Binding Modes and Water-Mediation of Polyelectrolyte\nAdsorption to a Neutral CaCO<sub>3</sub> Surface

Aqueous polyelectrolytes\nare effective mineralization inhibitors\ndue to their ability to template onto crystal surfaces and chelate\nions in solution. These additives have been shown to alter the morphology\nof calcium carbonate crystals, making them promising candidates for\nbiological and industrial applications. However, while key to designing\nmore effective mineralization inhibitors, the molecular mechanisms\ngoverning the interactions between polyelectrolytes and crystal surfaces\nremain poorly understood. In this study, we investigate the adsorption\nof poly(acrylic acid) (PAA) on the dominant calcite (101̅4) cleavage plane using all-atom\nmolecular\ndynamics simulations. Although the calcite slab is electrostatically\nneutral, its charge distribution induces a strong electrostatic potential\nin an aqueous solution, which leads to significant water structuring\nat the interface. We observe a very favorable adsorption affinity\nof the polyelectrolyte chain to the surface, yet the structure of\nthe interfacial water is not significantly affected. Direct interactions\nbetween the monomers on the polyelectrolyte and the calcite surface\nare infrequent, despite variations in chain length, charge density\nof the polyelectrolyte, and solution conditions. Intriguingly, the\npolyelectrolyte interaction with the calcite surface is dominantly\nmediated through bridging hydrogen bond interactions. As the polyelectrolyte\nadsorbs to the surface, the chain conformation adapts to the interfacial\nwater structure by increasing polyelectrolyte–water contacts\nand integrates into pre-existing hydrogen bond networks. We found\nthat water-mediated interactions are more dominant than direct interactions\nbetween the polyelectrolyte and the surface. This suggests an alternative\npathway to the widely accepted notion that entropic effects due to\nwater reorganization are the primary driving force. These results\nsuggest that the polyelectrolyte binding affinity can be tuned by\naltering the polymer chain interactions with the interfacial water\nstructure in addition to the surface itself.