Monitoring a Mechanochemical Reaction Reveals the Formation of a New ACC Defect Variant Containing the HCO3- Anion Encapsulated by an Amorphous Matrix
2020
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 firstand
fastreaction 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 secondand
slowreaction 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.
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