Abstract
Structural biocomposites found in nature often have a well-defined
organization on the nanometer scale. For mineralized materials, interactions
between organic and inorganic phases are important for controlling
crystal size, morphology, and spatial arrangement, which is a requirement
when structural biomaterials are designed. In this paper, we studied
influence of low concentrations of alginate on calcium carbonate
crystallization by seeded and unseeded experiments, at controlled
activity-based supersaturations. Crystal growth and nucleation were
characterized by scanning electron microscopy (SEM), calcium concentration
measurements, and crystal volume distribution measurements through
the crystallization experiments. Alginate concentrations as low as
10 ppm were found to have a significant effect on growth of vaterite
seeds, resulting in decreased growth rates and extensive agglomeration,
compared to the case without alginate. For increased alginate concentrations
(100 and 200 ppm), vaterite seed growth rates were decreased further.
The decreased growth rates were probably caused by adsorption of
alginate onto the active growth sites of the crystal surface. Alginate
with 65% G-units (HighG) reduced the growth rate more than alginate
with 43% G-units (LowG), which may be accounted for by the greater
G-block length, and thus higher affinity to calcium, in HighG alginate.
The unseeded experiments showed that mainly small vaterite crystals
nucleated with 100 ppm alginate present, after an induction time
of 50-80 min, while large calcite crystals were formed after sonic
time by transformation from vaterite. The decreased crystal growth
rates and higher nucleation rates caused by increased concentrations
of alginate explain how small size mineral particles can be formed
in alginate gel networks to form nanostructured composite materials.
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