(121d) Explicit Modeling of Polarization Effects Reveals Water-Mediated Ion Adsorption Mechanisms at Hexagonal Boron Nitride Interfaces

Ion adsorption phenomena at hexagonal boron nitride (hBN)/water interfaces significantly influence diverse technological applications including nanofluidic devices, desalination membranes, and electrochemical sensors. At any solid/water interface, water molecules, being inherently polar, and salt ions, carrying charges, generate strong electric fields, which can polarize the contacting solid surface. Previous studies on the thermodynamics and transport properties of salt ions on carbon-based materials, including graphene^[1-3] and carbon nanotubes^[4,5] , have shown that the polarization energy plays an important role in controlling interfacial properties. Despite the recognized importance, the precise molecular mechanisms governing ion adsorption behavior at the hBN/water interface remain elusive, particularly regarding the role of many-body polarization effects. Indeed, in contrast to the chemically uniform nature of graphene, hBN is a heteropolar 2D material composed of alternating boron and nitrogen atoms with distinct electronegativities, adding complexity to computational modeling.

With the above in mind, we present a detailed molecular-level investigation of ion adsorption phenomena utilizing an advanced polarizable force field-based molecular dynamics (MD) approach and building on our previous work on the wetting properties of hBN^[6]. Our study systematically investigates the adsorption behavior of five kosmotropic ions (SO₄²⁻, F⁻, Cl⁻, K⁺, and Na⁺) and five chaotropic ions (I⁻, SCN⁻, Li⁺, Ca²⁺, and Ba²⁺), representing distinct positions in the Hofmeister series, at the hBN/water interface. In vacuum, quantum chemical simulations based on symmetry-adapted perturbation theory (SAPT) reveal that all the ten ions considered in this study are strongly attracted to the hBN surface, with significantly negative binding energy minima and a substantial contribution of the polarization energy to the total binding energy. However, at the hBN/water interface, our MD simulations uncover an essential water-mediated screening mechanism, where the interfacial water molecules screen more than 90% of the hBN-ion polarization energies observed in vacuum. As a result, the thermodynamics of ion adsorption at the hBN/water interface are predominantly governed by the ion-water and water-water interactions, rather than by direct hBN-ion interactions.

Furthermore, our polarizable MD simulations accurately model ion-specific effects at the hBN/water interface. For example, our predictions regarding the repulsion of K⁺ and the attraction of I⁻ from/to the hBN/water interface, respectively, are consistent with the ab initio MD simulations reported by Joly et al.^[7] On the other hand, if the hBN-ion polarization energy is modeled implicitly using a non-polarizable force field, the model fails to account for the water-mediated screening of the hBN-ion interactions. This failure leads to the erroneous prediction that both the K⁺ and the I⁻ ions are strongly attracted to the hBN/water interface, including overestimating the adsorption free energies of both ions by more than 12 kcal/mol. Therefore, our findings underscore the critical importance of incorporating polarization effects into computational models to accurately predict ion adsorption at the hBN/water interface.

References

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