The microheterogeneity in ionic liquid mixtures: hydrogen bonding, dispersed
ions and dispersed ion clusters
Andrea Mezzetta, Maria J. Rodriguez Douton, Lorenzo Guazzelli, Christian Silvio Pomelli, Cinzia Chiappe*
Dipartimento di Farmacia, Università di Pisa, via Bonanno 33, 56126 Pisa
Abstract
Mixtures of ionic liquids (ILs) having a common ion, but differing in the identity of the anion or cation, represent highly interesting media. By varying the composition, one can fruitfully modulate specific physicochemical, structural, and biological properties. The molecular interactions (coulombic, hydrogen-bonding, van der Waals and π–π intermolecular forces) that determine the three-dimensional structure of pure ILs can indeed be modified by the addition of another IL. In this context, we present here a 1H NMR, FT-IR, thermogravimetric and solvatochromic study of the
structural features of ILs binary mixtures based on a common imidazolium cation ([CnC1im]+) and
anions of different size and hydrogen bond acceptor ability. For each mixture, the analyses were carried out at different molar ratios of the two components.
Ionic liquids (ILs), solvents that comprise entirely of ions, have been the object of an increasing interest in the last twenty years, attracting many researchers to investigate their unique properties by means of experiments, theory and molecular simulation methods.i The physicochemical and
structural properties of ionic liquids characterized by a single cation and anion have been largely investigated showing that they are governed by electrostatic interactions between the charged groups on the ions, by dispersion interactions between the nonpolar regions, and by hydrogen bonding.ii These interactions determine the bulk and the surface structure of ILs in the liquid state
and interesting phenomena of charge ordering and nanoscale segregation, depending on cation anion structure, have been described several times.iii,iv,v Nonetheless, one of the most important
features of ILs is their tunability, which can be achieved by modifying the identity of the ions or introducing specific functional groups on the cation or anion.vi However, the mixing of two ILs with
a common ion but differing in the identity of the anion or cation (or changing both components) can potentially provide a way to fine-tuning their desired physicochemical, structural, and biological properties.vii It is indeed well known that also the presence of residual amounts of chloride or
density amongst others) of ILs obtained by metathesis reactions. Recently, the interest of several research groups has been devoted to such IL mixtures. Thermodynamic, transport and other physicochemical properties (excess volume and enthalpy, density, viscosity, conductivity, diffusivity, surface tension, and refractive index) have been determined for several mixtures, which have been also studied through spectroscopic measurements. viii Two relevant reviews have been
published few years ago by Welton and co-workers and Rogers and co-workers.ix For most
physicochemical properties, it has been reported that IL mixtures present an approximating predictable or “simple” mixing behavior, i.e. they show a quasi-linear behavior with respect to the ratio between the two pure ILs. To explain this peculiarity, Welton and co-workers proposed that the ILs mixtures probably have random distributions of cations and anions and therefore are close to ideal mixtures.x However, non-linear (or non-ideal) behaviors have been also reported, in particular
for mixtures of at least two very different constituents and non-random microscopic distributions have been claimed for these systems. xi Recently, Fillion and Brennecke reported that some binary
ionic liquids can have a non-ideal behavior yielding a maximum in viscosity: a feature that has been attributed to the presence of attractive interactions, in addition to nonrandom mixing of the ions.xii
Indeed, although IL mixtures are often described as systems dominated by the random distribution of ions driven by Coulombic interactions, analogously to the situation of pure ILs, the amphiphilic nature of many IL ions can result in nano-segregation of the liquid into dynamic domains. These phenomena can arise from the interplay among Coulombic, hydrogen-bonding and dispersive (van der Waals and π–π) intermolecular forces, which are always present within these complex fluids. On the other hand, the preferential interaction of the cations for a specific anion can also be present within an essentially random distribution of cations and anions: it has been suggested that the latter occurrence can arise from subtle perturbations of the cation orientation within the ionic framework. Non-linear behaviors observed for some ideal IL mixtures when investigated by NMR have been attributed to phenomena of this kind.xiii
The impact of mixing two ILs on their solution structure, and consequently on the physico-chemical properties of the resulting systems, surely requires further studies. It has often been stressed that a large range of mixtures, properties and techniques have to be used to investigate these complex liquids. Some IL mixtures that display an ideal mixing behavior in some specific physical properties, exhibit on the other hand significantly non-linear behavior in others or, even more intriguing, the same properties can follow a different behavior depending on the used technique for the study in question.xiv For example, the effect of H-bonding interactions in imidazolium-based IL
mixtures can be examined by NMR and IR experiments. In some cases, it has been evidenced the prevalence of specific interactions of the imidazolium ring protons with selected anions through
NMR experiments, although practically linear behavior arose for the same ILs mixtures from IR data.13,14
Here, in order to obtain further information about the structural features that can affect the properties of IL mixtures, four different IL mixtures based on an imidazolium ([CnC1im]+) cation
and two anions of different size and having significantly different hydrogen bond acceptor ability were studied experimentally using NMR and IR. Furthermore, thermogravimetric analyses and solvatochromic measurements were performed on some selected mixtures.
Results and Discussion 1H NMR and IR spectra
The 1H NMR and IR spectra of the neat ILs ([C
4C1im]Cl, [C4C1im]Br, [C4C1im][PF6], [C4C1im]
[Tf2N], [C8C1im]Cl and [C8C1im][Tf2N]) and their mixtures [C4C1im]Clx[Tf2N](1-x),
[C4C1im]Brx[Tf2N](1-x), [C4C1im]Clx[PF6](1-x) and [C8C1im]Clx[Tf2N](1-x), previously accurately dried
at 70 °C for 24 h, were registered at room temperature: [C4C1im]Cl and [C4C1im]Br were used
under supercooling conditions for the measurements carried out on pure ILs. Furthermore, co-axial tubes, accurately closed after filling and containing DMSO-d6 as a lock and chemical shift reference
in the internal capillary, were employed to obtain the NMR spectra.
Scheme 1. Investigated ionic liquids as binary mixtures
As previously reported,13 only a single peak can be observed in the NMR spectra for each of the
three non-equivalent imidazolium protons of the two salts, due to the fact that although the chemical shift of these protons is affected by the strength of the anion-cation interaction the anion exchange is fast on the NMR timescale (Supplementary Material). Nonetheless, when the measured NMR chemical shifts where plotted against the binary mixtures composition deviations from linearity were observed, in particular when the imidazolium proton at C(2) (which preferentially interacts with the counteranion) is taken into account and the two ILs constituting the mixtures were
characterized by a strong difference in the cation-anion interaction strength.xv In figures 1-3, the
H(2) chemical shifts for the investigated binary systems have been plotted versus the mixtures composition. The related 1H NMR chemical shifts are provided in Supporting Material.
0.0 0.3 0.5 0.8 1.0 8.0 8.2 8.4 8.6 8.8 9.0 9.2 9.4 9.6 9.8 10.0 [C4C1im]X, c C (2 )-H C h e m ic a l s h ift ( p p m )
Figure 1. 1H NMR chemical shift for C(2)-H obtained for the mixtures [C
4C1im]Clx[Tf2N](1-x), red,
and [C4C1im]Brx[Tf2N](1-x), black.
0 0.25 0.5 0.75 1 7.6 7.8 7.9 8.1 8.2 8.4 8.6 8.7 8.9 9.0 9.2 9.4 9.5 9.7 9.8 10.0 [C4C1im]Cl, c C (2 )-H C h e m ic a l s h ift ( p p m )
Figure 2. 1H NMR chemical shift for C(2)-H obtained for the mixtures [C
4C1im]Clx[PF6](1-x).
Figure 3. 1H NMR chemical shift for C(2)-H obtained for the mixtures ([C
0.0 0.3 0.5 0.8 1.0 8 8.22 8.44 8.66 8.88 9.1 9.32 9.54 9.76 9.98 10.2 [C8C1im]Cl, c
C
(2
)-H
C
h
em
ic
al
s
h
if
(
p
p
m
)
The concentration of the strongest H-bond accepting anion in the mixture (chloride or bromide), expressed as molar fraction, is reported on the x-axis in all graphs to enable a more facile comparison between mixtures and previously reported data for analogous systems.13 For the mixture
[C4C1im]Clx[Tf2N](1-x), the sole already investigated, the here obtained values are practically
identical to those previously reported by Welton et al..13
As expected, the chemical shifts for the resonance of the imidazolium protons of these mixtures lie within the range defined by the IL components and the increase in the strength of the H-bonding interactions is associated with a downfield shift of the H-bond donating proton, corresponding to an increased chemical shift. In all examined mixtures, moreover, a deviation of the C(2)-H chemical shift above the line joining the chemical shifts of the pure ILs can be observed. The extent of this deviation depends on the hydrogen bond basicity of two anions, thus resulting moderate in the case of [C4C1im]Clx[PF6](1-x) but significantly higher for the [C4C1im]Clx[Tf2N](1-x) mixture. The deviation
of the chemical shift above the line joining the chemical shift of the pure ILs is generally attributed13,14,xvi to the preferential H-bonding between the more acidic cation hydrogens (C(2)-H for
imidazolium salts) and the stronger H-bond acceptor, which in our mixtures is chloride or bromide, whereas C(4)-H and C(5)-H possibly show a slight preference for the more weakly H-bonding anion (data reported in Supplementary Material).
Interestingly, the C(2)-H chemical shifts of the [C4C1im]Clx[Tf2N](1-x) and [C8C1im]Clx[Tf2N](1-x)
mixtures present a quite similar behavior although the longer alkyl chain on the imidazolium cation in [C8C1im]Clx[Tf2N](1-x), which favors the formation of polar and nonpolar domains in the IL bulk
with respect the butyl group, could modify the interplay among Coulombic, dispersive and hydrogen-bonding intermolecular forces.
Finally, it is worth remarking that in the first part of the graph, generally not investigated in detail in the previously reported papers on this topic,13,14,16 the four [C
4C1im]+-based ILs, show a clear linear
behavior (Table 1, Figure 4, added imidazolium halides are reported in %w/w).
Table 1. Parameters related to the linear behavior of mixtures reported in Figure 4.1
Mixture a 0 r Notes
[C4C1im]Clx[Tf2N](1-x) 0.0644 8.014 0.997 [C4C1im][Tf2N] arising from
[C4C1im]Cl
[C4C1im]Clx[Tf2N](1-x) 0.0653 8.010 0.9985 [C4C1im][Tf2N] arising from
[C4C1im]Br
[C4C1im]Brx[Tf2N](1-x) 0.0396 x 8.006 0.998 [C4C1im][Tf2N] arising from
[C4C1im]Cl
[C4C1im]Brx[Tf2N](1-x) 0.0395 8.010 0.995 [C4C1im][Tf2N] arising from
[C4C1im]Br
[C4C1im]Clx[PF6](1-x) 0.0529 7.732 0.998 [C4C1im][PF6] arising from
[C4C1im]Cl
[C4C1im]Clx[PF6](1-x) 0.0528 7.735 0.998 [C4C1im][PF6] arising from
[C4C1im]Br
[C4C1im]Brx[PF6](1-x) 0.0299 7.734 0.998 [C4C1im][PF6] arising from
[C4C1im]Cl
[C4C1im]Brx[PF6](1-x) 0.0302 7.735 0.995 [C4C1im][PF6] arising from
[C4C1im]Br 1 Fitting equation: = a C
[C4C1im]X + 0 . Concentration of added halide, C[C4C1im]X = %w/w
This linearity of this region is quite relevant: from an applicative point of view, it could be indeed exploited to evaluate the presence of residual halide in “pure” ILs. This scenario is of course relevant when the amount is not negligible, a situation however not so unusual. Related to this aspect, it is noteworthy that the starting chemical shift values obtained adding [C4C1im]Cl or
[C4C1im]Br at [C4C1im][Tf2N] or [C4C1im][PF6], these latter ones arising either from the
corresponding bromide or chloride, are practically identical. This behavior indicates that the possible residual amount of the starting halide was really negligible and surely unable to significantly affect the experiments.
0.0 2.8 5.5 8.3 11.0 7.7 7.9 8.1 8.3 8.6 8.8 %w/w, [C4C1m] X/[C4C1im][Tf2N] C (2 )-H C h e m ic a l sh if t, p p m 0.0 2.8 5.5 8.3 11.0 7.4 7.5 7.7 7.9 8.1 8.2 8.4 %w/w, [C4C1m] X/[C4C1im][PF6] C (2 )-H C h e m ic a l sh if t, p p m
Figure 4. Upper: 1H NMR chemical shift for C(2)-H obtained for the mixtures [C
4C1im]Cl/[C4C1im]
[Tf2N] (using a [C4C1im] [Tf2N] arising from[C4C1im]Cl or [C4C1im]Br) and[C4C1im]Br/[C4C1im]
[Tf2N] using a [C4C1im] [Tf2N] arising from[C4C1im]Cl or [C4C1im]Br. Lower: 1H NMR chemical
shift for C(2)-H obtained for the mixtures [C4C1im]Cl/[C4C1im][PF6] (using a [C4C1im][PF6] arising
from[C4C1im]Cl or [C4C1im]Br) and[C4C1im]Br/[C4C1im][PF6] using a [C4C1im][PF6] arising
from[C4C1im]Cl or [C4C1im]Br.
In order to obtain further information on the preferential H-bonding, the FT-IR spectra of the pure ILs and their mixtures were also registered. The region of the +C−H intramolecular vibrational
modes under the interactions with the counteranions, +C−H···A− (between 3000 and 3200 cm-1),
of +C(2)−H (and +C(4/5)−H) asymmetric stretching modes strongly red-shifted in the case of halide
anion (Cl > Br >I) with respect to other anions (i.e. [PF6]- and [Tf2N]-). Although the spectra of the
deuterium substituted imidazolium salts at C(2) are generally investigated by IR, due the richness in absorption bands of this region which makes the quantitative analysis of the C(2)–H stretching mode difficult, we decided to avoid the H/D exchange that could affect the hydrogen bonding. Thus, the entire spectra of the above listed mixtures were examined. The regions going from 2800 to 3200 cm-1 and from 725 to 925 cm-1, attributable respectively to the imidazolium C-H stretching
modes and to the in-plane and out-of-plane bending modes, presented the more significant differences with respect to the simulated absorptions obtained by the weighted combinations of the two salts constituting each mixture. The additivity parameters (AP), i.e. the difference between the simulated and the experimental results (eq. 1), previously introducedxvii by Quitevis et al., was
chosen for the analysis of our binary ionic liquid mixtures.
AP = Sexp(ω)−Scalc [¿( ω)2]
∑
ω ¿ √¿ (1)Scalc ( ω ) = AaSa ( ω ) + AbSb ( ω ) (2) where Ax is the molar concentration of IL x and Sx is the spectrum of the neat IL x.
Figure 5. Additivity parameters (AP), determined using eq. 1, of mixtures [C4C1im]Clx[PF6](1-x) (a),
[C8C1im]Clx[Tf2N](1-x) (b), [C4C1im]Brx[Tf2N](1-x) (c), [C4C1im]Clx[Tf2N](1-x) (d) vs their molar
fraction.
It is to note that the AP parameters for the [C4C1im]Cl/[C4C1im][PF6] mixtures (Fig. 5a) vary in a
small range, suggesting a high degree of additivity in the related spectra although the NMR data evidence a modest, but not negligible, preferential hydrogen bond for the chloride. This situation, however, is not unusual. It characterises also previously investigated mixtures based on spherical
top anions (Cl/I; Cl/BF4).14 Nonetheless, AP values significantly different from zero (in agreement
with the information arising from NMR measurements) have been obtained for all the other investigated mixtures, including [C4C1im]Cl/[C4C1im] [Tf2N]. In the case of this latter system, IR
spectra of the corresponding deuterated mixtures gave different results.14 Thus, for most of the
presently investigated mixtures IR and NMR results evidence deviations from a linear behavior when IL anions have significantly different interaction abilities.
To gain further insight into these latter systems we also evaluated the hydrogen bond donor ability of the [C4C1im]Clx[Tf2N](1-x) mixtures through solvatochromic measurements. In particular, the
Kamlet-Taft parameters (“polarizability-dipolarity”, *, “hydrogen bond acidity”, , and “hydrogen bond basicity”, ), which have been extensively applied to determine ionic liquid polarity, xviii were
determined. Interestingly, a clear deviation from the line joining the values of the pure ILs was observed for the parameter, Figure 6, whereas for the other two parameters differences were extremely low. The presence of the poor hydrogen bond acceptor [Tf2N]- anion, in agreement with
the other spectroscopic and spectrometric measurements, probably determines a variation in the IL mixture structure that determines an increased interaction of the chloride anion with the acidic proton at C2 of the imidazolium cation, evidenced by the NMR and IR measurements on the ionic liquid mixtures, or with the added probes, as shown by the solvatochromic measurements. It is noteworthy that the examined mixtures present quite high values under conditions in which the presence of relevant amounts of [C4C1im][Tf2N] reduces significantly viscosity.
0.0 0.3 0.5 0.8 1.0 0 0.25 0.5 0.75 1 [C4C1im]Cl, c
b
Figure 6. Differences in the hydrogen bond basicity, , for the [C4C1im]Clx[Tf2N](1-x) determined by
UV-vis spectroscopy through solvatochromic measurements.
At the microscopic level, for the IL mixtures the observed increased interactions of the imidazolium
ring protons with selected anions (for example, Cl- vs [Tf
the main anion–cation interaction for both pure ILs takes place through the most acidic proton at the C2 position of the imidazolium ring, with less pronounced contact through the two equivalent
C4/5–H protons at the back of the ring. In the mixtures, however, as reported by Kirchner et al.19 the
more basic anion preferentially interact with the most acidic proton. Thus, whereas the chloride ion maintains qualitatively (but not quantitatively) the situation (H2 > H4 ≈ H5), the less coordinating
anion ([Tf2N]-) increases its interactions with H4 and H5. Practically, the more hydrogen bond
acceptor ion replaces the [Tf2N]- anion at the most acidic position of the imidazolium ring. The
presence of preferential interaction between selected anion and specific position on the imidazolium ring not necessarily requires significant structural modifications of the IL networks. As stressed by
Welton et al, 13 the anions may be randomly distributed throughout the ionic liquid on the bulk
scale, but locally may favour one particular cation site thus inducing small modifications in the cation and/or anion orientation. This could guarantee an ideal behavior of these mixtures with respect to most of the generally investigated dynamic properties.
On the other hand, the mixtures components might largely maintain the three dimensional structure of the neat liquids, giving a random nanostructural organization with “blocks” of the two ILs along the network. A situation of this type could again guarantee an ideal trend of many physical
properties, as evidenced17 on the basis of the OHD-OKE spectra, but could also be in agreement
with the NMR data if “blocks” are relatively small and reorientation phenomena occur at the
interphases between “blocks”. Practically, addition of small amounts of [C4C1im]Clx to the other
IL, and vice versa, rarely will be able to give segregation phenomena resulting in nanostructural organization with “blocks” of the two salts: this latter situation instead should be more probable at comparable concentrations of the two ILs. The possibility to consider these two situations inside the same ionic media model can be useful, in our opinion, to explain the observed two peaks curves in the AP graphics related to the C-H stretching modes of mixtures characterized by anions having significantly different hydrogen bond ability.
Thermal Stability
Finally, the short-term thermal stability of the [C4C1im]Cl/[C4C1im][Tf2N], [C4C1im]Br/[C4C1im]
[Tf2N] and [C8C1im]Cl/[C8C1im][Tf2N] mixtures was investigated by thermogravimetric analysis.
The Tonset parameter was determined for all investigated mixtures performing the analyses under identical conditions in order to minimize discrepancies arising from the application of different conditions (heating ramp, atmosphere, gas flux, etc.).
Tonset and Tpeak are reported in Table 2 (GA thermographs for ILs mixtures are given in supplemetary material) .
Table 2. Tonset and Tpeak of studied mixtures.
[C8C1im][Tf2N]1-x Clx [C8C1im]Cl
Tpeak 1 Tpeak 2 Tonset 1 Tonset 2
0.23 287 483 259 440 0.38 289 474 259 435 0.70 299 472 267 431 0.85 303 457 272 418 0.95 315 437 280 402 [C4C1im][Tf2N]1-x Brx [C4C1im]Br
Tpeak 1 Tpeak 2 Tonset 1 Tonset 2
0.19 303 482 271 443 0.32 315 492 280 452 0.66 325 483 291 444 0.81 324 457 292 422 0.95 330 424 296 398 [C4C1im][Tf2N]1-x Clx [C4C1im]Cl
Tpeak 1 Tpeak 2 Tonset 1 Tonset 2
0.19 287 474 257 439
0.34 291 464 260 428
0.65 290 446 262 416
0.83 288 435 259 405
0.95 293 408 266 381
The TGA thermographs are always characterized by two distinct steps. The addition of [CnC1im]Cl
or [C4C1im]Br to the corresponding [Tf2N]-based ILs gives rise to a new mass loss process around
salts. Furthermore, the mass loss observed in these initial decomposition processes is in agreement with the weight percent of the added of [CnC1im]Cl or [C4C1im]Br. Thus, the first degradation step can be attributed to the well-known ability of nucleophilic anions, such as chloride or bromide, to give imidazole and alkyl halides through retro SN2 reactions. Although at relatively low concentrations of the added chloride and bromide salt the effect of the formed decomposition products (in particular, the basic imidazole) appears unable to significantly affect the decomposition temperature of the second degradation step (Tpeak 2), at higher halide concentrations a moderate but significative decrease in the decomposition temperature can be observed. This effect, which is more evident for [C4C1im]Clx[Tf2N](1-x) and more moderate in the case of [C8C1im]Clx[Tf2N](1-x), thus suggesting a role of the cation structure, is probably affected also by other parameters, such
viscosity, density and so on. However, any correlation between the structural organization of the
mixture and the thermal stability of the single ions appears premature on the basis of these few data.
Conclusions
Spectroscopic and spectrometric measurements carried out in the present investigation suggest that the investigated binary mixtures based on imidazolium cations and halide anions (chloride or bromide) associated to ILs having hydrophobic and low hydrogen bond donor anions are characterized by phenomena of preferential hydrogen bonding between the imidazolium cation and the more basic anion and, interestingly, between the more basic anion and possibly added hydrogen bond acceptor substrates. This latter feature is undoubtedly important for possible applications of [C4C1im]Cl: using proper mixtures, relatively high values of hydrogen bond basicity can be reach
under conditions of reduced viscosity.
Deviations from linearity have been observed for all investigated mixtures when the C(2)-H chemical shift was plotted against mixture composition and additivity parameters significantly different from zero were obtained analysing the FT-IR spectra. The extent of these deviations depend on the hydrogen bond basicity of two anions, thus resulting moderate in the case of [C4C1im]Clx[PF6](1-x) but significantly higher for the [C4C1im]Clx[Tf2N](1-x) mixture. Interesting,
deviations from the line joining the values of the pure ILs, having a clear linear behavior, were observed also when small amounts of imidazolium halides were added to the investigated hexaflorophosphate or bistriflimide based ILs: thus, NMR measurements could be used to evaluate the presence of residual halide in “pure” ILs. Finally, although the addition of [CnC1im]Cl or
of the more thermal stable IL, a trend attributable to the effect of formed decomposition product can be envisaged.
Experimental Section
Ionic liquids were synthesized as previously reported,15 and dried under a vacuum oven at 80 °C for
at least 8h. [C4C1im]Cl was used at the supercooled state. Infrared spectra were registered using an
ATR-FTIR Agilent 660 (Agilent Technologies, Santa Clara, CA, USA). NMR spectra were recorded at room temperature using a Bruker Instrument at 250 MHz using a co-axial tube, tube containing DMSO-d6 as reference. The thermal gravimetric analyses (TGA) were conducted using a
TA Instruments Q500 TGA. The IL mixture (15-18 mg) was heated in a platinum crucible. First, the heating mode was set to isothermal at 50 °C in N2 (100 mL/min) for 15 min. Then, IL was
heated from 40 to 700 °C with a heating rate of 5 °C min−1 under nitrogen (90 mL/min) and
maintained at 700 °C for 2 min. Mass change was recorded as a function of temperature. TGA experiments were carried out in triplicate. Kamlet-Taft parameters were registered using a Varian 300 UV-vis spectrophotometer, as previously reported.18
Acknowledgements Authors would like to acknowledge the kind support in the framework of the COST Action EXIL-Exchange on Ionic Liquids (CM1206).
Conflicts of interest
The authors declare “no conflicts of interest”.
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