Understanding the Control of Mineralization by Polyelectrolyte Additives : Simulation of Preferential Binding to Calcite Surfaces

Abstract

Understanding the mechanisms that govern the crystallization of natural minerals such as calcium carbonate, calcium oxalate, or hydroxyapatite and its control by biological and synthetic polymers can help to guide the design of new biomimetic materials. In this paper, the adsorption behavior of oligomers of polystyrene sulfonate (PSS) on calcite surfaces was investigated by molecular dynamics simulations. The binding strengths of PSS oligomers to different calcite surfaces were computed via potential of mean force calculations, and the binding modes were analyzed in detail. These results could be set in relation to and serve as a molecular-level explanation of the experimentally observed PSS-stabilized exposure of (001) surfaces during calcite mineralization. The simulations show that oligomers of PSS preferentially bind to the polar calcite (001) surface, much stronger than to the nonpolar (104) surface. While sharing in common a dominant role of solvent-induced forces, the mode of binding to the two surfaces is different. The interaction of the sulfonate group with the (001) surface is dominated by both direct and solvent-mediated binding, while the binding of the styrene sulfonate to the (104) surface is mediated by one or two layers of water molecules. Moreover, local solvent density variations at the interface impact the geometry of binding which vastly differs between the two surfaces. In particular, these last effects have important further implications for the preferential binding of PSS polymers (compared to monomers or oligomers) and specific material recognition by synthetic polymers and peptides in general.