Synthesis and composition modification of precipitate tubes in a confined flow reactor.

Precipitation reactions coupled to various transport phenomena, such as flow or diffusion, lead to the formation of different spatial gradients which can be influenced by tuning the experimental parameters (, reactant concentration, flow rate, reactor geometry, ). Thereby it gives us the opportunity to change the micro and macrostructure of the products. Herein, we investigate the precipitate tube formation in a flow-driven system applying a horizontal confined geometry for individual and composite alkaline earth metal (Mg(II), Ca(II), Sr(II), and Ba(II))-carbonate systems. First we attempted to achieve tube-like structures in each reactive system by increasing the reactant concentrations. It is found that the precipitate tubes are not present in the magnesium-carbonate system even at extremely high concentrations. Therefore, we doped the magnesium solution with other alkaline earth metal ions, which resulted in the desired precipitate patterns. Besides the macroscopic characteristics, the microstructure of the crystals (morphology, crystal phase, size, and composition) could also be modified by combining the ions and varying their concentration ratio. In addition, by varying the relative concentration of the alkaline earth metal cations, separated and composite crystals could also be produced as different extrema. These were spatially isolated due to the reactor geometry, and thus the products, which contain the metal ions either homogeneously or individually, can be easily separated from each other.