(594d) Uncovering a Universal Molecular Mechanism of Salt Ion Adsorption at Solid/Water Interfaces

Understanding the molecular mechanism of salt ion adsorption at solid/water interfaces is essential for all energy and membrane-based applications involving electrolytes. Because water is a polar solvent and salt ions are charged, a long-standing puzzle involving solid/water interfaces is how do the electric fields exerted by the salt ions and the interfacial water molecules polarize the charge distribution in the solid and how does this polarization, in turn, influence ion adsorption at any solid/water interface. Here, using state-of-the-art polarizable force fields derived from quantum chemical simulations, we perform all-atomistic molecular dynamics simulations to investigate the adsorption of various ions comprising the well-known Hofmeister series at the graphene/water interface, including comparing with available experimental data. Our findings reveal that, in vacuum, the ionic electric field - induced polarization of graphene results in a significantly large graphene - ion polarization energy, which drives all salt ions to adsorb to graphene. On the contrary, in the presence of water molecules, we show that the water molecules and the salt ions exert waves of molecular electric fields on graphene which destructively interfere with each other. This remarkable phenomenon is shown to cause a water-mediated screening of more than 85% of the graphene−ion polarization energy – a finding which remains consistent for all the salt ions of different types (e.g., kosmotropic, chaotropic) and valency (e.g., monovalent, divalent) considered in our study. Further, by investigating model superhydrophobic and superhydrophilic solid/water interfaces, which form asymptotic limits on the water density and structuring possible at any solid/water interface, we demonstrate that this phenomenon occurs universally at all solid/water interfaces. We also show that as a result of the significant water – mediated weakening of the ion – solid interactions, ion specific effects are governed primarily by a molecular mechanism involving an interplay between the ion – water and water – water interactions. Finally, we compare our simulation results to the classical theory of image charge interactions, including the Onsager – Samaras theory, and highlight the importance of discrete effects beyond the mean-field level on the energetics of salt ion adsorption at solid/water interfaces.