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Formation of Liquid-condensed mineral phases and the mechanisms of the PILP process: potential for a new morphosynthetic route to nanocomposite materials

Subject Area Synthesis and Properties of Functional Materials
Term from 2014 to 2022
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 251939425
 
Final Report Year 2023

Final Report Abstract

Under certain circumstances, liquid mineral precursors can form in solutions of inorganic solids, such as in solutions of chalk (i.e., chalk, thus in hard water). This peculiar phenomenon challenges our understanding of mineral formation, as standard concepts expect the immediate formation of a solid. Especially under biomimetic conditions, thus when polymers are added that imitate proteins associated with biominerals (such as seashells, corals, or bone), this phenomenon is often observed and known as the so-called “polymerinduced liquid precursor” (PILP) process. A thorough mechanistic understanding of such liquid/liquid phase separation processes is required to enhance our insight into biologically controlled mineralization processes and to tap the unlocked potential for bioinspired material synthesis. Up to this Emmy Noether project, the underlying physicochemical principles remained enigmatic. During this project, it turned out that the formation of liquid-condensed mineral phases arises from the spontaneous formation of coordination compounds in the mother solution. Prior to the extensive studies which form this Emmy Noether project, the capability of these compounds to phase separation was overlooked, or, in the case of coordination polymers of calcium carbonate, they were conceptionally reframed as prenucleation clusters. The results of this project especially highlight that these spontaneously forming coordination compounds, since they are nothing but molecules, can undergo phase separation. It turns out that the PILP process is driven by polymer-guided self-assembly of coordination polymers, fully in line with concepts of colloid chemistry. This mechanistic insight might also advance our understanding of how organisms exert excellent control over inorganic solid formation when growing biominerals such as bivalve seashells, corals, or even bone or teeth. With this new understanding, a class of biomimetic materials becomes better accessible in which organic and inorganic components are blended in a nanogranular ultrastructure that amends material properties (e.g., by imparting self-healing or damage-resistance). The project also pushed conceptual boundaries as it showed that under suitable reaction control, these coordination entities could even undergo so-called spinodal decomposition, a phase-separation pathway so far only well accessible for alloys or glasses. The feasibility of spinodal demixing of minerals in water falsifies misconceptions even taught in textbooks, and the convenient retrieval of the early bicontinuous, thus sponge-like structures on the nanoscale without hazardous ingredients or treatment provides a green synthesis route to materials with large surface areas.

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