Surface Structure of Alkyl/Fluoroalkylimidazolium Ionic–Liquid Mixtures

Abstract

The gas–liquid interface of ionic liquids (ILs) is critically important in many applications, for example, in supported IL phase (SILP) catalysis. Methods to investigate the interfacial structure in these systems will allow their performance to be improved in a rational way. In this study, reactive-atom scattering (RAS), surface tension measurements, and molecular dynamics (MD) simulations were used to study the vacuum interface of mixtures of partially fluorinated and normal alkyl ILs. The underlying aim was to understand whether fluorinated IL ions could be used as additives to modify the surface structure of one of the most widely used families of alkyl ILs. The series of ILs 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C n mim][Tf 2 N]) with n = 4–12 were mixed with a fixed-length, semiperfluorinated analogue (1 H ,1 H ,2 H ,2 H -perfluorooctyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C 8 mimF 13 ][Tf 2 N]), forming [C n mim] (1– x ) [C 8 mimF 13 ] x [Tf 2 N] mixtures, where x is the bulk mole fraction of the fluorinated component. The RAS-LIF method combined O-atom projectiles with laser-induced fluorescence (LIF) detection of the product OH as a measure of surface exposure of the alkyl chains. For [C 8 mim] (1– x ) [C 8 mimF 13 ] x [Tf 2 N] mixtures, RAS-LIF OH yields are below those expected from stoichiometry. There are quantitatively consistent negative deviations from linearity of the surface tension. Both results imply that the lower-surface-tension fluoroalkyl material dominates the surface. A similar deficit is found for alkyl chain lengths n = 4, 6, 8, and 12 and for all (nonzero) x investigated by RAS-LIF. Accessible-surface-area (ASA) analyses of the MD simulations for [C n mim] (1– x ) [C 8 mimF 13 ] x [Tf 2 N] mixtures qualitatively reproduce the same primary effect of fluoro-chain predominance of the surface over most of the range of n . However, there are significant quantitative discrepancies between MD ASA predictions and experiment relating to the strength of any n -dependence of the relative alkyl coverage at fixed x , and on the x -dependence at fixed ...

Direct Correlation of Surface Tension and Surface Composition of Ionic Liquid Mixtures—A Combined Vacuum Pendant Drop and Angle-Resolved X-ray Photoelectron Spectroscopy Study

“…At this point, we briefly want to come back to the very recent study by the McKendrick group, who addressed mixtures of ILs with different alkyl chain lengths and mixtures of non-fluorinated ILs with ILs carrying per-fluorinated alkyl chains using an overall very similar approach [ 17 , 18 ]. They employed reactive-atom scattering with laser-induced fluorescence (RAS-LIF) in combination with MD simulations and ST measurements with the pendant drop method under atmospheric conditions, all performed solely at room temperature.…”

“…The situation for ions at interfaces of the IL with a solid or with the gas phase is even more difficult to predict. A typical example is mixtures containing fluorinated chains, where a preferential enrichment of these moieties at the surface has been reported [ 15 , 16 , 17 , 18 ].…”

“…We also performed a first study addressing mixtures of ILs with the common [PF 6 ] − anion and imidazolium cations carrying either short butyl or partially fluorinated butyl chains [ 15 , 45 ] by combining conventional ST measurements (pendant drop measurements under 1 bar argon atmosphere) and ARXPS. Very recently, a similar approach was successfully employed by the McKendrick group using reactive-atom scattering with laser-induced fluorescence (RAS-LIF) in combination with MD and ST measurements [ 17 , 18 ]. From their RAS-LIF signals that originate mainly from alkyl units located at the outer surface, the authors derived a quantitative analysis of the alkyl content at the surface, which allowed them to describe the observed ST values of the mixture using the Langmuir concept as described above.…”

“…They studied mixtures of ILs with different alkyl chain lengths in the cation and also with partially fluorinated chains at room temperature. From the underrepresentation of the alkyl signals, the authors could indirectly deduce a preferential surface enrichment of the fluorine chains in the mixtures [ 17 , 18 ].…”

Small-angle neutron scattering from mixtures of long- and short-chain 3-alkyl-1-methyl imidazolium bistriflimides

“…As part of a study aimed initially at understanding the properties of imidazolium ILs at the gas-liquid interface using reactive atom scattering, 20,[26][27][28][29][30][31][32][33][34][35][36] a series of mixtures was prepared with the composition [C 12 mim] x [C 2 mim] 1Àx [Tf 2 N] (Fig. 1).…”