Non-Monotonic Interfacial Layering at Water/[C_nmim][TFSA] Interfaces Associated with Entropy-Enthalpy Compensation

Layering at air/room-temperature ionic liquid (RTIL) interfaces increases with alkyl chain length, whereas water/RTIL interfaces exhibit a distinct nonmonotonic trend. To clarify this difference, we investigated 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C_nmim][TFSA]; n = 4, 6, 8, 10, and 12) using infrared-visible sum-frequency generation (IV-SFG) spectroscopy, molecular dynamics simulations, and interfacial tension measurements. The SO₂ symmetric stretch intensity of [TFSA]^- maximizes at n = 8, while cation C-H signals are negligible. Simulations reveal a tail-to-tail bilayer at n = 8 that enhances anion ordering but cancels cation vibrational signals via destructive interference. Short chains (n = 4) fail to form stable layers, whereas longer chains (n = 12) show disrupted ordering due to conformational flexibility and bulk-like nanosegregation. The interfacial tension also peaks at n = 8, suggesting reduced configurational entropy compensated by structural coherence with the bulk, consistent with small-angle X-ray scattering periodicity. Taken together, these results suggest that the nonmonotonic structural evolution at the water/RTIL interfaces can be understood within a framework of entropy-enthalpy compensation associated with bulk-interface coupling, in contrast to the monotonic hydrophobic segregation observed at air/RTIL interfaces.