Ion Pairing & Ionicity in Ionic Liquids

Room-temperature ionic liquids are pure molten salts — no solvent, just cations and anions packed together — and their conductivity depends on a subtlety that trips up the naive picture: a lot of the ions that are happily diffusing around are doing it as tightly bound, charge-neutral cation–anion pairs that carry no net current. This simulator runs real overdamped Langevin dynamics on a 3D box of ions with soft-core Coulomb interactions and an applied field, then computes both the Nernst-Einstein conductivity predicted from each ion's self-diffusion and the actual current-driven conductivity, live. Their ratio — the "ionicity" — is the same number electrochemists read off a Walden plot to judge how much of a real ionic liquid's charge transport is lost to ion pairing.