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Mesoscopic structural and dynamic organization in ionic liquids
Journal of Molecular Liquids 210 (2015) 161–163
Contents lists available at ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Editorial
Mesoscopic structural and dynamic organization in ionic liquids
The present Special Issue of the Journal of Molecular Liquids is dedicated to the Mesoscopic Structural and Dynamic Organization in Ionic Liquids (ILs). These compounds represent one of the hottest research fields in the last two decades.[1] Being composed only of ionic species it can be expected that their properties are strongly affected by coulombic interactions. However, typically the cation of the ILs bears an apolar side chain - most commonly being a medium length alkyl tail - that interacts with the surrounding environment merely by dispersive forces. Such a chemical structure leads to an inherently amphiphilic nature for the resulting IL; as a consequence a strong tendency of the alkyl tail to segregate from the charged moieties is ubiquitously observed. This implies a distinct level of structural heterogeneity over the spatial scale of a few nanometers, due to the alternation between polar and apolar domains. [2–24] The consequences of this high level of structural architecture are manifold: they invest as different fields as separation, synthesis, catalysis, electrochemistry, nanoparticles manufacturing etc. The level of understanding of this spectacular peculiarity in IL’s morphology is now mature. Since the appearance of the first experimental[4] and computational[2,3] proposals for such an organization, numerous studies provided more and more compelling evidences of this specific architecture. Several aspects have been explored in the last few years, including the role of alkyl chain length[5,9,12–16, 25–30], charged moieties chemical nature [5,15,16], polarity of the tail[12,20,21,31–33], symmetry of the cation[34–38], effect of temperature [14,23,39] etc. It is stated that there is a strong correlation between structural complexity and performances of ILs. For example the solvation performance and the miscibility with other compounds are strongly dependent on the mesoscopic morphology of ILs. It solutions the ionic and apolar domains prevail and giving rise to a preferential solvation.[40,41] It should be noted that the domain structure in solutions must be distinguished from the fluctuations of the composition[42,43] that are the precursors of phase separation observed in many solutions of ILs [44,45]. In this Special Issue, we aimed at collecting the recent results and the opinions of several world- leading researchers in the field of mesoscopic organization in Ionic Liquids, providing in a unique volume a set of contributions that would make available to the general reader the current level of understanding in this field. The Issue is composed of sixteen contributions from authors from different laboratories who, using either experimental techniques or computational methods, either describe new original results or report an overview over specific aspects of the mesoscopic order in ILs. The first paper of Miller reports on the effect of the counterion nature on a series of physicochemical properties for 4-methyl-1-propyl-
http://dx.doi.org/10.1016/j.molliq.2015.08.025 0167-7322/© 2015 Published by Elsevier B.V.
1,2,4-triazolium ionic liquids, highlighting the role of the anion in influencing properties. [MILLER]. Greaves and coworkers report on the complex nanostructure of Fluorous protic ionic liquids upon water addition. This interesting new class of protic ILs opened the way to triphilic ILs, where simultaneously polar, lipophilic and fluorophilic domains coexist, despite their mutual incompatibility. In this work the authors explore the evolution of Lyotropic Liquid Crystalline Mesophases upon Water Addition. [GREAVES]. Using computational tools, Saielli and coworkers investigated the microscopic structure and dynamics of representative ILs where Xenon was added. The existence of IL cages around the probe Xe was explored. These studies are actual and well related to recent literature on the role of Xe in exploring IL’s morphology [46,47]. [SAIELLI] A thorough comprehensive characterization of the impact of cation side chain length and symmetry on water solubility in ILs was reported by Coutinho and coworkers. They report on water solubility in a series of asymmetric and symmetric [CnCxim][NTf2] ILs (where n is up to 21 and x = 1 or n), extending their study of symmetric cations [36–38]. In order to analyse their solubility data, Density Functional theory calculations were also performed. [Coutinho] Shimizu and coworkers used Molecular Dynamics simulations to probe the structural architecture in a series of 1-alkyl-3methylimidazolium hexafluorophosphate ionic liquid, [CnC1im][PF6] (n = 3, 6, 9, 12). They aim at exploring the mesoscopic order in these systems and compare it with the one they found in a similar series [28]. Also they show results for equimolar mixtures of ILs with different alkyl chain length. [Shimizu] In their contribution, Ribeiro and coworkers explore the highfrequency sound modes in ionic liquids using molecular dynamics simulations on [C2C1im][TFSI] and [C2C1im][FSI] and their mixtures with lithium salts. The outcome of the MD simulations is compared with recent experimental Inelastic X-ray Scattering data from Fujii et al. [48] [Ribeiro] Perera and collaborators report on structural features of low and high melting ILs, aiming at rationalising their differences in disorder in terms of particles’ shape and the ration of molecular size to temperature. Using both computer simulations and liquid state integral equation techniques, they clarify the differences between fluctuations and structural microheterogeneities, highlighting the different role of free and bound charges and identifying the existence of different forms of disorder. [Perera] Using both experimental (SAXS, Raman spectroscopy) and computational tools, Matic and coworkers report on the mesoscopic structural organization in solvated ionic liquids that are a class of IL formed by dissolving a Lithium salt in tetraglyme. They describe at atomistic detail how Lithium is solvated in this complex medium. [Matic]
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Idrissi and coworkers have described the nature of the intermolecular interactions in a technologically relevant IL (1-butyl-3methylimidazolium acetate) and its water mixtures. They used a large set of experimental (IR, Raman, NMR Spectroscopy) and Quantum Chemistry computation tools, exploring the role of water concentration on dynamics. Specific kinds of interactions are identified and the different findings are correlated. [Idrissi] Pulsed-field gradient 1H NMR has been used by Balevicius and coworkers to explore the diffusion and self-aggregation (micellization) processes in neat 1-decyl-3-methyl-imidazolium chloride and its aqueous solutions. An Arrhenius trend has been detected for the neat IL, thus prompting for a softening of the interactions in this long chained salt. The exploration of water mixtures led to the determination of critical aggregation concentration. [Balevicius] Mele and coworkers are reporting their recent progresses in the field of Nuclear Overhauser Enhancement (NOE), as a powerful tool to explore structure in ILs. The large potentialities of this experimental approach were recently described by Weigartner et al. [49]. In this contribution the authors use the technique to explore N-propyl-N-methyl pyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR13TFSI), its homologue with bis(fluorosulfonyl)imide (PYR13FSI), and their mixtures with LiTFSI. Discussion of the data in terms of either short range (first coordination shell) or long range (polar-apolar alternation) correlations is presented. [MELE] Recollecting some of their recent observations and presenting new results, Abe and coworkers used a wide range of experimental techniques to explore binary mixtures of ILs and water. They also used high pressure XRD and Raman spectroscopy to describe the complex polymorphism in ILs. Dracopoulos and coworkers employed spectroscopic techniques (FT-IR/ATR and FT-Raman) to explore the interactions in alkyl substituted imidazolium bis(trifluoromethanolsulfonyl)imide protic ionic liquids (PILs) HCnImNTf2 (n = 0-12) and aprotic (APILs) C1CnNTf2 (n = 1-12). They studied the role of alkyl chain length as well as the differences due to the enhanced hydrogen bonding interactions existing in PILs. [Dracopoulos] Varela and coworkers present a review of the existing work in the field of structural and dynamic properties of mixtures of ionic liquids with molecular cosolvents or salts. The major role of the nanoarchitecture in IL, with polar-apolar alternation in influencing the solvation mechanism of both charged and neutral species is discussed, leading to the proposal of nanostructured solvation [50,51]. [VARELA] By combining NMR spectroscopy with vibrational (IR and Raman) ones, Martinelli and coworkers investigated the consequences of increasing alkyl chain lenght in 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide on intermolecular interactions as well as rotational dynamics. The existence of nano-dynamical heterogeneities as a consequence of the mesoscopic structural heterogeneity in ILs is proposed. Yamamuro and coworkers used inelastic neutron scattering to explore low energy excitations in 1- alkyl-3-methylimidazolium ionic liquids: they focus mostly on the Boson peak and aim relating its energy to anion radius and glass transition, to conclude that Boson peak and glass transition are predominantly governed by the inter-ionic Coulomb interaction. [Yamamuro] Overall, we think that the papers contributed to this Special Issue can provide the interested reader with a broad overview of the most exciting issues related to the mesoscopic organization in ionic liquids and their binary mixtures. Of course the main goal of this work has been the one to stimulate further exciting work in this direction and we hope all these contributions will receive careful attention! References [1] H. Weingaertner, Understanding ionic liquids at the molecular level: Facts, problems, and controversies, Angew. Chem. Int. Ed. 47 (2008) 654–670, http://dx.doi. org/10.1002/anie.200604951.
[2] Y. Wang, G.A. Voth, Unique spatial heterogeneity in ionic liquids, J. Am. Chem. Soc. 127 (2005) 12192–12193, http://dx.doi.org/10.1021/ja053796g. [3] S.M. Urahata, M.C.C. Ribeiro, Structure of ionic liquids of 1-alkyl-3methylimidazolium cations: A systematic computer simulation study, J. Chem. Phys. 120 (2004) 1855–1863, http://dx.doi.org/10.1063/1.1635356. [4] A. Triolo, O. Russina, H.-J. Bleif, E. Di Cola, Nanoscale segregation in room temperature ionic liquids, J. Phys. Chem. B 111 (2007) 4641–4644, http://dx.doi.org/10. 1021/jp067705t. [5] C.S. Santos, N.S. Murthy, G.a. Baker, E.W. Castner, Communication: X-ray scattering from ionic liquids with pyrrolidinium cations, J. Chem. Phys. 134 (2011)http://dx. doi.org/10.1063/1.3569131. [6] T.L. Greaves, C.J. Drummond, Solvent nanostructure, the solvophobic effect and amphiphile self-assembly in ionic liquids, Chem. Soc. Rev. 42 (2013) 1096–1120, http://dx.doi.org/10.1039/c2cs35339c. [7] T.L. Greaves, D.F. Kennedy, S.T. Mudie, C.J. Drummond, Diversity observed in the nanostructure of protic ionic liquids, J. Phys. Chem. Phys. Chem. B. 114 (2010) 1002231, http://dx.doi.org/10.1021/jp103863z. [8] T.L. Greaves, D.F.D.F. Kennedy, A. Weerawardena, N.M.K. Tse, N. Kirby, C.J. Drummond, Nanostructured protic ionic liquids retain nanoscale features in aqueous solution while precursor Bronsted acids and bases exhibit different behavior, J. Phys. Chem. B 115 (2011) 2055–2066, http://dx.doi.org/10.1021/jp 1112203. [9] K. Fujii, R. Kanzaki, T. Takamuku, Y. Kameda, S. Kohara, M. Kanakubo, et al., Experimental evidences for molecular origin of low-Q peak in neutronx-ray scattering of 1- alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ionic liquids, J. Chem. Phys. 135 (2011) 244502, http://dx.doi.org/10.1063/1.3672097. [10] R. Atkin, G.G. Warr, The smallest amphiphiles: nanostructure in protic roomtemperature ionic liquids with short alkyl groups, J. Phys. Chem. B 112 (2008) 4164–4166, http://dx.doi.org/10.1021/jp801190u. [11] R. Hayes, S. Imberti, G.G. Warr, R. Atkin, Amphiphilicity determines nanostructure in protic ionic liquids, Phys. Chem. Chem. Phys. 13 (2011) 3237–3247, http://dx.doi. org/10.1039/c0cp01137a. [12] O. Russina, A. Triolo, New experimental evidence supporting the mesoscopic segregation model in room temperature ionic liquids, Faraday Discuss. 154 (2012) 97–109, http://dx.doi.org/10.1039/c1fd00073j. [13] O. Russina, A. Triolo, L. Gontrani, R. Caminiti, D. Xiao, L.G. Hines Jr., et al., Morphology and intermolecular dynamics of 1-alkyl-3-methylimidazolium bis{(trifluoromethane)sulfonyl}amide ionic liquids: structural and dynamic evidence of nanoscale segregation, J. Phys. Condens. Matter 21 (2009) 424121, http://dx.doi.org/10.1088/0953- 8984/21/42/424121. [14] O. Russina, A. Triolo, L. Gontrani, R. Caminiti, Mesoscopic Structural Heterogeneities in Room-Temperature Ionic Liquids, J. Phys. Chem. Lett. 3 (2012) 27–33, http://dx. doi.org/10.1021/jz201349z. [15] O. Russina, L. Gontrani, B. Fazio, D. Lombardo, A. Triolo, R. Caminiti, Selected chemical- physical properties and structural heterogeneities in 1-ethyl-3methylimidazolium alkyl- sulfate room temperature ionic liquids, Chem. Phys. Lett. 493 (2010) 259–262, http://dx.doi.org/10.1016/j.cplett.2010.05.042. [16] A. Triolo, O. Russina, B. Fazio, G.B. Appetecchi, M. Carewska, S. Passerini, Nanoscale organization in piperidinium-based room temperature ionic liquids, J. Chem. Phys. 130 (2009) 1645211–1645216, http://dx.doi.org/10.1063/1.3119977. [17] O. Russina, F. Lo Celso, M. Di Michiel, S. Passerini, G.B. Appetecchi, F. Castiglione, et al., Mesoscopic structural organization in triphilic room temperature ionic liquids, Faraday Discuss. 167 (2013) 499–514, http://dx.doi.org/10.1039/c3fd00056g. [18] O. Russina, M. Macchiagodena, B. Kirchner, A. Mariani, B. Aoun, M. Russina, et al., Association in ethylammonium nitrate-dimethyl sulfoxide mixtures: First structural and dynamical evidences, J. Non-Cryst. Solids 407 (2015) 333–338, http://dx.doi. org/10.1016/j.jnoncrysol.2014.08.051. [19] O. Russina, A. Mariani, R. Caminiti, A. Triolo, Structure of a Binary Mixture of Ethylammonium Nitrate and Methanol, J. Solut. Chem. 44 (2015) 669–685, http://dx.doi. org/10.1007/s10953-015-0311-7. [20] H.K. Kashyap, C.S. Santos, R.P. Daly, J.J. Hettige, N.S. Murthy, H. Shirota, et al., How does the ionic liquid organizational landscape change when nonpolar cationic alkyl groups are replaced by polar isoelectronic diethers? J. Phys. Chem. B 117 (2013) 1130–1135, http://dx.doi.org/10.1021/jp311032p. [21] J.J. Hettige, J.C. Araque, C.J. Margulis, Bicontinuity and multiple length scale ordering in triphilic hydrogen-bonding ionic liquids, J. Phys. Chem. B 118 (2014) 12706–12716, http://dx.doi.org/10.1021/jp5068457. [22] H.K. Kashyap, J.J. Hettige, H.V.R. Annapureddy, C.J. Margulis, SAXS anti-peaks reveal the length-scales of dual positive-negative and polar-apolar ordering in roomtemperature ionic liquids, Chem. Commun. 48 (2012) 5103–5105, http://dx.doi. org/10.1039/c2cc30609c. [23] H.K. Kashyap, C.S. Santos, H.V.R. Annapureddy, N.S. Murthy, C.J. Margulis, E.W. Castner, Temperature-dependent structure of ionic liquids: X-ray scattering and simulations, Faraday Discuss. 154 (2012) 133–143, http://dx.doi.org/10.1039/ C1FD00059D. [24] H.V.R. Annapureddy, H.K. Kashyap, P.M. De Biase, C.J. Margulis, What is the Origin of the Prepeak in the X-ray Scattering of Imidazolium-Based Room-Temperature Ionic Liquids? J. Phys. Chem. B 114 (2010) 16838–16846, http://dx.doi.org/10.1021/jp108545z. [25] M. Macchiagodena, L. Gontrani, F. Ramondo, A. Triolo, R. Caminiti, Liquid structure of 1- alkyl-3-methylimidazolium-hexafluorophosphates by wide angle x-ray and neutron scattering and molecular dynamics, J. Chem. Phys. 134 (2011) 114521, http:// dx.doi.org/10.1063/1.3565458. [26] E.W. Castner, C.J. Margulis, M. Maroncelli, J.F. Wishart, E.W.C. Jr, Ionic Liquids: Structure and Photochemical Reactions, Annu. Rev. Phys. Chem. 62 (2011) 85–105, http://dx.doi.org/10.1146/annurev-physchem-032210-103421. [27] H.K. Kashyap, C.S. Santos, N.S. Murthy, J.J. Hettige, K. Kerr, S. Ramati, et al., Structure of 1-Alkyl-1-methylpyrrolidinium Bis(trifluoromethylsulfonyl)amide Ionic Liquids
Editorial
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with Linear, Branched, and Cyclic Alkyl Groups, J. Phys. Chem. B (2013)http://dx.doi. org/10.1021/jp403518j. K. Shimizu, C.E.S. Bernardes, J.N. Canongia Lopes, Structure and Aggregation in the 1Alkyl-3-Methylimidazolium Bis(trifluoromethylsulfonyl)imide Ionic Liquid Homologous Series, J. Phys. Chem. B 118 (2014) 567–576, http://dx.doi.org/10.1021/ jp409987d. X. Song, H. Hamano, B. Minofar, R. Kanzaki, K. Fujii, Y. Kameda, et al., Structural heterogeneity and unique distorted hydrogen bonding in primary ammonium nitrate ionic liquids studied by high-energy X-ray diffraction experiments and MD simulations, J. Phys. Chem. B 116 (2012) 2801–2813, http://dx.doi.org/10.1021/jp209561t. S. Li, J.L. Banuelos, J. Guo, L. Anovitz, G. Rother, R.W. Shaw, et al., Alkyl Chain Length and Temperature Effects on Structural Properties of Pyrrolidinium-Based Ionic Liquids: A Combined Atomistic Simulation and Small-Angle X-ray Scattering Study, J. Phys. Chem. Lett. 3 (2012) 125–130, http://dx.doi.org/10.1021/jz2013209. A. Triolo, O. Russina, R. Caminiti, H. Shirota, H.Y. Lee, C.S. Santos, et al., Comparing intermediate range order for alkyl- vs. ether-substituted cations in ionic liquids, Chem. Commun. (Camb.) 48 (2012) 4959–4961, http://dx.doi.org/10.1039/ c2cc31550e. K. Shimizu, C.E.S. Bernardes, A. Triolo, J.N. Canongia Lopes, Nano-segregation in ionic liquids: scorpions and vanishing chains, Phys. Chem. Chem. Phys. 15 (2013) 16256–16262, http://dx.doi.org/10.1039/c3cp52357h. Y. Shen, D.F. Kennedy, T.L. Greaves, A. Weerawardena, R.J. Mulder, N. Kirby, et al., Protic ionic liquids with fluorous anions: physicochemical properties and selfassembly nanostructure, Phys. Chem. Chem. Phys. 14 (2012) 7981–7992, http:// dx.doi.org/10.1039/c2cp40463j. D. Xiao, L.G. Hines Jr., S. Li, R. a Bartsch, E.L. Quitevis, O. Russina, et al., Effect of Cation Symmetry and Alkyl Chain Length on the Structure and Intermolecular Dynamics of 1,3- Dialkylimidazolium Bis(trifluoromethanesulfonyl)amide Ionic Liquids, J. Phys. Chem. B (2009) 6426–6433, http://dx.doi.org/10.1021/jp8102595. W. Zheng, A. Mohammed, L.G. Hines, D. Xiao, O.J. Martinez, R. a Bartsch, et al., Effect of Cation Symmetry on the Morphology and Physicochemical Properties of Imidazolium Ionic Liquids. J. Phys. Chem. B 6572–6584 (2011)http://dx.doi.org/10. 1021/jp1115614. M. a a Rocha, C.M.S.S. Neves, M.G. Freire, O. Russina, A. Triolo, J.a.P. Coutinho, et al., Alkylimidazolium Based Ionic Liquids: Impact of Cation Symmetry on Their Nanoscale Structural Organization, J. Phys. Chem. B 48 (2013) 4959–4961, http://dx.doi. org/10.1021/jp406374a. M. a a Rocha, J.a.P.P. Coutinho, L.M.N.B.F. Santos, Evidence of nanostructuration from the heat capacities of the 1,3-dialkylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquid series, J. Chem. Phys. 139 (2013) 104502, http://dx.doi.org/10.1063/1. 4820825. M. a a Rocha, J. a P. Coutinho, L.M.N.B.F. Santos, Vapor Pressures of 1,3- Dialkylimidazolium Bis(trifluoromethylsulfonyl)imide Ionic Liquids with Long Alkyl Chains, J. Chem. Phys. 140 (2014) 134502, http://dx.doi.org/10.1063/1.4896704. B. Aoun, A. Goldbach, M. a Gonzalez, S. Kohara, D.L. Price, M.-L. Saboungi, Nanoscale heterogeneity in alkyl-methylimidazolium bromide ionic liquids, J. Chem. Phys. 134 (2011) 104509, http://dx.doi.org/10.1063/1.3 563540. O. Russina, A. Sferrazza, R. Caminiti, A. Triolo, Amphiphile Meets Amphiphile: Beyond the Polar-Apolar Dualism in Ionic Liquid/Alcohol Mixtures, J. Phys. Chem. Lett. 5 (2014) 1738–1742, http://dx.doi.org/10.1021/jz500743v. A.A.H. Padua, M.F. Costa Gomes, J.N. Canongia Lopes, Molecular solutes in ionic liquids: a structural perspective, Acc. Chem. Res. 40 (2007) 1087–1096, http://dx.doi. org/10.1021/ar700050q.
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Olga Russina Dept. of Chemistry, University Sapienza Universita di Roma, Piazzale Aldo Moro, Rome (Italy) Wolffram Schröer Institut fur Anorganische und Physikalische Chemie, University Bremen, Bremen (Germany) Alessandro Triolo Laboratorio Liquidi Ionici, Istituto Struttura della Materia, Consiglio Nazionale delle Ricerche, Rome (Italy)
Contents lists available at ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Editorial
Mesoscopic structural and dynamic organization in ionic liquids
The present Special Issue of the Journal of Molecular Liquids is dedicated to the Mesoscopic Structural and Dynamic Organization in Ionic Liquids (ILs). These compounds represent one of the hottest research fields in the last two decades.[1] Being composed only of ionic species it can be expected that their properties are strongly affected by coulombic interactions. However, typically the cation of the ILs bears an apolar side chain - most commonly being a medium length alkyl tail - that interacts with the surrounding environment merely by dispersive forces. Such a chemical structure leads to an inherently amphiphilic nature for the resulting IL; as a consequence a strong tendency of the alkyl tail to segregate from the charged moieties is ubiquitously observed. This implies a distinct level of structural heterogeneity over the spatial scale of a few nanometers, due to the alternation between polar and apolar domains. [2–24] The consequences of this high level of structural architecture are manifold: they invest as different fields as separation, synthesis, catalysis, electrochemistry, nanoparticles manufacturing etc. The level of understanding of this spectacular peculiarity in IL’s morphology is now mature. Since the appearance of the first experimental[4] and computational[2,3] proposals for such an organization, numerous studies provided more and more compelling evidences of this specific architecture. Several aspects have been explored in the last few years, including the role of alkyl chain length[5,9,12–16, 25–30], charged moieties chemical nature [5,15,16], polarity of the tail[12,20,21,31–33], symmetry of the cation[34–38], effect of temperature [14,23,39] etc. It is stated that there is a strong correlation between structural complexity and performances of ILs. For example the solvation performance and the miscibility with other compounds are strongly dependent on the mesoscopic morphology of ILs. It solutions the ionic and apolar domains prevail and giving rise to a preferential solvation.[40,41] It should be noted that the domain structure in solutions must be distinguished from the fluctuations of the composition[42,43] that are the precursors of phase separation observed in many solutions of ILs [44,45]. In this Special Issue, we aimed at collecting the recent results and the opinions of several world- leading researchers in the field of mesoscopic organization in Ionic Liquids, providing in a unique volume a set of contributions that would make available to the general reader the current level of understanding in this field. The Issue is composed of sixteen contributions from authors from different laboratories who, using either experimental techniques or computational methods, either describe new original results or report an overview over specific aspects of the mesoscopic order in ILs. The first paper of Miller reports on the effect of the counterion nature on a series of physicochemical properties for 4-methyl-1-propyl-
http://dx.doi.org/10.1016/j.molliq.2015.08.025 0167-7322/© 2015 Published by Elsevier B.V.
1,2,4-triazolium ionic liquids, highlighting the role of the anion in influencing properties. [MILLER]. Greaves and coworkers report on the complex nanostructure of Fluorous protic ionic liquids upon water addition. This interesting new class of protic ILs opened the way to triphilic ILs, where simultaneously polar, lipophilic and fluorophilic domains coexist, despite their mutual incompatibility. In this work the authors explore the evolution of Lyotropic Liquid Crystalline Mesophases upon Water Addition. [GREAVES]. Using computational tools, Saielli and coworkers investigated the microscopic structure and dynamics of representative ILs where Xenon was added. The existence of IL cages around the probe Xe was explored. These studies are actual and well related to recent literature on the role of Xe in exploring IL’s morphology [46,47]. [SAIELLI] A thorough comprehensive characterization of the impact of cation side chain length and symmetry on water solubility in ILs was reported by Coutinho and coworkers. They report on water solubility in a series of asymmetric and symmetric [CnCxim][NTf2] ILs (where n is up to 21 and x = 1 or n), extending their study of symmetric cations [36–38]. In order to analyse their solubility data, Density Functional theory calculations were also performed. [Coutinho] Shimizu and coworkers used Molecular Dynamics simulations to probe the structural architecture in a series of 1-alkyl-3methylimidazolium hexafluorophosphate ionic liquid, [CnC1im][PF6] (n = 3, 6, 9, 12). They aim at exploring the mesoscopic order in these systems and compare it with the one they found in a similar series [28]. Also they show results for equimolar mixtures of ILs with different alkyl chain length. [Shimizu] In their contribution, Ribeiro and coworkers explore the highfrequency sound modes in ionic liquids using molecular dynamics simulations on [C2C1im][TFSI] and [C2C1im][FSI] and their mixtures with lithium salts. The outcome of the MD simulations is compared with recent experimental Inelastic X-ray Scattering data from Fujii et al. [48] [Ribeiro] Perera and collaborators report on structural features of low and high melting ILs, aiming at rationalising their differences in disorder in terms of particles’ shape and the ration of molecular size to temperature. Using both computer simulations and liquid state integral equation techniques, they clarify the differences between fluctuations and structural microheterogeneities, highlighting the different role of free and bound charges and identifying the existence of different forms of disorder. [Perera] Using both experimental (SAXS, Raman spectroscopy) and computational tools, Matic and coworkers report on the mesoscopic structural organization in solvated ionic liquids that are a class of IL formed by dissolving a Lithium salt in tetraglyme. They describe at atomistic detail how Lithium is solvated in this complex medium. [Matic]
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Idrissi and coworkers have described the nature of the intermolecular interactions in a technologically relevant IL (1-butyl-3methylimidazolium acetate) and its water mixtures. They used a large set of experimental (IR, Raman, NMR Spectroscopy) and Quantum Chemistry computation tools, exploring the role of water concentration on dynamics. Specific kinds of interactions are identified and the different findings are correlated. [Idrissi] Pulsed-field gradient 1H NMR has been used by Balevicius and coworkers to explore the diffusion and self-aggregation (micellization) processes in neat 1-decyl-3-methyl-imidazolium chloride and its aqueous solutions. An Arrhenius trend has been detected for the neat IL, thus prompting for a softening of the interactions in this long chained salt. The exploration of water mixtures led to the determination of critical aggregation concentration. [Balevicius] Mele and coworkers are reporting their recent progresses in the field of Nuclear Overhauser Enhancement (NOE), as a powerful tool to explore structure in ILs. The large potentialities of this experimental approach were recently described by Weigartner et al. [49]. In this contribution the authors use the technique to explore N-propyl-N-methyl pyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR13TFSI), its homologue with bis(fluorosulfonyl)imide (PYR13FSI), and their mixtures with LiTFSI. Discussion of the data in terms of either short range (first coordination shell) or long range (polar-apolar alternation) correlations is presented. [MELE] Recollecting some of their recent observations and presenting new results, Abe and coworkers used a wide range of experimental techniques to explore binary mixtures of ILs and water. They also used high pressure XRD and Raman spectroscopy to describe the complex polymorphism in ILs. Dracopoulos and coworkers employed spectroscopic techniques (FT-IR/ATR and FT-Raman) to explore the interactions in alkyl substituted imidazolium bis(trifluoromethanolsulfonyl)imide protic ionic liquids (PILs) HCnImNTf2 (n = 0-12) and aprotic (APILs) C1CnNTf2 (n = 1-12). They studied the role of alkyl chain length as well as the differences due to the enhanced hydrogen bonding interactions existing in PILs. [Dracopoulos] Varela and coworkers present a review of the existing work in the field of structural and dynamic properties of mixtures of ionic liquids with molecular cosolvents or salts. The major role of the nanoarchitecture in IL, with polar-apolar alternation in influencing the solvation mechanism of both charged and neutral species is discussed, leading to the proposal of nanostructured solvation [50,51]. [VARELA] By combining NMR spectroscopy with vibrational (IR and Raman) ones, Martinelli and coworkers investigated the consequences of increasing alkyl chain lenght in 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide on intermolecular interactions as well as rotational dynamics. The existence of nano-dynamical heterogeneities as a consequence of the mesoscopic structural heterogeneity in ILs is proposed. Yamamuro and coworkers used inelastic neutron scattering to explore low energy excitations in 1- alkyl-3-methylimidazolium ionic liquids: they focus mostly on the Boson peak and aim relating its energy to anion radius and glass transition, to conclude that Boson peak and glass transition are predominantly governed by the inter-ionic Coulomb interaction. [Yamamuro] Overall, we think that the papers contributed to this Special Issue can provide the interested reader with a broad overview of the most exciting issues related to the mesoscopic organization in ionic liquids and their binary mixtures. Of course the main goal of this work has been the one to stimulate further exciting work in this direction and we hope all these contributions will receive careful attention! References [1] H. Weingaertner, Understanding ionic liquids at the molecular level: Facts, problems, and controversies, Angew. Chem. Int. Ed. 47 (2008) 654–670, http://dx.doi. org/10.1002/anie.200604951.
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Annapureddy, C.J. Margulis, SAXS anti-peaks reveal the length-scales of dual positive-negative and polar-apolar ordering in roomtemperature ionic liquids, Chem. Commun. 48 (2012) 5103–5105, http://dx.doi. org/10.1039/c2cc30609c. [23] H.K. Kashyap, C.S. Santos, H.V.R. Annapureddy, N.S. Murthy, C.J. Margulis, E.W. Castner, Temperature-dependent structure of ionic liquids: X-ray scattering and simulations, Faraday Discuss. 154 (2012) 133–143, http://dx.doi.org/10.1039/ C1FD00059D. [24] H.V.R. Annapureddy, H.K. Kashyap, P.M. De Biase, C.J. Margulis, What is the Origin of the Prepeak in the X-ray Scattering of Imidazolium-Based Room-Temperature Ionic Liquids? J. Phys. Chem. B 114 (2010) 16838–16846, http://dx.doi.org/10.1021/jp108545z. [25] M. Macchiagodena, L. Gontrani, F. Ramondo, A. Triolo, R. Caminiti, Liquid structure of 1- alkyl-3-methylimidazolium-hexafluorophosphates by wide angle x-ray and neutron scattering and molecular dynamics, J. Chem. 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Olga Russina Dept. of Chemistry, University Sapienza Universita di Roma, Piazzale Aldo Moro, Rome (Italy) Wolffram Schröer Institut fur Anorganische und Physikalische Chemie, University Bremen, Bremen (Germany) Alessandro Triolo Laboratorio Liquidi Ionici, Istituto Struttura della Materia, Consiglio Nazionale delle Ricerche, Rome (Italy)