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Perspective
Emerging Evidences of Mesoscopic-Scale Complexity in Neat Ionic Liquids and Their Mixtures. Olga Russina, Fabrizio Lo Celso, Natalia V Plechkova, and Alessandro Triolo J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b02811 • Publication Date (Web): 24 Feb 2017 Downloaded from http://pubs.acs.org on February 26, 2017
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Emerging Evidences of Mesoscopic-Scale Complexity in Neat Ionic Liquids and their Mixtures. Olga Russina1,*, Fabrizio Lo Celso2, Natalia V. Plechkova3, Alessandro Triolo4,* 1
Dipartimento di Chimica, Università di Roma Sapienza, Rome, Italy
2
Dipartimento di Fisica e Chimica, viale delle Scienze, ed. 17, 90128 Palermo, Italy
3
QUILL, The Queen’s University of Belfast, Stranmillis Road, Belfast, Northern Ireland, UK
BT9 5AG 4
Laboratorio Liquidi Ionici, Istituto Struttura della Materia, Consiglio Nazionale delle Ricerche,
Rome, Italy
AUTHOR INFORMATION Corresponding Author * Olga Russina:
[email protected]; Alessandro Triolo:
[email protected] ACS Paragon Plus Environment
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ABSTRACT
Ionic Liquids (IL) represent a blooming class of advanced materials continuously developing, aiming to the greening of 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 structuring. 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 out 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 potentiality to further expand their range of applications, by calling for a revisitation of the nature of interactions in these exciting media.
TOC GRAPHICS
KEYWORDS. X-ray and neutron scattering; Long range order; Hierarchical morphology; Structure and Dynamics; Structural heterogeneity.
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The 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 characterised by a number of appealing physicochemical properties, including a large liquid state temperature window, negligible vapour pressure, enhanced thermal and electrochemical stability. The nature of the IL’s ionic species can be varied, producing ~one million possible combinations e.g. by modifying cation/anion’s head, cation/anion alkyl chain length, nature of the side chain: a fine tuning of IL’s properties can thus be achieved, leading to identify them as designer solvents2, i.e. they are apt to best fit to 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 ionic liquids), 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 ILs’ 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 properties3. The joint use of X-ray/neutron scattering techniques and computational tools has provided in the last decade a unique level of physical insight into the micro- and meso-scopic correlations in ILs based systems.3–8 Nowadays, the existence of a distinct level of mesoscopic organisation in neat ILs as a direct consequence of their inherent amphiphilicity that leads to a spatially resolved mutual segregation of polar and apolar moieties
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at the nm-scale is well established7,9,10 and this organization is typically maintained in IL based mixtures. Recent studies addressing specific features of this structural organisation (e.g. its lifetime11, the effect of fluorinated tails7,12,13, 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 well-established belief that no larger structural correlations exist in neat ILs, above the spatial scale associated to 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 characterises most of the properties of ILs both as neat and solvent media.15,16 Evidences are presently emerging on the existence of enhanced structural complexity occurring in IL based 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 characterised 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 last 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 ago4, when a selection of imidazolium based ILs was studied. This work provided experimental ground for simulation studies that proposed the existence of such a kind of structural heterogeneities in neat ILs17–20. After that seminal study a wealth of
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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 IL’s mesostructure in determining this scattering peak and much is known nowadays about the origin and properties of this structural organisation. One of the issues that remain unclear and almost un-explored is the life-time of these structural heterogeneities. Aiming at extending previous work from the Yamamuro’s group11, we extended the Neutron Spin Echo (NSE) data set of a selected IL that we had previously explored21, namely deuteriated 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 characterised by the presence of three different peaks for Q318 K. The understanding of structural features in these mixtures can help in rationalising 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
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,
highlighting the very close to ideal behaviour of several mixing properties in these systems, at least from the structural point of view. The EAN-[C2mim][NO3] system is characterised by small excess volume, similarly to other IL-IL mixtures26,35 and current Molecular Dynamics 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 macroscopically as well as mesoscopically homogeneous distribution of ionic species in the system, over the whole
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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 microscopic level the ionic species tend to gradually organize to maintain the local electro-neutrality as well as excluded volume correlations without appreciable discontinuities. A detailed inspection will be reported in due time, but one can already observe that this kind of mixtures are characterised 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 organise 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; 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 behaviour and rather unexpectedly, examples of binary mixtures of ILs that are partially immiscible have been recently reported28,36: typical representatives of this behaviour 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 298
