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

Area of Science:

• Physical Chemistry
• Surface Science
• Materials Science

Background:

• Understanding interfacial organization of ionic liquids (ILs) is crucial for designing effective separation systems.
• Molecular-level insights into ion transport and selectivity at aqueous interfaces are needed.

Purpose of the Study:

• To investigate the interfacial organization of imidazolium-based ILs at the air/aqueous interface.
• To elucidate the roles of cation chain length and anion hydration in IL film formation.

Main Methods:

• Sum frequency generation (SFG) spectroscopy to probe vibrational regions of water, cations, and anions.
• Molecular dynamics (MD) simulations to analyze ion ordering and hydration shells.
• Frumkin isotherm analysis to quantify adsorption and lateral interactions.

Main Results:

• IL concentration affects water and ion ordering, suppressing free OH peaks and forming a distinct 3600 cm^-1 band attributed to TFSI hydration shells.
• MD simulations confirm the 3600 cm^-1 band originates from the asymmetric hydration shell of TFSI.
• Adsorption free energies range from -13 to -16 kJ/mol, with attractive lateral interactions strengthening with cation chain length.

Conclusions:

• Outer-sphere interactions and collective ion organization are key to interfacial IL film structure.
• TFSI anion organization can be observed through its direct vibrational signatures and its hydration shell.
• Cation chain length systematically influences adsorption and lateral interactions at the interface.