Impact of Sodium Polyacrylate
on the Amorphous Calcium
Carbonate Formation from Supersaturated Solution
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Abstract
A detailed in situ scattering study has been carried
out on the
formation of amorphous calcium carbonate (ACC) particles modulated
by the presence of small amounts of sodium polyacrylate chains. The
work is aiming at an insight into the modulation of ACC formation
by means of two polyacrylate samples differing in their molecular
weight by a factor of 50. The ACC formation process was initiated
by an in situ generation of CO<sub>3</sub><sup>2β</sup> ions
via hydrolysis of 10 mM dimethylcarbonate in the presence of 10 mM
CaCl<sub>2</sub>. Analysis of the formation process by means of time-resolved
small-angle X-ray and light scattering in the absence of any additives
provided evidence for a monomer addition mechanism for the growth
of ACC particles. ACC formation under these conditions sets in after
a lag-period of some 350 s. In the presence of sodium polyacrylate
chains, calcium polyacrylate aggregates are formed during the lag-period,
succeeded by a modulated ACC growth in a second step. The presence
of anionic polyacrylate chains changed the shape of the growing particles
toward loose and less homogeneous entities. In the case of low amounts
(1.5β7.5 mg/L) of the long chain additive with 97 kDa, the
size of the aggregates is comparable to the size of the successively
formed hybrid particles. No variation of the lag-period has been observed
in this case. Use of the short chain additive with 2 kDa enabled increase
of the additive concentration up to 100 mg/L and resulted in a significant
increase of the lag-period. This fact, together with the finding that
the resulting hybrid particles remained stable in the latter case,
identified short chain sodium polyacrylates as more efficient modulators
than long chain polyacrylates