Article pubs.acs.org/JPCB
Liquid Structure of Bis(trifluoromethylsulfonyl)imide-Based Ionic Liquids Assessed by FT-IR Spectroscopy Cassie Kimble and Christopher M. Burba* Department of Natural Sciences, Northeastern State University, 611 N Grand Avenue, Tahlequah, Oklahoma 74464, United States
ABSTRACT: Ionic liquids are a fertile and active area of research, in part, due to the unique properties these solvents offer over traditional molecular solvents. Because these properties are rooted in the fundamental ion−ion interactions that govern their liquid structure, there is a strong motivation to characterize the liquid structure of ionic liquids. Infrared spectroscopy is a standard analytical tool for assessing liquid structures, for the intramolecular vibrational modes of the ions composing the materials are often quite sensitive to their local potential energy environment. In this work, we demonstrate that the band asymmetry for the νa(SNS) anion mode of N(Tf)2−-based ionic liquids originates from the dynamic coupling of vibrationally induced dipole moments of anions across a quasilattice. The magnitude of TO−LO splitting is linearly correlated with the number densities of the ionic liquids; an observation that is in accord with the predictions of dipolar coupling theory. Dipole moment derivatives of νa(SNS) calculated from dipolar coupling theory, (∂μ/∂q)DCT, are lower than those obtained from independent measurements of (∂μ/∂q). The most likely explanation for this disparity is that although ionic liquids possess sufficient long-range structure to support TO−LO splitting of infrared-active modes, there is enough orientational and translational disorder in the quasilattice to partially disrupt the coupling of vibrationally induced dipole moments across the quasilattice. This will result in diminished amounts of TO−LO splitting than would be expected if the ionic liquid were a perfect crystal at 0 K. Impacts of cation molecular structure and the formation of a binary solution on the liquid structure are also explored.
■
INTRODUCTION
vibrational spectroscopic studies of ionic liquids, focusing on the core issues and controversies in the field. A defining characteristic of many ionic liquids is the presence of long-range charge organization among their constituent ions. There is now ample experimental and computational evidence for charge organization with some of the more convincing experiments coming from neutron scattering studies of ionic liquids.2 In those studies, cation−anion and cation−cation pair distribution functions are shown to be out of phase and persist across several solvation shells. Furthermore, neutron scattering data suggest that ion size is important in determining the degree of the charge ordering, with larger ions exhibiting some charge ordering but to a lesser extent than ionic liquids with smaller ions. A number of X-ray scattering studies have also confirmed the presence of charge organization. This is most clearly
Research in the field of ionic liquids has exploded over the past few decades, with ionic liquids now poised to make significant inroads into technological applications traditionally dominated by molecular solvents. Part of this success stems from the unique properties these materials offer over molecular solvents, namely, a combination of high ionic conductivities, low viscosities, and low vapor pressures. Moreover, the large number of cation and anion combinations available have led many to describe ionic liquids as designer solvents, wherein specific properties may be targeted and the corresponding compounds synthesized through judicious choices of constituent ions. The question of how molecular structure influences the liquid structure of ionic liquids and the various properties that make them attractive as solvents has dominated most of the work in this field. Elucidating these structure−property maps remains a critical need.1,2 Vibrational spectroscopy is well suited to identify the underlying ion−ion interactions that govern the liquid structure of ionic liquids. Paschoal and coworkers3 recently provided a comprehensive review of © XXXX American Chemical Society
Received: January 19, 2017 Revised: March 7, 2017 Published: March 17, 2017 A
DOI: 10.1021/acs.jpcb.7b00620 J. Phys. Chem. B XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry B
electric field at molecule i based on the electric dipole moment of molecule j. In dyadic notation, an element of the dipole field propagation matrix is given as Tij = −▽i▽j(1/rij) for the ith and jth molecular entities separated by a distance rij. After expanding the electric dipole moments in terms of translationally symmetrized normal coordinates, Decius14 was able to derive expressions for TO−LO splitting of IR-active vibrational modes. Although dipolar coupling theory was originally developed for crystalline materials, there are several instances where it has been successfully applied to disordered alkali nitrates,15 molten alkali nitrates,16 and ionic liquids.10−12 For our purposes, the important result from dipolar coupling theory is the predicted linear relationship between the difference in the squares of the TO and LO mode frequencies (in units of cm−1) and the product of the dipole moment derivative of the vibrational mode and number density of a compound
demonstrated in low-q regions of structure factor plots where two intense peaks roughly between 0.6 and 1.5 Å−1 are often observed (see, for example, the work of Kashyap and coworkers in ref 4). The peak at shorter distances has been attributed to cation−anion correlations, whereas the peak at longer distances is mainly due to anion−anion correlation, with some weaker cation−cation correlations and cation−anion anticorrelations.5 There is also a general consensus that the liquid structure for many ionic liquids resembles the crystal structure of the solid phase. Taken together, these observations have led some researchers to propose quasilattice models for the liquid phase, wherein ions are pictured as occupying lattice sites of a collapsed crystalline lattice with some degree of translational and orientational disorder introduced among the ion sites. In some cases, particularly for cations with long alkyl side chains, a first sharp diffraction peak (FSDP) is also observed in X-ray scattering and molecular dynamics simulations at ≲0.5 Å−1. The origin of FSDPs in ionic liquids has prompted lively debate and remains an unsettled question in ionic liquid research. One interpretation of FSDPs is a nanoscale organization model, wherein nonpolar side chains attached to the cations provide another source of molecular ordering apart from the charge ordering discussed above. In this view, hydrophobic interactions among the alkyl side chains drive ionic liquids to separate into polar and apolar moieties, which produce molecular organization over the mesoscopic spatial scale. Polar−apolar molecular segregation would be pronounced for ionic liquids with long side chains but virtually undetectable for shorter chain analogues.4−6 The FSDP linearly scales with the length of the alkyl side chain, prompting some researchers (e.g., ref 7) to propose that the alkyl chains of the cations adopt an arrangement similar to that of a micelle embedded within a charged network. An alternative explanation is that FSDPs are merely a manifestation of nearest neighbor interactions for asymmetric cations participating in a chargeordered liquid structure.4,5,8,9 Regardless of this issue, many room-temperature ionic liquids possess long alkyl side chains, and the effect of those groups is an important factor to consider when evaluating the liquid structure of ionic liquids. If the quasilattice of an ionic liquid is sufficiently organized, it may be possible for the intramolecular vibrational motions of the constituent ions to couple together, thereby producing long-wavelength optical phonons. In this case, the local electric field for an ion will be different for longitudinal optical (LO) and transverse optical (TO) phonons, resulting in different vibrational frequencies for LO and TO modes. TO−LO splitting of vibrational modes has been invoked to explain the lineshape of certain vibrational modes of 1-alkyl-3-methylimidazolium trifluoromethanesulfonates10−12 and 1-butyl-3methylimidazolium bis(trifluorosulfonyl)imide.13 Dipolar coupling theory14 is a useful framework for understanding the origin of TO−LO splitting for IR-active modes. In its general form, dipolar coupling theory adds a term to the potential energy expression to account for the dynamic coupling of vibrationally induced dipole moments V′ =
1 tr μ T(I + α T)−1μ 2
⎛ ∂μ ⎞2 νLO ̃ 2 − νTO ̃2 ∝ ⎜ ⎟ N ⎝ ∂q ⎠
(2)
If we view the liquid structure of an ionic liquid in terms of a quasilattice model, wherein the liquid structure is seen as a somewhat disordered crystalline lattice, it may be possible to use the relationships embodied by eq 2 to gain useful insights into the dynamic coupling of vibrational modes among ions. Of course, this assumes that the quasilattice is sufficiently ordered to support long-range coupling of the vibrational modes and produce measureable splitting of the TO and LO modes. Because ionic liquids undoubtedly possess some degree of orientational and translational disorder, we expect smaller TO− LO splittings than what might be found for comparable crystalline compounds. In this report, we test the linear dependence between TO−LO splitting and the number density predicted by dipolar coupling theory for bis(trifluoromethylsulfonyl)imide-based ionic liquids (the anion is hereafter abbreviated as N(Tf)2−). The role of ion polarizabilities in influencing TO−LO splitting is also explored. Finally, implications for the liquid structure of these materials are discussed.
■
EXPERIMENTAL METHODS We examined 17 ionic liquids, all embodying common heterocyclic cations; molecular structures are depicted in Figure 1. Table 1 summarizes the materials used along with the vendor. All of the ionic liquids were stored and manipulated under an argon atmosphere (VAC glovebox,
