  Amorphous calcium carbonate (ACC) has been widely found in biomineralization,
both as a transient precursor and a stable phase, but how organisms accurately
control its formation and crystallization pathway remains unclear. Here, we aim
to illu-minate the role of biologically relevant additives on the phase
behaviour of calcium carbonate solution by investigating their effects on the
formation of ACC. Results show that divalent cations like magnesium (Mg2+) ions
and negatively charged small organic molecules like aspartic acid (Asp) have
little/no effect on ACC formation. However, the particle size of ACC is
significantly re-duced by poly(Aspartic acid) (pAsp) with long chain-length,
but no effect on the position of the phase boundary for ACC formation was
observed. Phosphate (PO43-) ions are even more effective in reducing ACC
particle size, and shift the phase boundary for ACC formation to lower
concentrations. These phenomena can be explained by a cooperative
ion-association process where the for-mation of ACC is only influenced by
additives those are able to attract either Ca2+ ions or CO32- ions and, more
importantly, intro-duce an additional long range interaction between the CaCO30
complexes and promote the phase separation process. The findings corroborate
with our proposed model of ACC formation via spinodal decomposition and provide
a more realistic representation of how biology can direct mineralization
processes.
