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

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.