Nonlocal dielectric response and screening in ionic liquids

The dielectric response of room-temperature ionic liquids (RTILs) plays a central role in charge screening within these strongly correlated ionic fluids and thereby governs ion transport, electrochemical kinetics, solubility, and related electrochemical phenomena. Here, we present a comprehensive study of the nonlocal, wavenumber-dependent dielectric response function χk in representative RTILs using all-atom molecular dynamics simulations. We identified distinct features of this function χk, including a smeared solvation-induced peak of χk at ∼0.8 Å-1 and, notably, a higher wavenumber (∼2.25 Å-1) overscreening resonance-like peak underpinned by correlated anion-dominated collective structures. The latter points to a screening contribution that is distinct from simple solvation-layer ordering and is reminiscent of finite-wavevector screening features reported earlier in molten salts. To rationalize these features, we introduce a minimal phenomenological spectral representation containing a long-wavelength conducting background, a structural mode, and an additional anion-dominated resonance branch. We further connect the nonlocal dielectric response to screened potentials and solvation of simple ions in RTILs. A smeared Born-sphere representation of the charge density of such ions suppresses overscreening response and gives ion solvation energies in ionic liquids that are comparable to, but generally weaker than, hydration energies in water. These results provide a molecular picture of nonlocal screening in RTILs and highlight that dielectric response in such liquids is governed by several coupled structural modes rather than by a single screening length.