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_nmim][Tf₂N]) with n = 4-12 were mixed with a fixed-length, semiperfluorinated analogue (1H,1H,2H,2H-perfluorooctyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C₈mimF₁₃][Tf₂N]), forming [C_nmim]_(1-x)[C₈mimF₁₃]_x[Tf₂N] 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 [C₈mim]_(1-x)[C₈mimF₁₃]_x[Tf₂N] 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_nmim]_(1-x)[C₈mimF₁₃]_x[Tf₂N] 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 n. These discrepancies are discussed in the context of detailed examinations of the surface structures predicted in the MD simulations. Potential explanations, beyond experimental artifacts, include inadequacies in the classical force fields used in the MD simulations or the inability of simple ASA algorithms to capture dynamical factors that influence RAS-LIF yields.

Figure 1

Molecular structures of the components of the fluoroalkyl ionic–liquid mixtures, along with the reference liquid, squalane. x indicates bulk mole fraction of the partially fluorinated [C₈mimF₁₃][Tf₂N] component.

Figure 2

Schematic diagram of the RAS-LIF apparatus. The central, four-wheel carousel could be rotated about its central axis, which allowed each of the liquid wheels to be presented to the laser beams for study.

Figure 3

(a) ST data for [C₈mim]_(1–x)[C₈mimF₁₃]_x[Tf₂N] mixtures. The dashed line illustrates a linear-mixing model (eq 1). (b) Excess ST, the difference between ideal and observed behavior, which is negative here. All errors are 95% CL; derived from the repeatability of the measurements and not the accuracy of the instrument, which is a much smaller source of error.

Figure 4

Comparable relative measures of surface exposure of the alkyl component in [C_nmim]_(1–x)[C₈mimF₁₃]_x[Tf₂N] mixtures. ST = alkyl surface mole fraction (1 – x_s) as defined through eqs 2a and 2b for C8 mixtures only (green circles). RAS-LIF = normalized reactivity to produce OH as defined in eq 3 (red circles C12 mixtures; black squares C8 mixtures; blue triangles C6 mixtures; and magenta triangles C4 mixtures). MD-ASA = relative fraction of the surface area consisting of reactive secondary-hydrogen atoms for C8 mixtures only (yellow diamonds). The dashed line is the prediction for linear mixing in all cases. All experimental error bars 95% CL. A representative x-error bar, reflecting the precision of preparing the mixtures, is shown for x = 0.82. The confidence limits for the MD ASA analyses correspond to different assumptions about the alkyl chain positions which contribute, as described in the text, which makes only marginal differences here.

Figure 5

OH appearance profiles (OH density as a function of photolysis-probe delay) from [C₁₂mim]_(1–x)[C₈mimF₁₃]_x[Tf₂N] for varying x, as shown. Signals are normalized to those from the squalane reference liquid. ^aMeasurement of commercial sample, confirming reproducibility of data. Error bars are 95% CL.

Figure 6

(a) RAS-LIF reactivity relative to squalane for the neat liquids and for [C_nmim]_(1–x)[C₈mimF₁₃]_x[Tf₂N] mixtures, with chain lengths n as shown. The dashed lines show the behavior expected from a linear mixing law for C8 (black dashed line) and C12 (red dashed line). (b) Excess reactivity, which is the difference (not renormalized) between the reactivity of a mixture and the linear mixing prediction. All errors bars are 95% CL. A representative x-error bar, reflecting the precision of preparing the mixtures, is shown for x = 0.82.

Figure 7

(a) Relative reactivities from either RAS-LIF (OH signals, blue) or MD ASA (proportion of the exposed surface covered by secondary hydrogen, black) for pure alkyl liquids (x = 0, squares, solid lines) or [C₈mim]_0.75[C₈mimF₁₃]_0.25[Tf₂N] mixtures (x = 0.25, circles, dashed lines), as a function of alkyl chain length, n. Both RAS-LIF and MD ASA results have been normalized to those for pure C12 (i.e., x = 0, n = 12). (b) RAS-LIF and MD ASA results [symbols and lines as in (a)] for the same x = 0.25 mixtures renormalized at x = 0.25, n = 12. (c) Ratio of reactivity for x = 0.25 to x = 0 for RAS-LIF (blue) and ASA-MD (black). All experimental errors are 95% CL. The confidence limits for the ASA-MD analyses correspond to different assumptions about the alkyl chain positions which contribute, as described in the text. Note that these largely cancel in (c) because the effects of this assumption on x = 0.25 and x = 0 are correlated.

Figure 8

Color scheme used for the MD snapshots in Figures 9–12. (a) [C_nmim]⁺ (in this case n = 8), (b) [C₈mimF₁₃]⁺, and (c) [Tf₂N]⁻. For each species, the lower structure is a space-filling representation (as used in later figures) defined by van der Waals radii. The fluoroalkyl chains on the cation (C3–C8) are colored cyan, the equivalent chains on the alkyl cation are colored gray, and all other atoms are colored red. To ease rapid identification, the corresponding ball-and-stick representations are shown above in the same orientation and on the same scale. The same color scheme is used, with atom types (other than C, implied for unlabeled atoms) labeled explicitly.

Figure 9

Top-down view of representative single MD snapshots of [C₈mim]_(1–x) [C₈mimF₁₃]_x [Tf₂N] mixtures; x as indicated. Color scheme as in Figure 8.

Figure 10

Side view of representative single MD snapshots of [C₈mim]_(1–x) [C₈mimF₁₃]_x [Tf₂N] mixtures; x as indicated. Color scheme as in Figure 8.

Figure 11

Top-down view of representative single MD snapshots of pure [C_nmim][Tf₂N] (upper row) and [C₈mim]_0.75 [C₈mimF₁₃]_0.25 [Tf₂N] mixtures (lower row) for n = 4, 6, 8, and 12. Color scheme as in Figure 8.

Figure 12

Side view of representative single MD snapshots of pure [C_nmim][Tf₂N] (upper row) and [C₈mim]_0.75 [C₈mimF₁₃]_0.25 [Tf₂N] mixtures (lower row) for n = 4, 6, 8, and 12. Color scheme as in Figure 8.

Figure 13

Number density as a function of z for the 1600-ion-pair system with x = 0 (upper panel) and x = 0.5 (lower panel) in the [C₈mim]_(1–x)[C₈mimF₁₃]_x[Tf₂N] mixture system at 320 K. The densities of the same atom types for the [C₈mim]⁺ and [C₈mimF₁₃]⁺ cations are plotted: CT (or CT-F), the terminal carbon atom on the alkyl (or fluoroalkyl) chain and N (or N–F), the ring nitrogen to which the alkyl (or fluoroalkyl) chain is attached. The z = 0 position was defined by the center of mass of the slab. Bin widths were ∼0.09 nm, corresponding to 500 slices along the z-axis. Averaging is over the equilibrated periods of the MD trajectory, beginning 1 ns (to allow the surface to relax from the previous 500 K heating cycle) into each of the 320 K simulation blocks from 30 to 95 ns.

Figure 14

(a) Fraction of the total surface area (sum of all atom types) occupied by the H atoms attached to each C atom in the alkyl chain for pure [C_nmim][Tf₂N] liquids for n = 4–12, as indicated. Chain positions are numbered from the imidazolium ring. Note that the last member of each chain is a methyl group. (b) Corresponding MD ASA fractional areas for the same atom types in [C_nmim]_0.75[C₈mimF₁₃]_0.25[Tf₂N] mixtures. (c) Ratio of the fractional areas of the same atom types in the mixtures, (b), to those in the pure liquids, (a).