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The formation of amorphous calcium carbonate and its transformation mechanism to crystalline CaCO3 in laminar microfluidics Zeng Youpeng, Jianwei Cao, Zhi Wang, Jianwei Guo, and Jinshan Lu Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01634 • Publication Date (Web): 01 Feb 2018 Downloaded from http://pubs.acs.org on February 12, 2018
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Crystal Growth & Design
The formation of amorphous calcium carbonate and its transformation mechanism to crystalline CaCO3 in laminar microfluidics Youpeng Zenga,b, Jianwei Caoa,* , Zhi Wanga,*, Jianwei Guoa, Jinshan Lub a
National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Key
Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China b
School of Materials Science and Engineering, Nanchang Hangkong University, Nanchang
330063, PR China
Abstract For traditional experimental methods, the dynamic formation and transformation of amorphous calcium carbonate (ACC) are hard to be captured due to its metastability and ease of transforming rapidly to the more stable phase. The emergence of microfluidic technology provides an effective approach on that issue, thus attracting widespread researchers interest in crystallization research. In this study, based on laminar microfluidics, we demonstrate a microfluidic approach toward the study of the formation and transformation of ACC. Through the control of velocity and concentration of CaCl2 and Na2CO3, the crystallization process was observed on chip under the microscope. We show by in situ confocal Micro Raman spectroscopy and theoretical calculation based on fluid dynamics simulation that the phase transformation pathways are different under the different supersaturation level in laminar microfluidics. ACC doesn't always appear in the crystallization process, when the supersaturation above the critical supersaturation of ACC occurred, the crystalline CaCO3 phases appear after the metastable intermediates ACC through the transformation mechanism of dissolution-recrystallization. While the crystalline CaCO3 phases formed without the intermediates ACC when below the critical supersaturation of ACC occurred. Our work provides an approach for the investigation of metastable intermediates and support for one-step and multistep nucleation mechanisms. *
Corresponding author. Tel & Fax: 86-010-82544818, E-mail:
[email protected]. Corresponding author. Tel & Fax: 86-010-82544818, E-mail:
[email protected].
*
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Keywords: microfluidic; metastable intermediate; reactive crystallization; amorphous calcium carbonate; Flow field simulation 1. Introduction Crystallization, a common phenomenon in nature, is of a considerable important process for the manufactures of high-value-added chemicals in chemical engineering, pharmacy, and semiconductor. Although the history that crystallization has been studied up to hundreds of years, little is known about the mechanism of reaction crystallization in solution, and there is not an accurate theory that can completely replace the empirical method to study crystallization1. The reason behind the phenomenon lies in the complex process of reaction crystallization in solution. The complexity not only includes the polymorphism exhibited by crystals, such as calcium carbonate2, alumina hydrate3, calcium oxalate4, and different polymorphs of the same compound can transform each other5, but structural changes of environmental phase, including environment temperature, concentration, pH, viscosity, additives and so on, is not simple6. Therefore, reaction crystallization is a complex process that coupling multiphase reaction and crystallization. Nevertheless, It happens that metastable intermediates often appear in that process, its appearance makes the crystallization mechanism more anfractuous, leading to non-classical nucleation theories7, which have suggested that there is more than one pathway with respect to crystal nucleation8, have been proposed. Ji9 et al. demonstrated that there is a coexistence of direct and indirect pathways about CaCO3 crystal nucleation by in situ TEM imaging of CaCO3 nucleation process. In addition, metastable intermediates, as an amorphous precursor before the final crystalline phase formed, have been observed not only in the crystallization system of calcium carbonate, but also in many other substances10,11. However, metastable intermediates are normally transient and easily transformed to kinetically and thermodynamically favored stable products. Thus, these features make it challenging for the traditional experimental methods to capture them, but they are the key to uncover the formation and transformation mechanism of polymorphic crystals, to understand the mechanism has important significance for the development and design of advanced material12. -2-
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Fortunately, the emergence of microfluidic technology provides an effective way, and becoming an ideal platform for the investigation of crystallization13. Compared with time-consuming and labor-intensive conventional approaches, only trace reagents are needed and crystallization conditions for selecting specific polymorphs can be rapidly screened based-on microfluidic crystallization14. More importantly, in situ observation of the crystallization process can be achieved on that platform, making it more easy to capture the appearance of metastable intermediates during the crystallization process. Therefore, microfluidic systems have opened a door for the study of crystallization, and many applications of microfluidics in crystallization, such as polymorph and cocrystal screening15, preparation of nanocrystals16, data acquisition for crystallization17, and protein crystallization18 are applied. When fluid is operated at low Reynolds number and the flow is laminar, it tends to exhibit different crystallization behaviors in such a unique environment. Just as Yashina19 et al 's research on calcium carbonate polymorph control based-on droplet microfluidic, they have shown that when equal volumes of equimolar aqueous solutions of CaCl2 and Na2CO3 mixed in droplet, calcite was yielded in 4 mM reagents, only vaterite in 8 mM reagents, but a mixture of calcite and vaterite were achieved in 10 mM reagents. In the aspect of probing polymorphism, Laval20 et al detected the polymorphism of KNO3 by microfluidic devices. Shinohara21 et al observed more than one metastable polymorphic forms of C60 on the microfluidic platform. Ildefonso22 et al probed a metastable polymorphic form of lysozyme in a simple microfluidic device. All these indicate that the microfluidic is an ideal platform for the research insights into crystallization mechanism of metastable intermediates, thus the same with ACC in laminar microfluidics. calcium carbonate, the most abundant crystalline biomineral, is the standard model in biomineralization and of significant importance as industrial raw material23. To unravel the fundamental mechanisms of calcium carbonate crystallization may not only have great significance for understanding the mechanism of biomineralization, but for developing its potential applications in encapsulation and drug delivery24. However, more attentions were paid to research the effect of additives on the -3-
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polymorphism and shape of calcium carbonate25,26,27, or more efforts were devoted to the preparation of size or/and shape controlled of calcium carbonate crystals on microfluidic platforms28,29. While environment phase how to influence the crystallization process of amorphous calcium carbonate in microscale, and what new crystallization behaviors will appear when the crystallization process is completely diffusion-controlled, these researches are not investigated full enough and further. As Matsuoka30 et al 's research revealed, the supercooled micro flow can keep stable less than -20 ℃ in microchannels without freezing. Therefore, compared with the bulk system, the crystallization characteristics of calcium carbonate under microscale need further study. More importantly, what formation and transformation mechanism of the metastable intermediate ACC appeared in the crystallization process will present in microfluidic, it is worthy of further exploration. From the environmental phase aspect of crystal growth, we have demonstrated a microfluidic approach toward the study on these issues based-on “Y-shaped” microfluidic chips. A range of fluid velocity and concentration of reagents conditions were investigated on chip. The crystallization process, including the formation of ACC was observed on chip under the microscope, models about the evolution of concentration and supersaturation profiles on the chip have been established through fluid dynamics simulation, and the transformation process of metastable intermediates ACC has also been monitored on chip combined with in situ micro Raman. Our work not only provides a new method for the study of metastable intermediates of polymorphs, but also our results provide experimental evidence for the one-step nucleation and multistep nucleation mechanisms. 2. Experimental Section 2.1 Fabrication of microfluidic devices Two types of chips were used in experiment, one is reversibly sealed Y-junction microfluidic chip, which the glass substrate etched with microchannels is reversibly sealed with PDMS coverslip. The other is irreversibly sealed with glass, which has the same channel structures, this one needs combine with a chip clamp when using, and the fluid flows into microchannel from the side of the chip. Two types of chips were -4-
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fabricated using standard soft lithographic techniques31, and they were completed fabrication by Dalian Institute of Chemical Physics. The “Y-shaped” microfluidic chips just as figure 1 shows, with all microchannels have a uniform section of 200 µm (width) × 100 µm (height).
(a)
(b)
Figure 1 Pictures of the “Y-shaped” microfluidic chips: (a) Reversibly sealed chip. (b) Irreversibly sealed chip 2.2 Crystallization of calcium carbonate in laminar microfluidic CaCO3 crystals were formed on chip by mixing equal volumes of equimolar aqueous solutions of CaCl2 and Na2CO3. Different concentrations (between 2 and 50 mM) of CaCl2 and Na2CO3 solutions were prepared in deionized water. Then small volumes of CaCl2 and Na2CO3 solutions were delivered at the same flow rate through the two inlets using syringe pumps(LSP01-1A). In order to explore the formation and transformation laws of ACC in laminar microfluidic, experiments were performed under different fluid velocity and concentration of reagents conditions in laminar microfluidic. 2.3 Characterisation of reactive crystallization of calcium carbonate The formation and transformation process of metastable intermediates ACC was recorded in situ using Zeiss optical microscope(Imager.A1m), Raman spectrums were obtained on Renishaw inVia micro Raman spectrometer using a 532 nm excitation source, and the transformation process of metastable intermediates ACC was also monitored in situ using that Raman spectrometer, which has high spatial resolution (
