Article pubs.acs.org/JPCB
Macroscopic and Microscopic Study of 1‑Ethyl-3-methyl-imidazolium Acetate−DMSO Mixtures Asanah Radhi,† Kim Anh Le,‡ Michael E. Ries,† and Tatiana Budtova*,‡ †
Soft Matter Physics Research Group, School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, United Kingdom Mines ParisTech, Centre de Mise en Forme des Matériaux (CEMEF), UMR CNRS 7635, BP 207, 06904 Sophia Antipolis, France
‡
S Supporting Information *
ABSTRACT: Macroscopic (steady-state viscosity, density) and microscopic (NMR chemical shifts, 1H NMR relaxation times, and diffusion) properties of the 1-ethyl-3-methylimidazolium acetate ([EMIM][OAc])−dimethyl sulfoxide (DMSO) mixture were studied in detail as a function of DMSO molar fraction at various temperatures. Temperature dependencies were used to calculate the activation energies. NMR results indicate that at low molar fraction of DMSO (0.8) the peak then moved upfield. The other proton resonances in the [EMIM][OAc]−DMSO mixtures were instead consistently upfield (H4 and H5) or consistently downfield (H6, H7, H8, and the acetate ion) across the entire range of compositions. The mixture of DMSO with 1-butyl-3-methylimidazolium hexafluorophosphate also showed the lowest deviation of viscosity, density, speed of sound, and refractive index from Redlich− Kister prediction as compared to tetrahydrofuran, methanol, and acetonitrile.25 The goal of this work is to perform a comprehensive study of macroscopic and microscopic properties of [EMIM][OAc]− DMSO mixture, in view of the potential use of this binary solvent for cellulose processing and chemical modifications. The evaluation of thermophysical properties of binary mixtures of DMSO with two families of ILs, tetra-alkyl ammonium cation with hydroxide anion and 1-alkyl-3-methyl imidazolium cation with different anions (chloride, methyl sulfate, and tetrafluoroborate), showed that these properties depend, on one hand, on alkyl chain length of the cation of ammoniumbased ILs and, on the other hand, on the type of anion of imidazolium-based ILs.26 As far as most of the studies of ILs blends with DMSO were performed for 1-alkyl-3-methylimidazolium chloride ILs, a full characterization of [EMIM][OAc]−DMSO is needed. We took the same approach as was already used for the understanding of [EMIM][OAc]−water mixtures:19 we studied their macroscopic properties by analyzing density and viscosity, and we used NMR spectroscopy, relaxometry, and diffusion to investigate system microscopic properties. Each parameter obtained is analyzed as a function of temperature and mixture composition. We conclude on the interactions between the components, ions and DMSO, by comparing experimentally obtained characteristics with the ones predicted by the ideal mixing law.
atmospheric chamber under nitrogen, providing a dry environment, with the chamber being maintained at a dew point level between −70 and −40 °C with this corresponding to water levels of less than 1 ppm. Ready mixtures were transferred to 5 mm NMR tubes that had been dried in the oven at 50 °C prior to use. The tubes were subsequently sealed while still remaining within the chamber. At each temperature, the samples were left to equilibrate for 10 min before a measurement was taken. Density. The density of [EMIM][OAc] and DMSO and of their mixtures was measured using a pycnometer. All of the measurements were carried out at 25 °C. Viscosity. The Bohlin Gemini Advanced rheometer equipped with 4°−40 mm cone−plate was used to measure the viscosity of the neat components and of their mixtures. Steady-state viscosity was measured at temperatures from 20 to 90 °C with 10 °C increments. A thin film of low viscosity silicon oil was placed around the borders of the measuring cell to prevent moisture uptake. NMR Spectra. The 1H spectra were recorded using a simple single 90° pulse sequence on a Bruker Avance II 400 MHz spectrometer. In this work, the proton resonance 5, see Figure 1, was used as a reference point; the chemical shift δ of the
Figure 1. Chemical structure of [EMIM]+ and [Ac]− ions of 1-ethyl-3methyl-imidazolium acetate with the resonances (1−7) labeled.
other resonances was determined via their distances from this peak. This method follows several other 1H NMR studies on imidazolium-based ILs where the chemical shift of the methyl group corresponding to this peak has been shown to be independent of extrinsic variables: IL concentration in water/1alkyl-3-methylimidazolium bromide solutions; and cellobiose concentrations upon solvation in [EMIM][OAc].27,28 To calculate the change in position of the peaks Δδ upon the addition of DMSO, we used the δ for each resonance in the sample containing 0.5% by weight DMSO as a starting reference value, so that Δδ corresponds to the change in parts per million (ppm) of a peak from this starting mixture. NMR Relaxometry. The spin−lattice relaxation time T1 and spin−spin relaxation time T2 were measured for each mixture using a 20 MHz Maran Benchtop NMR spectrometer. The measurements of T1 and T2 were performed using inversion recovery and Carr−Purcell−Meiboom−Gill pulse sequences, respectively.29,30 Measurements were made from 20 to 80 °C inclusive in steps of 10 °C. NMR Diffusion. DMSO, anion, and cation diffusion coefficients in each mixture were determined with a pulsedfield gradient (PFG) technique using a Bruker Avance II 400 MHz spectrometer with a diffusion probe (Diff60) capable of producing field gradients of up to 24 T m−1 and temperature control of ±0.1 °C. The measurements were taken using a stimulated echo pulse sequence with bipolar gradients.31 The calibration of the gradient field strength was performed by measuring the self-diffusion coefficient of water at 20.0 ± 0.1 °C with subsequent calibration of sample temperature according to the temperature dependence of the diffusion coefficient of water.32 The recommendations set out by Annat et al.31 were followed: for example, sample depths were kept to
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EXPERIMENTAL SECTION Materials and Sample Preparation. The [EMIM][OAc] (97% purity) and DMSO (99.9% purity) were purchased from Sigma-Aldrich and were used without further purification. They were mixed in various proportions from 0 to 100 wt % DMSO. Mixing was performed for 2 h in an MBraun Labmaster 130 1634
DOI: 10.1021/jp5112108 J. Phys. Chem. B 2015, 119, 1633−1640
Article
The Journal of Physical Chemistry B
rates. Mean plateau values were taken and used to plot viscosity as a function of DMSO molar fraction for temperatures ranging from 20 to 100 °C; the representative examples are given in Figure 3. The figure also shows viscosity ηcalc calculated
less than 1 cm to minimize convection currents on heating in the NMR spectrometer. The uncertainty in our diffusion coefficient values is approximately 3%. The attenuation of the signal intensity in this PFG NMR experiment is described as follows:33 ln(Si /Si0) = − Diγ 2g 2δ 2(Δ − δ/3 − τ/2)
(1)
where Si is the measured signal intensity of species i and Di is the self-diffusion coefficient of that species, Si0 defines the initial signal intensity, γ is the proton gyromagnetic ratio, δ is the pulse duration of a combined pair of bipolar pulses, τ is the period between bipolar gradients, Δ is the period separating the beginning of each pulse-pair (i.e., diffusion time), and g is the gradient strength. In each experiment, the strength of the gradient pulse was incremented, while δ (2−5 ms), Δ (60 ms), and τ (2 ms) were kept constant. The 90° pulse width was 6.6 μs, g had maximum values between 200 and 600 G/cm, the number of scans was 16, and the repetition time was 6 s. Measurements were made from 10 to 60 °C in steps of 10 °C. To further reduce convection within the samples, the gradient coil itself was set to the same temperature as that of the air flow (up to a maximum of 40 °C), so as to minimize any temperature gradients across the NMR tube.
Figure 3. Viscosity versus DMSO mole fraction at T = 20, 40, and 90 °C. Lines are viscosities calculated according to eq 3 for the corresponding temperatures.
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RESULTS Density. Figure 2 shows the density of [EMIM][OAc]− DMSO mixtures at 298 K as a function of mole fraction of
according to the ideal mixing rule developed for binary mixtures:40−42 i=2
ln ηcalc =
∑ fi i
DMSO, f DMSO. The densities of both neat [EMIM][OAc] and DMSO are in a good agreement with the published data.34−39 The experimental values were compared to density ρcalc calculated according to the volume additive mixing rule: i=2
∑ fi i
1 ρi
(3)
where f i is a molar fraction of each component and ηi is the viscosity of the neat component i of the mixture. The addition of a low viscosity component, DMSO, to a rather viscous ionic liquid results in a significant decrease of mixture viscosity, as expected. However, the experimental values are slighly higher than the ones calculated according to the mixing rule (eq 3). It was reported that DMSO may disrupt the clusters that are formed within the ionic liquid.21,43,44 A small deviation of experimental viscosity from the theoretical mixing rule is an indication of weak interactions between the components in the system. This result will be discussed further together with chemical shifts, relaxometry, and diffusion coefficients obtained with NMR. Temperature dependence of viscosity at various mixure compositions is shown in Figure 4 as an Arrhenius plot. Neat [EMIM][OAc] shows a slightly concave dependence. This trend for imidazolium-based ionic liquids was already reported for cellulose−[EMIM][OAc] and cellulose−[BMIM][Cl] solutions.35,45 It was demonstrated that the deviation from the linear dependence diminished with the increase of cellulose concentration; Figure 4 shows that it also reduces with the increase of DMSO content. The activation energy of the viscous flow as a function DMSO concentration will be analyzed together with NMR results in the Discussion. NMR Chemical Shift. The changes of proton chemical shift Δδ for each proton resonance in the [EMIM][OAc] and DMSO mixtures as a function of DMSO mole fraction f DMSO are shown in Figure 5, and an example of the 1H spectrum for f DMSO = 0.59 in shown in Figure S1 of the Supporting Information. The Δδ measured here are small (as compared to results obtained for mixtures of [EMIM][OAc]−water18), indicating significantly weaker interactions between [EMIM][OAc] and DMSO than those of [EMIM][OAc] and water. In
Figure 2. Density of [EMIM][Ac]−DMSO mixtures as a function of molar fraction of DMSO, at 25 °C. The line corresponds to the ideal mixing law given by eq 2.
1 = ρcalc
× ln ηi
(2)
where f i is a molar fraction of each component in the mixture and ρi is the density of the neat component. The deviation of the experimental values from the additive mixing law prediction is within the experimental errors and less than 1% (Figure 2). Viscosity. The viscosity η for [EMIM][OAc]−DMSO system at various temperatures T and DMSO concentrations was measured as a function of shear rate. For all mixtures, a Newtonian plateau was found for at least 1−2 decades of shear 1635
DOI: 10.1021/jp5112108 J. Phys. Chem. B 2015, 119, 1633−1640
Article
The Journal of Physical Chemistry B
DMSO with the cation at low concentrations of DMSO could explain the increased solubility of cellulose that has been previously reported47 in the [EMIM][OAc]−DMSO solvent system. DMSO by associating with the cation “frees up” more anions, which can then dissolve more carbohydrate molecules. NMR Relaxometry. Relaxation times T1 and T2 were measured for various mixture compositions and temperatures from 20 to 80 °C using low field 1H NMR. In this study, T1 and T2 are equal, within the 5% experimental uncertainty, and thus only T1 will be considered. This means that the data are in the high temperature limit with motion occurring on a time scale much shorter than that set by the inverse of the NMR resonance frequency of 20 MHz. It also indicates that rotational motion, as opposed to translational, dominates the NMR response.48 As shown in ref 19, the molecular rotational correlation time τrot follows:
Figure 4. Arrhenius plot of viscosity−temperature dependence for [EMIM][OAc], DMSO, and three mixture compositions. Solid lines correspond to linear approximations.
τrot ∝
η 1 ∝ T1 T
(4)
The ideal mixing rule for the relaxation time T1,calc at a fixed temperature can thus be written, following eq 3, as ln
1 T1,calc
i=2
=
∑ fi ln i
1 T1, i
(5)
where T1,i are the relaxation times of the neat components. Experimental data for ln(1/T1) of [EMIM][OAc]−DMSO mixtures together with the mixing rule prediction are shown as a function of mixture composition in Figure 6.
Figure 5. Δδ as a function of DMSO mole fraction, at 20 °C. The solid lines are guides to the eye. The uncertainty in the ppm is approximately 0.05.
[EMIM][OAc]−water, the imidazolium peak 2 shows Δδ ≈ −2.0 ppm movement,19 whereas in [EMIM][OAc]−DMSO it has only Δδ ≈ −0.4 ppm. Our results agree well with those previously published by Chen et al.24 Typically the addition of another solvent to [EMIM][OAc] causes peak 2, the most acidic peak on the imidazolium ring, to display the largest Δδ.19,24,46 Here, this is not the case as peaks 1 and 3 move the most and do so in an upfield sense. At low mole fraction of DMSO (
