Polymorph selection and nanocrystallite rearrangement of calcium carbonate in carboxymethyl chitosan aqueous solution: Thermodynamic and kinetic analysis

In this article, the polymorph selection of calcium carbonate has been successfully achieved in water-soluble carboxymethyl chitosan aqueous solution at different temperatures (25-95 degrees C) Vaterite is formed in carboxymethyl chitosan solution 25 degrees C accompanied with trace of calcite, whereas pure aragonite is obtained at 95 degrees C. Scanning electron microscopy and transmission electron microscopy analyses show that the products are formed from the recrystallization of nanometer crystallites. Thermodynamic and kinetic analyses reveal that the polymorph of calcium carbonate is control led and selected by kinetics in various temperatures. As a heterogeneous nucleator and stabilizing agent carboxymethyl chitosan changes the nucleation and growth of calcium carbonate from thermodynamic into kinetic control. Under kinetic limitation, the reaction rate of aragonite increases along with the elevating of temperature and surpasses the rate of vaterite above 327 K. (C) 2009 Elsevier Ltd. All rights reserved.