Formation of thin calcium carbonate films with aragonite and vaterite forms coexisting with polyacrylic acids and chitosan membranes

CaCO 3 crystallization on a chitosan membrane was studied using diffusion of (NH 4) 2CO 3 vapors into a CaCl 2 solution containing differing added amounts of two polyacrylic acids (PAAs) with molecular weights of ca. 2.0 × 10 3 and ca. 4.5 × 10 4 . The coexistence of PAA and the chitosan membranes produced thin CaCO 3 island crystals, which developed into a continuous CaCO 3 film on the membranes. Continuous CaCO 3 films consisting of only aragonite formed on the chitosan membranes at the optimum amount of PAA. When the amount of PAA is not optimum, the polymorph of CaCO 3 switches from aragonite to vaterite, and the morphology has a tendency to become an isolated island structure. The formation of the aragonite and vaterite island crystals and the appearance of a range of added PAA suitable for their formation are explained by the action of two parallel phenomena: (a) the high concentration of Ca 2+ ions in the chitosan membrane vicinity is achieved by the interaction between the COO − groups of PAA adsorbed by the NH + 3 groups of the chitosan membrane through an electrostatic force and free Ca 2+ ions in the CaCl 2 solution, which produces the high supersaturation with CaCO 3 in the membrane vicinity during CO 2 diffusion; (b) PAA, remaining as mobile carboxylic anions in the CaCO 3 solution, inhibits the growth of the CaCO 3 island crystals by its adsorption. The CaCO 3 supersaturation in the membrane vicinity is controlled by regulating the balance of these phenomena, which leads to the formation of the desired CaCO 3 polymorph.