Charge Transport and Structural Dynamics in Carboxylic-Acid-Based

Article pubs.acs.org/JPCB

Charge Transport and Structural Dynamics in Carboxylic-Acid-Based Deep Eutectic Mixtures Philip J. Griffin,*,† Tyler Cosby,‡ Adam P. Holt,† Roberto S. Benson,§ and Joshua R. Sangoro*,‡ †

Department of Physics and Astronomy, ‡Department of Chemical and Biomolecular Engineering, and §Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996-1600, United States ABSTRACT: Charge transport and structural dynamics in the 1:2 mol ratio mixture of lidocaine and decanoic acid (LID−DA), a model deep eutectic mixture (DEM), have been characterized over a wide temperature range using broad-band dielectric spectroscopy and depolarized dynamic light scattering. Additionally, Fourier transform infrared spectroscopy measurements were performed to assess the degree of proton transfer between the neutral parent molecules. From our detailed analysis of the dielectric spectra, we have determined that this carboxylic-acid-based DEM is approximately 25% ionic at room temperature. Furthermore, we have found that the characteristic diffusion rate of mobile charge carriers is practically identical to the rate of structural relaxation at all measured temperatures, indicating that fast proton transport does not occur in LID−DA. Our results demonstrate that while LID−DA exhibits the thermal characteristics of a DEM, its charge transport properties resemble those of a protic ionic liquid.

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INTRODUCTION Deep eutectic mixtures (DEMs) are emerging as promising materials for environmentally friendly chemistry,1,2 electrochemical applications,3−5 and also as active pharmaceutical ingredients (APIs).6−8 It is hypothesized that DEMs form due to the presence of hydrogen bonding between the parent compounds, where molecular association reduces the crystal lattice energy and lowers the melting point of the mixture.9,10 Many DEMs appear at first glance to be similar to protic ionic liquids (PILs), which are formed when a Brønsted acid undergoes reversible proton transfer with a Brønsted base to produce an ionized acid−base pair. Depending on the magnitude of free-energy reduction due to proton transfer, these PILs may ionize completely, or they may exist as a mixture of ionized and neutral molecules.7,11 Determining the ionicity in DEMs and PILs is of prime importance because many of the potential applications depend on the degree of proton transfer between the acid and base parent compounds. Numerous APIs, for example, are delivered as salts, and controlling the degree of ionicity in these materials could potentially enhance cell membrane permeation rates.12 In order to be used as efficient electrolytes, PILs should have high electrical conductivity and be nearly 100% ionized.13,14 To this end, many studies have been performed to determine the degree of ionicity in PILs. On the basis of the comparisons of Walden products, Angell and co-workers have qualitatively assessed the ionicity in numerous PILs, establishing the concept of “good” versus “poor” ILs.15 MacFarlane and co-workers, using a combination of indicator tests, infrared spectroscopy, and Walden product analysis, have performed systematic studies of organic proton-transfer mixtures to understand the qualitative degree of ionicity in more detail.7,16 In order to directly quantify the degree of ionicity in PILs and DEMs, © 2014 American Chemical Society

however, it is essential to characterize the microscopic molecular transport properties in these materials, as has been done for aprotic ionic liquids.17−19 In this article, we present experimental studies of charge transport and structural dynamics of the 1:2 mol ratio mixture of lidocaine and decanoic acid (LID−DA) in a broad temperature range as measured via broad-band dielectric spectroscopy (BDS) and depolarized dynamic light scattering (DLS). We have also performed Fourier transform infrared measurements (FTIR) to qualitatively assess the degree of proton transfer between the neutral acid (DA) and base (LID) parent compounds. This mixture was recently shown by Bica et al. to be a DEM, for which they suggested that the elimination of the melting transition is strictly due to hydrogen bonding and not due to proton transfer.6 Our analysis of the FTIR data, as well as our detailed analysis of the dielectric spectra of LID− DA, indicates that this DEM in fact exhibits a measurable degree of proton transfer, with approximately 25% of the DA existing in the ionized state at room temperature. Second, we have found that the rate of proton transport is nearly identical to the rate of molecular reorientation, indicating that a fast proton-transport mechanism does not occur in this liquid. Our results demonstrate that LID−DA is a four-component mixture of neutral and ionized acid−base molecules that can be considered both a PIL as well as a DEM. Furthermore, our results demonstrate a potential new approach to quantify ionicity in other PILs and DEMs. Received: March 28, 2014 Revised: July 10, 2014 Published: July 15, 2014 9378

dx.doi.org/10.1021/jp503105g | J. Phys. Chem. B 2014, 118, 9378−9385

The Journal of Physical Chemistry B

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EXPERIMENTAL SECTION LID powder (MW = 234.34 g/mol) and DA (MW = 172.26 g/ mol, ≥98% purity) were purchased from Sigma-Aldrich and used as received. The chemical structures of these compounds are shown in Figure 1. The 1:2 mol ratio mixture of LID to DA

upon cooling and heating with a temperature ramp of 10 K/ min. Figure 1a shows the DSC thermograms for the LID−DA mixture and the parent compounds upon heating after cooling from 400 K down to 185 K. Figure 1b shows the thermal phase diagram for the LID−DA mixtures for all measured molar compositions. The melting transitions of LID (Tm = 341 K) and DA (Tm = 307 K) were clearly visible in the DSC thermograms, but no crystallization or melting was apparent for the LID−DA sample. Instead, the glass transition was observed at Tg = 207 ± 2 K, as was found in ref 6. FTIR spectra were recorded using a Varian FTS 6000 spectrometer in the wavelength range of 4000 to 400 cm−1 at a scan resolution of 2 cm−1. Measurements were conducted at room temperature in a nitrogen atmosphere. DA and LID samples were measured in KBr pellets for 512 interferometer scans. The LID−DA mixture was liquid at room temperature and was measured between NaCl plates for 1024 interferometer scans. BDS measurements were performed to characterize the dipolar relaxations and charge transport of LID−DA at temperatures ranging from 330 down to 200 K. The measurements were made using the Novocontrol Alpha-A dielectric analyzer in the frequency window of 10−1 Hz to 10 MHz. The sample was measured in a nitrogen atmosphere, and the temperature was controlled using the Novocontrol Quatro system with temperature stability of ±0.1 K. Depolarized photon correlation spectroscopy (DLS) measurements were performed to determine the structural reorientation rates of LID−DA over a broad temperature window (270 K down to Tg). The measurements were carried out in right angle geometry, with laser wavelength = 647 nm and laser power = 125 mW. Vertically polarized laser light was focused on the sample in an Oxford Optistat cryostat (temperature stability of ±0.1 K), and horizontally polarized scattered light was collected with a single-mode optical fiber, split between two photodiode detectors, and cross-correlated using the ALV-7004/FAST multitau digital correlator.

Figure 1. (a) DSC thermograms on heating at 10 K/min of the 1:2 LID-DA mixture, pure LID, and DA. The parent compounds both show melting transitions, while the LID−DA mixture forms a supercooled liquid with a calorimetric glass transition temperature of Tg = 207 ± 2 K. The chemical structures of the parent compounds are also shown. (b) The thermal phase diagram measured via DSC is presented for selected molar compositions of LID and DA mixtures.

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RESULTS AND DISCUSSION Figure 2 presents the room-temperature FTIR spectra of LID, DA, and the 1:2 LID−DA mixture in the whole spectral range as well as in the carbonyl stretching region (1400−1800 cm−1). It is immediately apparent that LID−DA shows a new vibrational mode centered near 1550 cm−1 that is not present in either of the parent compounds. This vibrational mode corresponds to the asymmetric stretching vibration of the carboxylate ion, that is, deprotonated DA.20,21 There is also a substantial signature of the carbonyl stretching mode near 1710 cm−1, and this indicates that the liquid consists of both neutral and ionized acid molecules. It is understandable that we observe clear peaks of both ionized and neutral DA in the IR spectrum of LID−DA because this is not the equimolar mixture, and there is certainly excess acid than what is necessary to ionize all LID. These results indicate that LID−DA might fall into the category of “poor” ionic liquids, much like other mixtures of carboxylic acids and tertiary amines.16 Unlike many other “good” pharmaceutical-based PILs such as LID HCl, however, LID−DA has a relatively low glass transition temperature, which makes it a much more electrically conductive ionic solvent at room temperature.22 The factors that control the degree of ionization in PILs are not well understood,16 and further studies are necessary to make any conclusions regarding this interesting puzzle.

(LID−DA) was prepared by weighing out the necessary amounts of the crystalline parent compounds into a cleaned, dried screw cap vial under a dry nitrogen atmosphere, which was quickly sealed after loading. Almost immediately, we observed partial liquefaction of the sample at room temperature. To ensure that the sample was thoroughly mixed, the LID−DA sealed sample was heated on a hot plate at 75 °C and shaken until there was no visible phase separation remaining in the melt. The 1:2 mol ratio liquid mixture was then cooled to room temperature and passed through a 0.22 μm Teflon filter into several cleaned, dried milliliter sized target vials for further characterization. This procedure was used to prepare mixtures of LID and DA across the entire range of molar compositions. The mass density of the LID−DA sample was determined at room temperature (295 K) by weighing a measured volume of liquid sample five times and averaging the results. The density of LID−DA was measured to be ρ = 0.84 g/mL, similar to the density of pure DA (ρ = 0.89 g/mL). Differential scanning calorimetry (DSC) measurements were performed on all mixtures of LID and DA as well as the two parent compounds using a Q2000 differential scanning calorimeter (TA Instruments). The samples were measured 9379



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range. As shown in Figure 3, the dielectric spectra appear to resemble those of neat aprotic ionic liquids.23−25 This is particularly evident in the ε″ representation where it is seen that dc conductivity dominates the spectra and obscures any apparent dielectric relaxation peak. The dielectric spectra show significant differences, however, when analyzed in further detail. We have analyzed the BDS spectra using a combination of the random barrier model (RBM)26,27 and a single Havriliak− Negami (HN) function. Within this framework ⎛ ⎞ iωτe iωε0ε*(ω) = σ *(ω) = σ0⎜ ⎟ ⎝ ln(1 + iωτe) ⎠ ⎛ ⎞ ΔεHN ⎟ + iωε0⎜⎜ε∞ + α β⎟ (1 + (iωτHN) ) ⎠ ⎝

(1)

with the RBM component accounting for ion motion and the HN component accounting for the dipolar reorientation in the liquid matrix. The RBM describes charge transport in disordered materials in terms of ion hopping that occurs in a random, spatially heterogeneous energy landscape. While this model has been shown to describe the dielectric spectra of many aprotic ionic liquids well,28 the RBM has yet to be used to model the spectra for PILs, such as LID−DA. Using eq 1, we have fit the complex dielectric spectra to extract the dc conductivity σ0 and the characteristic ion hopping rate 1/τe associated with ion conduction, as well as the dielectric strength ΔεHN, characteristic relaxation rate τHN, and shape parameters α,β of the dielectric relaxation process. The dielectric relaxation strength is plotted as a function of inverse temperature in the inset of Figure 4a, and the dc conductivity and the characteristic time scales are shown in Figure 6. Following the method proposed by some of the authors for determining free ion concentration in aprotic ILs, we have estimated the number density of mobile charge carriers in LID−DA as a function of temperature from the analysis of the dielectric data.35 In the case of LID−DA and other PILs, however, the charge carrying units are not permanently charged ions, but instead, they are presumably protonic defects that migrate through the acid−base matrix. Using the Einstein relation, σ0 = 1/kBT(n+D+q2+ + n−D−q2−), it is possible to calculate the number density of protonic defects from the dc conductivity σ0 and the characteristic hopping rate 1/τe from the RBM fits. If we assume that proton motion (ion motion) occurs via Brownian-like hopping as described by the RBM, one may calculate the self-diffusion coefficient of protonic defects such that DH+ = λ2/2τe, where λ is interpreted in this case as the length that a proton must hop in order to ionize an acid−base pair and 1/τe is the mean proton hopping rate. Assuming that DH+ = D+ = D−, nH+ = n+ = n−, and q+ = q− = e (the elementary charge), we have calculated the temperature-dependent number density of protonic defects, such that

Figure 2. (a) FTIR spectra are shown for the 1:2 LID−DA mixture (red line), LID (black line), and DA (blue line). (b) FTIR spectra are shown in the carbonyl stretching region for the 1:2 LID−DA mixture (open circles), LID (black line), and DA (blue line). The LID−DA spectrum was fit with a superposition of five Gaussian functions, with two components (filled) accounting for the ionized and neutral DA vibrations and the remaining components (unfilled) accounting for LID vibrations. It is seen that the LID−DA mixture has a new vibrational mode not found in the parent compounds centered near 1550 cm−1 that corresponds to the carboxylate stretching vibration of ionized carboxylic acid.21

As was previously reported in ref 6, we have also observed the presence of a peak in the FTIR spectrum centered near 1690 cm−1. This peak was hypothesized to be related to the strong hydrogen bonding that occurs between the carboxyl group of DA and the tertiary amine group of LID.6 Because the IR spectrum of the LID−DA mixture shows both neutral and ionized carbonyl/carboxylate moieties, it is likely that a fraction of the neutral DA moieties are participating in hydrogen bonding, which may result in peak splitting of the carbonyl stretching vibration.21 Our results run counter to the assertions made in ref 6, in which the authors claimed that the carboxylate vibrational mode (1550 cm−1) was not present in the FTIR spectra of LID−fatty acid mixtures. It is expected, however, that some degree of proton transfer should occur between DA and LID because ΔpKa ≈ 3, and our current results are in line with this expectation.15,16 In order to understand the microscopic nature of this ionic/ neutral acid−base mixture in more detail, we have used BDS to measure the complex conductivity σ*(f) and dielectric function ε*(f) of LID−DA at several temperatures. This technique provides detailed information regarding the structural (reorientational) dynamics of dipolar molecular moieties as well as ion transport by charged chemical species in a broad frequency

n H+ =

σ0kBTτe λ 2e 2

(2)

The mean hopping length λ used in eq 2 can be determined experimentally by complementary methods such as pulsed field gradient NMR. However, it can alternatively be estimated for the LID−DA sample investigated in the current work using known hydrogen bond lengths in similar materials. Recent NMR experiments of mixtures of LID and DA have demonstrated that the molecular moieties that participate in hydrogen bonding, and thus proton exchange, are primarily the 9380



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Figure 3. Real (a,b) and imaginary (c,d) parts of the complex conductivity σ*( f) (top) and permittivity ε*( f) (bottom) spectrum of 1:2 LID−DA measured at selected temperatures (225−255 K in steps of 5, 265, 275, and 285 K). The spectrum presented as open circles was measured at 225 K, and the red line is the fit of this spectrum using eq 1.

and the dipolar reorientational process are similar, this result indicates that LID−DA is approximately 25% ionized. Interestingly, there is quantitative agreement between the estimates of ionicity from the method outlined in the previous paragraphs as well as simple comparisons of the dielectric amplitudes of the dipolar and RBM processes. The agreement between these two approaches provides support for our assignment of the charge hopping length to the hydrogen bond−covalent bond length difference. Another important distinction between aprotic ILs and the PIL mixture LID−DA can be seen in the temperature dependence of the spectral shape of the complex permittivity. It is well established that for many aprotic ILs, the dielectric spectra obey the frequency−temperature superposition, that is, no changes in spectral shape occur with variation of temperature.29 This is because the mean ion jump rate as well as the dc conductivity have practically identical temperature dependences and the underlying mechanism of ion conduction (ion hopping within the framework of the RBM) does not change with temperature. LID−DA does not, however, exhibit frequency−temperature superposition, and it is observed in the ε′ data (Figure 3b) that the spectra change significantly as the temperature is decreased toward Tg. These changes are more readily observed in the ε′ derivative spectrum (ε′der. = −(π/2)(dε′/d(ln f)) shown in Figure 4b, where the spectrum is clearly seen to broaden substantially as Tg is approached. While the reasons for this strong spectral broadening are not yet well understood, the multicomponent nature of LID−DA may account for the apparent spectral broadening, similar to what has been observed for miscible polymer blends.30,31 Additionally, it is possible that as the temperature decreases, hydrophobic aggregation of the DA alkyl tails causes the formation of nanoscale insulating moieties, which lead to a Maxwell−Wagner−Sillars (MWS) interfacial polarization in the liquid matrix.32 This process would contribute to the dielectric

tertiary amine nitrogen of LID (acceptor) and the carboxyl hydrogen of DA (donor).6 It is known that the mean hydrogen bond length (H···N distance) for the O−H···N complex is approximately 2 Å.36 In order for the ionization of the acid− base pair to occur, the covalent O−H bond must be cleaved and replaced with the ionized H−N+ bond. When this occurs, the H atom must jump a distance d(H···N) − d(H−N+) ≈ 1 Å. Charge migration occurs when a proton jumps this distance, and it is our assumption that this process contributes to the dielectric function in the form described by the RBM. It should be noted that the calculation above is approximate, and experiments using pulsed field gradient NMR are necessary to verify our assessment of the mean hopping length. With the jump length determined, we have calculated the temperature-dependent number density of protonic defects in LID−DA using eq 2, and the result is shown in the inset of Figure 6. Taking this result one step further, we have calculated the mole fraction of protonic defects (which is equal to the mole fraction of deprotonated DA) using the measurements of the mass density of LID−DA at room temperature (295 K). The number density of total potentially ionizable protons was calculated to be nH = 1.75 × 1027 m−3, under the assumption that protons can be donated only by the DA carboxyl group. From this and the results of eq 2, the mole percent of protonic defects is determined to be approximately 23% at room temperature. The detailed fitting of eq 1 to the real part of the permittivity spectrum at 225 K (≈Tg + 15 K) is shown in Figure 4a. Unlike many aprotic ionic liquids, a majority of the spectral weight in the dielectric spectrum is contributed by the dipolar reorientation process, while a much smaller fraction is contributed by the ion hopping process. The Δε of the RBM contribution, that is, the ionic component of the permittivity spectrum, comprises approximately 25% of the cumulative dielectric strength at all measured temperatures. Assuming that the dipole moments associated with the ion hopping process 9381



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Figure 5. (a) ICFs measured in 1:2 LID−DA by DLS. From left to right, the ICFs were measured from 270 through 240 K in steps of 10 and 235 K and through 215 K in steps of 5 and 212 K. The ICF measured at 220 K is presented as open circles, and the red line is the fit using eq 3. (b) The field correlation function g1(t) (FCF) measured at 220 K (open circles) is presented with the fitting of the data to eq 3 (red solid line). The FCF is composed of a superposition of a faster process (blue dashed−dotted line) and a slower process (green dashed line) that is connected to the calorimetric glass transition of LID−DA.

Figure 4. (a) Real permittivity ε′ measured at 225 K (open circles). The red solid line is the cumulative fit using eq 1, the blue dashed line is the RBM contribution, and the green dashed−dotted line is the dipolar (HN) contribution to the spectrum. The inset shows the dielectric strength of the RBM (blue squares) process and the dipolar process (green circles). (b) The ε′ derivative spectra are shown for all measurement temperatures (except 285 K) depicted in Figure 3. Each spectrum has been scaled by the corresponding peak maximum. As the temperature of LID−DA decreases from 275 (orange line) to 225 K (black line), the spectrum broadens strongly and symmetrically.

In order to extract the characteristic time scales associated with the two relaxation processes, we have fit the ICFs with a superposition of two Kohlrausch−Williams−Watts (KWW) stretched exponential functions ICF = g 2(t ) − 1

spectrum on the low-frequency side of the charge-transport process and could lead to an apparent broadening. The presence of strong hydrophobic aggregation has been observed in aprotic quaternary ammonium ILs with large aliphatic groups, and it was shown that these regions could lead to a MWS contribution to the dielectric spectra.19,33,34 It is feasible that this contribution is present in LID−DA as well, but significant overlap of spectral features and the already complex form of the spectra (dipolar + ionic) prohibit further definitive analysis. Having assessed the ionicity of the LID−DA mixture via FTIR and BDS, the next question to address is how the characteristic rate of charge transport relates to structural relaxation in LID−DA. To understand this relationship in detail, we have measured the characteristic molecular reorientation times using DLS. The normalized intensity correlation functions (ICFs) measured via DLS at several temperatures for LID−DA are shown in Figure 5a. While it is not immediately evident from a visual inspection of the data, these ICFs are comprised of two superposed decays at all measured temperatures, as is demonstrated in the fitting of the field correlation representation in Figure 5b. The multistep decay of these correlation functions was confirmed by analyzing the derivative of the ICF, and the details of this analysis technique can be found elsewhere.37

= γ g 1(t )|2 ⎛ ⎛ ⎞ β2 ⎞ ⎛ ⎛ ⎞ β1⎞ t ⎟ t ⎜ = γ a1 exp⎜ −⎜ ⎟ ⎟ + a 2 exp⎜⎜ −⎜ ⎟ ⎟⎟ ⎝ ⎝ τ2 ⎠ ⎠ ⎝ ⎝ τ1 ⎠ ⎠

2

(3)

where γ is the coherence factor of the optical system, a1 and a2 are the relative relaxation strengths, τ1 and τ2 are the characteristic relaxation times, and β1 and β2 are the nonexponentiality (KWW) parameters of the faster (1) and slower (2) decay processes, respectively.38 Figure 5 depicts the ICF, as well as the field correlation function g1(t), measured at 220 K and fit using eq 3, and the relative contributions of the fast and slow relaxation processes are illustrated in Figure 5b. The characteristic relaxation times determined from these fits are plotted in Figure 6, and the stretching parameters were found to be temperature-independent at all measured temperatures, where βfast ≈ 0.35 and βslow ≈ 0.60. In order to determine which relaxation process is associated with the calorimetric glass transition, the relaxation time data were first fit with a Vogel−Fulcher−Tammann (VFT) function, τ = τ0 exp(A/(T − T0)), where τ0, A, and T0 are fit parameters,39 and then the apparent dynamic glass transition temperature (at τ = 100 s) was calculated for both the fast (Tg app. = 199 K) and slow processes (Tg app. = 205 K). Because 9382



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ments are necessary to establish any possible generalities of proton transport in these materials. It is quite surprising and interesting that the number density of protonic defects in LID−DA is temperature-independent over the entire measured temperature range, as seen in the inset of Figure 6. In proton-conducting liquids, charge transport occurs at the elementary level of the acid−base reaction and the proton hopping process. While the diffusion rate of protonic defects is apparently controlled by the rate of molecular reorientation, as is demonstrated in Figure 6, the exchange of protons across the hydrogen bond is known to occur rapidly.47 The effect of these competing time scales is that the exchange of protons between acid−base pairs can achieve equilibrium during structural rearrangement events. The state of this equilibrium depends more on the local electronic molecular properties48 and much less on thermally activated motion. Because these molecular properties are relatively insensitive to temperature changes, it follows that the number density (at the measured temperatures) of protonic defects in LID−DA should also be relatively insensitive to temperature, as we observe from analysis of the dielectric spectra. This is different from the case of aprotic ionic liquids, where the number density of free ions decreases with decreasing temperature with an Arrhenius type of thermal activation.18,19 Further studies are being pursued to understand this interesting distinction between aprotic ILs and PILs in more detail.

Figure 6. The characteristic time scales of charge transport and structural dynamics measured by DLS and BDS as well as the dc conductivity of 1:2 LID−DA are plotted against inverse temperature. The solid lines are fits using the Vogel−Fulcher−Tammann equation. The inset depicts the number density of protonic defects as a function of inverse temperature calculated using eq 2.

the apparent Tg associated with the slow process is nearly identical to the calorimetric glass transition temperature (Tg = 207 K), we assign this feature to the structural relaxation process. We speculate that the fast process in the ICF is connected to fluctuations of the more flexible alkyl tails of the DA, while the slow process is connected to fluctuations of the phenyl rings of the LID molecule. We conjecture that the flexibility of the alkyl chains allows for faster motions relative to the rotation of phenyl rings, which are rigid and require a concerted motion of the entire LID molecule. It should be noted that the DLS spectra of LID−DA differ substantially when compared to recent measurements of 2ethyl-4-methylimidazole, an intrinsic proton conductor, where a two-step DLS spectrum was also observed.37 In that liquid, however, the faster process was found to be related to structural relaxation, while the slow process (which was Debye-like) was demonstrated to be connected to the coherent motion of supramolecular chains comprised of 3−4 molecules. In LID− DA, we find no evidence of a Debye-like process associated with supramolecular structures, which have been suggested to be necessary to support a fast transport mechanism in protonconducting liquids.40 One of the key questions related to proton transport is whether or not a “super-protonic” transport mechanism exists such that protonic defects migrate much more rapidly than the host molecules can execute structural diffusion.41,42 This process is reminiscent of the Grotthuss mechanism in water,43 and it was recently suggested that fast proton transport may occur in phosphoric acid,44 as well as some pharmaceutical PILs.45 As is evident in Figure 6, the proton hopping rate of LID−DA is nearly identical to the rate of structural reorientation measured via DLS. The type of proton conduction mechanism observed for LID−DA is reminiscent of the so-called “vehicle mechanism”, in which protonic defects diffuse as passengers on the parent base molecule.46 It is clearly evident that fast proton transport does not occur in this PIL, which is in stark contrast to recent results reported for pharmaceutical-based PILs from analysis of the peak in the electrical loss modulus M″.45 It is worth noting that the mechanism of proton transport in LID−DA is different from that observed in water and phosphoric acid. Further experi-

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CONCLUSION The charge transport and structural dynamics in the 1:2 mol ratio DEM of LID and DA have been measured over a broad temperature range using BDS and depolarized DLS. Roomtemperature FTIR measurements were also performed to assess the ionicity of the mixture and the effects of mixing on the molecular vibrations of the parent compounds. Our analysis of the dielectric spectra demonstrates that while this mixture shows modest dc conductivity at higher temperatures, indicating some degree of proton transfer, the fraction of ionic species in this melt is relatively low, with approximately 25% ionization at 295 K. The degree of ionicity found from the dielectric data is corroborated by the FTIR spectrum of the mixture, in which a vibrational mode associated with anionic carboxylate has been observed that is not present in the parent compounds. Furthermore, we have found that the characteristic diffusion rate of free charge is nearly identical to the structural relaxation rate at all measured temperatures, which indicates that proton transport in this material is directly coupled to structural relaxation. Our results show that this liquid consists of a four-component mixture of primarily neutral acid and base with a modest addition of ionized parent compounds, and it straddles the boundary between “deep eutectic” and “PIL”.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.R.S.). *E-mail: pgriffi[email protected] (P.J.G.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.R.S. and T.C. thank the University of TennesseeKnoxville for financial support. We also thank A. P. Sokolov for granting us access to the dynamic light scattering system in his 9383



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laboratory. P.J.G. and A.P.H. gratefully acknowledge financial support from the NSF Chemistry program (Grant CHE1213444).

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