Modern Room Temperature Ionic Liquids, a Simple Guide to

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Modern Room Temperature Ionic Liquids, a Simple Guide to Understanding Their Structure and How It May Relate to Dynamics Juan C. Araque, Jeevapani J. Hettige, and Claudio J. Margulis* Department of Chemistry, University of Iowa, Iowa City, Iowa 52242, United States ABSTRACT: Modern room temperature ionic liquids are structurally defined by symmetries on different length scales. Polar−apolar alternation defines their nanoscale structural heterogeneity, whereas positive−negative charge alternation defines short length scale order. Much progress has been made in the past few years as it pertains to the theoretical interpretation of X-ray scattering experiments for these liquids. Our group has contributed to the development of theoretical interpretation guidelines for the analysis of their structure function. Perhaps less well developed is our understanding of how transport and dynamics in general couple to the very unique structure of ionic liquids which are often dynamically and structurally heterogeneous. This article attempts to present our most current understanding of ionic liquid structure in general and its coupling to transport and dynamics in minimally technical terms for the benefit of the broadest audience.

1. INTRODUCTION The past 10 years have witnessed tremendous expansion in the design and synthesis of modern room-temperature ionic liquids (ILs). We now have task specific ionic liquids,1−11 ionic liquid drugs,12−16 ionic liquids for energy applications such as batteries and supercapacitors,17−31 those that capture carbon dioxide,3,32−38 and those that dissolve cellulose39−44 and proteins.45−49 In terms of their structure, interesting work has surfaced on protic ionic liquids,50−59 ILs that are highly fluorinated,59−63 ILs with polar tails,64−67 ILs with paramagnetic ions,68,69 those containing silicon,70−74 and some that are triphillic59,61−63 just to mention a few examples. Not all ionic liquids but a vast number of them can be categorized as having both charged components and also apolar components.75 The balance between charge and other polar or apolar components defines properties such as viscosity, melting points, solubility, hydrophilicity, and nanoscale structure. Neutron56,76−84 and X-ray66,78,79,83,85−107 scattering experiments as well as computer simulations63−65,67,85,87,96,103,108−132 have provided tremendous insight into the structure of ionic liquids. One of the areas in which our group has contributed is in deriving some basic rules for the analysis of X-ray data. For the most part, such rules should also apply to neutron scattering which when used in combination with partial deuteration becomes a most powerful experimental technique.76,77,84,133 Because ILs are ultimately molten salts, their underlying structure is governed to a large extent by charge alternation. At larger distances (lower q values), many ILs display what is commonly known as a prepeak or a first sharp diffraction peak. In systems where this is observed, the phenomenon arises because charge networks are necessarily separated from other charge networks by apolar components. As the complexity of ILs increases, other features beyond charge and polarity alternation © 2015 American Chemical Society

manifest including apolar-fluorinated alternation and the analysis of experiments becomes very complex without the guide of detailed simulations. Because ILs have structural features on different length scales, they are at the molecular level structurally heterogeneous.50,56,61,88,90,95,102,126,134−137 They are also known to be dynamically heterogeneous;138−142 chemical143,144 and spectroscopic139,145−149 signatures of such heterogeneity are ubiquitous in the literature. More recently, a different manifestation of heterogeneity has been observed when probing solute diffusion.150,151 Motion of charged solutes can be much slower than hydrodynamic predictions and that of neutral solutes much faster depending on the volume of the solute as compared to that of the solvent. Are anomalous solute mobility and solvent structural heterogeneity related? If so, what is the underlying mechanistic connection? In sections 2−5, we describe our most current understanding of IL structure, and in section 6, we discuss connections between structure and dynamics.

2. THE BASICS ABOUT IONIC LIQUID STRUCTURE It is easy to tell a generic ionic liquid from many non-hydrogenbonding conventional solvents by simple inspection of the structure function (S(q)) defined in terms of the coherent X-ray scattering intensity Icoh (eq 1). In eq 1, xi and f i(q) correspond to the mole fraction and X-ray form factor152 of atoms of type i. S(q) =

Icoh(q) − ∑i xi fi 2 (q) [∑i xi fi (q)]2

(1)

Received: June 9, 2015 Revised: July 20, 2015 Published: August 5, 2015 12727

DOI: 10.1021/acs.jpcb.5b05506 J. Phys. Chem. B 2015, 119, 12727−12740

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The Journal of Physical Chemistry B

lower q values. The one at the lowest q value, the prepeak, is indicative of polar−apolar alternation and has been linked to structural heterogeneity. The peak at intermediate q values, higher than the prepeak but lower than the adjacency peak, is the molten salt signature and indicates charge alternation. A missing prepeak is often indicative of small apolar components and a lack of polar−apolar alternation. A missing intermediate peak is likely due to cancellations because at room temperature charge alternation is always present. Charge and polarity alternation are not unique to the liquid phase and can be clearly identified in crystal structures. Examples of this are depicted in Figure 2. In fact, one can think of ILs as having lost most crystal symmetries except for charge and polarity alternation.112

Figure 1a displays computationally derived S(q) for an eclectic set of imidazolium, alkylammonium, 64,85,113 phospho-

3. PEAKS AND ANTIPEAKS IN PARTIAL STRUCTURE FUNCTIONS ARE SIMPLE INDICATORS OF ALTERNATION ON THE MOLECULAR LANDSCAPE The experimental S(q) carries all information about the liquid landscape, but it does so in a way that is difficult to dissect, since its nature is collective. In contrast, in computer simulations, the situation is the opposite as we construct the collective S(q) from specific atomic pair information. This is done either as a sum in real space that gets Fourier transformed (eq 2) S(q) = ρo ∑i ∑j xixj fi (q) f j (q) ∫

0

L /2

4πr 2(gij(r ) − 1)

sin qr W (r ) qr

dr

[∑i xi fi (q)]2 (2)

or directly in reciprocal space (eq 3). In eq 2, gij(r) represents the radial distribution function between atomic species of type i and j, and a Lorch function W(r)154,155 is often utilized to mitigate finite truncation effects at large r. Angle brackets in eq 3 correspond to the ensemble average of atomic densities in reciprocal space defined in eq 4 where, for atomic species of types i and j, Ni and Nj are the total number of atoms.156 Each of these computational approaches provides different physical insight that will be discussed below.

Figure 1. Computationally derived S(q) for (a) a diverse collection of ionic liquids64,109,111,113 and (b) conventional solvents. Perhaps with the exception of methanol that forms hydrogen bonds, the conventional solvents show a single feature in the region of S(q) relevant to intermolecular interactions. Instead the ionic liquids show three features or peaks. The peak at larger q values is associated with adjacency correlations between neighboring atoms. These correlations are intermolecular and intramolecular in nature. The intermediate peak is the signature of charge alternation. The peak at the smallest q value is caused by polarity alternation and is associated with structural heterogeneity. Absence of a charge alternation peak as in the case of [Im1,8][BF4] does not mean this symmetry is absent; instead, it denotes perfect cancellations of peaks and antipeaks at this particular q value (see section 3).

S(q) =

ρqi =

1 ∑i ∑j fi (q)f j (q)⎡⎣ N ⟨ρqi ρ−j q ⟩ − xiδi , j ⎤⎦

[∑i xi fi (q)]2

(3)

Ni

∑ exp(−iq·rα) α=1 Nj

ρ−j q

nium,111,114 and pyrrolidinium109,110 based ILs. Figure 1b shows instead S(q) for a selected set of conventional liquids. It is clear from the comparison that, perhaps with the exception of methanol which forms hydrogen bonds, these conventional solvents only have one peak at q values below 2.5 Å−1, whereas the ionic liquids commonly have three peaks or features. The only peak in the case of conventional solvents and the peak at larger q values in the case of the ionic liquids account for a myriad of different correlations between adjacent atoms. These can be both intermolecular and intramolecular in nature. In the case of ILs, two extra peaks or shoulders are commonly observed at

=

∑ exp(iq·rβ) β=1

(4)

To learn about the specific liquid motifs that give rise to different features in S(q), eq 2 can be conveniently partitioned into subcomponents. Many partitionings are possible that give rise to identical S(q), but we learn the most when such partitionings are wavenumber dependent.85,108−110 As an example, to study low q phenomena, it is most natural to partition S(q) into contributions from polar−polar, polar−apolar, and apolar−apolar interactions disregarding the positive or negative nature of the charge. 12728


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Figure 2. Crystal structure of [Im10,1][PF6]153 showing (a) polarity alternation across Miller plane (0, 0, 1) and (b) charge alternation across Miller plane (1, 1, −1). These two symmetries, charge and polarity alternation, that are present in the crystalline state persist in the liquid phase and become the defining features of ionic liquid morphology (see ref 112).

Figure 3. On the left panel are idealized radial distribution functions g(r) for a liquid with patterns of alternation. The f1(r) function corresponds to the case of same-type objects, whereas f 2(r), to different-type objects. For example, f1(r) may represent positive−positive (++) correlations and f 2(r) may represent positive−negative (+−) correlations. Because the periodicity of the two functions is identical but f1(r) and f 2(r) are offset by half a period, their Fourier transforms (right-hand side panel) have a peak at identical q value but of different sign. Such peaks are always positive for same-type correlations and negative for different-type correlations. These are what we call peaks and antipeaks in the partial subcomponents of S(q). See ref 109 and the associated Supporting Information.

SPolar − Polar(q) = S a − a(q) + S c Head − c Head(q) + S c Head − a(q) + S a − c Head(q)

Instead, to study shorter range order, it is most appropriate to

S Apolar − Apolar(q) = S c Tail − c Tail(q)

positive, positive−negative, and negative−negative type con-

partition S(q) in a way that highlights and distinguishes positive− tributions, since such alternation is the phenomenon taking place

SPolar − Apolar(q) + S Apolar − Polar(q) = S c Head − c Tail(q) + S c Tail − c Head(q) + S c Tail − a(q) + S a − c Tail(q)

at length scales shorter than polarity alternation but longer than simple adjacency correlations. 12729


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The Journal of Physical Chemistry B When alternations in the liquid phase are present and a judicious partitioning of S(q) for a given wavenumber region is chosen, what we have coined as peaks and antipeaks naturally manifest as the hallmark of such alternations in the partial subcomponents of S(q). The rationale for this is simple; seen from an arbitrary origin, alternating liquid components have the same periodicity but a phase offset. This leads to peaks at the same wavenumber but with different signs. Figure 3 demonstrates this for a perfectly idealized system for which “same-type” components have a periodic real space separation of 2π and are intercalated by “opposite-type” species. Parts a and b of Figure 4 show the same phenomenon when considering realistic polarity and charge alternations in an IL. In the polarity alternation region, same type corresponds to polar−polar and apolar−apolar correlations, whereas, in the charge alternation region, same type corresponds to positive−positive and negative−negative correlations. For the proverbial IL in which the anion is small and the cation has charged and apolar subcomponents, there is a simplest and most convenient way to analyze charge and polarity alternations in the liquid phase.110,113 The cation head−anion subcomponent of S(q) is a “natural variable” both in the charge alternation regime as well as in the polarity alternation regime. A plot of this function alone tells us most of what we need to know in both wavenumber ranges. This is easy to understand; cation heads and anions correspond to the same group when discussing polarity alternation, as they both form part of the polar network that alternates with apolar components of the liquid at low q values. Instead, cation heads and anions form part of opposite groups when considering charge alternation at intermediate q values. In Figure 4c, we see that same-type elements are on average periodically separated by a distance nλ (λ is large for polarity alternation and small for charge alternation), whereas differenttype elements are separated by a distance (n + 1/2)λ. At low q, because they are same-type liquid elements, cation heads and anions contribute a peak to the overall S(q) (because of the same reason, in the same q region, apolar components also contribute a peak, but polar−apolar components contribute an antipeak). At intermediate q values (larger than polarity alternation but smaller than simple adjacency correlations), cationic head−head correlations contribute a peak, anion−anion correlations contribute a peak, and cationic head−anion correlations contribute an antipeak to the overall S(q). Simply put, a plot of the cationic head−anion subcomponent of S(q) often shows a peak at the low q range where polarity alternation occurs and always shows an antipeak where charge alternation occurs. Examples of this for two different ILs of the pyrrolidinium series are depicted in Figure 5. No peak in the low q region means that the apolar subcomponent is not signif icant enough to allow for polar−apolar alternations and is often interpreted as indicative of an absence of polar−apolar structural heterogeneity ([Pyrr1,4][NTf2] in Figure 5). Whereas the feature corresponding to polarity alternation can be missing, we have never observed a situation in which the charge alternation antipeak is absent in a prototypical IL. Charge alternation is the signature of a room-temperature molten salt. This is written with the caveat that such an antipeak in the case of protic ILs can look very different in shape and wavenumber, as now it reflects hydrogen bonding. Interestingly, whereas peaks and antipeaks in the charge alternation region are prominent in partial subcomponents of S(q), it is often the case that the total S(q) only shows a small shoulder; in other cases, not even that. Such a situation is due to complete interference cancellation and not caused by the absence of charge alternation

Figure 4. (a) (+/+), (+/−), and (−/−) subcomponents of S(q) for (2ethoxyethoxy)ethyltriethylphosphonium bis(trifluoromethylsulfonyl)amide64 exhibit peaks and antipeaks associated with charge alternation. Both (+/+) and (−/−) are same-type correlations and therefore result in peak contributions, whereas (+/−) are different-type correlations and result in an antipeak. Notice how (−/−) correlations are of large intensity; this is quite common because anions are often the most important reporters of X-ray structure in ILs. (b) Polar−polar, polar− apolar, and apolar−apolar subcomponents of S(q) for tetradecyltrihexylphosphonium bis(trifluoromethylsulfonyl)amide114 exhibit peaks and antipeaks associated with polarity alternation. Because they are sametype interactions, polar−polar and apolar−apolar correlations appear as peaks, whereas polar−apolar correlations appear as an antipeak. The polar contribution is large compared to the apolar contribution; anions often heavily weight in the polar contribution. (c) Schematic representation depicting the origin of peaks and antipeaks. Same-type species are separated by nλ periods, but different-type species are separated by (n + 1/2) λ periods. The 1/2λ offset is the cause for antipeaks in correlations of different-type species.

in the liquid (a typical example is the case of [Im1,8][BF4] in Figure 1a). So far, we have discussed the benefits of using eq 2 to interpret IL structure. The route for this approach involves generating all possible radial distribution functions between atomic species and their properly weighted Fourier transformed sum. There are many advantages to this procedure but some limitations as well. Radial distribution functions are collective functions that report all spatial correlations between atomic classes (all carbon against all fluorine, for example). However, what if, for example, one is interested in teasing out the contribution to S(q) of a certain 12730


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in Figure 6a, the prepeak anomalously increases with increasing temperature. Parts b and c of Figure 6 show that indeed the

Figure 5. For typical ionic liquids where cationic components have charged and apolar components and anions are small and symmetrically charged, a plot of the cationic head−anion subcomponent of S(q) highlights the most important features associated with liquid structure. Because the cationic head and the anion belong to same-type elements in the region associated with polarity alternation, a peak is expected from a plot of this subcomponent of S(q). Instead, because the cationic head and anion belong to opposite-type groups, in the case of charge alternation, an antipeak is expected at lower q value. The figure shows exactly this in the case of two pyrrolidinium based ILs.109 The system with a long alkyl tail shows both a polarity alternation peak at low q and a charge alternation antipeak at larger q. Instead, in the case of the cation with a shorter alkyl tail, polarity alternation is almost completely abolished but charge alternation is still present as an antipeak at about 0.8 Å−1.

specific liquid pattern involving only some cationic carbon atoms in tails and fluorinated anionic components? This is much more easily achieved with a sum over specific atomic contributions in reciprocal space as in eq 3. Reference 63 shows in detail how starting from eq 3 one can seek to identify atomic pair contributions that at a wavenumber of interest q contribute largely to S(q) and that at the same time are consistent with the condition that the atomic distance is d = 2π/q. Examples of the use of the approach discussed in ref 63 will appear in section 5.

Figure 6. (a) Total S(q), (b) polar−polar subcomponent of S(q), and (c) apolar−apolar subcomponent of S(q) for [P14,6,6,6][NTf2] at different temperatures. At the prepeak region which is indicative of polarity alternation and structural heterogeneity, the temperature dependence is anomalous. One would expect that higher temperatures would result in a less intense first sharp diffraction peak in the top panel, but the opposite is true. As expected, the increase in temperature coincides with a more disorganized apolar subcomponent but with a more organized polar component. Since anions are very important reporters of structure in ILs, the signal due to the polar component overwhelms that of the apolar part. A larger prepeak does not mean in this case that apolar domains are better defined; instead, it is a reflection of the predominant role of anions in these types of experiments (see ref 113).

4. CATIONS DEFINE THE LANDSCAPE BUT ANIONS REPORT Cations used in ILs are often more structurally rich and asymmetric than their anionic counterparts. This is by no means a general rule, but it applies to a large number of ILs. Cations are often the species that contribute both polar and apolar components, whereas anions are frequently smaller and of higher symmetry and charge is more evenly distributed. It is therefore reasonable to envision cation heads and tails as defining the landscape of polarity alternation. X-ray diffraction experiments probe electronic density which manifests in eq 2 via the atomic form factors. Because of their higher electronic density, anions tend to be better scatterers than cations. It is not uncommon for two systems that have similar pair correlation functions to have significant differences in S(q) depending on the identity of the anion.113 In other words, cations may determine the liquid landscape, but we only learn about them from the scattering of the less structurally rich anions. An example where this clearly manifests is in the case of the temperature dependence of the prepeak in long-tail phosphonium ILs.111,114 Since nanoscale order in the form of polarity alternation is expected to decrease with an increase in temperature, so is the prepeak intensity. In reality, as is shown

apolar contribution to S(q) diminishes as temperature increases; the loss of organization of the apolar component allows for a moderate increase in organization of the charged subcomponent. The apolar subcomponent disorder decreases the prepeak intensity, but the enhancement in positive−negative interactions results in an intensity increase. Why do these two opposite effects not cancel? The answer is simple: Because of their X-ray scattering form factors, anions tend to be the most prominent reporters of structure. This is why the enhancement to the purely polar component of the prepeak dominates what is seen in X-ray experiments. In this case, a more intense prepeak does not imply 12731


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Figure 7. (a) Comparison of S(q) for phosphonium and ammonium isoelectronic analogues where the cation has either alkyl or ether containing tails. (b) Anionic first solvation shell around an ether-based cation shown in terms of a spatial density distribution. When comparing the ether substituted analogues with their isoelectronic alkyl based systems, we find that the prepeak indicative of polarity alternation and polar−apolar structural heterogeneity is completely abolished. This can be understood from part b where cationic tails curl and anions cap both ends of the cation. Anions close to the more polar tail occupy the space that apolar subcomponents would have taken in the case of the alkyl based cation. Because of this, polarity alternation is disrupted and the symmetry reflected as a prepeak in S(q) is completely absent from these ionic liquids with tails that are more polar (see ref 64).

the assignment of the prepeak on the center panel in Figure 8 as well as in the rendering in the bottom panel. Other features in S(q) highlight apolar-fluorinated cationic tail−anionic tail alternations and hydrogen bonding as well as adjacency correlations that are nonperiodic. To tease out detailed information on liquid patterns in complex systems like this one, the method based on Fourier transforms of pair distribution functions (eq 2) is less advantageous than the direct sum in reciprocal space in eq 3. In ref 63, we proposed a methodology to highlight patterns of atomic interactions in the liquid phase that not only contribute significantly at a given q value to the overall S(q) but that are also resonant with the Bragg condition d = 2π/q. Such a methodology has allowed us to assign all features in S(q), as highlighted in the top and center panels in Figure 8.

a better defined apolar domain. The opposite is true. The anomalous behavior is instead associated with the highly weighted role that anions play on scattering. In other words, if you want to learn about the cations, pay attention first to the anions.

5. BREAKING THE CONVENTIONAL PARADIGM. BEYOND APOLAR TAILS Ionic liquids do not have to be comprised of cations with polar heads and apolar tails combined with small symmetric anions. In fact, the paramount feature of ILs, charge alternation, can become hydrogen bonding in the case of protic ILs. Anions can have apolar or fluorinated tails, they can be large and asymmetric and cations can have tails that are polar or no tails at all. All of these possible changes and combinations dramatically alter the prototypical three-featured S(q) function. Figure 7a shows how when ethers replace alkyl tails charge alternation and adjacency correlations persist but polarity alternation characterized by a prepeak is completely vanished from S(q). Figure 7b shows that, when apolar tails are replaced by more polar ether containing tails, the morphology of said tails changes (tails curl) and anions efficiently solvate them. Since anions can solvate both the cationic charged head as well as the tail, the characteristic length scale corresponding to charged groups separated from other charged groups by apolar components disappears and so does the prepeak in S(q) at low q values.64 Among the most interesting systems that we have studied which break with the biphilic paradigm are protic ILs with large anionic fluorinated tails. 59 Some of these systems are continuously percolated by a network of hydrogen bonds. This network takes the form of continuous filaments decorated both by fluorinated anionic components and by alkyl cationic components (bottom of Figure 8). Several symmetries are present in the liquid phase. Because of charge alternation caused by hydrogen bonding along filaments, tails of cations and anions that decorateand are perpendicular tothe hydrogen bond filaments also alternate. This causes apolar-fluorinated alternation within each filament with interesting helical-like structures. The prepeak in this case highlights the typical separation between percolating hydrogen bond filaments spaced by decorating cationic and anionic tails. This is highlighted with

6. IONIC LIQUID STRUCTURE DEFINES SOLUTE DYNAMICS Do mechanisms of transport, anomalous diffusion, and dynamical heterogeneity have an underlying structural origin in ILs? Or are structural and dynamical heterogeneities fundamentally unrelated phenomena? A comparison between parts a and b of Figure 1 clearly highlights that many common ILs have at least two symmetries that are absent in most conventional solvents, one at shorter distances associated with charge alternation and the other at longer nanoscale distances (lowest q) linked to polar−apolar alternation. This lowest q feature associated with polarity alternation defines what is commonly associated with structural heterogeneity. Since the prepeak is indicative of nanoscale polarity alternations, one can envision regions of the liquid that are charge-dominated and others that are dominated by apolar interactions. It has been a long ongoing debate whether such structural heterogeneity may be related to other phenomena such as red edge effects commonly associated with dynamical heterogeneity or distributed dynamics in general.139 Recently, a set of intriguing articles157−168 discussing electron transfer reactivity at anomalous rates (too slow for charged species but too fast for neutral species) have surfaced in the literature. A comprehensive study150 on the mobility of neutral and charged species in comparison to hydrodynamic predictions indicates that when the molecular volume of the solute is small 12732


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It is not difficult to see how stiff environments which are more polar and soft environments which are more apolar will also be linked to energy heterogeneities.169 Stiff or soft environments are associated with the distance to charge and should certainly affect the solvent contribution to the HOMO−LUMO gap for spectroscopic processes causing distributed dynamics and absorption wavelength dependent phenomena.139 The IL structure as well as the size and neutral or charged nature of the solute strongly couple to different mechanisms of diffusion. A pictorial explanation of the reason why Stokes−Einstein behavior fails for small solutes of different charge in ILs is presented in Figure 9. Figure 9a shows that, at 400 K, on a 600 ps time scale, a trajectory for methane in an ionic liquid with very modest apolar components is comprised of many cage and jump events linked to enhanced and diminished local friction. Cages and jumps have typical time scales much smaller than the overall trajectory which samples a significant volume. Instead, Figure 9b shows very different behavior in the case of ammonium where only a single jump event is observed on the same time scale and the overall volume sampled by the trajectory is much more modest. Both methane and ammonium have similar radii, and their diffusivity should be similar based on Stokes−Einstein predictions. We see that, as opposed to the case of methane, ammonium is not an innocent spectator of solvent environments but instead an integral part of the IL charge network. In general, the motion of ionic solutes is concerted with that of other ions in the liquid, apparent hydrodynamic radii become larger because of this, and the effective friction becomes much larger than could be predicted by the bulk viscosity. In general, liquid regions with local charge depletion are softer and more mobile than regions in which there is charge enhancement. However, the reader should understand that apolar tails are chemically bonded to charge and therefore it is not that the overall diffusion of apolar liquid components is faster. The softer nature of the more apolar liquid component plays an important dynamical role as it pertains to the dynamical coupling between neutral solute and solvent motion. In the case of neutral solutes, the fastest solute motion (solute jump) is associated with small variations in local density that are provided mostly by the soft apolar component. For example, apolar tails “flood” the space left behind by the diffusion of neutral solutes while also providing space for arrival at a new location. This is depicted in Figure 10a, where for a fixed distance of 1 Å the time dependent pair distribution function between the solute and different solvent components is shown. As the solute diffuses away, the vacated hole is promptly filled by cationic tails. Instead, charged liquid components only fill the hole on much longer time scales as memory of the presence of an apolar solute is lost. The dynamical coupling between charged solutes and solvent is dominated by charge−charge interactions, as the fastest motion is associated with the shedding of a significant portion of the solvation shell, the transport of the partial solvate, and the reassembly of a full solvation environment that is an integral part of the charge network.151 Figure 10b shows that the space left behind by small positively charged solutes is initially flooded by anions. These anions, which are associated with the solute, swipe the space it leaves behind as they follow its motion. Other liquid components only fill the hole on much longer time scales associated with randomization.

Figure 8. Assignment of structural features for BAOF IL (butylammonium pentadecafluorooctanoate). The top panel shows computationally derived S(q) and its assignment based on eq 3 and methodology described in ref 63. The prepeak is due to polar-fluorinated tail alternations, an intermediate peak due to alkyl cationic tail-fluorinated anionic tail alternations, and the peak at around 1.25 Å−1 due to hydrogen bonding associated with charge alternation. The bottom panel shows that this liquid is continuously percolated by a network of hydrogen bonded filaments which are decorated by tails. Because fluorinated tails are attached to negative charge and alkylated tails attached to positive charge that alternate, within each of the filaments the tails also alternate in a quasi-helical shape. The prepeak corresponds to the typical separation between adjacent filaments. Filaments are shown in yellow. Figure reprinted in part with permission from ref 63. Copyright 2014 American Chemical Society.

compared to that of the solvent deviations from Stokes−Einstein behavior are very large in ILs, more so than in other solvents.150 A recent study from our group151 has shed some light on the relation between anomalous dynamics and structure. A solute of comparable size to the solvent or smaller will experience local friction that is very different in different liquid locations. For example, a neutral gas molecule will be caged in regions of excess local charge and its mobility will be enhanced when in regions of charge depletion. In other words, a small neutral solute will be an innocent spectator of high and low electrostriction regions associated with stiff and soft environments that slow or enhance its mobility. In the case of small neutral solutes, this is at the origin of solute dynamical heterogeneity; structure defines dynamics. 12733


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Figure 9. Comparison of 600 ps trajectories for (a) methane and (b) ammonium in [Pyrr1,4][NTf2] at 400 K. Part a shows that, in the case of methane, on this time scale, a trajectory is composed of many cage regimes separated by jumps. The overall volume visited by the probe is significant. Instead, part b shows that, on the same time scale, ammonium which has a very similar molecular volume undergoes a single jump event and visits a much smaller configuration space volume. Such disparity in diffusive behavior is common in ILs when the size of the tracer is smaller than the average size of the solvent. In such cases, diffusion predictions based on the bulk viscosity can fail by orders of magnitude. When a solute is small, its charge and volume become key components that define mechanisms of transport. Figure reprinted in part with permission from ref 151. Copyright 2015 American Chemical Society.

Figure 10. As a function of time, the probability of finding a solvent component (head, tail, or anion) at 1 Å from the position the solute held at time zero for (a) methane and (b) ammonium. In each case, color coded is a solute trajectory segment and a solvent interaction that qualitatively helps visualize a particular mechanism of transport. In the case of methane, we see from gmethane−tail(r − r0 = 1 Å, t − t0) that apolar tail components promptly fill the space left behind by the diffusing solute. This can also be qualitatively seen from the color coded trajectory, as a cationic tail initially opens up for solute passage and later closes filling the void left by the solute. In the case of ammonium, it is the anionic component that most promptly fills the space left by the diffusing cation. This can be seen quantitatively from gammonium−anion(r − r0 = 1 Å, t − t0) and qualitatively from the color coded trajectory coupled with one selected anion. Both parts a and b show that other liquid components only fill the hole left by the solute on a time compatible with randomization.151

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7. CONCLUSIONS We see conventional solvents as often willing to forego most symmetries present in the solid state. Ionic liquids do too, except for polarity alternation and charge alternation. These two symmetries present both in the crystal and liquid phases are their hallmark and make them somewhat different from other liquids. When trying to interpret the structure of ILs via X-ray scattering, we learn that not all peaks are built the same way. Because polarity and charge symmetry are to an extent periodic, they are characterized by peaks and antipeaks in the partial subcomponents of S(q); instead, adjacency correlations are nonperiodic. From an experimental perspective, the analysis of X-ray liquid structure is difficult, as one is left to attempt an inversion of a single S(q) function to derive many pair correlations that interfere and often cancel out. The analysis is much facilitated when molecular simulations are available. A q-dependent partition of the simulated S(q) enormously simplifies the analysis, but in some cases, techniques involving direct sums in reciprocal space prove more advantageous particularly when trying to identify complex liquid structural patterns. Charge and polarity alternation not only dictate liquid structure but also affect local dynamics. If a solute is small, the friction felt in charge enhanced (stiff) and charge depleted (soft) regions can be very different. It is reasonable to expect soft and stiff environments (i.e., structure) to correlate with solvation environments of different local charge density which may be related to distributed chemical dynamics. This energy heterogeneity was recently highlighted by Hu and co-workers as correlated with dynamical heterogeneity of the type that gives rise to excitation wavelength dependent spectroscopy.169 From a transport perspective, small neutral solutes are to some extent innocent spectators of different stiff and soft frictional environments, whereas charged solutes become intrinsic components of the charge network. The motion of small charged solutes can be very slow, since their effective volume becomes very large. Instead, motion of small neutral solutes can be very fast. Because of this, solutes of similar volume can have vastly different diffusivities and significant deviation from Stokes− Einstein behavior can be expected. There is also strong coupling between solute motion and solvent motion. When small neutral solutes undergo fast motional jumps, it is the soft apolar component of the IL that fills the void left behind. By time reversal symmetry, it is the soft apolar components of the IL that vacate the space needed for fast neutral solute diffusion. Instead, a small cationic solute will see the space it leaves behind promptly filled by anions that swipe the space as they follow its motion. In other words, mechanisms of transport can show strong solute size and charge dependence. It would appear that ionic liquid structure and dynamics are strongly intertwined and one needs to understand one to fathom the other.

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Juan Carlos Araque earned his B.S. degree in Chemical Engineering from the University of the Andes in Venezuela, and then worked for 2 years as a R&D engineer at PDVSA’s Petroleum Research Center. After completing his Ph.D. in Chemical Engineering at Rice University he did post-doctoral work at Cornell University. He is currently a post-doctoral scholar at the University of Iowa working on ionic liquids with emphasis on the relation between their structural and dynamical properties.

Jeevapani Hettige earned her B.S. in Chemistry at the University of Colombo, Sri Lanka, in 2008. Since 2010, she has been a Ph.D. student under the mentorship of Professor Claudio J. Margulis at the Department of Chemistry of the University of Iowa. Her research interests are mainly focused on investigating the nanoscale structure of ionic liquids using molecular dynamics simulations.

AUTHOR INFORMATION

Corresponding Author

Claudio J. Margulis is a professor of chemistry at the University of Iowa. He received his Licenciado en Ciencias Quı ́micas degree from the University of Buenos Aires; he later obtained his Ph.D. from Boston University working with David Coker. He did postdoctoral work at

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 12735


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Columbia University with Bruce J. Berne. His research is theoretical and computational in nature with a strong focus on the structure and dynamics of ionic liquids.

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ACKNOWLEDGMENTS This article provides an overview of selected work done in the Margulis group in the past five years. Some of this work was carried out by current group members, but a significant portion was also carried out by prior group members. In particular, Harsha V. R. Annapureddy and Hemant Kashyap were instrumental to many of the developments here presented. Almost all of these works were done in collaboration with the groups of Edward W. Castner Jr. at Rutgers University, James Wishart at BNL, and Mark Maroncelli at Penn State University to whom we are greatly indebted. This work was supported by the US Department of Energy under grant DE-SC0008644 and by the US National Science Foundation under grant CHE-1362129.

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