Kinetics of Calcite Nucleation onto Sulfated Chitosan Derivatives and Implications for Water–Polysaccharide Interactions during Crystallization of Sparingly Soluble Salts

Abstract

Anionic macromolecules are found at sites of CaCO 3 biomineralization in diverse organisms, but their roles in crystallization are not well-understood. We prepared a series of sulfated chitosan derivatives with varied positions and degrees of sulfation, DS(SO 3 − ), and measured calcite nucleation rate onto these materials. Fitting the classical nucleation theory model to the kinetic data reveals the interfacial free energy of the calcite−polysaccharide−solution system, γ net , is lowest for nonsulfated controls and increases with DS(SO 3 − ). The kinetic prefactor also increases with DS(SO 3 − ). Simulations of Ca 2+ −H 2 O−chitosan systems show greater water structuring around sulfate groups compared to uncharged substituents, independent of sulfate location. Ca 2+ −SO 3 − interactions are solvent-separated by distances that are inversely correlated with DS(SO 3 − ) of the polysaccharide. The simulations also predict SO 3 − and NH 3 + groups affect the solvation waters and HCO 3 − ions associated with Ca 2+ . Integrating the experimental and computational evidence suggests sulfate groups influence nucleation by increasing the difficulty of displacing near-surface water, thereby increasing γ net . By correlating γ net and net charge per monosaccharide for diverse polysaccharides, we suggest the solvent-separated interactions of functional groups with Ca 2+ influence thermodynamic and kinetic components to crystallization by similar solvent-dominated processes. The findings reiterate the importance of establishing water structure and properties at macromolecule−solution interfaces.