FRONTIERS ARTICLEDynamics and structure of room temperature ionic liquids
Graphical abstract
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
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