Control Of Crystalline Polymorphs And Morphologies For CaCO₃ With Sodium Citrate

In this paper, calcium carbonate crystals were prepared by urea hydrolytic method, using sodium citrate as additives. Calcium carbonate crystals with different polymorphs and morphologies had been obtained in the aqueous and mixed solvent of ethanol and water system. XRD and SEM were used as characterization methods to investigate the influence of sodium citrate concentration, volume fraction of ethanol, concentrations of Ca2+ and ionic liquids on controlling the polymorph and morphology of calcium carbonate crystals. The experiments and results were as follows:The polymorph of calcium carbonate crystals was well controlled using sodium citrate as additives. Uniform needle-like aragonite crystals were obtained in aqueous solution at 90℃. In contrast, the addition of sodium citrate resulted in the formation of novel morphologies of calcite crystals. Sodium citrate also had significant influence on polymorphs and morphologies of calcium carbonate in the ethanol-water mixed solution. Calcite crystals were acquired with the volume fraction of ethanol increasing from 0 to 50%, while hollow spherical vaterite crystals were obtained in the volume fraction of ethanol is 60%. Possible reasons,including complexation between citrate and Ca2+, adsorption between citrate and crystal faces were simulated and discussed.Aragonite crystals with different morphologies such as need-like and butterfly-like were obtained with the increase of Ca2+ concentration in aqueous solution at 120 ℃.While changing the volume fraction of ethanol and the concentration ratio of calcium acetate to urea, aragonite crystals also were obtained in the ethanol-water mixed solution. Novel morphologies of aragonite, including bundle-shaped, honoka-shaped and dumbbell-shaped were found. The change of crystal growth microenvironment caused different morphologies of aragonite.The nucleation and growth of calcium carbonate crystals in the presence of sodium citrate were investigated at 120℃.The as-obtained vaterite with cake-shaped, shell-shaped and hexagonal structures were fabricated by changing concentrations of Ca2+ and sodium citrate in aqueous solution. The possible formation mechanisms were discussed. The synergistic effects of citrate ions, Ca2+ and NH4were believed to play an important role on the morphologic selectivity and phase transformation from vaterite to calcite. It was also found that the ethanol / water volume ratio could change the morphologies of vaterite.In this paper, we also studied the effect of sodium citrate and ionic liquids concentrations on calcium carbonate crystals in aqueous solution. Calcite crystals with special morphologies were synthesized by changing the concentration of sodium citrate when sodium citrate existed alone. With changing the concentration of sodium citrate and ionic liquids, calcite crystals with different morphologies were synthesized when sodium citrate and ionic liquids coexisted. The interactions between citrate ions, ionic liquids and crystal surfaces were the key points, which might change the formation rate and growth rate of calcium carbonate crystal nucleus, and lead to form different polymorphs and morphologies of calcite.