Structure of 1-Alkyl-1-methylpyrrolidinium Bis(trifluoromethylsulfonyl

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Structure of 1‑Alkyl-1-methylpyrrolidinium Bis(trifluoromethylsulfonyl)amide Ionic Liquids with Linear, Branched, and Cyclic Alkyl Groups Hemant K. Kashyap,† Cherry S. Santos,‡ N. Sanjeeva Murthy,§ Jeevapani J. Hettige,† Kijana Kerr,∥ Sharon Ramati,⊥ JinHee Gwon,⊥ Masao Gohdo,∥ Sharon I. Lall-Ramnarine,⊥ James F. Wishart,∥ Claudio J. Margulis,*,† and Edward W. Castner, Jr.*,‡ †

Department of Chemistry, University of Iowa, Iowa City, Iowa 52242, United States Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, Piscataway, New Jersey 08854, United States § New Jersey Center for Biomaterials, Rutgers, The State University of New Jersey, Piscataway, New Jersey 08854, United States ∥ Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973-5000, United States ⊥ Department of Chemistry, Queensborough Community College-CUNY, Bayside, New York 11364, United States ‡

S Supporting Information *

ABSTRACT: X-ray scattering and molecular dynamics simulations have been carried out to investigate structural differences and similarities in the condensed phase between pyrrolidinium-based ionic liquids paired with the bis(trifluoromethylsulfonyl)amide (NTf2−) anion where the cationic tail is linear, branched, or cyclic. This is important in light of the charge and polarity type alternations that have recently been shown to be present in the case of liquids with cations of moderately long linear tails. For this study, we have chosen to use the 1-alkyl-1-methylpyrrolidinium, Pyrr1,n+ with n = 5 or 7, as systems with linear tails, 1-(2ethylhexyl)-1-methylpyrrolidinium, Pyrr1,EtHx+, as a system with a branched tail, and 1-(cyclohexylmethyl)-1-methylpyrrolidinium, Pyrr1,ChxMe+, as a system with a cyclic tail. We put these results into context by comparing these data with recently published results for the Pyrr1,n+/NTf2− ionic liquids with n = 4, 6, 8, and 10.1,2 General methods for interpreting the structure function S(q) in terms of q-dependent natural partitionings are described. This allows for an in-depth analysis of the scattering data based on molecular dynamics (MD) trajectories that highlight the effect of modifying the cationic tail.

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INTRODUCTION Modern room-temperature ionic liquids (RTILs) are novel materials that are being pursued for a variety of applications including energy storage in solar cells, supercapacitor devices, lithium ion batteries, as well as catalysis and separation processes.3−8 In the past decade, a significant number of Xray1,9−34 and neutron35−37 scattering experiments as well as MD simulations23,28,31,32,38−51 have been carried out to unravel the local and mesoscale structure of RTILs. As our group has indicated in several recent publications,2,23,28,50 the X-ray structure function S(q) of RTILs typically has either two or three peaks in the relevant intermolecular region at q values Pyrr1,8+ > Pyrr1,10+. This ordering will become more clear as we discuss the different partitionings of S(q) in subsequent sections. Since we plan to interpret the experimental S(q) by using computer simulations, we first show in Figure 3 the excellent agreement between computed and experimental results. The high level of agreement between simulations and experiments permits us to use the computational structure function and different C

dx.doi.org/10.1021/jp403518j | J. Phys. Chem. B XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry B

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partitioning of S(q) sums the contributions from the cationic and anionic charged parts together in the polar component. This results in destructive interference in the charge alternation region of S(q). In essence, the “natural partition” to study polarity alternation is that for which polar and apolar groups are separately identified as different contributors to the overall S(q). Conversely, the natural partitioning to study charge alternation is one for which cations and anions are not grouped together but instead constitute mutually exclusive S(q) subcomponents. Figure 5 shows that for the case of Pyrr1,10+ a large peak near q = 0.35 Å−1 is observed both in the polar−polar and apolar− apolar subcomponents of S(q). An antipeak at the same value of scattering vector is observed for the polar−apolar subcomponent. This is the signature of polarity alternation. The signature of alternation becomes less clear when the cationic alkyl groups are shorter, branched, or cyclic, since both peaks and antipeaks become broad and poorly defined for this partitioning of S(q). Ionic Partitioning of S(q). Figure 6 depicts a Pyrr1,7+/ NTf2− simulation box color coded according to the ionic

Figure 4. (top) A snapshot of the Pyrr1,7+/NTf2− system colored based on the polarity partitioning of S(q). (bottom) A single ion pair with the same color coding. Green corresponds to polar and white to apolar.

should be positive going, whereas the cross term (polar− apolar) + (apolar−polar) contribution to S(q) should be negative going.2,54 Peaks and antipeaks in the structure function S(q) are a signature of alternation because they correspond to features of the same spatial periodicity but with an origin offset.2 Figure 5

Figure 6. (top) A snapshot of the Pyrr1,7+/NTf2− system colored on the basis of the ionic partitioning of S(q). (bottom) A single ion pair with the same color coding. Blue corresponds to cation and green to anion.

partitioning scheme. The most obvious use of the ionic partitioning of S(q) is to identify the region in reciprocal space in which charge alternation occurs. We expect all ionic liquids to show some type of charge alternation in the ionic partitioning of S(q) independent of their nonpolar tails even when this is absent because of cancellations in the overall S(q). Figure 7 shows exactly this for q ≈ 0.85 Å−1. The cation−cation and anion−anion contributions in Figure 7 lead to peaks for all systems studied, whereas the (cation−anion) + (anion−cation) contributions lead to antipeaks. This is the ubiquitous signature of charge alternation in ionic liquids.

Figure 5. Polar−apolar partitioning of the structure function S(q). Each of the partial structure functions is spaced by +2 units vertically.

shows a plot of the polarity partitioning of S(q) highlighting this effect.54 When analyzing Figure 5, the reader should only focus on the prepeak region (i.e., q < 0.5 Å−1) for which the polar/apolar partitioning of S(q) holds special significance. On the contrary, the polar/apolar partitioning is not useful to interpret features such as charge alternation that occur at around q = 0.85 Å−1 in S(q). This is because the polar/apolar D



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In the same way, Pyrr1,ChxMe+ appears similar to Pyrr1,5+ and not to cations with longer linear tails. Sub-Ionic Partitioning of S(q). In the previous subsection, we indicated that an ionic partitioning of S(q) was advantageous in order to better understand the charge alternation region of S(q). However, with the exception of the anionic component, the ionic partitioning is of little value to study polarity alternation in the prepeak region of S(q). The appreciation of the importance of anionic contribution to the prepeak region and the apparent lack of importance of cationic contribution to the same region has been reported before by our group.50 The underlying reason for this correct but perhaps misleading observation is that the cationic contribution to the prepeak in the ionic partitioning of S(q) is convoluted. This is because both polar and apolar components of S(q) project onto the correlations of cations with cations and those of cations with anions, leading to net cancellations in S(q). In order to deconvolute this information, one must resort to a subionic partitioning of S(q). Figure 9 depicts a Pyrr1,7+/NTf2− simulation box color coded according to the subionic partitioning scheme. As one would

Figure 7. Cation−cation (left), cation−anion (middle), and anion− anion (right) partial structure functions for all the ionic liquids studied here. Curves are offset vertically by 2 units for visual clarity.

Note that, while we cannot extract much useful information from this partitioning in the case of cation−cation or (cation− anion) + (anion−cation) interactions in the prepeak region, much can be learned instead by looking at the anion−anion subcomponent of S(q). This is because both polar and apolar components of S(q) are included in the cationic and (cation− anion) + (anion−cation) partitions but only purely polar components of S(q) are projected onto the anionic partition. Therefore, in this particular partitioning of S(q), information related to polarity alternation that occurs at the prepeak region is not convoluted only for the case of the purely anionic subcomponent of S(q). In the ionic partitioning of S(q), if one is interested in the prepeak, one should only analyze Figure 7 (right). At the value of q for the prepeak, the left and center graphs show data that is convoluted. Figure 7 (right) shows as expected a positive going peak due to polarity alternation. This peak is better defined and more prominent when tails are longer. The purely anion−anion component of the prepeak in Figure 8a and b reveals the reciprocal space value at which polarity alternation occurs. These figures show that at this q value Pyrr1,EtHx+ is much more comparable to Pyrr1,6+ (similar in length) and not to Pyrr1,8+ (same number of tail carbon atoms). Figure 9. (top) A snapshot of the Pyrr1,7+/NTf2− system colored on the basis of the subionic partitioning of S(q). (bottom) A single ion pair with the same color coding. Blue corresponds to cation head, green to anion, and white to cation tail. Notice that head is defined as including the ring, the methyl group, and up to the second methylene group of the longer alkyl tail.

predict, for the case of the longer cations, Figure 10 unambiguously demonstrates that cation−cation polar subcomponents (i.e., head−head) show positive going peaks at the prepeak region. At the same time, polar−apolar antipeaks can be seen in the same region when considering head−tail correlations. Furthermore, the apolar−apolar correlation of tails also gives rise to positive going peaks. These findings are again the signature of polarity alternation and are in complete agreement with matching observations in Figure 5. With increasing length of the cationic alkyl groups, cancellations in the prepeak region due to convolution of

Figure 8. The anion−anion subcomponents of total S(q) for (a) Pyrr1,ChxMe+ (black), Pyrr1,5+ (red), and Pyrr1,7+ (green) and (b) Pyrr1,EtHx+ (black), Pyrr1,6+ (red), and Pyrr1,8+ (green) RTILs. E



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Figure 10. Cation−cation partial structure function split into head− head (left), head−tail (middle), and tail−tail (right) subcomponents. Curves are vertically offset by 2 units for visual clarity.

Figure 11. Cation−anion contributions to the structure function S(q) partitioned into cation head−anion and cation tail−anion subcomponents. Curves are offset vertically by 2 units for visual clarity.

polar and apolar components in the ionic partitioning of S(q) are partially overcome. However, it is much better to examine the polarity partitioning or the subionic partitioning of S(q) to properly understand longer-range polarity alternations in the prepeak region. Because of its striking simplicity, perhaps the most elegant way73 to concurrently check for the existence of charge and polarity alternations in ionic liquids is to plot the cationic head−anion subionic partitioning of S(q). Since both the cationic head and the anion are polar, their correlation should always give rise to a positive going peak in the prepeak region if polarity alternation is indeed present in the system. Furthermore, because cation heads and anions carry charges of opposite sign, their contribution to S(q) should always appear as an antipeak in the charge alternation region. This function, as opposed to others described in previous sections, does not suffer from convolution problems either at the prepeak region or at the charge alternation region in reciprocal space. We expect these general principles to hold true for most if not all ionic liquids. A caveat is in place here; our line of reasoning in this and previous articles is based on the well established model of cations with nonpolar tails and anions that are completely polar. The situation will become more complex for ILs comprising cations and anions that both have longer alkyl tail substituents such as 1-octyl-1-methylpyrrolidinium octylsulfate or for systems in which tails are polar.55 Figure 11 (left) demonstrates that the cationic head−anion subionic partitioning of S(q) carries all the desirable information needed to establish polarity alternation and charge alternation regions. Systems with longer cationic alkyl groups show both charge and polarity alternation, whereas those with shorter tails only show charge alternation. Figure 11 (right) only teaches us about polarity alternation but not about charge alternation. Systems with longer cationic alkyl groups show antipeaks in the cation-tail−anion subionic partitioning of S(q). Figures 12 and 13 provide a clear comparison of the subionic contributions of cyclic and branched systems against those with linear tails. In all cases, the left panel shows that for branched and cyclic tailed cations polar−polar components scatter in a way that is similar to systems with linear tails of approximately the same spatial length but not of the same number of carbons.

Figure 12. Cation−cation partial structure function split into head− head, head−tail, and tail−tail terms for (a) Pyrr1,ChxMe+ (black), Pyrr1,5+ (red), and Pyrr1,7+ (green) and (b) Pyrr1,EtHx+ (black), Pyrr1,6+ (red), and Pyrr1,8+ (green) RTILs.

This analogy is not as clear for the other subionic components of S(q). Bulk Densities. Figure 14 shows that, in the case of the systems in which cations have linear alkyl tail groups, experimental and simulated densities (see Table S1, Supporting Information) follow very similar trends with density decreasing F



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with increasing number of tail carbons. For these liquids, computational densities are about 5% higher than the corresponding experimental densities. Density deviations between simulation and experiment in the case of the branched and cyclic cationic systems are less than 3.5%. In our experience from previous studies and from the data presented in this article, excellent agreement between experimental and computational S(q) can be obtained even if small density discrepancies are observed. S(q) does not appear to be sensitive to these small density differences. For fixed n = 7, the density of the system with the cyclic chain is significantly larger by about 5 or 6% with respect to that of the linear chain cation system. Instead, the density difference between the system with linear cationic tail of n = 8 and the branched system is much less significant. Notably, an earlier study75 found that the branched sec-butyl isomer of Pyrr1,4+/NTf2− was denser than the linear isomer (1.46 vs 1.39 g/cm3), in keeping with the trend observed here.

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CONCLUSIONS In this article, we compare the structure of linear, branched, and cyclic alkyl tail pyrrolidinium bis(trifluoromethylsulfonyl)amide RTILs. From a methodological perspective, we found that the ideal way to study these and other ionic liquids is by using qdependent partitionings of the structure function. These qdependent partitionings enable us to clearly identify the physical origin of the different intermolecular features of S(q). As expected, ILs having cations with long linear alkyl groups show both the signs of polarity alternation and charge alternation. ILs comprising cations with shorter alkyl tails show charge alternation but no polarity alternation. The behavior of our branched and cyclic systems at the prepeak region is from the perspective of the charged groups similar to that observed for systems where the alkyl tail in the cations is of similar length but not necessarily of similar number of carbon atoms. The behavior of the polar−apolar components and apolar−apolar components in these two systems is somewhat more complex and not necessarily similar to that of systems with the same tail length.

Figure 13. Cation−anion partial structure function split into cation head−anion and cation tail−anion terms for (a) Pyrr1,ChxMe+ (black), Pyrr1,5+ (red), and Pyrr1,7+ (green) and (b) Pyrr1,EtHx+ (black), Pyrr1,6+ (red), and Pyrr1,8+ (green) RTILs.

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ASSOCIATED CONTENT

S Supporting Information *

Details on the synthesis of the compounds and a table of measured and simulated densities. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (C.J.M.); ed.castner@ rutgers.edu (E.W.C.). Author Contributions

H.K.K. and C.J.M. completed the molecular simulations and theoretical analysis of the structure factor. J.J.H. worked with H.K.K. in checking the correctness of all force field parameters in various simulation input files. C.S.S., N.S.M., and E.W.C. carried out the X-ray experiments at APS and analyzed the Xray data. K.K., S.R., J.G., M.G., S.L.-R., and J.F.W. prepared and characterized three ionic liquids. H.K.K. drafted most of the manuscript in collaboration with C.J.M. and E.W.C.

Figure 14. Experimental and simulated bulk densities of the RTILs as a function of the number of alkyl chain carbon atoms, n. The densities of Pyrr1,n+/NTf2− RTILs are shown with black circles and those of Pyrr1,ChxMe+/NTf2− and Pyrr1,EtHx+/NTf2− with red circles. All simulated densities are at 295 K. Reported experimental densities74−76 were measured between 293 and 303 K, as indicated in Table S1 of the Supporting Information.

Notes

The authors declare no competing financial interest. G



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ACKNOWLEDGMENTS The authors thank Prof. Mark Maroncelli for the sample of Pyrr1,5+/NTf2−, Dr. Tomasz Szreder for measuring the density of Pyrr1,4+/NTf2−, Dr. Alison Funston for measuring the density of Pyrr1,5+/NTf2−, and Jasmine Hatcher for assistance preparing scattering samples. We thank Dr. Chris Benmore and Dr. Yang Ren for help in the X-ray measurements and data analysis at APS beamline 11-ID-C. This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences under contracts DE-SC0001780 (E.W.C.), DE-SC0008644 (C.J.M.), and DE-AC02-98CH10886 (J.F.W.). Preparation of Pyrr1,ChxMe+ and Pyrr1,7+ salts by K.K. was supported at BNL by the DOE Office of Nuclear Energy. Researchers (S.L.-R., S.R., J.G.) from Queensborough Community College were supported by PSC-CUNY Research Award Grants and the Louis Stokes Alliance for Minority Participation. Use of the Advanced Photon Source at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC0206CH11357.

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