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

Abstract

The structural dynamics of planar ∼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 (NTf 2 − ), but have different chain length cations: 1-butyl-3-methylimidazolium (Bmim + ), 1-hexyl-3-methylimidazolium (Hmim + ), and 1-decyl-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 CaF 2 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 ∼50 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 .

The effect of dilution on induced free charge density gradients in room temperature ionic liquids

“…The Israelachvilli group reported order on the nanometer length scale based on force measurements and suggested based on that finding that the free ion concentration in RTILs was very low. 7,11–13 The Fayer group, in a series of elegant experiments using small molecules as probes, has identified organization with a persistence length on the order of tens to hundreds of nm in RTILs, 14–19 and the Shaw group has identified the evolution of structural order on the micrometer length scale in thin RTIL films. 20–23 Very recently, the Welton group has identified spatial variation in n , the refractive index, over distances of tens to hundreds of nm using Raman scattering.…”

Local and Long-Range Organization in Room Temperature Ionic Liquids

“…As noted above, there is abundant evidence for structural organization and for the influence of charged surfaces on dynamics in RTILs. Most of these reports place such organization up to multiple tens of nanometers, ,,− ,− and in some instances the organization is limited by the thickness of the RTIL films examined. − Our group has found evidence for induced charge density gradients (ρ f ) in RTILs with a spatial extent of tens of micrometers. − We discuss our findings and their implications below, with an eye toward understanding how the free charge density gradients we find may be influenced by structural organization in RTILs. One important consideration is that the structural order reported for thin RTIL films may exhibit longer-range effects in thicker samples, bringing structural organization and free charge density gradients into closer agreement.…”

“…The ρ f data we report can thus not be explained in the context of a depth-dependent effective viscosity of the RTIL, a quantity that will depend sensitively on structural organization. We note that the change in relaxation dynamics seen by the Fayer group − ,− must have some effect on the rotational diffusion dynamics of larger chromophores, such as the ones we use, but the spatial extent of the slowed dynamics they report is less than the depth of focus of the optics we use for the rotational diffusion measurements and we would not expect to be sensitive to the effect they report.…”

“…The Fayer group has used surface charge density as a means of controlling the properties of RTIL thin films − ,, and confined environments . In thin films of RTILs (50–250 nm thick), they time-resolved the band position and shape of the CN stretch of SCN – and SeCN – and CO 2 reorientation to assess the evolution of local environment as a function of surface charge density and RTIL imidazolium aliphatic chain length. − ,, They found that dynamics within the RTIL thin were slowed by the presence of surface charge for films up to ca . 250 nm, and that the characteristic length scale of the influence of interfaces on RTIL dynamics was ca .…”

“…− anion demonstrated ordering that appeared to template from the support surface while RTILs with a N(CN) 2 − anion demonstrated ordering that was templated from organization of the air−IL interface. 23 The Fayer group has used surface charge density as a means of controlling the properties of RTIL thin films [24][25][26]42,43 and confined environments. 44 In thin films of RTILs (50−250 nm thick), they time-resolved the band position and shape of the CN stretch of SCN − and SeCN − and CO 2 reorientation to assess the evolution of local environment as a function of surface charge density and RTIL imidazolium aliphatic chain length.…”