Formulation of Polarizable Force Fields to Model Simple Ionic Liquid/Graphite Interfaces

Force fields that are specifically parametrized for performing molecular dynamics simulations of liquid/solid interfaces are in high demand. Such interfaces are broadly present in nature and applied in, for example, batteries, membranes, or catalysts, and therefore, an accurate modeling is required, including the explicit treatment of electronic polarization effects. In molecular dynamics simulations, the liquid/solid interactions are most often modeled by combining existing force fields for both phases using mixing rules. Ionic liquids, a very versatile class of compounds that can be utilized in the aforementioned applications, exert strong electric fields on the surrounding molecules, and therefore, are best described using polarizable force fields. In this work, the derivation of polarizable force fields for the simple, model ionic liquid ([C₁C₁Im][BF₄]) at a graphite interface is presented. In doing so, we explicitly parametrize the ionic liquid/graphite interactions instead of relying on mixing rules. Implementing the derived force fields, we observe that the explicit parametrization significantly affects the structure of the interface, resulting in the formation of a sharply defined contact layer and about 30 to 50 pm reduced ion-graphite distances when compared to mixing rules-based force fields. Upon calculating the work of adhesion of the ionic liquid on graphite, we show that the liquid/solid interactions are dominated by dispersive interactions while induction effects only play a minor role. Additionally, we find that using mixing rules strongly underpredicts the overall work of adhesion, highlighting the demand for and justifying the computational efforts of explicitly parametrizing the ionic liquid/graphite interactions.