In Situ Characterization of Calcium Carbonate Crystallization on Nano-Textured Surfaces

Natural and technological interfaces influence the crystal nucleation and growth of calcium carbonate, including the crystallization pathways, the thermodynamic/kinetic barriers, and the crystal orientation. Predictions show that the heterogeneous nucleation rate on functionalized surfaces of the calcium carbonate polymorph calcite are 20 orders of magnitude higher compared to homogeneous nucleation. Hence, the crystallization pathway can be redirected from complex free energy landscapes towards the classical nucleation theory, allowing to compare different functionalized surface. This is exemplified by higher nucleation rates of calcite for carboxyl groups on surfaces than for thiol groups on surfaces under the same conditions. While smooth functionalized surfaces have been studied, the effect of nano-textured surfaces on calcium carbonate crystallization is unknown. Based on research for other phase change materials, it can be either: promoting or inhibiting. If calcium carbonate crystallization is promoted or inhibited by nano-textured surfaces is relevant to the understanding of calcium carbonate crystallization and facilitates rationally designed interfaces.<br/>Here we show the effect of nano-textured functionalized surfaces on calcium carbonate crystallization. We fabricated and quantified a rationally designed nano-textured surfaces using blockcopolymer lithography, ensuring a significant amount of surface curvature features in an order of magnitude to the critical nucleus size for calcite. The fabricated nano-textured surfaces are functionalized with carboxyl groups and compared to smooth functionalized surfaces to isolate the effect of the nano-texture onto the crystallization. The investigation of the crystallization at different supersaturation conditions provides a holistic understanding of the effect on the thermodynamic as well as the kinetic barrier. <i>In situ</i> optical transmittion microscopy coupled with digital holographic microscopy characterize the nucleation and single-crystal growth rates in a microfluidic cell with controlled calcium carbonate supersaturation through mixing. The analysis of the acquired microscopy images is using trained instance segmentation algorithms for accurate crystal detection.<br/>This work shows that nano-textured surfaces promote the calcium carbonate nucleation by more than one order of magnitude compared to a smooth surface at the same supersaturation. This difference can be described by the classical nucleation theory, explaining an increasing kinetic barrier of nucleation and a decreasing thermodynamic barrier. For the tested supersaturations and nano-textures, the decreasing thermodynamic barrier outperforms the increasing kinetic barrier. While the increasing kinetic barrier is affected by a decreased monomer collision probability in the confined volumes of the nano-texture pits, the thermodynamic barrier is lowered by surface curvatures in the pits within an order of magnitude to the critical nucleus size. Besides higher nucleation rates, calcium carbonate crystallization on nano-textured surfaces shows lower induction times, higher site densities, and lower single-crystal volume growth rates. We expect that this work will provide guidance for the design of interfaces, which are needed to advance technologies relying on the understanding of the crystallization behavior of calcium carbonate and other inorganic minerals.<br/>This project has received funding from the European Research Council (ERC), Starting Grant, under the European Union's Horizon 2020 research and innovation programme (Grant agreement No. 853257).