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

Anionic macromolecules are found at sites of CaCO3 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(SO3 −), 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, γnet, is lowest for nonsulfated controls and increases with DS(SO3 −). The kinetic prefactor also increases with DS(SO3 −). Simulations of Ca2+−H2O−chitosan systems show greater water structuring around sulfate groups compared to uncharged substituents, independent of sulfate location. Ca2+−SO3 − interactions are solvent-separated by distances that are inversely correlated with DS(SO3 −) of the polysaccharide. The simulations also predict SO3 − and NH3 + groups affect the solvation waters and HCO3 − ions associated with Ca2+. Integrating the experimental and computational evidence suggests sulfate groups influence nucleation by increasing the difficulty of displacing near-surface water, thereby increasing γnet. By correlating γnet and net charge per monosaccharide for diverse polysaccharides, we suggest the solvent-separated interactions of functional groups with Ca2+ 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.