Letter pubs.acs.org/JPCL
Dynamic-Mechanical and Dielectric Evidence of Long-Lived Mesoscale Organization in Ionic Liquids Tyler Cosby,† Zachariah Vicars,† Yangyang Wang,‡ and Joshua Sangoro*,† †
Department of Chemical and Biomolecular Engineering, University of Tennessee, Knoxville, Tennessee 37996, United States Center for Nanophase Materials Science, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States
‡
S Supporting Information *
ABSTRACT: Experimental evidence of the dynamics of mesoscopic structure in roomtemperature ionic liquidsa feature expected to correlate with many physicochemical properties of these materialsremains limited. Here, we report the observation of slow, sub-α relaxations corresponding to dynamics of nanoscale hydrophobic aggregates in a systematic series of 1-alkyl-3-methylimidazolium-based ionic liquids from detailed analysis of dynamic-mechanical and broad-band dielectric spectra. The emergence of the sub-α relaxations correlates with increases in the zero-shear viscosity and static dielectric permittivity, constituting direct evidence of the influence of mesoscale aggregation on the physicochemical properties of ionic liquids.
onic liquids (ILs) are a versatile class of fluids consisting entirely of cations and anions conventionally categorized as having melting temperatures below 100 °C. The molecular structures of the constituent ions and, consequently, the resultant intermolecular interactions, are central to tuning their physicochemical properties such as viscosity, ionic conductivity, density, thermal and electrochemical stability, and polarity. There has been a concerted effort to understand the impact of molecular organization on physicochemical properties of ILs during the past 2 decades.1,2 A recent advance in this respect is the observation of mesoscale organization in bulk ILs arising from the hydrophobic aggregation of extended alkyl tails located on the ionic head groups of a variety of imidazolium, pyridinium, pyrrolidinium, phosphonium, and ammonium cations.3−13 Due to their ability to solvate both polar and nonpolar molecules, the formation of such nanoscale aggregates could provide ILs with possible advantages for a variety of solvent applications such as polymerization, organic synthesis and catalysis, as well as nanoparticle growth.3,12,14−16 Until now, experimental evidence of mesoscale organization has been mainly provided by observation of structural features from lowmomentum transfer (q) peaks in X-ray and neutron scattering functions. Coupled with molecular dynamics simulations, the structure functions reveal the existence of complex architectures ranging from micellar-like spherical aggregates to extended bicontinuous structures as a function of the alkyl chain length. 4−13 The influence of these structures on the physicochemical properties of ILs depends strongly on the lifetimes of the nanoscale aggregates.15−17 However, neutron spin echo (NSE) spectroscopy is one of the few techniques currently known to be capable of probing such dynamics, albeit at rather short time scales.15,18,19 These measurements are
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© XXXX American Chemical Society
tedious to carry out as they require deuterated IL samples and access to suitable neutron facilities. To date, NSE data for only two ILs, namely, 1-octyl-3-methylimidazolium and 1-hexyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide, have been reported in the literature.15,18,19 Other studies involving optical Kerr effect and dielectric spectroscopy at time scales faster than nanoseconds have reported slow, sub-α relaxation modes, attributed to the existence of mesoscale aggregation, in the recent past.20,21 The question regarding whether the nanostructures observed in imidazolium-based ILs have lifetimes beyond a few nanoseconds has remained unanswered. Despite the rapid and significant advances in our understanding of mesoscale organization in bulk ILs, experimental techniques capable of directly probing the dynamics of such aggregates have thus far been limited to fast time scales. How long-lived are these mesoscopic aggregates and how do they influence the physicochemical properties of ILs? To address these questions, it is necessary to find other experimental techniques that can probe detailed signatures of the mesoscale aggregates and their dynamics over much broader time scales.15 In this Letter, we report the observation of slow, sub-α dynamics in the shear-mechanical and dielectric spectra of a series of alkyl-imidazolium ILs with tetrafluoroborate and bis(trifluoromethylsulfonyl)imide anions. These features are not present in the imidazolium ILs with alkyl spacer lengths equal to or shorter than butyl but rather emerge only for ILs that have been shown to be capable of hydrophobic aggregation, as evidenced by previous X-ray and neutron Received: June 14, 2017 Accepted: July 17, 2017 Published: July 17, 2017 3544
DOI: 10.1021/acs.jpclett.7b01516 J. Phys. Chem. Lett. 2017, 8, 3544−3548
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The Journal of Physical Chemistry Letters ⎡ ⎛ ⎞ 1 G′ = Re⎢G∞⎜1 − γ⎟ ⎢⎣ ⎝ (1 + iωτα) ⎠
scattering experiments complemented by molecular dynamics simulations.3−5,7,9 The slow, sub-α relaxation is attributed to the motion of extended mesoscale aggregates. This is the first time, to our knowledge, that evidence of long-lived mesoscopic aggregates in ILs is reported from dynamic-mechanical spectroscopy. In addition, the rates of the slow dielectric relaxation agree well with those obtained by NSE, indicating that this is a complementary approach to probe the interplay of aggregate formation, morphology, and dynamics in these complex fluids. The complex shear modulus, G* = G′(ω) + iG″(ω), of glassforming 1-alkyl-3-methylimidazolium tetrafluoroborate ILs (AlkylMIm BF4) with alkyl spacer lengths of butyl, hexyl, and octyl are shown in Figure 1a. The shift factors are provided in
⎛ ⎞⎤ 1 ⎥ + G∞ ,aggregate⎜1 − γ⎟ (1 + iωτslow ) ⎠⎥⎦ ⎝
(1)
where G∞ and G∞,aggregate are the high-frequency limiting shear moduli of the two relaxations, τα and τslow are the relaxation times of the structural α-relaxation and sub-α relaxation, respectively, and γ is a parameter associated with the spectral shape.25 The relaxation rates ωα = 1/τα and ωslow = 1/τslow, extrapolated over a broad temperature range using the shift factors, are presented in Figure 4b and discussed later in the current work. In low-molecular-weight ILs, the primary structural relaxation is coupled to the translational motion of ions, as evidenced by numerous studies utilizing dielectric spectroscopy, dynamic light scattering, differential scanning calorimetry, and rheology.2,26 Accordingly, the slow, sub-α relaxation mode must arise from structures significantly larger than those responsible for the primary structural relaxation. X-ray and neutron scattering experiments coupled with molecular dynamics simulations have indicated that 1-alkyl-3-methylimidazolium tetrafluoroborate ILs begin to form distinct hydrophobic aggregates when the alkyl spacer is six carbons long, and nonpolar domains percolate the entire material at spacer lengths of approximately eight and longer.3−5,9 The emergence of the new terminal relaxation is therefore attributed to dynamics of these mesoscale aggregates. Similar rheological responses have also been observed in glassforming monohydroxy alcohols (MAs), systems in which aggregation occurs by intermolecular hydrogen bonding interactions.25,27,28 Recently, compressibility measurements on two representative MAs revealed a similar low-frequency response in the complex adiabatic bulk modulus, thereby linking the terminal relaxation to density fluctuations.28 A signature of the terminal relaxation is also observed in the broad-band dielectric spectra of MAs, referred to as the slow Debye relaxation due to its approximately single-exponential relaxation time distribution. It is therefore of interest to investigate whether such slow dielectric relaxations associated with dynamics of supramolecular aggregates occur in 1-alkylimidazolium-based ILs as well. Despite extensive investigations of ILs utilizing dielectric spectroscopy, only two research groups have reported slow, sub-α relaxation dynamics.20,21,29 The origin of the observed slow dynamics remains unclear. To elucidate a possible link between these slow relaxations and mesoscale aggregation, we have revisited the dielectric spectra of a series of 1-alkyl-3methylimidazolium ILs with varying alkyl spacer lengths and bis(trifluoromethylsulfonyl)imide and tetrafluoroborate anions because imidazolium ILs have been the most widely investigated category using several complementary experimental and computational techniques. The complex dielectric permittivity, ε*(f) = ε′(f) − iε″(f), and conductivity, σ*( f) = σ′( f) + iσ″( f), spectra for the IL 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (OctylMIm NTf2) are presented in Figure 2. Application of the derivative analysis, ε″der = (−π/2)[∂ε′/∂ ln( f)], reveals the existence of two distinct dielectric loss peaks. Fitting the real part of the complex dielectric function, ε′(f), using eq 2, a combination of two Havriliak−Negami functions with a power law function included to account for low-frequency electrode polarization
Figure 1. (a) Real (G′, open symbols) and imaginary (G″, closed symbols) parts of the complex shear modulus, G*, of 1-alkyl-3methylimidazolium tetrafluoroborate ILs with alkyl spacer lengths of butyl, hexyl, and octyl. The spectra are normalized with respect to the high-frequency shear modulus, G∞, and the structural relaxation rate, ωα. (b) Real part of the complex viscosity normalized by the viscosity contribution of structural α-relaxation.
the Supporting Information. The real and imaginary parts of the complex modulus, G′ and G″, are scaled by the highfrequency shear modulus, G∞, and plotted versus frequency normalized by the structural relaxation rate at the glass transition, ωα. Simple molecular glass-forming liquids are expected to display a single mechanical relaxation corresponding to the glass transition with low-frequency slopes of 2 and 1 for G′ and G″, respectively.22 As shown in Figure 1, this response is observed for 1-butyl-3-methylimidazolium tetrafluoroborate (ButylMIm BF4). However, upon increasing the alkyl spacer length to octyl, a substantial departure from a simple Maxwell relaxation occurs at time scales slower than the structural relaxation rate. This terminal relaxation in G′ is reminiscent of chain relaxations in the rheological response of short-chain polymers.23,24 The complex shear modulus, G*, of 1-octyl-3-methylimidazolium tetrafluoroborate (OctylMIm BF4) is fit by a combination of two Cole−Davidson-modified Maxwell relaxation models 3545
DOI: 10.1021/acs.jpclett.7b01516 J. Phys. Chem. Lett. 2017, 8, 3544−3548
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Figure 2. Real, ε′ and σ′, and imaginary, ε″ and σ″, parts of the complex permittivity and conductivity, ε* and σ*, of OctylMIm NTf2. The derivative spectra, ε″der = (−π/2)[∂ε′/∂ ln( f)] (solid symbols), reveal a slow relaxation that is obscured by conductivity in the dielectric loss.
Figure 3. Derivative spectra, ε″der = (−π/2)[∂ε′/∂ ln(f)], of 1-alkyl-3methylimidazolium ILs with (a) bis(trifluoromethylsulfonyl)imide and (b) tetrafluoroborate anions and indicated alkyl spacer lengths at a fixed mean ion hopping rate. Insets: Data are shifted vertically for clarity.
provides an excellent description of the complex permittivity, conductivity, and ε″der spectra30 ⎛ ⎞ ⎛ ⎞ Δεe Δεslow ⎟ + ⎜⎜ ⎟ ε*(ω) = ε∞ + ⎜⎜ β γ⎟ β γ⎟ ⎝ (1 + (iωτe) ) ⎠ ⎝ (1 + (iωτslow ) ) ⎠ + A(ω)n
(2)
where ω = 2πf, ε∞ is the high-frequency limiting permittivity, Δεα and Δεslow are the dielectric strengths, τe and τslow are the relaxation times, and β and γ are stretching parameters. The pre-exponential factor, A, and exponent, n, of the power-law function account for the low-frequency dispersion due to electrode polarization. The mean rate of the faster relaxation corresponds closely to the frequency of the peak in the imaginary part of the complex dielectric modulus, M″( f), as well as rates for fits by the random barrier model and is therefore attributed to ion hopping conduction.31,32 It has previously been reported, and is also illustrated in Figure 4, that the mean ion hopping rates in these imidazolium-based ILs agrees well with those of the structural relaxation.26 The slower relaxation is not readily observable in any representation other than the derivative of the real part of the dielectric function, presumably due to the dominant contribution of the ionic conductivity to the dielectric and conductivity spectra. The ε″der spectrum of OctylMIm NTf2 is compared with that of 1-alkyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide (AlkylMIm NTf2) ILs with alkyl lengths of propyl, butyl, and hexyl in Figure 3a; a similar comparison is made for the tetrafluoroborate-based series in Figure 3b. The derivative spectra of each IL are presented at temperatures where the ion hopping rates are approximately equal. In this representation, a distinct dielectric relaxation, approximately 10−20 times slower than ωe, is observed at spacer lengths of hexyl and octyl. The dielectric spectra of the ILs with shorter spacer lengths are described by a single Havriliak−Negami function due to the disappearance of the slow relaxation, although for butyl there appears to be a weak additional spectral contribution at lower frequencies. The mean relaxation rates of the ion hopping and slow, sub-α dielectric relaxations are presented in Figure 4a,b for the bis(trifluoromethylsulfonyl)imide and tetrafluoroborate IL series, respectively. Interestingly, the rates obtained from NSE
Figure 4. (a) Temperature dependence of the dielectric relaxation rates, ωe,BDS = 1/τe and ωslow,BDS = 1/τslow, for AlkylMIm NTf2 ILs with alkyl spacer lengths of hexyl and octyl, shown as circle and square symbols, respectively. The rates previously reported by NSE, at the indicated q-values, are provided as crossed and half-filled symbols.15,19 Lines correspond to fits with the Vogel−Fulcher−Tammann equation, and parameters provided in the Supporting Information. (b) Temperature dependence of the dielectric relaxation rates, ωe,BDS and ωslow,BDS, and dynamic-mechanical relaxation rates, ωα,DMS and ωslow,DMS, for 1-octyl-3-methylimidazolium tetrafluoroborate. The crossed and half-filled squares correspond to rates obtained by eq 1 and extrapolated using the shift factors. Dotted squares are structural relaxation rates obtained from the crossover frequency of G′ and G″.
by Kofu et al. for the low-q peak in OctylMIm NTf2 and by Russina et al. in HexylMIm NTf2 are in good agreement with those of the slow dielectric relaxation, as illustrated by a fit using the Vogel−Fulcher−Tammann equation, ω = ω∞ exp[B/ (T − T0)], where ω∞ is the high-temperature limit of the relaxation rate, B is the curvature, and T0 is the Vogel temperature; see the solid lines in Figure 4a.15,18,19 Structural 3546
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interfaces of polar and nonpolar domains of the aggregates. These results support the interpretation of the low-momentum transfer (q) peak in the X-ray and neutron scattering functions as evidence of mesoscale organization in numerous ILs. The observed influence of these aggregate dynamics on transport properties illustrates the importance of mesoscale aggregation in developing structure−property relationships in ILs. The ability to probe dynamics of mesoscale aggregation with these experimental techniques provides a new and complementary approach to elucidate the role of aggregate formation and morphology on the physicochemical properties of bulk ILs.
and slow, sub-α relaxation rates obtained from the dynamicmechanical spectra for 1-octyl-3-methylimidazolium tetrafluoroborate are compared with the dielectric relaxation rates in Figure 4b. The slow, sub-α relaxation rates in the rheological response are slower and more sensitive to the alkyl spacer length than the corresponding dielectric relaxation, as seen in Figures 4b, 1a, and 3b. This discrepancy might be due to sensitivity of the terminal relaxation to the percolation of hydrophobic domains at the octyl spacer length. The time scales associated with the slow process observed in the broad-band dielectric spectra correlate with those from NSE and dynamic-mechanical data, confirming the existence of long-lived mesoscopic structures. The slow dielectric relaxation is attributed to interfacial polarization at the interfaces of the polar and apolar domains. This polarization mechanism may arise from the motion of counterions that surround the aggregate or alternatively by the deformation of aggregates themselves. Further studies are required to assign the definitive molecular mechanism of interfacial polarization in these systems, but it is worth noting that similar interfacial polarizations have been observed in a variety of heterogeneous systems such as aqueous micelles and biopolymers in electrolyte solutions.33−37 The rates obtained by NSE correspond to the time scale of the decay in the correlation responsible for the low-q peak. It is therefore directly related to the motion and lifetimes of the aggregates. The fact that the dielectric relaxation occurs at similar rates indicates that it is the deformation of aggregates that is responsible for the slow, sub-α dielectric relaxation. The emergence of slow, sub-α dynamics in this series of 1alkyl-3-methylimidazolium ILs has two direct and noticeable effects on the physicochemical properties. The first is an increase in the zero-shear viscosity due to the additional contribution from the slower dynamic-mechanical relaxation, as seen at low frequencies in Figure 1b. The second is an increase in the low-frequency, static dielectric permittivity due to contributions from the slow dielectric relaxation, presented in Figure 5. These effects on the transport properties in bulk ILs
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b01516. Experimental details, shear modulus shift factors, highfrequency permittivity, and additional dielectric relaxation rates (PDF)
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AUTHOR INFORMATION
ORCID
Joshua Sangoro: 0000-0002-5483-9528 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Z.V. and J.S. acknowledge support by the National Science Foundation, Division of Materials Research, Polymers Program through DMR-1508394. T.C. gratefully acknowledges financial support of the U.S. Army Research Office under Contract No. W911NF-17-1-0052. The rheology measurements were conducted at the Oak Ridge National Laboratory’s Center for Nanophase Materials Sciences, which is sponsored by the Division of Scientific User Facilities, Office of Basic Energy Sciences, U.S. Department of Energy.
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