The role of the symmetry and the flexibility of the anion on the characteristics of the nanostructures and the viscosities of ionic liquids

doi:10.1016/j.cjche.2015.04.013

Abstract

Abstract: The transport properties of ionic liquids (ILs) are crucial properties in view of their applications in electrochemical devices. One of the most important advantages of ILs is that their chemical-physical properties and consequently their bulk performances can be well tuned by optimizing the chemical structures of their ions. This will require elucidating the structural features of the ions that fundamentally determine the characteristics of the nanostructures and the viscosities of ILs. Here we showed for the first time that the “rigidity”, the order, and the compactness of the three-dimensional ionic networks generated by the anions and the cation head groups determine the formation and the sizes of the nanostructures in the apolar domains of ILs. We also found that the properties of ionic networks are governed by the conformational flexibility and the symmetry of the anion and/or the cation head group. The thermal stability of the nanostructures of ILswas shownto be controlled by the sensitivity of the conformational equilibrium of the anion to the change of temperature. We showed that the viscosity of ILs is strongly related to the symmetry and the flexibility of the constitute ions rather than to the size of the nanostructures of ILs. Therefore, the characteristics of the nanostructures and the viscosities of ILs, especially the thermal stability of the nanostructures, can be fine-tuned by tailoring the symmetry and the conformational flexibility of the anion.

Key words: Ionic liquids, Viscosity, Nanostructure, Symmetry and flexibility of anion

摘要： The transport properties of ionic liquids (ILs) are crucial properties in view of their applications in electrochemical devices. One of the most important advantages of ILs is that their chemical-physical properties and consequently their bulk performances can be well tuned by optimizing the chemical structures of their ions. This will require elucidating the structural features of the ions that fundamentally determine the characteristics of the nanostructures and the viscosities of ILs. Here we showed for the first time that the “rigidity”, the order, and the compactness of the three-dimensional ionic networks generated by the anions and the cation head groups determine the formation and the sizes of the nanostructures in the apolar domains of ILs. We also found that the properties of ionic networks are governed by the conformational flexibility and the symmetry of the anion and/or the cation head group. The thermal stability of the nanostructures of ILswas shownto be controlled by the sensitivity of the conformational equilibrium of the anion to the change of temperature. We showed that the viscosity of ILs is strongly related to the symmetry and the flexibility of the constitute ions rather than to the size of the nanostructures of ILs. Therefore, the characteristics of the nanostructures and the viscosities of ILs, especially the thermal stability of the nanostructures, can be fine-tuned by tailoring the symmetry and the conformational flexibility of the anion.

关键词: Ionic liquids, Viscosity, Nanostructure, Symmetry and flexibility of anion

Jianguang Qi, Yufeng Hu, Yamei Zhao, Jiguang Li. The role of the symmetry and the flexibility of the anion on the characteristics of the nanostructures and the viscosities of ionic liquids[J]. , 2015, 23(9): 1565-1571.

References

[1] J.F. Wang, C.X. Li, C. Shen, Z.H. Wang, Towards understanding the effect of electrostatic interactions on the density of ionic liquids, Fluid Phase Equilib. 279 (2009) 87-91.
[2] H.Y.Wang, B. Tan, J.J.Wang, Z.Y. Li, S.J. Zhang, Anion-based pH responsive ionic liquids: design, synthesis, and reversible self-assembling structural changes in aqueous solution, Langmuir 30 (2014) 3971-3978.
[3] Y.F. Hu, Z.C. Liu, C.M. Xu, X.M. Zhang, The molecular characteristics dominating the solubility of gases in ionic liquids, Chem. Soc. Rev. 40 (2011) 3802-3823.
[4] X.H. Li, D.B. Zhao, Z.F. Fei, L.F. Wang, Applications of functionalized ionic liquid, Sci. China Ser. B 49 (2006) 385-401.
[5] M. Armand, F. Endres, D.R. MacFarlene, H. Ohno, B. Scostati, Ionic-liquid materials for the electrochemical challenges of the future, Nat. Mater. 8 (2009) 621-629.
[6] M.Montanino,M. Carewska, F.Alessandrini, S. Passerini, G.B. Appetecchi,The roleof the cation aliphatic side chain length in piperidinium bis(trifluoromethansulfonyl)imide ionic liquids, Electrochim. Acta 57 (2011) 153-159.
[7] K. Iwata, H. Okajima, S. Saha, H. Hamaguchi, Local structure formation in alkylimidazolium- based ionic liquids as revealed by linear and nonlinear Raman spectroscopy, Acc. Chem. Res. 40 (2007) 1174-1181.
[8] L.M.N.B.F. Santos, J.N. Canongia Lopes, J.A.P. Coutinho, J.M.S.S. Esperança, L.R. Gomes, I.M. Marrucho, L.P.N.J. Rebelo, Ionic liquids: first direct determination of their cohesive energy, J. Am. Chem. Soc. 129 (2007) 284-285.
[9] Y. Wang, W. Jiang, T. Yan, G.A. Voth, Understanding ionic liquids through atomistic and coarse-grained molecular dynamics simulations, Acc. Chem. Res. 40 (2007) 1193-1199.
[10] Z. Hu, C.J. Margulis, Room-temperature ionic liquids: Slow dynamics, viscosity, and the red edge effect, Acc. Chem. Res. 40 (2007) 1097-1105.
[11] W. Xu, C.A. Angell, Solvent-free electrolytes with aqueous solution-like conductivities, Science 302 (2003) 422-425.
[12] L.M. Martinez, C.A. Angell, A thermodynamic connection to the fragility of glassforming liquids, Nature 410 (2001) 633-667.
[13] W. Lu, A.G. Fadeev, B.H. Qi, E. Smela, B.R. Mattes, J. Ding, G.M. Spinks, J. Mazurkiewicz, D.Z. Zhou, G.G. Wallace, D.R. MacFarlane, S.A. Forsyth, M. Forsyth, Use of ionic liquids for π-conjugated polymer electrochemical devices, Science 297 (2002) 983-987.
[14] L. Fang, Y.F. Hu, J.G. Qi, Y.F. Chen, H.R. Zhang, H.Z. Huang, The physical and electrochemical properties of the ionic liquids based on N-ethylpiperidinium cations and TFSI anion, Electrochim. Acta 133 (2014) 440-445.
[15] P. Wang, S.M. Zakeeruddin, P. Comte, I. Exnar, M. Grätzel, Gelation of ionic liquidbased electrolytes with silica nanoparticles for quasi-solid-state dye-sensitized solar cells, J. Am. Chem. Soc. 125 (2003) 1166-1167.
[16] A.A.H. Pádua, M.F. Costa Gomes, J.N.A. Canongia Lopes, Molecular solutes in ionic liquids: a structural perspective, Acc. Chem. Res. 40 (2007) 1087-1096.
[17] J. Eastoe, S. Gold, S.E. Rogers, A. Paul, T. Welton, R.K. Heenan, I. Grillo, Ionic liquid-inoil microemulsions, J. Am. Chem. Soc. 127 (2005) 7302-7303.
[18] Z.H. Hu, C.J. Margulis, Heterogeneity in a room-temperature ionic liquid: Persistent local environments and the red-edge effect, Proc.Natl. Acad. Sci. 103 (2006) 831-836.
[19] A. Paul, P.K. Mandal, A. Samanta, On the optical properties of the imidazolium ionic liquids, J. Phys. Chem. B 109 (2005) 9148-9153.
[20] Y.T.Wang, G.A. Voth, Unique spatial heterogeneity in ionic liquids, J. Am. Chem. Soc. 127 (2005) 12192-12193.
[21] S. Shigeto, H. Hamaguchi, Evidence for mesoscopic local structures in ionic liquids: CARS signal spatial distribution of C_nmim[PF₆] (n = 4, 6, 8), Chem. Phys. Lett. 427 (2006) 329-332.
[22] A. Triolo, O. Russina, B. Fazio, R. Triolo, E. Di Cola, Morphology of 1-alkyl-3- methylimidazolium hexafluorophosphate room temperature ionic liquids, Chem. Phys. Lett. 457 (2008) 362-365.
[23] D. Xiao, L.G. Hines Jr., S. Li, R.A. Bartsch, E.L. Quitevis, O. Russina, A. Triolo, 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 113 (2009) 6426-6433.
[24] O. Russina, A. Triolo, L. Gontrani, R. Caminiti, D. Xiao, L.G. Hines Jr., R.A. Bartsch, E.L. Quitevis, N. Plechkova, K.R. Seddon, 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.
[25] O. Russina, M. Beiner, C. Pappas, M. Russina, V. Arrighi, T. Unruh, C.L. Mullan, C. Hardacre, A. Triolo, Temperature dependence of the primary relaxation in 1- hexyl-3-methylimidazolium bis {(trifluoromethyl) sulfonyl} imide, J. Phys. Chem. B 113 (2009) 8469-8474.
[26] S.G. Raju, S. Balasubramanian, Emergence of nanoscale order in room temperature ionic liquids: simulation of symmetric 1,3-didecylimidazolium hexafluorophosphate, J. Mater. Chem. 19 (2009) 4343-4347.
[27] J.C. Lassègues, J. Grondin, R. Holomb, P. Johansson, Raman and ab initio study of the conformational isomerism in the 1-ethyl-3-methyl-imidazolium bis (trifluoromethanesulfonyl) imide ionic liquid, J. Raman Spectrosc. 38 (2007) 551-558.
[28] D. Xiao, J.R. Rajian, A. Cady, S. Li, R.A. Bartsch, E.L. Quitevis, Nanostructural organization and anion effects on the temperature dependence of the optical Kerr effect spectra of ionic liquids, J. Phys. Chem. B 111 (2007) 4669-4677.
[29] 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) 164521.
[30] L.C. Branco, J.N. Rosa, J.J. Moura Ramos, C.A.M. Afonso, Preparation and characterization of new room temperature ionic liquids, Chem. Eur. J. 8 (2002) 3671-3677.
[31] K.R. Harris, M. Kanakubo, L.A. Woolf, Temperature and pressure dependence of the viscosity of the ionic liquids 1-hexyl-3-methylimidazolium hexafluorophosphate and 1-butyl-3-methylimidazolium bis (trifluoromethylsulfonyl) imide, J. Chem. Eng. Data 52 (2007) 1080-1085.
[32] M.A. Abraham, L. Moens, Clean solvents, Alternative Media for Chemical Reactions and Processing, American Chemical Society, Washington DC, 2002.
[33] H. Tokuda, K. Hayamizu, K. Ishii, M.A.B.H. Susan, M. Watanabe, Physicochemical properties and structures of room temperature ionic liquids. 2. Variation of alkyl chain length in imidazolium cation, J. Phys. Chem. B 109 (2005) 6103-6110.
[34] C. Hardacre, S.E.J. McMath, M. Nieuwenhuyzen, D.T. Bowron, A.K. Soper, Liquid structure of 1, 3-dimethylimidazolium salts, J. Phys. Condens. Matter 15 (2003) S159-S166.
[35] R. Holomb, A.Martinelli, I. Albinsson, J.C. Lassègues, P. Johansson, P. Jacobsson, Ionic liquid structure: The conformational isomerism in 1-butyl-3-methyl-imidazolium tetrafluoroborate ([bmim][BF₄]), J. Raman Spectrosc. 39 (2008) 793-805.
[36] B.L. Bhargava, R. Devane, M.L. Klein, S. Balasubramanian, Nanoscale organization in room temperature ionic liquids: a coarse grained molecular dynamics simulation study, Soft Matter 3 (2007) 1395-1400.
[37] K.S. Pitzer, Electrolytes. From dilute solutions to fused salts, J. Am. Chem. Soc. 102 (1980) 2902-2906.
[38] C. Wakai, A. Oleinikova, M. Ott, H.Weingärtner, How polar are ionic liquids? Determination of the static dielectric constant of an imidazolium-based ionic liquid by microwave dielectric spectroscopy, J. Phys. Chem. B 109 (2005) 17028-17030.
[39] S.V. Dzyuba, R.A. Bartsch, Influence of structural variations in 1-alkyl (aralkyl)-3- methylimidazolium hexafluorophosphates and bis (trifluoromethylsulfonyl) imides on physical properties of the ionic liquids, ChemPhysChem 3 (2002) 161-166.
[40] A.E. Bradley, C.Hardacre, J.D. Holbrey, S. Johnston, S.E.J.McMath, M. Nieuwenhuyzen, Small-angle X-ray scattering studies of liquid crystalline 1-alkyl-3-methylimidazolium salts, Chem. Mater. 14 (2002) 629-635.
[41] C.M. Gordon, J.D. Holbrey, A.R. Kennedy, K.R. Seddon, Ionic liquid crystals: hexafluorophosphate salts, J. Mater. Chem. 8 (1998) 2627-2636.
[42] E. Gómez, B. González, Á. Domínguez, E. Tojo, J. Tojo, Dynamic viscosities of a series of 1-alkyl-3-methylimidazolium chloride ionic liquids and their binary mixtures with water at several temperature, J. Chem. Eng. Data 51 (2006) 696-701.
[43] J.G. Li, Y.F. Hu, S. Ling, J.Z. Zhang, Physicochemical properties of [C₆mim][PF₆] and [C₆mim][(C₂F₅)₃PF₃] ionic liquids, J. Chem. Eng. Data 56 (2011) 3068-3072.
[44] J.G. Huddleston, A.E. Visser, W.M. Reichert, H.D. Willauer, G.A. Broker, R.D. Rogers, Characterization and comparison of hydrophilic and hydrophobic room temperature ionic liquids incorporating the imidazolium cation, Green Chem. 3 (2001) 156-164.
[45] P.A. Hunt,Why does a reduction in hydrogen bonding lead to an increase in viscosity for the 1-butyl-2,3-dimethyl-imidazolium-based ionic liquids? J. Phys. Chem. B 111 (2007) 4844-4853.