Emerging Evidences of Mesoscopic-Scale Complexity in Neat Ionic

Perspective pubs.acs.org/JPCL

Emerging Evidences of Mesoscopic-Scale Complexity in Neat Ionic Liquids and Their Mixtures Olga Russina,*,† Fabrizio Lo Celso,‡ Natalia V. Plechkova,§ and Alessandro Triolo*,⊥ †

Dipartimento di Chimica, Università di Roma Sapienza, Rome 00185, Italy Dipartimento di Fisica e Chimica, Università degli Studi di Palermo, viale delle Scienze, ed. 17, 90128 Palermo, Italy § QUILL, The Queen’s University of Belfast, Stranmillis Road, Belfast, Northern Ireland, United Kingdom BT9 5AG ⊥ Laboratorio Liquidi Ionici, Istituto Struttura della Materia, Consiglio Nazionale delle Ricerche, Rome 00133, Italy ‡

ABSTRACT: Ionic liquids (ILs) represent a blooming class of continuously developing advanced materials, with the aiming of a green chemical industry. Their appealing physical and chemical properties are largely influenced by their micro- and mesoscopic structure that is known to possess a high degree of hierarchical organization. High-impact application fields are largely affected by the complex morphology of neat ionic liquids and their mixtures. This Perspective highlights new arising research directions that point to an enhanced level of structural complexity in several IL-based systems, including mixtures. The latter represent a change in paradigm in the approach to formulate new, task-specific IL-based media, and the reported phenomenology has the potential to further expand their range of applications by calling for a revisitation of the nature of interactions in these exciting media.

T

the existence of a distinct level of mesoscopic organization in neat ILs as a direct consequence of their inherent amphiphilicity that leads to a spatially resolved mutual segregation of polar and apolar moieties at the nanometer scale is well established,7,9,10 and this organization is typically maintained in IL-based mixtures.

he term ionic liquids (ILs) is conventionally used to address the class of compounds composed solely of ionic species with a melting point below 100 °C.1 They are characterized by a number of appealing physicochemical properties, including a large liquid-state temperature window, negligible vapor pressure, and enhanced thermal and electrochemical stability. The nature of the IL’s ionic species can be varied, producing ∼106 possible combinations, for example, by modifying the cation/anion’s head, cation/anion alkyl chain length, or nature of the side chain; a fine-tuning of the IL’s properties can thus be achieved, leading to identify them as designer solvents,2 that is, they are apt to best fit a wide range of applications, including synthesis, catalysis, separation, electrochemistry, materials science, and others. One way to further expand the already large number of solvent/reaction media based on ILs is by mixing them with different kinds of compounds, such as other ionic compounds (including ILs, which would produce binary, ternary, etc. mixtures of ILs), molecular liquids, and macromolecules with known properties. This approach (rather than inventing new materials) can lead to media with enhanced fine-tuned properties that are different from starting neat IL ones, thus expanding the potential range of their applications. While several approaches exist to explore the properties of these mixed systems, tools accessing the micro- to mesoscopic scales (both spatially and temporally) are the most suited to find correlations between molecular features and macroscopic properties.3 The joint use of X-ray/neutron scattering techniques and computational tools has provided in the past decade a unique level of physical insight into the micro- and mesoscopic correlations in ILs-based systems.3−8 Nowadays, © 2017 American Chemical Society

Probing inherently mesoscopic structural and dynamics features in ILs and their mixtures will require synergic use of advanced experimental and computational tools. Recent studies addressing specific features of this structural organization (e.g., its lifetime,11 the effect of fluorinated tails,7,12,13 the role of pressure in structural disruption14) highlighted the existence of new facets of the mesoscopic organization in neat ILs; on the other hand, it is also a wellestablished belief that no larger structural correlations exist in neat ILs above the spatial scale associated with the segregated domains. The latter in turn play a major role in hosting guest compounds that distribute into the polar or apolar IL domains according to their polarity, giving rise to a polar/apolar dualism that characterizes most of the properties of ILs both as neat and Received: December 1, 2016 Accepted: February 24, 2017 Published: February 24, 2017 1197

DOI: 10.1021/acs.jpclett.6b02811 J. Phys. Chem. Lett. 2017, 8, 1197−1204

The Journal of Physical Chemistry Letters

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solvent media.15,16 Evidence is presently emerging on the existence of enhanced structural complexity occurring in ILbased binary mixtures, leading to hierarchical complex morphologies that play a role in bulk properties. In this Perspective contribution, we will describe selected examples that directly reflect such a complexity both in neat ILs and in their mixtures and highlight the synergistic role of experimental (X-ray and neutron scattering) and computational tools in addressing them. Neat ILs are characterized by a distinct level of mesoscopic organization reflecting the local microseparation between polar and apolar moieties that leads to the formation of segregated polar and apolar domains in the bulk. This phenomenology has been thoroughly investigated in the past decade, and its most direct experimental fingerprint is the existence of a low momentum transfer (Q) peak in X-ray/neutron scattering patterns. The first experimental evidence of such a feature dates to a decade ago,4 when a selection of imidazolium-based ILs was studied. This work provided experimental grounds for simulation studies that proposed the existence of such a kind of structural heterogeneities in neat ILs.17−20 After that seminal study, a wealth of experimental and computational investigations focused on different aspects and implications of this feature. Joint experimental and computational studies highlighted the role played by polar/apolar alternation in the IL’s mesostructure in determining this scattering peak, and much is known nowadays about the origin and properties of this structural organization. One of the issues that remains unclear and almost unexplored is the lifetime of these structural heterogeneities. Aiming at extending previous work from Yamamuro’s group,11 we extended the neutron spin echo (NSE) data set of a selected IL that we had previously explored,21 namely, deuterated 1-hexyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl}amide, [C6mim][Tf2N]. The technique directly probes the lifetime of a structural correlation identified by a neutron diffraction peak at a given Q value. The diffraction pattern of [C6mim][Tf2N] is characterized by the presence of three different peaks for Q < 2 Å−1 (namely, at 0.3, 0.9, and 1.4 Å−1, hereinafter indicated as QI, QII, and QIII, respectively) that are related to intermolecular correlations.21,22 We have shown that the peaks at higher Q values (QII and QIII) that fingerprint structural correlations related to charge ordering and close contacts,23 relax with strongly non-Debye and non-Arrhenius behavior, following a diffusive trend.21 Recently, Yamamuro’s group explored [C8mim][Tf2N] using NSE at Q = QI and QII. Their analysis of data collected at QII agrees with our findings; the structural correlation decay is bimodal, with a fast, Debye-like and Arrhenius process and a slower process that shows non-Debye and non-Arrhenius behavior, following the viscosity trend. It is of much relevance to probe the features of the process occurring at QI; the lifetime of these structural correlations as well as their temperature dependence can provide useful hints on further understanding of mesoscopic order in ILs. Yamamuro et al. described their NSE data on [C8mim][Tf2N] at QI, using a model accounting for three Debye-like processes, envisaging the existence of two slow processes that are related to the relaxation of organized and disordered regions of the bulk liquid. Our presently reported data (work done in collaboration with Prof. Hardacre’s team; a more extended analysis will be reported elsewhere) on [C6mim][Tf2N] at QI, which were collected independently on two different spectrometers, indicate the existence of just one slow process that follows a distinctly

Arrhenius trend not related to the viscosity trend (see Figure 1). Preliminary analysis of NSE data on other deuterated ILs

Figure 1. (top) NSE data collected on deuterated [C6mim][Tf2N] at Q = 0.3 Å−1. The lines represent a fit in terms of a fast exponential process and a slower stretched exponential one. (bottom) Characteristic average times for the slow process obtained from the above fit compared to the slower relaxation time obtained by Kofu et al.11 for [C8mim][Tf2N], assuming the existence of three exponential processes and the rheology trend observed for [C6mim][Tf2N]. Continuous lines represent Arrhenius-like fits of the data.

leads to a similar conclusion. These results indicate that a larger number of data sets is required to fully understand the nature of relaxation dynamics of mesoscopic correlations in ILs. The latter is found to follow a temperature dependence that does not correspond to the rheological trend that is usually found for the slow process at higher Q values in conventional glass formers.24 So far, a full rationalization of the activated process responsible for the decay of this structural correlation is not achieved yet. One can speculate on the existence of breathing modes or complex formation/dissolution of the segregated domains, but a careful comparison with state-of-the-art molecular dynamics (MD) simulations (using large simulation boxes and simulation times) will be required to validate these mechanisms; probing inherently mesoscopic structural and dynamic features in ILs and their mixtures will require synergic use of advanced experimental and computational tools. Another feature that has been largely unexplored is the role played by pressure on the IL’s mesoscopic organization; while phase diagrams and local conformation studies have taken great advantage of high-pressure (HP) Raman/IR spectroscopies, very few diffraction studies exist on these materials under HP conditions. Yoshimura et al. reported the first small-angle X-ray scattering (SAXS) data set on [C8mim][BF4],25 highlighting 1198



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IL (DSILs), rather than simply an IL mixture.32 Binary mixtures of ionic compounds with different or common anions/cations behaving as DSILs have been the topic of several studies in the past.26,33 Being composed solely of ionic species, one can expect that the main correlations driving the structure of these mixtures are Coulombic ones, and several reports indicate that DSILs composed of ionic species with comparable sizes behave in an almost ideal way, while the role of dispersive and more complex interactions in affecting microscopic properties can be fingerprinted by the emergence of deviations from the ideal behavior, especially when considering DSILs where ion sizes are quite different. Binary mixtures with a common ion (either the cation or the anion) are being explored in order to decrease the number of variables and access useful details on the way multiple ions mutually organize when mixed. An example is the case of ethylammonium nitrate (EAN) and [C2mim][NO3], whose binary mixtures are liquid over the whole concentration range at T > 318 K. Understanding of structural features in these mixtures can help in rationalizing thermodynamic and dynamic properties as well as foreseeing bulk properties for other DSILs. Binary mixtures of EAN and [C2mim][NO3] share a common anion, the nitrate one, that is paired to two different cations, ethylammonium and [C2mim] (which are protic and aprotic, respectively). Similar mixtures (EAN/ [C2mim][BF4]) have been recently investigated,34 highlighting the very close to ideal behavior of several mixing properties in these systems, at least from the structural point of view. The EAN/[C2mim][NO3] system is characterized by small excess volume, similarly to other IL−IL mixtures,26,35 and current MD approaches can account for this effect in an essentially quantitative way. Our MD simulations for the EAN/[C2mim][NO3] mixtures at 320 K are consistent with a macroscopically as well as mesoscopically homogeneous distribution of ionic species in the system, over the whole concentration window. Simulated X-ray diffraction patterns illustrate that upon adding EAN to neat [C2mim][NO3], a mesoscopic structural evolution occurs as EAN tends to arrange into its typical locally lamellar organization that is fingerprinted by a low-Q peak. Such a progressive evolution is however smooth, and also at the microscopic level, the ionic species tend to gradually organize to maintain the local electroneutrality as well as excluded volume correlations without appreciable discontinuities. A detailed inspection will be reported in due time, but one can already observe that these kinds of mixtures are characterized by a rather homogeneous ionic species distribution: (a) Different cationic species (ethylammonium and [C2mim]) are found to intermix without the development of specific segregation; (b) anions organize themselves, interacting through hydrogen bonding with both cation species, and no major changes are detected in the geometrical features related to this interaction when varying the cation species ratio; (c) the ethyl chains stemming from either the ammonium or imidazolium heads intermix without specific preference related to the nature of the cation head; and (d) a very specific interaction such as the hydrogen bonding between the cation polar heads (imidazolium and ammonium) and the nitrate anions is not found to be affected by the changing ratio between imidazolium and ammonium cations. Overall, similarly to the mentioned case of EAN/[C2mim][BF4],34 the present mixtures can be considered as essentially ideal, from the structural point of view. In contrast with this simple behavior and rather unexpectedly, examples of binary mixtures of ILs that are partially

the fact that upon increasing pressure the low Q peak, fingerprinting the mesoscopic organization in the system, progressively disappears. Typically, application of HP conditions to ILs determines either access to a glassy state or formation of crystalline phases;25 therefore, it is somehow puzzling that pressure application induces a disruption in the mesoscopic organization of ILs. Inspired by Yoshimura’s results, we recently developed a series of MD simulations on [C8mim][BF4] at different pressure conditions, aiming at achieving an atomistic rationalization for this behavior.14 Our simulations nicely reproduce the experimental findings (see Figure 2), indicating that upon increasing pressure the low Q

Figure 2. Simulated X-ray weighted static structure factor, S(Q), obtained for a series of thermodynamic states of [C8mim][BF4] at different pressures and 320 K. The progressive disappearance of the low-Q peak can be observed, in agreement with the experimental findings from Yoshimura et al.25 (Inset) Computed average distances between the terminal methyl group in the octyl chain and the nitrogen of the imidazolium ring bound to this tail at the different investigated pressures. Adapted from ref 14 with permission from the PCCP Owner Societies.

peak disappears; the polar/apolar alternation is then drastically influenced not only by temperature (see, e.g., refs 4 and 6) but also by pressure. The latter parameter has been found to affect interionic correlations only marginally, but it plays a large role in the side-chain conformation, triggering folding of the chain with the overall effect to reduce the polar/apolar alternation in the bulk system; accordingly, a decrease of the intramolecular distance between the cation’s ring and terminal methyl group in the octyl chain is observed (see the inset of Figure 2). Related studies appear, providing evidence of further specific features characterizing the evolution of mesoscopic order in ILs due to pressure application; one can envisage that hydrostatic pressure will receive further attention as a tool to explore polymorphism in ILs and to access refined insight into intermolecular forces acting in these materials. Binary (as well as ternary and so forth) mixtures of ionic compounds that show a melting point lower than 100 °C are identified as ILs and are presently attracting a great deal of attention due to the possibility of fine-tuning IL properties by varying the chemical nature and concentration of the mixed ionic species. Several studies,26−30 including reviews,31,32 appeared in the past few years witnessing the importance and new opportunities provided by this class of compounds. Of course, when mixing ionic species that lead to a new IL system, the origin of each ion in the mixture is lost and new structural organization as well as chemical processes can be triggered by the mixture composition, thus leading to the term double salt 1199



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immiscible have been recently reported;28,36 typical representatives of this behavior are mixtures of [Cnmim]Cl and [P66614]Cl (where [P66614] represents the trihexyltetradecyl-phosphonium cation). These binary mixtures with n ≤ 5 were found to have a miscibility gap in the range of 298 < T (K) < 458 that was explored in ref 36 with an upper critical solution temperature; on the other hand, mixtures with n ≥ 6 are fully miscible over the whole concentration window in the same temperature range.36 Such a phenomenology has been rationalized in terms of the establishment of liquid−liquid separations driven by entropic effects, leading to an upper critical solution temperature.36,37 These results are very interesting because they can provide smart opportunities for separation and other specific applications. A current description of these mixtures is that in the range where they are miscible, their structure is characterized by a homogeneous distribution of the ionic species. While on one hand, the demixing process can be accounted for in terms of large structural differences between the constituent ions and/or due to the development of specifically organized entities that decrease the entropic dissolution entropy,36 on the other hand, homogeneous mixtures are simply considered as conventional DSILs, with no preferential structural organization. Of course, should this not be the case, the existence of complex mesoscopic morphology in these macroscopically homogeneous systems can be expected to largely affect their bulk performances, such as solvent capability and so forth. Using SAXS, we explored a series of homogeneous mixtures of [C6mim]Cl and [P66614]Cl at ambient conditions, aiming at understanding how this system behaves as compared to the very similar [C5mim]Cl and [P66614]Cl mixtures that, at the same conditions, tend to demix into two phases. Figure 3 shows the SAXS data collected for a

approach a low-Q limit related to the compound compressibility; no large-scale structural correlations exist in the neat state of common ILs at spatial scales larger than the one related to the polar/apolar alternation in ILs.22 Upon adding [C5mim] Cl to [P66614]Cl, no major change occurs until ∼25 mol % [C5mim]Cl is added; at that concentration, a distinct increase of the low-Q (Q < 0.1 Å−1) scattering amplitude is observed as a consequence of the development of structural heterogeneities that are precursors for the phase separation in this mixture. A similar set of measurements was collected on [C6mim]Cl/ [P66614]Cl mixtures. The corresponding SAXS patterns for [C6mim]Cl/[P66614]Cl over the whole concentration range at ambient temperature are reported in Figure 4. Also in this case,

Figure 4. Experimental SAXS data collected on neat [P66614]Cl (red dashed line), [C6mim]Cl (blue dashed line), and their mixtures with the indicated molar content in [C6mim]Cl at ambient conditions. For the case of [C6mim]Cl molar content = 0.8 (black dashed line), a fit in terms of the Fisher−Burford expression39 that is commonly used to describe density fluctuations related to the formation of aggregates (including two peaks centered at 0.3 and 1.4 Å−1) is shown. In the inset, the scattering amplitude measured at Q = 0.02 Å−1 is shown, indicating that the excess scattering reaches its maximum amplitude for [C6mim]Cl molar content = 0.8. The differently colored symbols indicate two different, yet fully compatible data sets.

the two neat compounds are characterized by SAXS patterns showing the existence of the low-Q peaks, QI, and at lower Q values, no relevant diffraction features can be detected. However, when adding [C6mim]Cl to [P66614]Cl, one can observe that while the polar/apolar segregated morphology is maintained essentially unaffected (as indicated by the unchanged peak’s amplitude and position at Q ≈ 0.35 Å−1), a progressive increase of the SAXS intensity at low Q (Q < 0.1 Å−1) occurs. This feature witnesses the progressive development of large-scale structural heterogeneities over a spatial range that is far larger than the one related to first-neighbor correlations or even polar/apolar alternation. In the inset of Figure 4, the scattering amplitude at 0.02 Å−1 is plotted vs [C6mim]Cl content; it reaches a maximum at a 0.8 molar fraction [C6mim]Cl content. The excess low-Q scattering profiles were modeled using the Fisher−Burford expression39 that is commonly used to describe density fluctuations related to the formation of aggregates (see the fit in Figure 4 for the case of χ[C6mim]Cl = 0.8), and it emerges that these structural heterogeneities correspond to density fluctuations over a spatial

Figure 3. Experimental SAXS data collected on neat [P66614]Cl, [C5mim]Cl, and their mixtures with the indicated molar content in [C5mim]Cl at ambient conditions. For concentrations above 25 mol %, a distinct increase in the low-Q scattering is observed as a consequence of approaching the liquid−liquid separation.36

series of [C5mim]Cl/[P66614]Cl samples in the phosphoniumrich concentration range, where the mixtures are stably homogeneous (Arce et al. claim that the limit solubility of [C5mim]Cl in [P66614]Cl is ∼40 mol % at 300 K36). The diffraction patterns from the two neat compounds are shown with dashed lines and are characterized by the ubiquitous low-Q peak (QI) that has been reported for these4,38 and several other ILs that is centered at ∼0.35−0.4 Å−1. It can be observed that, similarly to all of the so far explored ILs, for Q values lower than QI, the diffraction patterns are featureless and tend to 1200



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scale larger than nanometers. These correspond to clusters with both fluctuating shape and size, and while no information exists on the phase equilibrium of these mixtures at temperatures below ambient, one can envisage that the present behavior is a precursor for phase demixing at lower temperatures. These results indicate that, in contrast to the previous examples where DSILs can homogeneously intermix, leading to macroscopically as well as mesoscopically homogeneous mixtures, the present systems are characterized by a high level of structural heterogeneity at the mesoscopic level that is however fully compatible with a stably homogeneous macroscopic organization. Although macroscopically homogeneous, an intricate hierarchy of structural heterogeneities can be established over the mesoscopic spatial scale in DSIL systems, with potentially large, yet unexpected, applicative consequences. Similar X-ray scattering behavior has been recently observed on DSILs formed by [P66614][Tf2N] and [C3mpyr][Tf2N] (N,N-propyl,methylpyrrolidinium bistriflamide) whose phase diagram has similar features to the one of [C5mim]Cl/[P66614]Cl.28

Figure 5. Experimental SAXS data collected on neat [C1mim][Tf2N] (black dashed line), [C12mim][Tf2N] (black dotted line), and their mixtures at ambient conditions. In the bottom-right inset, the scattering patterns measured for [C12mim][Tf2N] molar fractions equal to 0.08 and 0.30 are shown together with their fit in terms of monodisperse dilute pseudospherical aggregates (red line) and monodisperse hard-sphere interacting pseudospherical aggregates (green line), respectively; in both cases, a Voigt-like peak has been included in the fit to account for the diffraction feature at 9 nm−1. In the top-right inset, the dependence of the low-Q peak position on the cubic root of the [C12mim][Tf2N] volume fraction is shown, highlighting the progressive approach of the aggregates as the [C12mim][Tf2N] content increases.

Although macroscopically homogeneous, an intricate hierarchy of structural heterogeneities can be established over the mesoscopic spatial scale in DSIL systems, with potentially large, yet unexpected, applicative consequences.

contrary, [C12mim][Tf2N] is, to our knowledge, the IL with the longest alkyl side chain that is liquid at ambient conditions, and because of the polar/apolar differentiation between its polar head and apolar tail, it is characterized by a distinct structural segregation fingerprinted by the low-Q peak at ∼0.25 A−1 (dotted curve). These two components are fully compatible, and the mixture can be imagined as composed by the mixing of an ideal ionic compound ([C1mim][Tf2N]) and a surfactantlike one ([C12mim][Tf2N]), the latter bearing a polar head identical to the former component. The common imidazolium head and anion make these kinds of mixtures ideal in order to investigate how the mesoscopic order develops in long-chained ILs and the nature of large-scale order in such a class of DSILs. A gradual progression of the structure can be observed when adding [C12mim][Tf2N] to [C1mim][Tf2N]; first, low-Q excess scattering develops, and then, at approximately XC12 = 0.15 (yellow line), a distinct peak begins to develop for 1 < Q (nm−1) < 3; eventually, this peak transforms into the low-Q peak that is found in neat [C12mim][Tf2N] and progressively shifts toward its final position (dotted line). Related small-angle neutron scattering (SANS) data on mixtures of [Cnmim]Cl (with n = 14, 16, and 18, three solid ionic compounds) in [C2mim][FeCl4], over a narrower concentration range than the one explored herein, showed similar behavior and were modeled in terms of micellar aggregates interacting as hard spheres.40 This structural model nicely accounts for our present set of SAXS data over a large concentration range, as well. In the lower inset of Figure 5, such descriptions are reported for two selected examples; when dissolved in [C1mim][Tf2N], [C12mim][Tf2N] tends to organize in pseudospherical aggregates with a characteristic radius on the order of 8 Å that essentially do not interact with each other in the dilute regime (red line, X C12 = 0.08). Upon increasing the [C12mim][Tf2N] content, a shoulder, progressively trans-

Of course, a variety of examples of DSILs composed by largely different ionic species that still remain compatible, that is, do not demix, exists. Among them, we would like to mention the class of DSILs composed by mixing [Cnmim][Tf2N] and [Cpmim][Tf2N], with n ≠ p; they share the anion but are characterized by cations with potentially very different chain lengths. A similar kind of DSILs has been explored in the past (e.g., see refs 7, 23, and 26), showing that this class of homogeneous mixtures is characterized by quasi-ideal behavior (with larger and larger deviations from this trend when increasing the difference between n and p) due to the lack of large cross-interactions between the mixed components (as both polar and apolar interactions remain appreciably similar upon mixing). Moreover, an intermixing of the chains with different length building up apolar domains has been observed both experimentally and by simulations.7 Such effects can be brought to extreme conditions by increasing the difference in chain length of the mixed salts; in particular, we choose to mix two salts that, both being liquid at ambient conditions, show the largest difference in chain length; this is achieved with mixtures of [C1mim][Tf2N] and [C12mim][Tf2N] that share a common anion ([Tf2N]) and are characterized by two different cations with very short (methyl) and very long (dodecyl) alkyl chains. These are three ionic component mixtures that remain liquid and homogeneous over the whole concentration range at ambient conditions. In Figure 5, SAXS data from these mixtures over the whole concentration window are shown. [C1mim][Tf2N] (dashed curve) does not show any indication of polar/ apolar differentiation (i.e., no low-Q peak can be detected in the range of 1−5 nm−1) and accordingly behaves as a common molten salt, where the structure is dominated by Coulombic interactions that mainly determine a successive alternation of positive and negative species in a liquid environment. On the 1201



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forming into a distinct peak, develops, fingerprinting the occurrence of long-range intermicelle interactions (green line, XC12 = 0.30). Eventually, this peak, which is correlated with the average intermicellar correlation, evolves into the low-Q peak of neat [C12mim][Tf2N]; the concentration evolution of the peak position linearly scales with Φ1/3 (Φ = [C12mim][Tf2N]’s volume fraction) (see the upper inset of Figure 5), thus signifying that the peak originates from correlations between nearest neighbors of aggregates dispersed in the homogeneous solvent phase. This study is important because [C1mim][Tf2N] has common polar moieties to [C12mim][Tf2N] but essentially no alkyl moieties, thus allowing comfortable dissolution of [C12mim][Tf2N]’s polar moieties and leading to a segregation of its alkyl tails. Upon increasing the [C12mim][Tf2N] content, the alkyl chain content increases, thus enhancing the trend toward their segregation in domains that are a characteristic feature of neat, long-chain ILs. It is interesting to note that no major discontinuity is observed when approaching the limit of neat [C12mim][Tf2N]; the low-Q peak in neat ILs then turns out to be the natural evolution of the interaggregates’ correlation peak that progressively develops and shifts from the condition of dilute aggregates to that of neat [C12mim][Tf2N]. Another topic that we would like to highlight in this Perspective is the complex mesoscopic behavior of binary mixtures of protic ILs, such as EAN, and linear alcohols. Several reports described the observation that these macroscopically homogeneous mixtures are characterized by the existence of large-scale structural heterogeneities that are fingerprinted by excess low-Q X-ray scattering.41−43 These findings, represented in the inset of Figure 6 for the case of EAN/n-pentanol mixtures, 43 have been related to the exciting preouzo phenomenology that has been described in the past few years in several papers.44 The latter studies provide extensive experimental evidence of the existence of surfactant-free microemulsion-like aggregates in systems such as water/ ethanol/octanol, where two components are immiscible (e.g.,

water/octanol) and a third one (e.g., ethanol) is mutually soluble with the others. In these three-component systems, complex structural organization has been proposed; strong adsorption of the water- and oil-philic component (in the mentioned case, ethanol) at the interface between oil-rich and water-rich domains stabilizes the macroscopically homogenenous but mesoscopically segregated morphology. In the present case of the binary IL−alcohol systems, related structural evidence was first accounted for in terms of aggregates45 or, more specifically, in terms of micellar or microemulsion entities.41 More recently, Murphy et al. described a large set of neutron diffraction data sets from propyl ammonium nitrate (PAN)/n-octanol mixtures (in the range of 10−50 v.v % alcohol in PAN),46 stressing the important role played by PAN’s underlying nanostructure that acts as a guest environment for the segregating solvophobic alcohol, leading to the formation of weakly defined alcohol aggregates as compared, for example, to micelles. Another point raised by Murphy et al. is that the IL’s cation plays a dual role in this kind of mixture; it acts as both a solvent and a co-surfactant. Clearly, these puzzling observations are very interesting; they provide insight into a potentially new kind of mesoscopic organization in relatively simple mixtures (even binary ones) that could lead to a large number of useful applications and technological exploitations by developing structured systems without requiring the presence of potentially polluting surfactants. While the kind of neutron studies reported by Murphy et al. on differently isotopically labeled mixtures are extremely informative for the microscopic organization of probed systems,46 the complexity revealed by SAXS data on this class of mixtures would suggest that in order to fully rationalize the mesoscopicscale evidence, low-Q (either X-ray or neutron) scattering data have to be explicitly taken into consideration (especially when using the scattering data to drive the simulation, as in the case of the work reported in ref 46). So far, PIL−alcohol binary mixtures have never been quantitatively described in terms of such techniques, apart from our recent report on EAN/ pentanol that highlighted the outstanding effect played by temperature on the low-Q scattering.42 Such a kind of description would provide robust validation for currently proposed structural models. In Figure 6, we show SANS data collected at room conditions on a 1:1 binary mixture of EAN and n-butanol for three different isotopic substitutions (namely, data for d8-EAN/h10-butanol, d3-EAN/d10-butanol, and h8EAN/d10-butanol are reported). All of the data sets show indication of a distinct low-Q scattering that fingerprints the existence of structural heterogeneities with characteristic size in the mesoscopic regime, in agreement with other recently published SAXS data on this kind of mixtures.41−43 Previously, SAXS data from the related system EAN/pentanol were described in terms of the Ornstein−Zernike function that is commonly employed for description of the distribution of the molecular species in complex systems with strong concentration fluctuations.43 In that study, the emerging low-Q diffraction features at ambient conditions have been rationalized in terms of fluctuating clusters that represent the precursors for the phase separation process that occurs at lower temperature. Upon decreasing the temperature and approaching the phase separation point, the excess scattering dramatically increases and the diverging characteristic size of the structural heterogeneities nicely follows the trends indicated by commonly accepted theoretical models for phase separation phenomenology.43 The SANS data reported in Figure 6 were

Figure 6. Experimental SANS data collected on three differently isotopically labeled equimolar mixtures of EAN and n-butanol at ambient conditions. The continuous lines correspond to modeling of the data in terms of the Ornstein−Zernike formalism. In the inset, SAXS patterns from EAN/butanol mixtures as a function of concentration at ambient conditions are shown. Reprinted with permission from ref 43. Copyright (2016) American Chemical Society. 1202



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of magnitude in space and time. It can be envisaged that scattering tools such as small, as well as ultrasmall, X-ray and neutron scattering will be more and more required to access the relevant spatial scale of these structures. Similarly, techniques such as NSE will be required to access the scale of tens (or larger) nanoseconds over which relaxation processes and diffusive dynamics occur in these complex systems. In parallel, it will be required to use MD simulation tools pushing the current limits of size and time scales covered in order to compare and rationalize the experimental data.

simultaneously described in terms of the Ornstein−Zernike formalism; the latter provides an excellent description for the different SANS patterns. The three fitting descriptions of the SANS data converge in indicating the existence of structural heterogeneities with a characteristic size, ξ, of 4 Å. It is important to be aware that this parameter does not identify the size of monodisperse clusters but rather a distribution of aggregate dimensions correlated to exp(−r/ξ), which implies the existence of clusters whose sizes span a range that is much larger than ξ. Overall, the simultaneous fit of the SANS data sets with a unique model accounting for mesoscopic-scale structural heterogeneities provides a robust indication that the morphology of these mixtures is more complex and intricate than expected on the basis of well-organized and regular entities such as micelles or microemulsions. So far, a common approach to foresee the thermodynamic and structural behavior of mixtures of ILs with other compounds was to consider that polar compounds would evenly distribute in IL polar domains while apolar compounds would fit into the alkyl chain domains. Similarly, amphiphilic compounds would organize to adapt their moieties to fit into the nanoscale segregated morphology that characterizes neat ILs. However, emerging evidence supports the speculation that this is not always the case; the polar vs apolar dualism that is considered to account for distribution of compounds and moieties in ILs does not succeed in describing experimental evidence for the case, for example, of PILs−alcohols. In these systems, indications of mesoscale-segregated clusters, which cannot be accounted for in terms of micellar aggregates but rather as weakly defined, structureless entities, are systematically found. These entities are envisaged to represent the mesoscale precursors for subsequent phase separation. It is important to stress that understanding of mixtures of ILs with alcohols, and expectedly with other amphiphilic compounds, will require careful consideration of structural correlations over mesoscopic scales. The expanding spectrum of investigated properties and proposed applications for ILs and their mixtures allows spotting of more and more features of structural complexity in these systems. This emerging phenomenology encompasses a wealth of mesoscopic scale, hierarchically organized entities ranging from well-shaped micellar aggregates to ill-defined fluctuating aggregates. This structuring at the nanometer scale is envisaged to largely determine bulk properties such as solvation capability or diffusive properties that in turn affect application performances of these complex solvent media. Moreover, this emerging phenomenology prompts an evolution of the polar/apolar dualism paradigm used to foresee and model the behavior of IL-based mixtures; the conventionally accepted polar/apolar dualism so far used to interpret miscibility and distribution of compounds in ILs might not be ubiquitously adequate to account for experimental evidence.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (O.R.). *E-mail: [email protected] (A.T.). ORCID

Alessandro Triolo: 0000-0003-4074-0743 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The access to Large Scale Facilities (ESRF, NIST, ILL, HZB) through different programs is acknowledged. In particular, the role of instrument local contacts (C. Jafta (HZB), E. Di Cola (ESRF), B. Farago (ILL), and A. Faraone (NIST)) is gratefully acknowledged. N.V.P. thanks the industrial advisory board of QUILL for their support. A.T. acknowledges Prof. C. Hardacre (Queens’s University, Belfast, U.K.) for providing the deuterated sample used in the NSE experiment in the framework of collaborative work.

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REFERENCES

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