On the crystallization of calcium carbonate modulated by anionic surfactants

Crystallization experiments of calcium carbonate (CaCO3) in the presence of different anionic surfactants are carried out by rapidly mixing CaCl2 with Na2CO3 in aqueous solutions. The selected anionic surfactants are sodium dodecylsulfate (SDS), sodium dodecylsulfonate (DDS), and sodium dodecylbenzenesulfonate (SDBS), respectively. The concentration and the head group of the anionic surfactants intensively influence the crystalline phase and morphology of the precipitated crystals. When the concentration is as low as 0.05 mM, the crystal particles obtained in the presence of all the investigated anionic surfactants are rhombohedral calcite crystals. As the surfactant concentration is increased to 0.5 mM, vaterite crystals are produced in SDBS system, while for SDS and DDS systems, there is no obvious change both for the crystalline phase and morphology of CaCO3. As the surfactant concentration is further increased to as high as 5 mM, vaterite particles are produced in SDBS system, and hollow-spherical calcite crystals obtained in the presence of SDS, whereas rhombohedral calcite crystals in DDS system. Possible formation mechanism of different CaCO3 polymorphs from different anionic surfactants solutions is proposed based on the experimental results.