Growth of Aragonite CaCO3 Whiskers in a Microreactor with Calcium

Jun 29, 2015 - This study presents a novel technique for the controllable preparation of aragonite CaCO3 whiskers using a microdispersion process. Cal...
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Growth of Aragonite CaCO3 Whiskers in a Microreactor with Calcium Dodecyl Benzenesulfonate as a Control Agent Le Du,† Yujun Wang,‡ Kai Wang,‡ and Guangsheng Luo*,‡ †

The State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of Membrane Science and Technology, Beijing University of Chemical Technology, Beijing 100029, China ‡ The State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China ABSTRACT: This study presents a novel technique for the controllable preparation of aragonite CaCO3 whiskers using a microdispersion process. Calcium dodecyl benzenesulfonate [Ca(Ar−SO3)2] was first used as an additive for controlling the crystalline form. A homogeneous concentration distribution in the system was achieved due to the efficient micromixing. The effects of various operation parameters on the whiskers were determined, and the reaction conditions were optimized. The mechanism of the additive was discussed. The dynamic adsorption of Ca(Ar−SO3)2 on the surface of whiskers was experimentally demonstrated to be in charge of maintaining the one-dimensional growth and the high purity of the crystals. Aragonite CaCO3 with a crystalline purity of 98−99.5% was controllably synthesized at room temperature. The whiskers were approximately 27 μm in length and had an aspect ratio of about 58.

1. INTRODUCTION Calcium carbonate (CaCO3), one of the most widely existing minerals in nature, has been extensively used in rubber, plastic, printing ink, dope, toothpaste, cosmetics, and food industries.1−5 CaCO3 has three different crystalline forms: calcite, aragonite, and vaterite.6,7 Aragonite CaCO3 in the form of a needle-like crystal is regarded as a functional inorganic material that can alter the optical and mechanical properties of pigment, plastics, and paints.8−10 Especially in biologic systems, the crystal lattice of aragonite can be stable with the existence of amino acids in microenvironments, which makes aragonite CaCO3 a possible biomedical material in the future.11−13 However, among these three anhydrous polymorphs, calcite is the most stable, while aragonite and vaterite are the metastable and unstable forms of CaCO3, respectively.14−16 To keep the continuous growth of aragonite CaCO3, it is necessary to ensure that the concentrations of the ions are within the metastable region (between the saturation and supersaturation curves), as shown in Figure 1. QC is the corresponding ion product, which is represented as Q C = [Ca 2 +] × [CO32 −] 2+

[CO32−]

Figure 1. Schematic illustration of synthesis conditions for different crystalline phases of CaCO3.

the solution of soluble calcium salts for controlling the growth of aragonite CaCO3 at high temperatures.23 The corresponding carbonates of these additives are usually sparingly soluble and thus can be utilized to control the supersaturation and slow growth of aragonite CaCO3. On the other hand, needle-like aragonite CaCO3 whiskers have also been obtained in water− organic solvent systems. For instance, the aragonite whiskers could be prepared using 50% water and 50% pyridine as the solvents.24 High purity aragonite whiskers have also been synthesized in an aqueous solution (the ratio of ethanol to water was 1:3) without any additives.2 Of all these approaches, the aims are to control the supersaturations of the systems. Higher temperatures, lower concentrations, and longer reaction time are required for the formation of aragonite.25,26 Thus,

(1) 2+

where [Ca ] and are the concentrations of Ca and CO32−, respectively. As QC exceeds the supersaturation level, the generation of calcite occurs. Thus, the precise control of the synthesis of aragonite CaCO3 is of particular importance. Normally, high-quality aragonite CaCO3 crystals have a whisker length of larger than 20 μm, a high aspect ratio of larger than 35, and a phase purity of larger than 98%.17,18 Some techniques have been employed to realize the controllable growth of aragonite CaCO3. The existing synthetic approaches can be divided into two categories: (1) slow precipitation in the presence of an additive as the control agent, and (2) synthesis in water−organic solvent systems.19−22 Inorganic additives containing ions such as Sr2+, Ba2+, Pb2+, and Mg2+ are added to © XXXX American Chemical Society

Received: April 9, 2015 Revised: June 6, 2015 Accepted: June 29, 2015

A

DOI: 10.1021/acs.iecr.5b01338 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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achieved in this homogeneous reaction system. Therefore, we suggest introducing an alcohol soluble additive, calcium dodecyl benzenesulfonate [Ca(Ar−SO3)2], as the control agent, and employing homogeneous ethanol−water solvent mixture as the working system. Ca(Ar−SO3)2 could be not only dispersed in the system, but also adsorbed on the whiskers by the electrostatic forces,28 which is considered as a dynamic adsorption. The additive and the solvent mixture might also be recycled to dissolve raw materials after the solid−liquid separation. Furthermore, the ability to create homogeneous reaction environment with microfluidic devices has led to the development of novel synthesis and fabrication techniques of nanoparticles and other powder materials.29−31 In our previous study, a membrane dispersion microreactor was developed and has been successfully used to prepare nanometer and submicrometer particles.32−35 Especially for the synthesis of β-Ca2(PO4)3 particles, the process was controllable, and the reaction environment remained homogeneous during a relatively long period of time.35 In this study, a novel preparation technique of aragonite CaCO3 whiskers using a membrane dispersion microreactor has been developed. Ca(Ar−SO3)2 was first used as an additive for controlling the crystalline form. High-quality aragonite CaCO3 was successfully prepared using the new technique. The effects of the operating conditions were investigated, and the growth mechanism with Ca(Ar−SO3)2 as the control agent was also discussed.

some serious challenges remain in most of the processes such as the controllability and efficiency of the mass transfer. Under the conditions of higher local concentrations caused by poor mixing, calcite crystals tend to form, and the purity of the aragonite is relatively low. Even the generated aragonite CaCO3 whiskers also tend to dissolve and transform into the calcite particles, as shown in Figure 2, panel a. Both homogeneous

Figure 2. Schematic representation of the preparation CaCO3 whiskers in different systems: (a) traditional technique without additives; (b) preparation with dissoluble additives.

reaction environment and control agents for preventing the crystal phase transformation are required to realize the controllable preparation of the aragonite CaCO3 whiskers. It has been confirmed that different biomineralization processes of CaCO3 could be conducted by utilizing organic matrices or templates.19,20,27 For example, alginate that forms one-dimensional framework could be used to promote the nucleation of CaCO3 and lead to a controllable crystallization.36 If the organic additives are uniformly dispersed in the system and dynamically adsorbed on the CaCO3 surfaces, the organic chains will link with the whiskers on the side and keep the onedimensional growth, as shown in Figure 2, panel b. In the case of this condition, an efficient mixing process could also be

2. EXPERIMENTAL SECTION 2.1. Materials. Dodecyl benzenesulfonic acid (Ar−SO3H, 90 wt % analytical purity, purchased from Aladdin Industrial Co.) and ethanol (99.5 wt % analytical purity, purchased from Beijing Modern Eastern Fine Chemical Co., Ltd.) were used. The other main raw materials, namely, Ca(OH)2 powder (analytically pure) and CO2 mixed gas (10.0 vol %, 29.8 vol % and 99.9 vol %, mixed with N2), were purchased from Beijing Chemical Works and Beijing Hua Yuan Gas Chemical Industrial Co., Ltd., respectively.

Figure 3. Experimental setup for preparing CaCO3 whiskers. B

DOI: 10.1021/acs.iecr.5b01338 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research 2.2. Synthesis of Aragonite Whiskers. The experimental setup for the microreaction is shown in Figure 3. A 1-μm microfiltration membrane with a working area of 12.5 mm2 was used as the dispersion medium in the microreactor. The geometric size of the main channel was 20 mm × 2 mm × 0.5 mm (length × width × height). Ca(OH)2 solutions or suspensions (0.008−0.34 mol/L) were mixed with different concentrations and amounts of prepared Ca(Ar−SO3)2 (the molar ratio of Ca(Ar−SO3)2 to Ca(OH)2 was from 0−0.2) as the continuous phase. The mixed CO2 gas (0.3 MPa) was then dispersed into the continuous phase in the form of microbubbles. To realize the complete reaction of Ca(OH)2, the continuous phase was circulated at a certain flow rate. At the beginning, the pH of the system was 12.3. The reaction required 2−6 h under different conditions and was stopped when the pH value was 8. Then the precipitates were separated from the solution with a centrifugal separator (LD5−2A, Beijing Medical Centrifugal Separator Factory). The prepared whiskers were washed twice with distilled water, washed once with ethanol, and dried in a drying cabinet at 100 °C for 24 h. The preparation was also carried out in a stirred tank reactor for comparison. The solution of Ca(Ar−SO3)2 was prepared first by mixing the ethanol solution of Ar−SO3H and the aqueous solution of Ca(OH)2. Ca(OH)2 solution (0.008 mol/ L) was mixed with the prepared Ca(Ar−SO3)2 (the molar ratio of Ca(Ar−SO3)2 to Ca(OH)2 was from 0−0.1) in a 1000 mL beaker equipped with a propeller agitator (300 r/min). The mixed CO2 gas (0.3 MPa) was then bubbled through an orifice with a diameter of 0.5 mm into the beaker (2 mL/min). The reaction process was stopped when the pH was 8. The subsequent procedures were the same as those of the microreaction. 2.3. Characterization Methods. The morphologies of the prepared whiskers were recorded by transmission electron microscope (TEM) and scanning electron microscope (SEM). Fourier transform infrared (FTIR) spectroscopy (Bruker Corporation, TENSOR 27) was used to identify the molecular structures. The weight loss was measured by thermogravimetric analysis (TGA; STA 409 PC). The crystalline forms of the whiskers were characterized by X-ray diffraction analysis (XRD; Rigaku Corporation D/max-RB).

Figure 4. SEM images of CaCO3 whiskers at different synthesis conditions (a, c) in the stirred tank or (b, d) in the microreactor: (a) [Ca(Ar−SO3)2] = 0 mol/L; (b) [Ca(Ar−SO3)2] = 0 mol/L, FD = 2 mL/min, FC = 20 mL/min; (c) [Ca(Ar−SO3)2] = 0.0008 mol/L; (d) [Ca(Ar−SO3)2] = 0.0008 mol/L, FD = 2 mL/min, FC =20 mL/min. Other conditions: T = 20 °C, V(CO2) = 29.8%, [Ca(OH)2] = 0.008 mol/L. The scale bar corresponds to 10 μm.

found. Upon addition of Ca(Ar−SO3)2, the whisker growth was significantly improved, as shown in Figure 4, panel d. The continuity of the growth might be caused by the dynamical adsorption of Ca(Ar−SO3)2 on the surfaces of CaCO3 whiskers and the change of the mass transfer process, which should be verified in the growth experiments. Figure 5 shows the effects of the molar ratio of Ca(Ar−SO3)2 to Ca(OH)2. Without the addition of Ca(Ar−SO3)2, the

3. RESULTS AND DISCUSSION 3.1. Influence of Ca(Ar−SO3)2 and Microreaction Process on CaCO3 Whiskers. The effects of the microreaction and Ca(Ar−SO3)2 as the control agent were first investigated. Figure 4 shows the SEM images of the CaCO3 whiskers prepared by using the microreactor (b, d) and the stirred tank reactor (a, c). The CaCO3 whiskers are uniform in size using the microreactor at the same conditions. In contrast, CaCO3 whiskers prepared in the stirred tank reactor were of low quality. The whiskers were uneven in average length and apparently lacked growth. Especially with the addition of Ca(Ar−SO3)2, large particles with an average size of 1.8 μm were generated, which might be caused by the agglomeration of the calcite CaCO3 nanoparticles. The effect of Ca(Ar−SO3)2 as the control agent could also be confirmed by comparing Figure 4, panel a with panel c and panel b with panel d, respectively. In the stirred tank reactor, the whiskers are approximately twice as large as those prepared without Ca(Ar−SO3)2 and have a higher average aspect ratio. For the microreaction samples, even greater differences were

Figure 5. Effects of the molar ratio of Ca(Ar−SO3)2 to Ca(OH)2 on CaCO3 whiskers. [Ca(OH)2] = 0.008 mol/L, V(CO2) = 29.8%. Other conditions: T = 20 °C, FD = 2 mL/min, FC =20 mL/min.

whisker length was about 6 μm, and the aspect ratio was about 22. An increase in molar ratio resulted in increasing of the whisker length and aspect ratio to their maximum values at a molar ratio of 0.05. CaCO3 whiskers of approximately 27 μm in length and an aspect ratio of 58 were synthesized. 3.2. Influence of Operating Conditions on CaCO3 Whiskers. A series of experiments were performed to optimize the operating conditions. The concentrations of the reactants, feed flow rates, and reaction temperatures were optimized. To C

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Figure 6. Effects of the raw material ratios and concentrations on the whiskers: (a) Ca(OH)2 concentration varying, V(CO2) = 29.8%; (b) CO2 concentration varying, [Ca(OH)2] = 0.008 mol/L; (c) volume ratio of ethanol to H2O varying, [Ca(OH)2] = 0.008 mol/L, V(CO2) = 29.8%. Other conditions: T = 20 °C, FD = 2 mL/min, FC =20 mL/min.

Figure 7. Effects of reaction temperature and feed flow rates on the whiskers: (a) without the addition of Ca(Ar−SO3)2, FD = 2 mL/min, FC =20 mL/min; (b) with the addition of Ca(Ar−SO3)2, FD = 2 mL/min, FC =20 mL/min; (c) FD phase ratio varying, T = 20 °C, FC = 20 mL/min; (d) FC varying, T = 20 °C, FD = 2 mL/min. Other conditions: [Ca(OH)2] = 0.008 mol/L, V(CO2) = 29.8%.

Figure 6, panel a shows the influence of Ca(OH) 2 concentration on the whiskers. A significant growth was observed when the concentration was less than 0.024 mol/L,

obtain a statistically representative measurement of sizes and aspect ratios of the CaCO3 whiskers, more than two hundred whiskers were acquired from SEM images for each sample. D

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Figure 8. TEM images of CaCO3 whiskers at different synthesis conditions: (a, c) [Ca(OH)2] = 0.024 mol/L, [Ca(Ar−SO3)2] = 0.0024 mol/L; (b, d) [Ca(OH)2] = 0.008 mol/L, [Ca(Ar−SO3)2] = 0.0008 mol/L, the SAED pattern was taken on a typical single fiber. Other conditions: T = 20 °C, V(CO2) = 29.8%, FD = 2 mL/min, FC =20 mL/min.

3.3. Characterization of CaCO3 Whiskers. The CaCO3 whiskers were characterized by various techniques. The lengths and aspect ratios of the prepared whiskers were comparable to those of high quality products. The characterization of the crystalline forms also confirmed the high purity of the whiskers. More information on the whiskers could be further provided by TEM images, as shown in Figure 8. Clearly, the whiskers had a relatively uniform morphology. The width of effectively grown whiskers was approximately 300−450 nm. No obvious impurity could be found at the high magnification view. In addition, the CaCO3 whiskers are highly single crystalline, which is confirmed by selected area electron diffraction (SAED) pattern, and have a preferential growth along (001), as shown in Figure 8, panel d. FTIR spectra of CaCO3 whiskers are shown in Figure 9. The absorption peaks at 854 cm−1 (out-of-plane bending) and 706 cm−1 (in-of-plane bending) were attributed to the ν2 and ν4 vibrations to −CO− in aragonite CaCO3, which could be observed in all the samples. However, the absorption peaks [848 cm−1 (ν2) and 714 cm−1 (ν4)] corresponding to calcite CaCO3 were also found in whiskers synthesized without Ca(Ar−SO3)2, even at the low Ca(OH)2 concentration of 0.008 mol/L. By contrast, upon addition of Ca(Ar−SO3)2, the purity of whiskers was significantly improved. No absorption peaks of calcite were found when the Ca(OH)2 concentration was lower than 0.024 mol/L. In addition, the absorption peak of Ar−SO3− (1210 cm−1) was not observed in all these samples, which indicated no Ca(Ar−SO3)2 adsorbing on the surface of final whiskers. XRD patterns of the CaCO3 crystals obtained in the presence and absence of Ca(Ar−SO3)2 are presented in Figure 10. Although the growth was relatively effective at low supersaturation conditions and no particles were observed in SEM images, the calcite CaCO3 was still obtained (curve b). The

which was also the solubility of Ca(OH)2 in H2O. The growth was not significantly affected by the CO2 concentration (Figure 6b) because of the low feed flow rate and thus the relatively low supersaturation of the system. Figure 6, panel c shows the influence of the volume ratio of ethanol to water. The whisker growth was enhanced with the increase in the volume fraction of ethanol, which might be attributed to a higher dispersity of Ca(Ar−SO3)2 and hence the higher adsorption affinity on the crystal seeds. Reaction temperature influences solubilities, diffusion coefficients, and supersaturations of reactants. Figure 7, panel a shows the effects of temperature on the synthesis without the addition of Ca(Ar−SO3)2. Although higher temperatures were conducive to the growth of CaCO3 whiskers, the whisker length was only 14 μm, and the aspect ratio was 35 at 70 °C. The growth behavior was apparently changed after Ca(Ar−SO3)2 was added into the system, as shown in Figure 7, panel b. A decrease in temperature was accompanied by the enhancement of the stability of ethanol and the dispersibility of Ca(Ar− SO3)2. Even at room temperature, the growth was also sufficient according to the standard of the high-quality products. The whisker lengths under different dispersed-phase flow rates (FD) are presented in Figure 7, panel c. The mixing performance was not apparently affected by the dispersed-phase flow rate, and there was little difference among the products at these conditions. The influence of the continuous-phase flow rate (FC) is shown in Figure 7, panel d. The enhancement of whisker growth became stable when FC was increased to 20 mL/min. The increased FC reduced the mixing scale and shortened the mass transfer distance. Thus, relative uniform concentration distributions that were dominant in this slow reaction were achieved.37 E

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(curves c, d). Thus, the enhancement of the growth of aragonite CaCO3 by Ca(Ar−SO3)2 was confirmed. 3.4. Mechanism Analysis of Growth Process. For the purpose of understanding the mechanism of the control agent, dynamic evolution of the reaction and the crystalline forms was investigated in the presence and absence of Ca(Ar−SO3)2. In the Ca(OH)2−CO2 system, the pH value could represent the reaction process. To determine whether or not the addition of Ca(Ar−SO3)2 would affect the reaction process, the pH value was recorded during the synthesis under different conditions. Figure 11, panel a shows the influence of FD on pH values. Clearly, the changes of pH values were similar at the same flow rate, and hence the influence of Ca(Ar−SO3)2 on the mass transfer could be ignored. Figure 11, panel b shows the influence of the ratio of Ca(Ar−SO3)2 on the reaction process. The pH value was not significantly influenced by the dosage, indicating that Ca(Ar−SO3)2 had little influence on the mass transfer. Therefore, we could conclude that the control agent, Ca(Ar−SO3)2, was not practically involved in the reaction. However, Ca(Ar−SO3)2 was able to enhance the stability of aragonite CaCO3, as mentioned previously. Thus, it is also necessary to analyze the influences of Ca(Ar−SO3)2 on the whisker growth and the crystalline forms. The TGA curves in Figure 12 show the weight losses of CaCO3 whiskers under different conditions. The results of the

Figure 9. FTIR spectra of whiskers at different synthesis conditions: (a) [Ca(OH)2] = 0.024 mol/L, [Ca(Ar−SO3)2] = 0 mol/L; (b) [Ca(OH)2] = 0.008 mol/L, [Ca(Ar−SO3)2] = 0 mol/L; (c) [Ca(OH)2] = 0.067 mol/L, [Ca(Ar−SO3)2] = 0.0067 mol/L; (d) [Ca(OH)2] = 0.024 mol/L, [Ca(Ar−SO3)2] = 0.0024 mol/L. Other conditions: T = 20 °C, V(CO2) = 29.8%, FD = 2 mL/min, FC =20 mL/ min.

Figure 10. XRD patterns of whiskers at different synthesis conditions: (a) [Ca(OH)2] = 0.024 mol/L, [Ca(Ar−SO3)2] = 0 mol/L; (b) [Ca(OH)2] = 0.008 mol/L, [Ca(Ar−SO3)2] = 0 mol/L; (c) [Ca(OH)2] = 0.067 mol/L, [Ca(Ar−SO3)2] = 0.0067 mol/L; (d) [Ca(OH)2] = 0.024 mol/L, [Ca(Ar−SO3)2] = 0.0024 mol/L. Other conditions: T = 20 °C, V(CO2) = 29.8%, FD = 2 mL/min, FC =20 mL/ min.

Figure 12. Thermogravimetric curves of whiskers at different synthesis conditions: (a) [Ca(OH)2] = 0.024 mol/L, [Ca(Ar−SO3)2] = 0 mol/ L; (b) [Ca(OH)2] = 0.008 mol/L, [Ca(Ar−SO3)2] = 0 mol/L; (c) [Ca(OH)2] = 0.067 mol/L, [Ca(Ar−SO3)2] = 0.0067 mol/L; (d) [Ca(OH)2] = 0.024 mol/L, [Ca(Ar−SO3)2] = 0.0024 mol/L; (e) [Ca(OH)2] = 0.024 mol/L, [Ca(Ar−SO3)2] = 0.0024 mol/L, the products had not been washed before drying. Other conditions: T = 20 °C, V(CO2) = 29.8%, FD = 2 mL/min, FC =20 mL/min.

crystalline peaks corresponding to calcite were even stronger with the increasing supersaturation in the synthesis process (curve a). In comparison, sharp crystalline peaks corresponding to aragonite were observed in the presence of Ca(Ar−SO3)2

Figure 11. Change of pH values with reaction time: (a) FD varying; (b) molar ratio of Ca(Ar−SO3)2 to Ca(OH)2 varying, FD =2 mL/min. Other conditions: [Ca(OH)2] = 0.008 mol/L, T = 20 °C, V(CO2) = 29.8%, FC =20 mL/min. F

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Figure 13. (a) Effects of QC on Xa in absence or presence of Ca(Ar−SO3)2, T = 20 °C. (b) Experimentally determined synthesis conditions for different crystalline phases of CaCO3: Ksp is the solubility product of aragonite CaCO3.

crystal morphologies: the whisker region, the whisker−particle transition region, and the particle region, as shown in Figure 14.

samples synthesized with or without Ca(Ar−SO3)2 were similar. No obvious weight loss could be observed except that of the decomposition of CaCO3. In comparison, for the samples that had not been washed before the drying process, a decrease in weight occurred at temperatures ranging from 420− 460 °C. The weight loss was caused by the Ca(Ar−SO3)2 adsorbed on whiskers. These results indicated that Ca(Ar− SO3)2 played a role as the additive for controlling of the crystalline form, which also verified the assumption we proposed previously. The fractions of different crystalline forms in the precipitated CaCO3 can be calculated based on the results of XRD.19,23 The equation is presented as 3.4(Ia /Ic) Xa = 1 + 3.4(Ia /Ic)

Figure 14. Different precipitation regions of CaCO3 crystals: additive content refers to the molar ratio of Ar-SO3H to CaCO3 products.

(2)

The boundaries between the regions are roughly determined based on the results of operating condition experiments with different ion products and contents of the additive. The ion product of boundaries is about 7.59, 18.98, and 46.32 ( × 10−9 mol−2 L−2), respectively. On the basis of the division of CaCO3 crystal morphologies, we could get a preliminary conclusion that the precise control of homogeneous mixing is the critical factor to synthesize aragonite whiskers of high qualities. A higher local concentration of the reactants would cause the generation of calcite particles and reduction of the product purity. 3.5. Growth Mechanism of CaCO3 Whiskers. Whisker growth depends on step growth in axial direction and kink growth in radial direction, whose rates could be usually denoted as G and v, respectively.38 A high G corresponds to thermally activated attachment of ions onto the top surface, while a low v corresponds to thermally activated detachment of ions from the side surface. On the basis of the whisker morphologies by SEM, the growth process of CaCO3 whiskers could be quantified, and both G and v could be obtained, as shown in Figure 15, panel a. Upon addition of Ca(Ar−SO3)2, both the growth rates were increased. Especially, the difference between G and v was significantly enlarged, leading to a high aspect ratio of the CaCO3 whiskers. According to the step and kink growth kinetics raised by Chernov,39 G could be calculated by the following equation:

where Xa corresponds to the fraction of aragonite, and Ia/Ic is the ratio of the integrated peak areas corresponding to aragonite (111) reflection (2θ = 26.1°) and calcite (104) reflection (2θ = 29.2°). To compare the differences of crystalline forms under similar conditions, the relationships between QC (eq 1) and Xa are summarized. [CO32−] can be derived by eq 3 as follows: ρ [CO32 −] = HpCO = p 2 EMS CO2 (3) where E is the Henry coefficient, and the value is 1.66 × 105 kPa at 25 °C. The calculations are shown in Figure 13, panel a. Clearly, the purity of aragonite CaCO3 in the precipitates increased significantly when Ca(Ar−SO3)2 was added. Even when QC was 4−5-times larger than that of the system without Ca(Ar−SO3)2, Xa was still larger than 98%. On the basis of the measurements and calculations of Xa, the metastable range of aragonite CaCO3 could be experimentally determined. The results are shown in Figure 13, panel b, where the supersaturation curves (denoted as QC0) are determined by the inflection point of Xa ≥ 98%. The metastable range for aragonite form was significantly extended with the addition of Ca(Ar−SO3)2. Especially at lower temperatures, the effective operating region for aragonite was extended, which was favorable for the controllable preparation of aragonite CaCO3. In our previous study, hydrophobic CaCO3 nanoparticles were synthesized using the same reactants including CO2, Ca(OH)2, and Ar−SO3H.34 By integrating the results of the previous study and investigating CaCO3 whiskers, precipitation of CaCO3 could be divided into three regions with different

G = b[chBC − λ ∑ ni exp( −Ei /kT )] = Ωβ(C − Ce) (4)

where Ω is the volume per crystal lattice; C is the concentration of solution (in terms of [Ca2+]); and Ce is the equilibrium G

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Figure 15. (a) Growth rate changes in the growth process. T = 20 °C; the subscript “0” refers to the absence of Ca(Ar−SO3)2. (b) Effects of the reactant concentrations on G; the solid lines refer to experimental results, and the dash lines refers to the theoretical data.

Table 1. Comparison of Different Methods for Synthesizing CaCO3 and Kinetic Coefficients batch reactor form control agent β (10−3 m/s) length or diameter (μm) aspect ratio

particle 0.2−0.9 0.05 1

batch reactor21

microreactor whisker Ca(Ar−SO3)2 3−5 24−28 50−58

concentration at QC0 when the attachment of ions equals to the detachment. The parameter β is commonly regarded as the kinetic coefficient. v could be calculated by the following equation:

whisker 0.2−1 10−15 16−30

batch reactor20

batch reactor22

whisker SDS 1.6−3 8−16 22−30

whisker MgCO3 1.6−2.5 8−12 12−20

Ca(Ar−SO3)2, plays an important role in maintaining the axial growth of CaCO3 whiskers. In brief, the assumption that the stabilization of aragonite CaCO3 whiskers by introducing Ca(Ar−SO3)2 as a control agent (as mentioned in Figure 2) is verified. The analysis above could adequately explain the increase of the purity in the presence of Ca(Ar−SO3)2. However, this approach requires further research because the interaction between the additive and the whisker is still unclear.

v = Ω[Cef (n1) − β(C − Ce)] = Ω[2γa /σd − β(C − Ce)] (5)

where γa refers to step edge energies per crystal lattice (a equals the lattice spacing), and σd refers to the critical supersaturation below which no crystal growth occurs (a socalled “dead zone”).40 Among all these parameters for the assigned CaCO3 precipitation, β, which represents the attempt frequency and probability of raw material onto the crystal surface, is considered to be determinant in the growth degree for axial and racial directions. By combining the experimental data of the CaCO3 whiskers, β under different growth conditions was calculated. The calculation was based on the step and kink growth kinetics as well as the presented parameters. Figure 15, panel b presents the comparison between the experimental results and the theoretical data predicted by using eqs 4 and 5. With the addition of Ar−SO3H, the growth behaviors of the whiskers are approximated to the linear trend as predicted. β is close to the theoretical value at high reactant concentrations. Without Ar− SO3H, the growth rates are not as high as the predicted value when the concentration increases. For all these cases, although high growth rate was maintained, the nucleation still consumes most raw materials when the concentration keeps increasing, as mentioned in Figures 13, panel b and 14. In addition, β under the conditions used in other studies has also been calculated for comparison, as shown in Table 1. These results show conclusively that β is almost five-times as high as that without Ca(Ar−SO3)2 and is twice as high as that with sodium dodecyl sulphate (SDS) or MgCO3, causing the different behaviors in whisker growth. It is noteworthy that under the same solution condition, the coefficients are similar in systems for both particles and whiskers, indicating that β is mainly affected by the solution condition. The control agent,

4. CONCLUSIONS In this study, we have proposed a novel microreaction process for preparing aragonite CaCO3 whiskers with Ca(Ar−SO3)2 as a control agent. Aragonite CaCO3 with a crystalline purity of 98−99.5% could be controllably prepared at room temperature. The whiskers were approximately 27 μm in length and had an aspect ratio of about 58. The effects of operating conditions were examined and optimized. The possible mechanism of utilizing Ca(Ar−SO3)2 as the control agent was discussed. The organic chains might dynamically adsorb on the surface of whiskers and prevent the aragonite whiskers from dissolving and transforming into calcite particles since the mass transfer process was not apparently affected by the addition of Ca(Ar− SO3)2. The obtained results suggest that using soluble organic additives has a potential for controlling the one-dimensional growth of whiskers and being highly recyclable and reusable. Future studies should focus on investigating the interaction between the control agent and the whisker, and corresponding mechanisms of controlling the crystalline form.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. H

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ACKNOWLEDGMENTS

We gratefully acknowledge the support of the National Natural Science Foundation of China (Grant No. 2013CB733600) and the National Natural Science Foundation of China (91334201, U1463208, 21276140).

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NOTATION [M] or CM = molar concentration (mol/L), M corresponds to the specific material a = the lattice spacing (m) E = Henry coefficient (kPa) FC = flow rate of the continuous phase (mL/min) FD = flow rate of the dispersed phase (mL/min) FT = total flow rate (mL/min) G = growth rate in axial direction (m/s) H = solubility coefficient [mol/(m3 kPa)] Ix = intensity of the XRD peaks, x corresponds to the specific crystalline phase MS = molecular weight of the solvent (kg/mol) p = partial pressure (kPa) QC = ion product (mol2/L2) T = temperature (°C or K) t = time (h) V = volume fraction (%) v = growth rate in radial direction (m/s) Xa = the fraction of aragonite (%) Ω = the volume per crystal lattice (m3) σd = the critical supersaturation (mol2/L2) ρ = density of Ca(OH)2 solution (kg/m3) γ = interfacial tension (N/m)

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