Structural Dynamics in Ionic Liquid Thin Films: The Effect of Cation Chain Length

The structural dynamics of planar similar to 100 nm thin films of three ionic liquids (ILs) were investigated using ultrafast two-dimensional infrared (2D-IR) spectroscopy. The ILs share the same anion, bis-(trifluoromethylsulfonyl)imide (NTf2-), but have different chain length cations: 1-butyl-3-methylimidazolium (Bmim(+)), 1-hexy1-3-methylimidazolium (Hmim(+)), and 1-decy1-3-methylimidazolium (Dmim(+)). The CN stretching mode of SeCN- dissolved in the ILs served as the vibrational probe. For each IL thin-film sample, the vibrational probe cation was the same as the cation in the corresponding liquid. The films were made by spin coating the IL on CaF2 substrates with a 100 nm silica layer on top, which was functionalized with an ionic monolayer that mimics the structure of the corresponding IL. The thicknesses of the IL films ranged from similar to 50 to similar to 250 nm and were controlled by the concentration of the IL in the spin-coating solution. The structural dynamics in the films are slower than those in the corresponding bulk IL, and the dynamics are slower for thinner films. Relative to the dynamics in the corresponding bulk IL, the slowing of the dynamics decreases as the cation ion alkyl chain length increases, that is, the DmimNTf(2) film dynamics slow down significantly less relative to bulk DmimNTf(2) than BmimNTf(2) thin films compared to bulk BmimNTf(2).