Quantum State Resolved Scattering from Room-Temperature Ionic Liquids: The Role of Cation versus Anion Structure at the Interface

We present results on state-resolved scattering studies for seeded CO2 supersonically cooled molecular beams (E-inc = 61.9(40) kJ/mol) from a series of room-temperature ionic liquids (RTILs). These RTILs are composed of C-n-methylimidazolium cations with BF4- or Tf2N- counter-anions. The final rovibrational quantum state distributions from these nonequilibrium surface scattering collisions are monitored by high-resolution diode laser absorption spectroscopy as a function of (i) cation alkyl chain length and (ii) anion size, and analyzed to yield the propensity for thermal desorption (TD) versus impulsive scattering (IS) dynamics. For a fixed BF4- or Tf2N- counteranion, the distributions reveal an increase in the TD fraction (a) with the C atom number (n) in the alkyl side chain, which provides evidence for selective preference of nonpolar groups at the gas liquid interface with increasing chain length. Conversely, for short carbon chains (n = 4), the thermal fraction decreases when the anion is changed from a compact and less polarizable BF4- to the bulkier and more polarizable Tf2N-, whereas any sensitivity to anion identity essentially vanishes for longer alkyl chains (n = 8, 12). These combined data illustrate a number of interesting trends in anion versus cation competition for interfacial sites, specifically (i) the presence of interfacial anions at the surface layer for sufficiently short alkyl headgroups, (ii) inertial "stiffening" due to increasing average surface mass, as well as (iii) a propensity for larger anion sizes in the interfacial region. Finally, the TD probabilities follow the exact opposite trend in "bulk" Henry's Law solubility constants with respect to anion size, which further highlights the intrinsically nonequilibrium dynamics sampled by hyperthermal collisions at the gas-liquid interface.