Asymmetric Hydration Shell Reveals Interfacial TFSI Organization in Imidazolium Ionic Liquid Films

Understanding how ionic liquids (ILs) organize and restructure a solvent at aqueous interfaces is critical for developing molecular-level descriptions of ion transport and selectivity in separation systems. Here, we investigate interfacial organization of imidazolium-based [BMIM], [HMIM], and [OMIM][TFSI] ILs at the air/aqueous interface using sum frequency generation (SFG) spectroscopy and molecular dynamics (MD) simulations. Simultaneous measurements across the CH, OH, SO, and CF vibrational regions directly probe water and both cation and anion ordering. An increasing IL concentration suppresses the dangling free OH peak while leading to a distinct 3600 cm^-1 band. Sub-ensemble analysis of MD simulations identifies this band as the asymmetric hydration shell of TFSI. This provides a unique opportunity to observe TFSI organization through both its direct vibrational signatures and its hydration shell. Global Frumkin isotherm analysis of all vibrational reporters yields systematic trends in adsorption free energy (-13 to -16 kJ/mol) and confirms attractive lateral interactions that strengthen with cation chain length. These results demonstrate that outer-sphere interactions and collective ion organization play central roles in interfacial IL film organization.