(551d) Molecular Interactions at Calcite-Aerosol OT Interface As a Model for Surfactant Mediated Mineralization

Surfactants have long enabled the synthesis of fine particles of controlled size and crystalline morphology. However, surfactant-mediated particle formation is a complex interplay of elementary processes which remain incompletely understood. A key step during surfactant-mediated particle formation is the adsorption of surfactants onto the facets of the growing particle. While prior studies have unraveled the nature of surfactant aggregates (micelles, bilayers, etc.) in aqueous solutions, our understanding of aggregate morphologies at mineral/water interfaces is still incomplete. As a first step towards determining the morphologies of surfactant aggregates at mineral/water interfaces, we employ multiscale modeling to probe interactions between single surfactant molecules and a prototypical mineral surface. In this study, we study the adsorption of Aerosol-OT (AOT) on calcite. First, we use molecular dynamics (MD) and metadynamics to probe the orientation and strength of surfactant adsorption at the crystal surface. Further, we study cooperativity among AOT molecules in the adsorption process to determine the how surfactant aggregates differ from single surfactant molecules in their interactions with the mineral surface. Second, we supplement our MD-based findings with first principles density functional theory (DFT) to understand precisely how AOT and calcite interact at the electronic level. This multiscale view gives detailed information about the thermodynamics of AOT adsorption and the electronic structures that drive adsorption.

The MD study reveals that AOT molecules are physisorbed 0.42 nm away from the calcite surface with an interaction energy of about 3.5 kJ/mol. Furthermore, a layer of water molecules mediates the adsorption of AOT onto the surface. Determining the adsorption configuration of AOT is complicated by the fact that AOT is twin-tailed and contains both sulfonate and ester functional groups. Using DFT calculations, we find that AOT preferentially adsorbs on calcium atoms within calcite in a bidentate configuration through the sulfonate group. The first principles analysis reveals that water molecules bind more strongly than AOT to calcite, thus confirming the presence of a water-layer which is also observed in MD and metadynamics simulations. Metadynamics results indicate that aggregate structures are disrupted upon adsorption to the calcite interface, and aggregate dissolution/rearrangement energy contributes to the binding process. Ultimately the findings of this study can be used to improve surfactant-mediated mineralization for the controlled growth of crystalline nanoparticles.