Preparation and formation mechanism of vaterite calcium carbonate microspheres by PEG/Na2CO3 aqueous two-phase emulsion method
The emulsion method with micro-compartmentalization is an effective means to prepare spherical materials.In this paper,calcium carbonate(CaCO3)microspheres were prepared by using water-in-water emulsion system composed of polyethylene glycol(PEG)and sodium carbonate(Na2CO3).Firstly,the formation conditions of the water-in-water emulsion were studied and the phase diagram was drawn.Then,the calcium carbonate(CaCO3)microspheres with high dispersion and high sphericity were prepared by mixing CaCl2 solution with PEG/Na2CO3 emulsion under stirring in the two-phase emulsion method.The effects of water-in-water emulsion system composition and metathesis reaction conditions on the preparation of CaCO3 were explored and the optimal preparation conditions of CaCO3 microspheres were also investigated.The product of CaCO3 microspheres was characterized using XRD,SEM,laser particle size distributor and static particle image analyzer.The results showed that the CaCO3 microspheres had the most morphology and highly dispersed when PEG8000 and Na2CO3 were 33%and 7%of the mass of in the water-in-water emulsion system,respectively,the volume ratio of CaCl2 solution to PEG/Na2CO3 emulsion was 1∶15,the reaction time was 1min and the aging time was 0.5h.The CaCO3 microspheres mainly composed of vaterite(up to 95.72%),containing a small amount of calcite.The spheroidization mechanism wasthat Ca2+penetrated into the interior of the tiny spherical droplet and mineralized with CO32-in a fine cavity with micro-compartmentalization,forming crystal nuclei and rapidly generates vaterite.The PEG water environment prevented the dissolution recrystallization of vaterite,inhibited the development trend of vaterite to calcite,and then the crystal nuclei grew in the lower limit of the macromolecule crowding microenvironment of the aqueous two-phase system,finally obtaining spherical structure.