Authors: Knight, Brenna M.
Source: VTechWorks (Virginia Tech) (article, 2024-12-18)
Field: Materials Science — Biomaterials

Effect of ionic strength on heterogeneous nucleation of calcite during biomineralization Biominerals often form within a matrix of biomacromolecules in high-salinity environments, yet the relationships for how macromolecules and ionic strength influence the crystallization of sparingly soluble salts (e.g., CaCO₃) are not established. Developing a physical picture of these controls is hindered by the traditional assumption that background electrolytes are inert. In this study, we investigate calcite nucleation onto two model organic matrix polysaccharides, chitosan and alginate, in a series of ionic strength solutions (65 – 600 mM NaCl). Chitosan is near-neutral (at pH 8.5) and analogous to the structural polysaccharide chitin. In contrast, alginate has a strong negative charge akin to the many anionic biopolymers in the organic matrix. By measuring the rate of calcite nucleation onto these materials and fitting classical nucleation theory to the data, we find the interfacial free energy (γ<sub>net</sub>) and the kinetic prefactor depend upon ionic strength for both polysaccharides. The thermodynamic barrier to nucleating calcite onto alginate strongly depends on ionic strength, while calcite nucleation onto chitosan shows a similar but weaker dependence. Parallel molecular dynamics (MD) simulations were conducted to examine ion (Ca²⁺, Na⁺, HCO₃⁻, Cl⁻) and water interactions with models of a carboxylated polysaccharide and a chitosan material. The MD predictions indicate that at higher ionic strength, the polysaccharide-solution interface is increasingly stabilized by progressively higher concentrations of Na⁺ and Cl⁻. Stronger Na⁺ interactions with the polysaccharide are observed in the carboxylated system. The numbers of H₂O and HCO₃⁻ in the Ca²⁺ hydration sphere decrease with increasing ionic strength, while the number of Cl⁻ increases for both polysaccharides. The evidence suggests the increase in interface stabilization by Na⁺ and Cl⁻ increases γ<sub>net</sub> through reductions in the polysaccharide-solution interfacial energy. We predict the effect of higher salinity is enhanced for alginate because Na⁺ interactions with COO⁻ groups make it more difficult for Ca²⁺ to displace near-surface water and/ or Na⁺. Relatively weak Na⁺-chitosan molecular interactions lead to a lesser dependence on ionic strength. Calcite nucleation rates were also measured onto chitosan in a series of sodium halide solutions (NaCl, NaBr, NaI) and onto alginate in a series of chloride salts (LiCl, NaCl, CsCl) at constant ionic strength. CaCO₃ nucleation in the presence of electrolytes with the strongest hydration properties presents the lowest γ<sub>net</sub>. Values of γ<sub>net</sub> increase in the order Cl⁻<Br⁻<I⁻ and Li⁺<Na⁺<Cs⁺ for nucleation onto chitosan and alginate, respectively. The findings demonstrate that background electrolytes can modulate the energy barrier to CaCO₃ nucleation through tunable effects at the polysaccharide-solution interface. Nucleation ; Ionic strength ; Calcite ; Ionic bonding ; Crystallization ; Biomineralization ; Electrolyte ; Chitosan Calcium Carbonate Crystallization and Inhibition ; Microbial Applications in Construction Materials ; Chemical and Physical Properties in Aqueous Solutions