Elsevier

Chemical Physics Letters

Volumes 616–617, 25 November 2014, Pages 259-274
Chemical Physics Letters

FRONTIERS ARTICLE
Dynamics and structure of room temperature ionic liquids

https://doi.org/10.1016/j.cplett.2014.09.062Get rights and content

Highlights

  • Dynamics of room temperature ionic liquids (RTIL) studied with several techniques.

  • Fluorophore orientational relaxation in organic and ionic regions give friction.

  • OHD-OKE experiments on pure RTILs show dynamics like simple organic liquids.

  • Addition of small amounts of water drastically change RTIL dynamics and structure.

Abstract

Room temperature ionic liquids (RTIL) are intrinsically interesting because they simultaneously have properties that are similar to organic liquids and liquid salts. In addition, RTILs are increasingly being considered for and used in technological applications. RTILs are usually composed of an organic cation and an inorganic anion. The organic cation, such as imidazolium, has alkyl chains of various lengths. The disorder in the liquid produced by the presence of the alkyl groups lowers the temperature for crystallization below room temperature and can also result in supercooling and glass formation rather than crystallization. The presence of the alkyl moieties also results in a segregation of the liquid into ionic and organic regions. In this article, experiments are presented that address the relationship between RTIL dynamics and structure. Time resolved fluorescence anisotropy measurements were employed to study the local environments in the organic and ionic regions of RTILs using a nonpolar chromophore that locates in the organic regions and an ionic chromophore that locates in the ionic regions. In the alkyl regions, the in plane and out of plane orientational friction coefficients change in different manners as the alkyl chains get longer. Both friction coefficients converge toward those of a long chain length hydrocarbon as the RTIL chains increase in length, which demonstrates that for sufficiently long alkyl chains the RTIL organic regions have properties similar to a hydrocarbon. However, putting Li+ in the ionic regions changes the friction coefficients in the alkyl regions, which demonstrates that changes of the ion structural organization influences the organization of the alkyl chains. Optical heterodyne detected optical Kerr effect (OHD-OKE) experiments were used to examine the orientational relaxation dynamics of RTILs over times scales of a hundred femtoseconds to a hundred nanoseconds. Detailed temperature dependent studies in the liquid and supercooled state and analysis using schematic mode coupling theory (MCT) show that RTILs have bulk liquid orientational relaxation dynamics that are indistinguishable in their nature from common nonpolar organic liquids that supercool. This behavior of the RTILs occurs in spite of the segregation into ionic and organic regions. However, when small amounts of water are added to RTILs at room temperature, novel dynamics are observed for the RTILs with long alkyl chains that have not been observed in OHD-OKE experiments on organic liquids. The results are interpreted as water induced structure in the ionic regions that causes the long alkyl chains to organize and ‘lock up.’ The dynamical measurements indicate that this lock up is involved in the formation of RTIL gels that occur over a narrow range of water concentrations.

Introduction

Over the last decade, room temperature ionic liquids (RTILs) have generated a great deal of interest because of their very low volatility, enormous variability, and good thermal stability. RTILs are salts with a melting point below 25 °C. They are often composed of an inorganic anion paired with an asymmetric organic cation that contains one or more pendant alkyl chains. The asymmetry of the cation frustrates crystallization, causing the salt's melting point to drop substantially compared to a common salt such as NaCl. RTILs have applications in organic synthesis, electrochemistry, and separation processes [1], [2], [3], [4], [5]. They are also being investigated for a variety of other applications, such as us in batteries [6].

For a number of years, the general consensus was that there was significant mesoscopic structure, with the long, hydrocarbon tails aggregating to form non-polar domains and with the cationic head groups and the anions forming tortuous ion channels that percolated through the pure liquid on relatively long length scales. This view was supported by a number of theoretical studies [7], [8], [9], [10], [11]. X-ray scattering experiments initially seemed to support such mesoscopic organization [12], [13], [14]. Within that framework, many experiments, including fluorescence [15], [16], [17], optical Kerr effect [18], [19], [20], [21], [22], and dielectric spectroscopies [21], [23]; NMR [24], [25]; and small-angle neutron scattering (SANS) [26] were interpreted by invoking the extended structure concept.

However, in 2010, H/D isotope labeled SANS experiments [27] and MD simulations [28] called into question the interpretation of the X-ray data. The SANS results showed that the X-ray data could be explained by anisotropic solvation, in which the alkyl-cation is preferentially solvated on the alkyl tail side by other alkyl tails and on the imidazolium side by anions. Recent X-ray scattering experiments on binary mixtures of RTILs and on RTILs with polar polyether tails were again interpreted as supporting the long range correlations [29]. Both mesoscopic structure and more local nanoscopic heterogeneity have been invoked in recent papers [30], [31], [32], [33], [34]. Detailed X-ray scattering experiments and theoretical analysis demonstrated that there is nanoscopic heterogeneity of the ionic and organic regions, but no extensive mesoscopic structuring [30]. A recent study of nanostructure using electronic excitation transfer experiments is consistent with the X-ray results [35].

While the question of mesoscopic heterogeneity may not be fully settled, there is no doubt that RTILs contain nanoscopic heterogeneity with hydrophobic regions of alkyl tails, and charge-ordered, hydrophilic head group regions that are distinct at least when the alkyl tails are sufficiently long, greater in length than ethyl. In the context of this heterogeneous liquid structure, highly selective solvation is possible [36]. The resulting diversity of local environments can have a direct impact on the dynamics of processes where solvation environment is important. The effect of environment has been observed in the context of a photoisomerization [37]. Anomalous reaction rates in RTILs have been attributed to this nanoscale structural heterogeneity [38], [39]. Dynamic heterogeneity could influence transport properties and availability of reagents and catalysts. One appeal of RTILs as a reaction medium is the breadth of molecules they can solvate because of the presence of multiple chemical environments. Dynamic heterogeneity may have a non-trivial impact on the reaction dynamics, and therefore make possible task-specific organic ionic liquids [40]. The design of task-specific RTILs may depend on the dynamics of molecules in distinct environments in addition to the specific substrate interactions.

The addition of co-solvents to RTILs is an important area of study [41]. To reduce the often high viscosity of most RTILs, low viscosity co-solvents are frequently added [42], [43]. One of the most common co-solvents, water, is especially important, in part due to the hygroscopic nature of many RTILs and the significant effect a small amount of water can have on properties such as viscosity and conductivity [44]. In applications, RTILs will have some water in them because of the difficulties and expense of producing dry ionic liquids. Thus, the question of the role of water in RTILs on the liquid's structure and dynamics is of importance. Simulations of the addition of water to various RTILs indicate that adding water promotes the alkyl tail aggregation up to a water mole fraction of ∼0.75 and then proceeds to break it up as the volume fraction of water becomes larger [45], [46]. It has been observed that 1-decyl-3-methylimidazolium bromide or nitrate (DmImBr or DmImNO3) RTILs gelled at a similar mole fraction of water [47] and the same behavior has been observed for 1-methyl-3-octylimidazolium chloride (OmImCl) [48]. The proposed hexagonal mesophase structure for the gel [47] has been observed in simulations [49]. The structural changes in water/RTIL co-solvent mixtures should significantly alter reaction rates and thus the dynamics of such binary mixtures are important.

Here a variety of experiments will be presented that illuminate many of the properties and features of RTILs. First, the orientational relaxation dynamics of fluorescent probes were studied by measuring their time dependent orientational anisotropy [36], [50]. Two probe molecules were employed. One is nonpolar and was located in the organic alkyl regions of the RTILs. As the alkyl chains got longer, the friction coefficients approach those of a long chain hydrocarbon. The other fluorescent probe was an anion and was located in the ionic regions. Its orientational relaxation was strongly affected by interactions with the surrounding cations. The results are consistent with different elements of rotational friction available to solutes depending on their chemical nature because of the ionic liquids’ nanoscopic heterogeneous structures.

Information on the influence of lithium cations on RTIL structure was obtained from time resolved fluorescence anisotropy measurements of a nonpolar chromophore that is located in the alkyl regions of the RTIL. As the concentration of Li+ was increased, the rotational friction experienced by the chromophore changes, demonstrating that the addition of lithium cation to the ionic regions changes the structural arrangement of the alkyl chains in the organic regions.

Optical heterodyne detected optical Kerr effect (OHD-OKE) experiments were used to study the bulk orientational relaxation dynamics of several pure dry RTILs over a wide range of temperatures [51], [52]. The experiments followed the orientational relaxation from hundreds of femtoseconds to a hundred nanoseconds. The results were analyzed using schematic mode coupling theory. The orientational relaxation is complex, consisting of several power laws at short time and ending with an exponential decay, which is the final complete randomization of the orientations. The power laws reflect the dynamics on time scales during which a molecule is ‘caged’ by the surrounding molecules and is not undergoing orientational diffusion. The functional form of the RTIL dynamics and their temperature dependence are indistinguishable from a wide range of organic liquids. These results show that in spite of the nanoscopic heterogeneity of RTILs, they have dynamical properties that are no different from organic liquids such as dibutylphthalate [53], [54] or acetylsalicylic acid [54].

In contrast to the pure dry RTILs, the OHD-OKE experiments reveal dramatic difference in dynamics when even small amounts of water are added [55], [56]. For chain lengths of hexyl and longer, there is clear evidence that water in the ionic regions produces structuring of alkyl chains that dramatically slows orientational relaxation and changes its fundamental nature. For RTILs that gel, the results indicate that gelation is preceded by water induced changes in structural dynamics.

Section snippets

Time resolved fluorescence probes of the organic and ionic regions of RTILs

The nanoscopic heterogeneity picture of RTILs implies distinct organic and ionic regions. An important question is how closely do the organic regions, which consist of the alkyl tails of the cation head groups, resemble a hydrocarbon liquid, and how does an ionic solute interact with the ions in the ionic regions. To examine these questions, the orientational relaxation dynamics of perylene and sodium 8-methoxypyrene-1,3,6-sulfonate (MPTS) (see Figure 1) were determined using time dependent

Pure RTILs

The last section clearly demonstrated the nanoscopic heterogeneity of the RTILs and explicated aspects of interactions with solutes that selectively locate in the alkyl and ionic regions. A remarkable aspect of pure RTILs is that they have bulk dynamical properties that in many respects are indistinguishable from those of relatively simple organic liquids. The dynamics of RTILs were investigated using optical heterodyne detected optical Kerr effect experiments [88], [89], [90]. The results are

Concluding remarks

Time dependent experiments were employed to explicate aspects of the relationship between dynamics and structure in RTILs. The mesoscopic heterogeneity of RTILs was shown clearly by the distinct solvation environments of the non-polar fluorescent probe perylene and the charged fluorescent probe MPTS (see Figure 1). For the RTILs with sufficiently long alkyl chains, the friction coefficients obtained from the orientational relaxation of perylene in RTILs converged toward those of perylene in

Acknowledgements

I would like to thank the many members of the Fayer Group who contributed to this research over many years. The research presented here was supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through Grant # DE-FG03-84ER13251, the Division of Chemistry, Directorate of Mathematical and Physical Sciences, National Science Foundation Grant # CHE-1157772, and the Air Force Office of Scientific Research Grant

Michael D. Fayer is the David Mulvane Ehrsam and Edward Curtis Franklin Professor of Chemistry at Stanford University. Fayer received his Ph.D. in chemistry from the University of California at Berkeley in 1974. He went directly to Stanford, where he has been a faculty member for forty years. Fayer is a member of the National Academy of Sciences, and has won a number of awards for his work developing and applying ultrafast laser techniques to the study of the dynamics of complex molecular

References (149)

  • M. Sureshkumar et al.

    J. Mol. Catal. B: Enzym.

    (2009)
  • A. Triolo et al.

    Chem. Phys. Lett.

    (2008)
  • L.A.S. Ries et al.

    Polyhedron

    (2008)
  • S. Feng et al.

    Fluid Phase Equilib.

    (2010)
  • M.A. Firestone et al.

    Inorg. Chim. Acta

    (2004)
  • N. Ito et al.

    Chem. Phys. Lett.

    (2004)
  • J.R. Mannekutla et al.

    Chem. Phys.

    (2007)
  • S.W. Pauls et al.

    J. Chem. Phys.

    (1998)
  • A. Chaumont et al.

    J. Mol. Liq.

    (2007)
  • K.G. Spears et al.

    J. Chem. Phys.

    (1978)
  • M. Galiński et al.

    Electrochim. Acta

    (2006)
  • Y. Kai et al.

    J. Mol. Liq.

    (1995)
  • S.A. Forsyth et al.

    Aust. J. Chem.

    (2004)
  • W.J. Li et al.

    J. Phys. Chem. B

    (2007)
  • A. Paczal et al.

    Monatsh. Chem.

    (2007)
  • J.R. Harjani et al.

    Curr. Org. Synth.

    (2007)
  • R. Hagiwara et al.

    Electrochemistry

    (2007)
  • S.M. Urahata et al.

    J. Chem. Phys.

    (2004)
  • Y. Wang et al.

    J. Am. Chem. Soc.

    (2005)
  • B.L. Bhargava et al.

    Soft Matter

    (2007)
  • J.N. Canongia Lopes et al.

    J. Phys. Chem. B

    (2006)
  • M.F.C. Gomes et al.

    Top. Curr. Chem.

    (2009)
  • O. Russina

    J. Phys.: Condens. Matter

    (2009)
  • A. Triolo et al.

    J. Phys. Chem. B

    (2007)
  • A. Sarkar et al.

    J. Phys. Chem. B

    (2008)
  • J. Guo et al.

    Phys. Chem. Chem. Phys.

    (2011)
  • A.M. Funston et al.

    J. Phys. Chem. B

    (2007)
  • D. Xiao et al.

    J. Phys. Chem. B

    (2007)
  • D. Xiao et al.

    J. Phys. Chem. B

    (2008)
  • D. Xiao et al.

    J. Phys. Chem. B

    (2006)
  • D.A. Turton

    J. Am. Chem. Soc.

    (2009)
  • P. Yang et al.

    J. Chem. Phys.

    (2011)
  • M.M. Mizoshiri et al.

    J. Chem. Phys.

    (2010)
  • M. Imanari et al.

    Phys. Chem. Chem. Phys.

    (2010)
  • C. Chiappe

    Monatsh. Chem.

    (2007)
  • R. Atkin et al.

    J. Phys. Chem. B

    (2008)
  • C. Hardacre et al.

    J. Chem. Phys.

    (2010)
  • H.V.R. Annapureddy et al.

    J. Phys. Chem. B

    (2010)
  • O. Russina et al.

    Faraday Discuss.

    (2011)
  • B. Aoun et al.

    J. Chem. Phys.

    (2011)
  • M. Macchiagodena et al.

    J. Chem. Phys.

    (2011)
  • H.K. Kashyap et al.

    Faraday Discuss.

    (2011)
  • C.S. Santos et al.

    J. Chem. Phys.

    (2011)
  • C.S. Santos et al.

    J. Chem. Phys.

    (2011)
  • K. Fruchey et al.

    J. Phys. Chem. B

    (2012)
  • K. Fruchey et al.

    J. Phys. Chem. B

    (2010)
  • H. Jin et al.

    J. Phys. Chem. B

    (2007)
  • S. Tiwari et al.

    J. Org. Chem.

    (2008)
  • E.W. Castner et al.

    Ann. Rev. Phys. Chem.

    (2011)
  • G. Douhéret et al.

    Phys. Chem. Chem. Phys.

    (2004)
  • Cited by (72)

    • Ionic liquids in extraction techniques: Determination of pesticides in food and environmental samples

      2021, TrAC - Trends in Analytical Chemistry
      Citation Excerpt :

      In the case of imidazolium-based ILs, a longer substituent, regardless of the anion type, decreases the melting point. For various anions, a longer IL alkyl chain results in a lower melting point [14]. Surface tension is considered an important factor in multiphase processes.

    View all citing articles on Scopus

    Michael D. Fayer is the David Mulvane Ehrsam and Edward Curtis Franklin Professor of Chemistry at Stanford University. Fayer received his Ph.D. in chemistry from the University of California at Berkeley in 1974. He went directly to Stanford, where he has been a faculty member for forty years. Fayer is a member of the National Academy of Sciences, and has won a number of awards for his work developing and applying ultrafast laser techniques to the study of the dynamics of complex molecular systems. He has published two books, ‘Elements of Quantum Mechanics,’ Oxford University Press (2001), a graduate level text book, and ‘Absolutely Small – How Quantum Theory Explains Our Everyday World,’ AMACOM (2010), a quantum book for laymen, high school and undergraduate students, which explains quantum theory, atoms, and molecules with no math.

    View full text