The growth of calcium carbonate by precipitation was shown experimentally to produce different morphology and surface areas in the presence of carboxylic acids (succinic acid and glutaric acid) and polyelectrolytes (Polyacrylic acid, PAA) and polyaspartic acid (PASPA) . The polymers having a much stronger influence (specific surface areas 40-80 m2/g) than the simple acids (5-10 m2/g) producing nanostructured particles.
|Facetted calcite particles (5m2/g) precipitated in the presence of succinic acid||Nanostructured CaCO3 (calcite, 70 m2/g) in the presence of PASPA (Mw 2000) |
Molecular dynamics atomistic scale simulations were used to investigate possible explanations for experimentally observed differences in the growth modification by polyacrylic acid (PAA) and polyaspartic acid (p-ASP). The more rigid backbone of p-ASP was found to inhibit the formation of stable complexes with counter-ions in solution, resulting in a higher availability of p-ASP compared to PAA for surface adsorption. Furthermore the presence of nitrogen on the p-ASP backbone yields favorable electrostatic interactions with the surface, resulting in negative adsorption energies, in an upright (brush conformation). This leads to a more rapid binding and longer residence times at calcite surfaces compared to PAA, which adsorbed in a flat (pancake) configuration with positive adsorption energies. The PAA adsorption occurring despite this positive energy difference can be attributed to the disruption of the ordered water layer seen in the simulations and hence a significant entropic contribution to the adsorption free energy.
|Water density from molecular dynamic atomistic scale simulations (a) above the clean surface, (b) in presence of PAA and (c) in presence of p-ASP. Color code: Ca = red, water = blue, molecules = yellow/orange/green.|
Attempts at calculating reliable adsorption energies for short succinic acid (SA) and glutaric acid (GA) molecules proved to be difficult as these molecules are found to rapidly explore a manifold of configurations around the step. The rapid transition between configurations shows that short molecules most likely will not anchor to the surface and block growth sites for a time sufficiently long to noticeably affect growth, hence the bigger crystals and lower surface areas.
 U. Aschauer, J. Ebert, A. Aimable and P. Bowen, “Growth Modification of Seeded Calcite by Carboxylic Acids Oligomers and Polymers: Towards an Understanding of complex growth mechanisms ” Crystal Growth & Design, 2010, 10 (9), pp 3956–3963.
. Aschauer U., Spagnoli D., Bowen P., Parker S. C., Growth modification of seeded calcite using carboxylic acids: Atomistic simulations, Journal of Colloid and Interface Science, 346(1), (2010), 226-231, (Link to article)