Hiding the Headgroup? Remarkable Similarity in Alkyl Coverage of the Surfaces of Pyrrolidinium- and Imidazolium-Based Ionic Liquids

Tesa-Serrate, Maria Antonia; Smoll, Eric J.; D'Andrea, Lucia; Purcell, Simon; Costen, Matthew L.; Bruce, Duncan W.; Slattery, John M.; Minton, Timothy K.; McKendrick, Kenneth G.

doi:10.1021/acs.jpcc.6b09315

Cited by 14 publications

(31 citation statements)

References 61 publications

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“…49 − 51 , 54 , 56 We have shown the power of the RAS-LIF and RAS-MS methods to analyze the extreme outer surfaces of a number of IL systems, including homologous series of imidazolium- and pyrrolidinium-based liquids, the coupled effects of variation of the anion, and even liquid-crystalline materials. 47 − 53 The majority of this work has used ground-state oxygen, O( 3 P), atoms as the projectile, with the detection of OH (RAS-LIF) or inelastically scattered O, OH, and H 2 O (RAS-MS). Recently, we have also shown that RAS-MS can be extended to F( 2 P) projectiles, combined with the detection of HF, or DF for isotopically labeled samples, giving detailed site-specific information on the occupancy and orientation of cations at the surface.…”

Section: Introductionmentioning

confidence: 99%

Surface Structure of Alkyl/Fluoroalkylimidazolium Ionic–Liquid Mixtures

et al. 2022

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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 ...

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Section: Introductionmentioning

confidence: 99%

Surface Structure of Alkyl/Fluoroalkylimidazolium Ionic–Liquid Mixtures

et al. 2022

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“…Comparisons of pyrrolidinium-based ILs with C nmim + -based ILs found the similarity of the two kinds of cations in the dependence of the surface structure on the alkyl chain length. 42,63,64 In the nonpolar region at the surface, alkyl chains are not fully packed. Hence, other parts of IL ions, such as polar parts of the cation and the polar/nonpolar parts of the anion, are also exposed at the surface.…”

Section: ■ Introductionmentioning

confidence: 99%

Surface Structure of Quaternary Ammonium-Based Ionic Liquids Studied Using Molecular Dynamics Simulation: Effect of Switching the Length of Alkyl Chains

et al. 2019

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Electric double layer structure at the electrode interface has been studied by using molecular dynamics simulation on four quaternary ammonium-based ionic liquids (QaILs) to investigate the effect of switching the alkyl chain length of the Qa cation. These four QaILs are composed of a common anion, bis(trifluoromethanesulfonyl)amide (TFSA −) and different cations: butyltrimethylammonium (N + 1114 , k = 1), dibutyldimethylammonium (N + 1144 , k = 2), tributylmethylammonium (N + 1444 , k = 3), and tetrabutylammonium (N + 4444 , k = 4), where k represents the number of butyl chains. The difference in k affects the potential dependence for the composition of the first ionic layer and the orientation of butyl chains in the layer. For the case of k = 1, 2, 3, the butyl chains parallel to the interface increases as the potential becomes negative, but further negative potential results in the increase in perpendicular ones. In the case of k = 1, all the cations in the first ionic layer show the perpendicular orientation at the negative potentials, forming a honeycomb lattice consisting of only cations. On the other hand, in the case of k = 4, no change in orientation has been observed due to the geometrical restrictions. The difference in k also affects the differential capacitance. The potential dependence of differential capacitance shows bell shape for the smaller two (k = 1, 2) and camel shape for the larger two (k = 3, 4). The camel shape for larger IL cations agrees with the prediction from the mean-field lattice gas model and recent experimental results. The differential capacitance at negative potentials deviated to the values higher than the model prediction and the discrepancy becomes greater for smaller k. The results indicate that the potential dependence of ionic orientation significantly affects the differential capacitance. Even for k = 4, which does not show the orientational change, the discrepancy has been observed, indicating that not only the orientational change but also the densification of ions in the first ionic layer are the factors we should take into account beyond the lattice gas model.

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“…Experiments capable of quantifying the relative populations of preferred ion conformations are crucial to the development and verification of realistic models of the liquid–vacuum interface of RTILs and other molecular liquids. Methods like sum-frequency generation (SFG) and angle-resolved X-ray photoelectron spectroscopy (ARXPS) have made important contributions to our understanding of the average orientation of specific ions at the liquid–vacuum interface of common RTILs. − A smaller number of studies using molecular dynamics (MD) simulations and low-energy ion scattering (LEIS) have addressed the distribution or relative populations of ion conformations at such interfaces. − Our own investigations of the RTIL–vacuum interface with the use of reactive-atom scattering have revealed general features of the interface through signatures in the scattering dynamics. − Unlike studies of inelastic scattering on interfaces with noble gas atoms, reactive-atom scattering is intrinsically chemically specific because the scattered reaction product carries an interfacial atom that the probe atom must have come into contact with before the product exited the interface. Thus, the resulting reactive-atom scattering signals sum over all sites that expose the interfacial atom to the probe atom.…”

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confidence: 99%

Probing Conformational Heterogeneity at the Ionic Liquid–Vacuum Interface by Reactive-Atom Scattering

Smoll

Purcell

D'Andrea

et al. 2018

J. Phys. Chem. Lett.

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The atomic-level description of liquid interfaces has lagged behind that of solid crystalline surfaces because existing experimental techniques have been limited in their capability to report molecular structure in a fluctuating liquid interfacial layer. We have moved toward a more detailed experimental description of the gas–liquid interface by studying the F-atom scattering dynamics on a common ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. When given contrast by deuterium labeling, the yield and dynamical behavior of reactively scattered HF isotopologues can resolve distinct signatures from the cation butyl, methyl, and ring groups, which help to quantify the relative populations of cation conformations at the liquid–vacuum interface. These results demonstrate the importance of molecular organization in driving site-specific reactions at the extreme outer regions of the gas–liquid interface.

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Hiding the Headgroup? Remarkable Similarity in Alkyl Coverage of the Surfaces of Pyrrolidinium- and Imidazolium-Based Ionic Liquids

Cited by 14 publications

References 61 publications

Surface Structure of Alkyl/Fluoroalkylimidazolium Ionic–Liquid Mixtures

Surface Structure of Alkyl/Fluoroalkylimidazolium Ionic–Liquid Mixtures

Surface Structure of Quaternary Ammonium-Based Ionic Liquids Studied Using Molecular Dynamics Simulation: Effect of Switching the Length of Alkyl Chains

Probing Conformational Heterogeneity at the Ionic Liquid–Vacuum Interface by Reactive-Atom Scattering

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