New clues about carbonate crystallization kinetics at interfaces

The formation of minerals from aqueous solutions is a widespread natural phenomenon that controls mass transfers within the lithosphere, impacting elemental cycling –mostly through interactions with living organisms. Crystallization phenomena have also a high industrial relevance, e.g., for the development of new anti-scaling agents or the synthesis of biomimetic materials.

The process of mineral formation usually involves two consecutive steps –nucleation and growth- that are controlled by a complex interplay of different environmental factors. In recent years, abundant evidence of the widespread occurrence of the so-called ‘non-classical nucleation pathway’ has emerged. This multistep pathway involves the formation of disordered or nanocrystalline phases during the early stages of the crystallization process, which subsequently aggregate and/or recrystallize forming the final crystal phase. Multi-step pathways are commonly observed during the early stages of biomineral formation. In the case of calcium carbonate biominerals, an amorphous intermediate –amorphous calcium carbonate, ACC– is formed at early stages of development, allowing the organisms to conform the intricate shapes of their shells and skeletons. This amorphous intermediate crystallizes then forming calcite (e.g., sea urchin) or aragonite (e.g., nacre) biominerals.

For the biomineralization process to be effective, the crystallization kinetics of the amorphous calcium carbonate precursor need to be exquisitely controlled. This is done via the interactions with organic and inorganic molecules and ions –additives–, and with organic interfaces that provide loci for the nucleation and growth of the mineral phase. However, the precise molecular mechanism controlling the crystallization kinetics of ACC remain unknown.

The EC2CO ‘Nucleation’ project (funded by INSU) has addressed this problem. The project included a group of researchers from ISTerre (Grenoble), in collaboration with the PHENIX laboratory (Paris) and the Complutense University from Madrid, Spain. The goal of the project was to determine the influence of the hydrophobicity of a substrate over the heterogeneous formation of CaCO3. To that end, a set of phlogopite mica samples with different hydrophilic behavior (modulated by F- by OH- substitutions in their structure) were obtained from Jussieu Collection, National Museum of Natural History (Paris, France) and from H. C. Materials Corporation (IL, USA). The samples are model systems that provide nucleation loci for CaCO3, with contrasting wetting properties: OH- samples are hydrophilic and F- bearing samples are hydrophobic. In situ experiments were performed using a surface-sensitive synchrotron technique that allowed the observation of the early stages of ACC formation on the mica substrates. These experiments were combined with atomic force microscopy imaging and infrared spectroscopy experiments that allowed characterizing the crystallization kinetics of the system.

The results show a contrasting behavior between the hydrophilic (OH- phlogopite) and hydrophobic (F- phlogopite) substrates: the amorphous calcium carbonate precipitate shows a longer stability on the hydroxylated (hydrophilic) surface than on the hydrophobic (fluorinated) one. Indeed, AFM images show that, whereas isolated calcite crystals are formed on the hydrophobic phlogopite, a layer of ACC persists for longer times onto the hydrophilic one, delaying the crystallization. This can be rationalized by the highly hydrated character of the ACC precipitate, which could be responsible of its longer lifetime via the formation of hydrogen bonds with the hydrophilic substrate.

These results highlight the importance of the wetting properties of a substrate during the crystallization process. They provide a baseline for further studies on biomineralizing organisms: organic interfaces with contrasting wetting properties are present in the organic matrices, which could provide a way to regulate crystallization kinetics during the biomineralization process. These results are also relevant to abiotic crystallization processes, such as the transport of reactive fluids in geologic and industrial settings.

Reference

Surface wetting controls calcium carbonate crystallization kinetics, Ayumi Koishi, Alejandro Fernandez-Martinez, Alexander E. S. Van Driessche, Laurent J. Michot, Carlos M. Pina, Carlos Pimentel, Byeongdu Lee, German Montes-Hernandez, Chemistry of Materials (2019) ,
doi: 10.1021/acs.chemmater.9b00417