; (b) (Ca(OH)2 ) = 1 g/L; and (c) (Ca(OH)2 ) = 0.5 g/L. Table 2 Comparison with different methods to synthesize hydrophilic CaCO3 particles. Device

Microreactor

Stirred tank

Stirred tank

Stirred tank

Operation Modification Reactants Particle size (nm) Dispersion Contact angle (◦ ) Reference

Continuous/batch In situ CO2 , Ca(OH)2 , sodium stearate 30 Monodisperse 117.7 This study

Batch Follow-up CO2 , Ca(OH)2 , sodium stearate 40–60 Monodisperse 127 Tran et al. (2010)

Batch In situ CO2 , Ca(OH)2 , oleic acid 40–60 Agglomerated 108.77 Wang et al. (2010)

Batch Follow-up CO2 , Ca(OH)2 , sodium stearate, ethanol 200–300 Monodisperse 125 Ma et al. (2008)

average particle sizes are smaller than those obtained with other methods. Agglomeration, the disadvantage of in situ modification, can also be avoided for the precise control in the microreactor. The contact angles are large enough to ensure the dispersibility of CaCO3 nanoparticles in polymer materials.

4. Conclusions In situ surface modified CaCO3 nanoparticles were prepared using a multiple-orifice dispersion microreactor. The monodispersed particles were highly hydrophobic with the average size of 30 nm. An efficient mixing and fast transfer rate were achieved in the microreactor with both high supersaturation and uniform reaction environment. The dosage of the sodium stearate was optimized. The effects of operation conditions on mixing performance and particle size were investigated, confirming that the particle size could be controlled by varying the flow rate, reactant concentration and temperature. The possible mechanism has been presented to explain the effect of surfactant in suspension system.

Acknowledgements

Fig. 12. Mass transfer flux densities of CO2 and particle sizes at different Ca(OH)2 concentrations.

We gratefully acknowledge the supports of the National Natural Science Foundation of China (21036002 and 20876084) and National Basic Research Program of China (2007CB714302) to this work.

Please cite this article in press as: Du, L., et al. In situ preparation of hydrophobic CaCO3 nanoparticles in a gas–liquid microdispersion process. Particuology (2013), http://dx.doi.org/10.1016/j.partic.2012.07.009

G Model PARTIC-503; No. of Pages 7

ARTICLE IN PRESS L. Du et al. / Particuology xxx (2013) xxx–xxx

References Cao, W. L., Wang, Z., & Zhang, J. C. (2003). Analysis of particle size control in the preparation of nano-size CaCO3 particles. Chinese Journal of Chemical Engineering, 11(5), 589–593. Chen, G. G., Luo, G. S., Li, S. W., Xu, J. H., & Wang, J. D. (2005). Experimental approaches for understanding mixing performance of a minireactor. AIChE Journal, 51(11), 2923–2929. Chen, G. G., Luo, G. S., Yang, X. R., Sun, Y. W., & Wang, J. D. (2004). Preparation of ultra-fine TiO2 particles using micro-mixing precipitation technology. Journal of Inorganic Materials, 19(5), 1163–1167. Chen, X. H., Li, C. Z., Xu, S. F., Zhang, L., Shao, W., & Du, H. L. (2006). Interfacial adhesion and mechanical properties of PMMA-coated CaCO3 nanoparticle reinforced PVC composites. China Particuology, 4(1), 25–30. Ding, H., Lu, S.-C., Deng, Y.-X., & Du, G.-X. (2007). Mechano-activated surface modification of calcium carbonate in wet stirred mill and its properties. Transactions of Nonferrous Metals Society of China, 17(5), 1100–1104. Duraiswamy, S., & Khan, S. A. (2009). Droplet-based microfluidic synthesis of anisotropic metal nanocrystals. Small, 5(24), 2828–2834. Guenther, P. M., Gross, G. A., Wagner, J., Jahn, F., & Koehler, J. M. (2008). Introduction of surface-modified Au-nanoparticles into the microflow-through polymerization of styrene. Chemical Engineering Journal, 135, S126–S130. Guo, W., Luo, G. S., & Wang, Y. J. (2003). Synthesis and coating of ordered mesoporous silica. China Particuology, 1(4), 151–155. Katz, E., & Willner, I. (2004). Integrated nanoparticle-biomolecule hybrid systems: Synthesis, properties, and applications. Angewandte Chemie-International Edition, 43(45), 6042–6108. Koehler, J. M., Held, M., Huebner, U., & Wagner, J. (2007). Formation of Au/Ag nanoparticles in a two step micro flow-through process. Chemical Engineering & Technology, 30(3), 347–354. Kong, S., Zhang, P., Wen, X., Pi, P., Cheng, J., Yang, Z., et al. (2008). Influence of surface modification of SrFe12 O19 particles with oleic acid on magnetic microsphere preparation. Particuology, 6(3), 185–190. Konopacka-Lyskawa, D., & Lackowski, M. (2011). Influence of ethylene glycol on CaCO(3) particles formation via carbonation in the gas-slurry system. Journal of Crystal Growth, 321(1), 136–141. Lee, W.-B., Weng, C.-H., Cheng, F.-Y., Yeh, C.-S., Lei, H.-Y., & Lee, G.-B. (2009). Biomedical microdevices synthesis of iron oxide nanoparticles using a microfluidic system. Biomedical Microdevices, 11(1), 161–171. Li, S. W. (2009). Controllable preparation of nano-particles in micro-structured systems. Doctoral dissertation. Department of Chemical Engineering, Tsinghua University, China. Li, S. W., Xu, J. H., Wang, Y. J., & Luo, G. S. (2008). Controllable preparation of nanoparticles by drops and plugs flow in a microchannel device. Langmuir, 24(8), 4194–4199. Luo, G. S., Du, L., Wang, Y. J., Lu, Y. C., & Xu, J. H. (2011). Controllable preparation of particles with microfluidics. Particuology, 9(6), 545–558. Ma, X., Liu, Y., Yu, Y., Lei, H., Lv, X., Zhao, L., et al. (2008). The influence of the different modifying agents on the synthesis of poly(methylmethacrylate)-calcium carbonate nanocomposites via soapless emulsion polymerization. Journal of Applied Polymer Science, 108(3), 1421–1425.

7

Novokshonova, L. A., Meshkova, I. N., Ushakova, T. M., Grinev, V. G., Ladigina, T. A., Gultseva, N. M., et al. (2003). Modification of properties of CaCO3 polymerization-filled polyethylene. Journal of Applied Polymer Science, 87(4), 577–583. Raschke, G., Kowarik, S., Franzl, T., Sonnichsen, C., Klar, T. A., Feldmann, J., et al. (2003). Biomolecular recognition based on single gold nanoparticle light scattering. Nano Letters, 3(7), 935–938. Sahebian, S., Zebarjad, S. M., Khaki, J. V., & Sajjadi, S. A. (2009). The effect of nano-sized calcium carbonate on thermodynamic parameters of HDPE. Journal of Materials Processing Technology, 209(3), 1310–1317. Shen, C., Wang, Y. J., Xu, J. H., Lu, Y. C., & Luo, G. S. (2012). Preparation and ion exchange properties of egg-shell glass beads with different surface morphologies. Particuology, 10(3), 317–326. Shui, M. (2003). Polymer surface modification and characterization of particulate calcium carbonate fillers. Applied Surface Science, 220(1–4), 359–366. Song, M. G., Kim, J. Y., & Kim, J. D. (2003). Effect of sodium stearate and calcium ion on dispersion properties of precipitated calcium carbonate suspensions. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 229(1–3), 75–83. Sun, Q. H., & Deng, Y. L. (2004). Synthesis of micrometer to nanometer CaCO3 particles via mass restriction method in an emulsion liquid membrane process. Journal of Colloid and Interface Science, 278(2), 376–382. Tran, H. V., Tran, L. D., Vu, H. D., & Thai, H. (2010). Facile surface modification of nanoprecipitated calcium carbonate by adsorption of sodium stearate in aqueous solution. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 366(1–3), 95–103. Wang, C., Piao, C., Zhai, X., Hickman, F. N., & Li, J. (2010). Synthesis and character of super-hydrophobic CaCO3 powder in situ. Powder Technology, 200(1–2), 84–86. Wang, C., Sheng, Y., Bala, H., Zhao, X., Zhao, J., Ma, X., et al. (2007). A novel aqueousphase route to synthesize hydrophobic CaCO3 particles in situ. Materials Science & Engineering C: Biomimetic and Supramolecular Systems, 27(1), 42–45. Wang, C., Zhao, X., Zhao, J., Liu, Y., Sheng, Y., & Wang, Z. (2007). Biomimetic nucleation and growth of hydrophobic vaterite nanoparticles with oleic acid in a methanol solution. Applied Surface Science, 253(10), 4768–4772. Wang, H., Tang, L., Wu, X., Dai, W., & Qiu, Y. (2007). Fabrication and anti-frosting performance of super hydrophobic coating based on modified nano-sized calcium carbonate and ordinary polyacrylate. Applied Surface Science, 253(22), 8818–8824. Wang, K., Wang, Y. J., Chen, G. G., Luo, G. S., & Wang, J. D. (2007). Enhancement of mixing and mass transfer performance with a microstructure minireactor for controllable preparation of CaCO3 nanoparticles. Industrial & Engineering Chemistry Research, 46(19), 6092–6098. Wang, Y., Lu, J., & Wang, G. H. (1997). Toughening and reinforcement of HDPE/CaCO3 blends by interfacial modification interfacial interaction. Journal of Applied Polymer Science, 64(7), 1275–1281. Yang, L. M., Wang, Y. J., Luo, G. S., & Dai, Y. Y. (2008). Preparation and functionalization of mesoporous silica spheres as packing materials for HPLC. Particuology, 6(3), 143–148. Ye, J., & Zhang, X. F. (2004). Microwave-assisted surface modification of calcium bicarbonate. China Particuology, 2(1), 37–40.

Please cite this article in press as: Du, L., et al. In situ preparation of hydrophobic CaCO3 nanoparticles in a gas–liquid microdispersion process. Particuology (2013), http://dx.doi.org/10.1016/j.partic.2012.07.009