How Does Electronic Polarizability or Scaled-Charge Affect the Interfacial Properties of Room Temperature Ionic Liquids?

The room temperature ionic liquid (RTIL) air–liquid interface plays an important role in many applications. Herein, we present molecular dynamics simulation results for the air–liquid interface of a common RTIL, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide, [C₄mim][NTf₂]. To elucidate the effects of electronic polarizability and scaled-charge ions on the properties of the RTIL air–liquid interface, we employ three different kinds of force fields: a nonpolarizable force field (FF) with united ion charges (FixQ), a nonpolarizable FF with scaled-charge by 0.8 (ScaleQ), and a polarizable FF (Drude). To identify whether the ions reside at the interface or not, the method of identification of the truly interfacial molecules is used. The structural and dynamical properties in the interfacial, subinterfacial, and central layers are evaluated. In general for bulk liquids, the FixQ model predicts too-ordered structures and too-sluggish dynamics, while the ScaleQ model can serve as a simple cure. However, the ScaleQ model cannot reproduce the results of the Drude model at the interface, due to an inappropriate scaled-down charge near the interface.