Superstructures of temporarily stabilized nanocrystalline CaCO3 particles:: Morphological control via water surface tension variation

In this paper, the formation of different complex morphologies of nanocrystalline CaCO3 under the control of double hydrophilic block copolymers (DHBCs) carrying phosphate groups is described. The DHBCs consist of a poly(ethylene glycol) (PEG) block and a pendant poly [2-(2-hydroxy ethyl)ethylene] block with different degrees of phosphorylation up to 40%, some of which show surface activity. The polymers furthermore temporarily stabilize CaCO3 nanocrystals, which are formed by slow CO2 evaporation from a supersaturated Ca(HCO3)(2) solution (Kitano method). The polymers are active down to concentrations of 10(-4) g/L. In dependence of the nature and concentration of the DHBC, tunable complex shuttlecock flowerlike and other superstructures are formed, which are aggregates of CaCO3 vaterite nanoparticles with an enhanced stability of at least 2 months. It is shown that the aggregation starts around template CO2 gas bubbles at the air/water interface. The size and morphology of the growing aggregates depends on the polymer concentration, phosphorylation degree, and water surface tension. The latter determines when the aggregate sinks to the bottom, interrupting the further growth process. Variation of the water surface tension by addition of the nonionic surfactant Antharox CO880 also allows a variation of the aggregate morphology, thus implying the described method as simple and versatile for the generation of complex CaCO3 morphologies.