Monitoring a Mechanochemical Reaction Reveals the Formation of a New ACC Defect Variant Containing the HCO3– Anion Encapsulated by an Amorphous Matrix

Phil Opitz,Anke Kabelitz,Alejandro Fernandez-Martinez,Wolfgang Tremel,Maria P Asta,Martin Panthöfer,Franziska Emmerling,Mihail Mondeshki

doi:10.1021/acs.cgd.0c00912

Abstract

Amorphous calcium carbonate (ACC) is an important precursor in the biomineralization of crystalline CaCO3. In nature, it serves as a storage material or as a permanent structural element, whose lifetime is regulated by an organic matrix. The relevance of ACC in materials science is primarily related to our understanding of CaCO3 crystallization pathways and CaCO3/(bio)polymer nanocomposites. ACC can be synthesized by liquid–liquid phase separation, and it is typically stabilized with macromolecules. We have prepared ACC by milling calcite in a planetary ball mill. Phosphate “impurities” were added in the form of monetite (CaHPO4) to substitute the carbonate anions, thereby stabilizing ACC by substitutional disorder. The phosphate anions do not simply replace the carbonate anions. They undergo shear-driven acid/base and condensation reactions, where stoichiometric (10%) phosphate contents are required for the amorphization to be complete. The phosphate anions generate a strained network that hinders ACC recrystallization kinetically. The amorphization reaction and the structure of BM-ACC were studied by quantitative Fourier transform infrared spectroscopy and solid state 31P, 13C, and 1H magic angle spinning nuclear magnetic resonance spectroscopy, which are highly sensitive to symmetry changes of the local environment. In the first—and fast—reaction step, the CO32– anions are protonated by the HPO42– groups. The formation of unprecedented hydrogen carbonate (HCO3–) and orthophosphate anions appears to be the driving force of the reaction, because the phosphate group has a higher Coulomb energy and the tetrahedral PO43– unit can fill space more efficiently. In a competing second—and slow—reaction step, pyrophosphate anions are formed in a condensation reaction. No pyrophosphates are formed at higher carbonate contents. High strain leads to such a large energy barrier that any reaction is suppressed. Our findings aid in the understanding of the mechanochemical amorphization of calcium carbonate and emphasize the effect of impurities for the stabilization of the amorphous phases in general. Our approach allowed the synthesis of new amorphous alkaline earth defect variants containing the unique HCO3– anion. Our approach outlines a general strategy to obtain new amorphous solids for a variety of carbonate/phosphate systems that offer promise as biomaterials for bone regeneration.

Highlights

Mechanochemistry has become a tool for the synthesis of new inorganic,[1,2] organic,[3,4] and metal–organic compounds.[5]
In an earlier study we have shown that amorphous CaCO3 could be prepared by ball milling only, when Na2CO3 was added to stabilize the metastable ball-milled amorphous calcium carbonate phase (BM-ACC) by cationic defects.[70]
The full width at half maximum of the nuclear magnetic resonance (NMR) signals in crystalline solids is a sensitive measure of the uniformity of the local field[73] and suitable to follow the changes during the amorphization process

Summary

Introduction

Mechanochemistry has become a tool for the synthesis of new inorganic,[1,2] organic,[3,4] and metal–organic compounds.[5]. Ball milling can induce an alloying of metals[17] or polymorph changes (e.g., a transformation from the anatase to the rutile phase of TiO2).[18] organic compounds mixtures of organic and inorganic compounds[19,20] undergo structural changes during mechanochemical treatment This is of particular interest for pharmaceuticals as the higher solubility of amorphous phases enhances the uptake of drugs with poor water solubility.[21,22] Mechanochemistry is of particular interest for large applications due to its scalability and the reduced use of solvents, which make it a “green chemistry” technique.[23] Continuous mechanical stress during ball milling was proposed to trigger unusual reactivity and lead to the formation of transient phases that are different from those accessible in conventional solid state reactions.[24] There is limited evidence for or against this hypothesis. These approaches start from the Ca2+ and CO32- ion constituents in aqueous solution, and the crystallization process is stopped at the ACC stage by stabilizing the product kinetically

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