TY - JOUR
T1 - Effect of pH on amorphous calcium carbonate structure and transformation
AU - Tobler, Dominique Jeanette
AU - Rodriguez Blanco, Juan Diego
AU - Sørensen, Henning Osholm
AU - Stipp, Susan Louise Svane
AU - Dideriksen, Knud
PY - 2016/8/3
Y1 - 2016/8/3
N2 - A number of organisms produce crystalline calcium carbonate via a metastable precursor phase termed amorphous calcium carbonate (ACC). ACC also forms during production of CaCO3 for industrial purposes, e.g., paper manufacturing and synthesis of fillers for polymers. Previous studies suggest that the local structure of ACC controls crystallization kinetics and pathways, i.e., the crystalline polymorph(s) that form(s) in the process. We used pair distribution function (PDF) analysis to provide evidence that the local structure of ACC gradually changes as the pH of the synthesis solutions is increased from 10.6 to 12.7, at ambient conditions. These changes correlate with the mole fraction of incorporated hydroxide ions, which varies gradually from negligible at pH 10.6 to 0.12 at pH 12.7. At lower pH (10.5), vaterite and calcite formed in less than 2 min, but as the pH increased, the lifetime of ACC increased, and it transformed directly to calcite (i.e., no vaterite intermediate). Although higher pH led to a preferable transformation into calcite with decreasing crystal size, variations in ACC local structure cannot be linked to development of a calcite like motif, as has been suggested. On the basis of comparing the measured vaterite PDF to a range of structural models to determine Ca-Ca distances, we found no obvious structural similarity between vaterite and the ACC precursor either, although such analysis is complicated by the ambiguity of the vaterite structure. Presumably, the absence of vaterite and the prolonged ACC stability with increasing synthesis pH could indicate inhibition of crystal nucleation and growth by hydroxide ions.
AB - A number of organisms produce crystalline calcium carbonate via a metastable precursor phase termed amorphous calcium carbonate (ACC). ACC also forms during production of CaCO3 for industrial purposes, e.g., paper manufacturing and synthesis of fillers for polymers. Previous studies suggest that the local structure of ACC controls crystallization kinetics and pathways, i.e., the crystalline polymorph(s) that form(s) in the process. We used pair distribution function (PDF) analysis to provide evidence that the local structure of ACC gradually changes as the pH of the synthesis solutions is increased from 10.6 to 12.7, at ambient conditions. These changes correlate with the mole fraction of incorporated hydroxide ions, which varies gradually from negligible at pH 10.6 to 0.12 at pH 12.7. At lower pH (10.5), vaterite and calcite formed in less than 2 min, but as the pH increased, the lifetime of ACC increased, and it transformed directly to calcite (i.e., no vaterite intermediate). Although higher pH led to a preferable transformation into calcite with decreasing crystal size, variations in ACC local structure cannot be linked to development of a calcite like motif, as has been suggested. On the basis of comparing the measured vaterite PDF to a range of structural models to determine Ca-Ca distances, we found no obvious structural similarity between vaterite and the ACC precursor either, although such analysis is complicated by the ambiguity of the vaterite structure. Presumably, the absence of vaterite and the prolonged ACC stability with increasing synthesis pH could indicate inhibition of crystal nucleation and growth by hydroxide ions.
U2 - 10.1021/acs.cgd.6b00630
DO - 10.1021/acs.cgd.6b00630
M3 - Journal article
SN - 1528-7483
VL - 16
SP - 4500
EP - 4508
JO - Crystal Growth & Design
JF - Crystal Growth & Design
IS - 8
ER -