Preparation and in-situ surface modification of CaCO3 nanoparticles with calcium stearate in a microreaction system
Graphical abstract
Introduction
Nano calcium carbonate(nano-CaCO3) has been widely used in various industrial fields based on the advantages of high specific surface area, non-toxicity, biocompatibility, high whiteness and low price [1]. For example, it's usually utilized as functional filler in rubber, plastics, paper-making and painting, which can not only greatly improve product performance, but also reduce production costs [[2], [3], [4], [5]]. In the biomedical field, nano-CaCO3 can be used for drug delivery, protein immobilization, disease detection and bone regeneration [[6], [7], [8]]. In the food industry, it can be applied in biosensors to test the freshness of seafood, as well as for enzyme immobilization or as a catalyst [9,10]. Moreover, nano-CaCO3 can also be used in environmental protection, such as water treatments, toxicant detection, etc. [[11], [12], [13]]. By combining with various compounds, CaCO3 particles are expected to play more significant role in catalysis, separation, biomedicine, and so on. Many of the above applications involve the interaction of nano-CaCO3 with organics, requiring good compatibility between them. Surface modification, as an important method to improve the compatibility, has attracted extensive attention and research. Through surface modification, CaCO3 nanoparticles can not only mix well with organic substrates, but also show hydrophobicity, which greatly broadens its applications.
In-situ surface modification of calcium carbonate is based on the premise that nano-CaCO3 can be controllably synthesized. CaCO3 nanoparticles are primarily prepared by three methods, including double decomposition method (Ca2+ − H2O − CO32−), emulsion liquid membrane technology (Ca2+ − R[organic phase] − CO32−) and carbonation method (Ca(OH)2/CO2) [[14], [15], [16]]. Carbonation method, by contrast, is preferred in terms of economy and feasibility, which are significant for industrial production, but it is hard to control the morphology and size of CaCO3 nanoparticles. According to the crystallization kinetics, the key to preparing nano-sized CaCO3 is to provide high supersaturation, because higher supersaturation is beneficial to the nucleation process, and lower supersaturation promotes the crystal growth [[17], [18], [19]]. Intensifying mixing and mass transfer process is conductive to increasing the supersaturation of the system.
Since the 1990s, microfluidic technology has been widely used in reactions, separation processes, chemical analysis and material preparation owing to excellent performances in mixing, mass transfer, safety and controllability [20,21]. According to the key factor mentioned above, microdispersion device has significant advantages in the preparation of nanoparticles [22,23]. By providing uniform reaction condition and rapid mixing, the formation process of nanoparticles can be precisely regulated, which is helpful to synthesize particles with specific morphology and size. In our previous work, microreactor has been successfully used in the preparation of a variety of nanomaterials, including ultra-fine TiO2 particles, hydrophilic nano-CaCO3, BaSO4 nanoparticles, ZrO2, ZnO, FePO4 as well as some polymer nanoparticles such as polystyrene and polysulfone [[24], [25], [26], [27], [28]].
In the last few decades, several methods have been developed to modify the surface property of CaCO3 nanoparticles, mainly including dry modification, wet modification and in-situ modification [[29], [30], [31]]. The first two methods are post-modification, which has the characteristics of high energy consumption, low efficiency and uneven modification, mainly because the modification is carried out by high temperature, vigorous stirring or mechanical grinding after the formation of CaCO3 nanoparticles. Compared with post-modification, in-situ surface modification is of great interest, which realizes the synthesis and modification of particles simultaneously [32,33]. The in-situ surface modification method can not only shorten the production cycle, reduce production cost, but also improve the effect of modification. To date, a variety of modifiers have been employed to modify CaCO3 nanoparticles, including surfactants, coupling agents, inorganics and polymers, which have great differences in the modification mechanism [[34], [35], [36], [37]]. Fatty acids and their salts, a kind of surfactant, are the most commonly used modifiers. However, for the process of in-situ surface modification, the addition of most surfactants is not conducive to the dissolution of Ca(OH)2 solids and the mass transfer between gas and liquid phases, thereby significantly affecting the morphology and particle size of CaCO3 nanoparticles. Therefore, developing a controllable in-situ surface modification method with high mass transfer efficiency makes great sense for the production of high-quality hydrophobic CaCO3 nanoparticles.
In this study, CaCO3 nanoparticles were prepared by the carbonation method and modified in situ, with calcium hydroxide slurry and carbon dioxide/nitrogen mixed gas (30% volume fraction CO2) as the reactants and calcium stearate(CaSt2) as the modifier. A membrane dispersion microreactor was applied to intensify the mixing and mass transfer between the gas phase and the liquid phase. The effects of gas and liquid flow rates, reaction temperature on the morphology, particle size and size distribution of CaCO3 particles were investigated in detail. Meanwhile, the effect of dosage of calcium stearate on the surface modification was studied based on the tests of active ratio and contact angle. CaCO3 nanoparticles with hydrophobic surface and good dispersity were controllably prepared.
Section snippets
Materials
Calcium hydroxide (A.R) was supplied by Beijing Tongguang Fine Chemical Co. Ltd. (China). Calcium stearate with a Ca content of 6.6–7.4% was purchased from Aladdin Industrial Corporation (China). Carbon dioxide/nitrogen mixed gas (30% volume fraction CO2) was purchased from Beijing Zhaoge Gas Co. Ltd. (China). All chemicals were used without any further purification. The deionized water was used throughout the study.
Preparation procedure
Fig. 1 shows the experimental apparatus. The microreactor was a key component.
Results and discussion
Before surface modification, we studied the effects of operating parameters on the morphology and particle size of CaCO3 particles, including gas-liquid two-phase flow rates and reaction temperature. Fig. 2 shows TEM images of the CaCO3 nanoparticles prepared at different continuous phase flow rates and the corresponding particle size distributions. Fig. 3(A) gives the variations of average particle size and the specific surface area of CaCO3 nanoparticles with the continuous phase flow rate.
Conclusions
In this study, an energy-saving and efficient approach was developed for the controllable preparation and in-situ surface modification of high-quality CaCO3 nanoparticles using a membrane dispersion microreactor. The effects of the continuous phase and dispersed phase flow rates, temperature and the dosage of CaSt2 surfactant on morphology, size and modification effect of CaCO3 nanoparticles were investigated. It was found that the particle size of CaCO3 nanoparticles could be regulated by
Acknowledgements
We gratefully acknowledge the support from the National Natural Science Foundation of China (Nos. 91334201, U1463208 and 21506110).
Nomenclature
- a
- specific interfacial area, 1/m
- FC
- continuous phase flow rate, mL/min
- FD
- dispersed phase flow rate, mL/min
- HCO2
- solubility coefficient of CO2, mol/(L∙kPa)
- kL
- liquid phase mass transfer coefficient, m/s
- kLa
- volumetric mass transfer coefficient, 1/s
- ΔMCO2
- mass transfer flux of CO2, mol/L
- p1
- partial pressure of CO2 at the inlet of microreactor, kPa
- p2
- partial pressure of CO2 at the outlet of microreactor, kPa
- P2
- total pressure of the gas phase at the outlet of microreactor, kPa
- p1∗
- equilibrium partial pressure of CO2
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