Fast Ions, Ordered Layers: Chain-Length Control of Ionic-Liquid Layering on Graphite

The nanoscale organization of ionic liquids (ILs) on graphitic electrodes governs interfacial transport and stability, yet whether ion mobility can drive extended interfacial layering remains insufficiently understood. Here, a structurally related series of ILs spanning approximate ion lengths from 0.64 to 2.9 nm was studied on highly oriented pyrolytic graphite to test whether chain-length-tuned mobility dictates interfacial ordering. Shortening the ions increases interfacial mobility, evidenced by a monotonic decrease in the nanoscale friction coefficient (≈0.0036 to ≈0.0019) measured by atomic force microscopy (AFM) using a sharp silicon probe, together with a reduced diffusion resistance reflected by a smaller Warburg contribution. This mobility increase transforms the interfacial morphology from thicker, less laterally coherent films to thinner, more ordered layered architectures. Notably, the shortest-chain IL forms terraces epitaxially registered with the graphite lattice and remains essentially unchanged under biased voltages. Consistently, colloid probe AFM force-distance plateaus and simulation-derived density oscillations indicate compact near-surface layers for the high-mobility system, whereas longer-chain ILs exhibit weaker early layering. These results support a mobility-mediated assembly mechanism in which faster interfacial reorganization enables surface-guided packing into robust layered structures, offering a practical molecular design rule for tuning IL-carbon interfaces in electrochemical systems.