Mixing divalent ionic liquids: effects of charge and side-chains

Article abstract of DOI:10.1039/d1cp00208b

We have prepared novel divalent ionic liquids (ILs) based on the bis(trifluoromethylsulfonyl)imide anion where two charged imidazolium groups in the cations are either directly bound to each other or linked by a single atom. We assessed the influence of the side-chain functionality and divalency on their physical properties and on the thermodynamics of mixing. The results indicate that shortening the spacer of a divalent IL reduces its thermal stability and increases its viscosity. Mixtures of divalent and monovalent ILs show small but significant deviations from ideality upon mixing. These deviations appear to depend primarily on the (mis)match of the nature and length of the cation side-chain. The non-ideality imposed by mixing ILs with different side-chains appears to be enhanced by the increase in formal charge of the cations in the mixture.

Full text of DOI:10.1039/d1cp00208b

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Mixing divalent ionic liquids: effects of charge and
side-chains†
Cite this: Phys. Chem. Chem. Phys.,
2021, 23, 4624
a
b
Eduards Bakis,
Thomas Chang,a Marianne Kjellberg,
Margarida Costa Gomes *b and Tom Welton
Adriaan van den Bruinhorst,
Laure Pison,‡b Ivan Palazzo,a
a
c
Cameron C. Weber,
a
*
We have prepared novel divalent ionic liquids (ILs) based on the bis(trifluoromethylsulfonyl)imide anion
where two charged imidazolium groups in the cations are either directly bound to each other or linked
by a single atom. We assessed the influence of the side-chain functionality and divalency on their
physical properties and on the thermodynamics of mixing. The results indicate that shortening the
spacer of a divalent IL reduces its thermal stability and increases its viscosity. Mixtures of divalent and
monovalent ILs show small but significant deviations from ideality upon mixing. These deviations appear
to depend primarily on the (mis)match of the nature and length of the cation side-chain. The non-ideality
imposed by mixing ILs with different side-chains appears to be enhanced by the increase in formal charge
of the cations in the mixture.
Received 15th January 2021,
Accepted 17th February 2021
DOI: 10.1039/d1cp00208b
rsc.li/pccp
liquid phase shows more complex behaviour due to differences
in ion size, shape, conformation, flexibility, or specific interactions,
Introduction
Estimates place the number of possible ionic liquids (ILs) such as hydrogen bonding and p-stacking. Consequently, the liquid
available at considerably over one million.1 Amongst such a cohesive energies might depend as much upon van der Waals
vast number of alternatives there should exist an ideal candidate interactions as electrostatics.11 The molar volumes of IL mixtures
for every application. Rather than endeavouring to prepare a new typically follow nearly ideal behaviour, since the liquid phase of
IL for each application, we2–5 and others6–8 have explored the ILs generally has low compressibility and reduced free volume.
possibility of mixing ILs in order to control the physicochemical The molar volumes of IL mixtures have been, so far, relatively easy
properties of these fluids and to determine whether unique to predict from the pure IL molar volumes. The excess volumes
systems can be accessed by simply combining already established being often less than one percent of the magnitude of the property
ILs. Most salts mix as molecular compounds, but— depending on itself.12 However, the same is not true for the energetics of
the nature of the salts—the properties of the resulting mixtures mixing.13
can be radically different from those of the pure constituents. For
The first IL mixtures studied showed slightly endothermic or
example, mixing salts can significantly lower the melting point or exothermic mixing enthalpies (less than 1 kJ molꢀ1), but more
result in complex speciation. In this work, the ILs used to form the recent studies revealed stronger deviations from ideal mixing
mixtures are well characterised and are expected to mix as with reported excess enthalpies as low as ꢀ3 kJ molꢀ1 8
. The
molecular compounds. Understanding their mixing behaviour prediction of the excess enthalpy is not straightforward, even
enables the properties of the resultant mixtures to be predicted.
when the IL mixtures are regular.14 The excess enthalpy macro-
Interactions between the individual ions in the ILs are scopically reflects the microscopic effects of adding a third or
dominated by long-range Coulombic forces.9,10 However, the fourth ion to the liquid phase. Furthermore, precisely mea-
sured partial molar enthalpies can be extrapolated to infinite
dilution conditions, hence providing unique experimental
insight into the interactions between the two salts.
a Department of Chemistry, Imperial College London, White City Campus
80 Wood Lane, London W12 0BZ, UK. E-mail: t.welton@imperial.ac.uk
b
Mixing ILs also allows the control of their transport properties,
for example by reducing the IL viscosity upon dilution with a
second, less viscous, salt. Such mixtures have been used to
improve the performance of several IL-based processes, such as
CO2 capture15,16 in terms of the kinetics of gas absorption and
absorbent regeneration.17 The use of IL mixtures has also led to
´
Laboratoire de Chimie de l’ENS Lyon, CNRS and Universite de Lyon,
´
46 allee d’Italie, 69364 Lyon, France. E-mail: margarida.costa-gomes@ens-lyon.fr
c School of Chemical Sciences, University of Auckland, 23 Symonds St, Auckland,
New Zealand
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1cp00208b
‡ Present address: Laboratoire Magmas et Volcans, CNRS and Clermont Auvergne
`
University, 6 Avenue Blaise Pascal, TSA 60026 – CS 60026, 63178 Aubiere, France. breakthroughs in the capacitive performance of electrical double
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layer (EDL) capacitors18,19 outperforming systems consisting of ranging from 40 K to approximately 150 K.30–32 These findings
simple ILs or molecular liquids. So far, relatively high melting have focused on DILs that contain two singly charged head
points limit IL application at low temperatures while relatively low groups linked by an aliphatic spacer of at least 3 carbons in
diffusion coefficients, implied by the high viscosity, reduce their length, i.e. headgroups that are largely independent of each
conductivity. High temperature devices using surfactant ILs20 or other. Several groups have successfully utilized such DILs as
IL mixtures have been shown to resolve both these issues,19 whilst media for high-temperature organic transformations.22 While
taking advantage of the properties of these liquid salts which the mechanism of the improved DIL thermal stability is not
are inherently conductive, often possess broad electrochemical clear, it is important to note that such liquids are substantially
windows and have negligible vapour pressures. These aspects more viscous than their monovalent counterparts, which may
provide ILs with safety benefits over volatile, flammable organic influence the rate of decomposition. When used in gas
electrolytes combined with markedly improved electrochemical chromatography (GC) stationary phases, whether as individual
windows as compared to aqueous solutions.
ILs or mixtures thereof,33 IL thermal stability is of utmost
Given the importance of Coulomb forces in ILs, it might be importance and determines the upper column operation
speculated that increasing the charge density on one or more of temperature boundary.34
the ions in a mixture of ILs could lead to greater effects than
when all of the ions are singly charged.
The [NTf2]ꢀ anion is the archetypal anion for air- and water-
stable ILs with low viscosities and excellent electrochemical
Most of the known divalent ILs (DILs, where either the properties, hence it has been chosen for this study. The lowest-
cation, anion or both are divalent) in the condensed phase melting [NTf2]ꢀ DIL with a single 2+ charge centre remains
suffer from high melting points due to their high lattice [(C4im)4Cu][NTf2]2, which melts at 46 1C, although lower melting
energies,21,22 or are too viscous for practical use. The majority DILs based on this anion where two singly charged heads are
of room temperature ILs that comprise a formal divalent ion joined via a linker were reported before.35 Armstrong and co-
contain two formal monovalent ions that are joined by an workers have introduced a family of ILs with formally divalent
aliphatic or aromatic linker.22,23 Accordingly, the two charged cations (Fig. 1A), among which there are [NTf2]ꢀ based salts that
sites of the cation are distant from one another and do not are fluid at room temperature.32 The corresponding cations
localise the charge density.
consist of two charged heads that are joined with (CH2)m (m Z 3)32
Shortening the size of the aliphatic linker has proven to have or oligoether linkers. Molecular dynamics studies by Yeganegi et al.
important advantages, as far as the properties and performance show that anion coordination around the charged heads closely
of the divalent salt are concerned. In liquid chromatography, resembles that in classical monovalent imidazolium ILs.36
sorbents functionalised with DIL cations exhibited enhanced Xiao et al. reported37 ILs containing monovalent and divalent
performance over their monovalent counterparts.24 Decreasing (Fig. 1B) cations formed from 2,20-biimidazole with [NTf2]ꢀ and
the aliphatic linker led to an enhanced band separation,24 their application in Suzuki cross-coupling reactions. There have
which might be related to the increased localisation of the been no subsequent studies of this unusual IL-forming divalent
charge density in the cation. Also, EDL capacitance is affected cation.
by the DIL cation linker size. Molecular dynamics of spatially
Herein, we aim to develop an understanding of the physico-
isolated DILs, such as [(C1im)2C6][BF4]2, suggest these provide chemical properties of DILs, both with and without oligoether
similar capacitance to the monovalent [C6C1im][BF4].25 However, functionalisation, and their mixtures with both di- and mono-
shortening the linker length to C3 in [(C1im)2C3][BF4]2 brings valent ILs. To this end, several novel DILs containing divalent
the capacitance above that of the [(C1im)2C6]2+ analogue.25 cations have been prepared (Table 1) where we have used the
Moreover, Matsumoto et al. recently demonstrated that using structural motifs discussed above but shortened the distance
oligoether linkers to join together four imidazolium cations between the charged heads to a minimum in order to provide
(with [NTf2]ꢀ as counterion) to form a tetravalent ion leads to an increase in charge density, and/or modified the cation
‘‘exceptionally high’’ EDL capacitance, exceeding that of symmetry. These combined changes allowed these ILs to
[C4C1im][NTf2] by more than 6 times.26 The authors hypothe- remain liquid at ambient temperature. Moreover, we have
sised that a relatively smaller entropy loss upon self-organization assessed the effect of both aliphatic and oligoether side chains
of the tetravalent cation around the electrode, as well as multi- on the DIL properties.
valent electrostatic interactions account for this remarkable
Mixing these ILs allows the tuning of their properties.
boost in capacitive performance. Oligoether-functionalised ILs, For example, it is possible to improve their viscosity without
depending on the anion,27 are usually less viscous than their
alkylated counterparts.28,29 It is therefore of great interest
whether the desirable electrochemical effects arise from the
increased cation charge, conformational flexibility of cations
introduced by ether linkages, or a combination thereof.
Another important physical property with respect to the use
of ILs as electrolytes, materials, and solvents is their thermal
stability. It has been argued that DILs are more thermally stable
Fig. 1 General structure of imidazolium-based dications.
than monovalent ILs, with improvements in thermal stability
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Table 1 Structures, charge (Chg), abbreviations, and melting points (m.p.)
of the mono- and divalent imidazolium-based cations in the prepared ILs
with [NTf2]ꢀ anions
whenever it does not affect the understanding of the data, the
mole fraction and/or the common anion will be omitted.
Density measurements
No. Structure
Chg Abbreviation
m.p.
Densities were measured at atmospheric pressure and in
the temperature range of 293.15–353.15 K using a U-shaped
vibrating-tube densimeter (model DMA 5000M; Anton Paar).
The calibration of the equipment was verified with air and
tridistilled water before and after each measurement. The
estimated uncertainties of this equipment are 0.00001 g cmꢀ3
and 0.01 K for density and temperature, respectively.
1
2
3
4
5
1+ [C2C1im]1+
o20 1C
o20 1C
o20 1C
o20 1C
o20 1C
1+ [C10C1im]1+
1+ [(C3O)C1im]1+
1+ [(C5O2)C1im]1+
1+ [(C7O3)C1im]1+
1+ [C10C1C1biim]1+
o20 1Ca
Molar volume
6
Molar volumes of the pure ILs were determined experimentally
from the density data and predicted using a group contribution
method:38
7
8
2+ [C10C1C1C1biim]2+ 60.0–63.0 1Ca
2+ [C10imC1imC1]2+
o20 1Ca
2
X
X
i
VmGCM
¼
nj
CiðdTÞ
(1)
9
2+ [(C7O3)imC1imC1]2+ o20 1Ca
j
i¼0
10
11
2+ [(C4im)2C1]2+
58.6–59.3 1C
where VGmCM denotes the IL molar volume calculated using the
group contribution method (GCM), nj are the number of groups
j listed in Table S5 (ESI†) for the studied ILs, Ci are temperature-
dependent coefficients listed in Table S6 (ESI†), and dT = T ꢀ
298.15 K such that C0 corresponds to the group’s molar volume
at 298.15 and 0.1 MPa.
The excess molar volumes of the IL mixtures were inter-
polated using a zero-order expansion of the Redlich–Kister
polynomial:
2+ [((C3O)im)2C1]2+
o20 1Ca
a
Novel compounds.
significantly affecting their density or thermal stability. In order
to understand how the properties of the mixtures depend upon
those of the pure ILs and of their composition in the mixture,
we conducted a thorough and original study of the thermo-
dynamic properties of mixing. After characterisation of the
properties of the pure ILs, both the excess molar volumes and
the excess enthalpies were measured. These investigations
represent the first calorimetric studies of mixtures of divalent
salts with monovalent ILs and present a valuable insight into
the properties of ILs with high charge densities. The ion
dynamics and glass transition temperatures of mixtures containing
divalent and a monovalent IL have been studied previously23 but
only at a few compositions and including families of ILs that
contained divalent salts having large spacers between the
charged head-groups.
VE = (1 ꢀ xIL2)xIL2A0
(2)
where xIL2 is the mole fraction of second IL in the binary
mixture. Even though higher order expansions would be justified
for some of the studied mixtures, we decided not to include
these for the sake of comparison with mixtures containing
limited data at different compositions.
Viscosity measurements
Viscosity measurements were performed at atmospheric
pressure and in the temperature range 293.15–353.15 K using
a falling-ball-based microviscosimeter Lovis 2000 M/ME from
Anton Paar. The temperature was controlled to within 0.005 K
and measured with an accuracy better than 0.02 K. A capillary
tube of 1.8 mm diameter previously calibrated, as a function of
temperature and angle of measurement, with reference oils was
used for the measurements. The overall uncertainty on the
viscosity was estimated to be 2%.
In the current manuscript we explore a conceptually novel
family of DILs that comprise ions with high charge density in
the divalent headgroup. Furthermore, we studied the effect of
the functionalisation of the DILs and monovalent ILs upon
mixing over wide composition ranges.
Experimental
Thermogravimetric analysis
The synthetic procedures for all of the ILs prepared are provided Temperature-ramped Thermogravimetric Analysis (TGA)
in the ESI.† Mixtures of ILs were prepared gravimetrically experiments were performed on a PerkinElmer ‘Pyris 1 TGA’
with the uncertainty on the mole fraction composition being thermogravimetric analyser, using platinum sample pans of
estimated as ꢁ0.0001. Hereinafter, the following nomenclature 6 mm diameter. Samples of 5–15 mg were heated at 10 1C min-1
is used for the mixtures: [cation1]+(1ꢀx)[cation2]+ [anion]ꢀ and from 70 to 600 1C under a nitrogen flow of 20 ml minꢀ1. Each IL
x
[cation1]2+(1ꢀx)[cation2]+ [anion]ꢀ(2ꢀx), where x stands for the was dried isothermally at 70 1C for one hour prior to
x
mole fraction composition. For the sake of simplicity and measurement.
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Isothermal titration calorimetry
The enthalpy of mixing can be represented as a function of
mole fraction composition by a Redlich–Kister equation:
An isothermal titration nanocalorimeter (TA Instruments)
equipped with a thermal activity monitor—TAM III from TA
Instruments—was used for measuring the excess molar enthalpies
(HE, or mixing enthalpies, DmixH) at atmospheric pressure and at
313.15 K. The temperature of the thermostat was controlled to
within 10ꢀ5 K.
Two 1 mL Hastelloy cells were used. Both the measuring cell
and the reference cell were initially filled with identical
amounts of the same liquid. During the titration experiments,
small volumes (between 3 and 10 mL) of a pure IL or a mixture
of ILs at a known composition were injected (from a 250 mL or
100 mL gastight Hamilton syringe) into the measuring cell
containing an accurately known quantity of approximately
0.8 mL of liquid. To ensure effective mixing, the liquid in the
cell was stirred at 60 or 100 rpm. A motor-driven pump
(thermometric 3810 syringe pump) was used for the automatic
injections (around 5 per experiment), with each injection taking
either 10 or 60 s. Between injections the system was allowed to
equilibrate for 25–60 min to re-establish the baseline.
After each injection, a peak corresponding to the heat
generated or absorbed by the mixing process is recorded. The
area of the peak, Qi, is proportional to the heat involved and is
used to calculate the excess molar enthalpy. The integration of
the peaks recorded during the titration experiments was done
using the TAM assistant software. The consistency of the results
was checked by measuring the mixing enthalpies at several mole
fraction concentrations along the whole composition range.
The partial molar excess enthalpy of IL2 added to IL1, can be
calculated as described previously,39 through the measured
heat effects (QIL2) when small quantities of IL2 (DnIL2) are
added to larger quantities of IL1:
n
X
i
DmixHIL1þIL2 ¼ ð1 ꢀ xIL2ÞxIL2
Aið1 ꢀ 2xIL2
Þ
(4)
i¼0
where xIL2 is mole fraction of IL2 in the mixture. The partial
excess enthalpies are then obtained by the appropriate deriva-
tives with respect to composition:
ꢀ
ꢁ
@DmixHIL1þIL2
@xIL2
ꢀE
HIL2 ¼DmixHIL1þIL2 þð1ꢀxIL2
Þ
p;T;xIL1
!
n
n
X
X
2
iꢀ1
i
¼ðxIL2 ꢀ1Þ xIL2
ꢀ2iAið1ꢀ2xIL2
Þ
þ
Aið1ꢀ2xIL2
Þ
i¼0
i¼0
(5)
Eqn (5) can be fitted directly to the experimental data to obtain
the Redlich–Kister parameters (Ai) which can then be used to
calculate the enthalpy of mixing using eqn (4) or to assess the
partial molar excess enthalpies at infinite dilution as follows:
n
X
ꢀE;1
ꢀE
i
HIL1 ¼ lim HIL1
ðꢀ1Þ Ai
(6)
xIL1!0
i¼0
n
X
ꢀE;1
ꢀE
HIL2 ¼ lim HIL2
Ai
(7)
xIL2!0
i¼0
Results and discussion
Ionic liquid selection and synthesis
In order to assess the effect of the side-chain and the linker
between imidazolium cationic groups, we prepared the
compounds listed in Table 1 using the procedure outlined in
Scheme 1. A graphical overview of these compounds and the
properties that have been characterised is given in Fig. 2. Three
key structural motifs were employed. The first is the presence of
symmetric side-chains on each imidazolium headgroup for the
divalent cation (No 10 and 11 Table 1, 5th row Fig. 2).
ꢀ
ꢁ
@ðnIL1 þ nIL2ÞDmixHIL1þIL2
@nIL2
QIL2
ꢀE
HIL2
¼
ꢂ
(3)
DnIL2
p;T;nIL1
Syntheses of these ILs were straightforward and scalable,
proceeding through the preparation of the dibromide salts
(via the reaction of alkylimidazoles with dibromomethane) fol-
lowed by an aqueous anion exchange (Scheme 1). In addition, a
multi-gram alkylation protocol for the preparation of various
where nIL1 and nIL2 denote the amounts of IL1 and IL2,
respectively; DmixHIL1+IL2 is the enthalpy of mixing of the two
ILs. DnIL2 is the molar quantity of IL2 added per injection
calculated from the injected volumes and the experimental
densities.
Scheme 1 Synthesis of geminal dicationic ILs. (1) NaOH, PhMe, 24 h reflux with Dean–Stark, drying at 100 1C; (2) R-Cl, MeCN, r.t./17 h reflux, vac.
distillation; (3) CH2Br2, MeCN, 96 h reflux, recryst.; (4) LiNTf2 (aq), H2O/EtOAc extraction; (5) NaOH, Bu4NCl, DCM, 24 h reflux, sublimation; (6) MeI (0.55
eq.), THF, 2 h, 80 1C, re-precipitation) R0–X, MeCN, 1–8 days, 80–100 1C.
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Fig. 2 Overview of synthesised ionic liquids for which the physicochemical
properties were evaluated. The ILs have an alkyl or oligoether sidechain R,
where Cn and C2n+1On correspond to a –(CH2)nꢀ1CH3 and –(CH2CH2O)n
CH3 chain, respectively. Symbols correspond to those used in Fig. 3 and 5.
For ionic liquid 10, only the decomposition temperature was studied.
Numbers correspond to entry in Table 1.
Scheme 2 Synthesis of 2,20-biimidazolium-based ILs. (1) CuCl, NaO2Py,
air, xylenes, 8–15 h reflux, sublimation; (2) n-C10H21Br, 6 h, 120 1C, recryst.;
(3) LiNTf2 (aq), H2O/DCM extraction; (4) Me2SO4, 21 h, 130 1C, (5) LiNTf2
(aq), H2O/EtOAc extraction.
alkylimidazoles, such as 1-(2-methoxyethyl)imidazole, from readily
available precursors was developed and is discussed in greater
detail in the ESI.† Both ether and aliphatic sidechains were chosen
to have approximately the same sidechain length in terms of
number of consecutive atoms to enable a direct comparison of
the role of the ether functionality on the properties of the DIL.
The second motif was the formation of an asymmetric DIL
featuring a methylimidazolium cation directly attached to an
imidazolium cation with a larger sidechain (no 8 and 9 Table 1,
4th row Fig. 2). These were prepared using the mono-
quaternisation protocol of bis-imidazolylmethane established
by Weiss et al.40 We also prepared an [NTf2]ꢀ salt of a divalent
cation without carbon linkers between the imidazolium rings
(no 7 Table 1, 3rd row Fig. 2), leading to a DIL with the greatest
charge density in the cationic ‘head-group’ amongst the ILs
studied in this work. The monovalent analogue with only one of
the imidazole rings quaternised was also prepared (No 6
Table 1, 2rd row Fig. 2), providing access to similar structures
that differ only with the formal charge of the cation. Preparation
of these 2,20-biimidazolium cations was performed from
1,10-dimethyl-2,20-biimidazole, which was straightforward to
mono-quaternise using bromodecane (Scheme 2). The second alky-
lation required the use of dimethylsulfate, perhaps because of the
reduced nucleophilicity and steric hindrance around the remaining
neutral imidazole moiety. For the synthesis of this series of cations
we also developed a multi-gram protocol for 1,10-dimethyl-2,20-
biimidazole (see ESI†), based on catalytic oxidative homocoupling
of azoles and using readily available precursors.
the thermodynamic equilibrium between their liquid and solid
phase.41,42 As a result, simply extending a side-chain by one
extra carbon atom can have a significant effect on the melting
temperature in a way that is difficult to predict. Nonetheless,
the melting temperatures of the studied divalent ILs revealed
some interesting trends.
[(C1im)2C1][NTf2]2 ILs, those with a divalent cation containing
a methylene linker and methyl substituents on each imidazolium
ring (i.e. R1 = R2 = C1), are solids at room temperature with a
melting point of 90–94 1C (Table S1, ESI†). Similarly to mono-
valent ILs, the melting point is reduced when either the linker or
side chains are substituted for more flexible butyl chains
although they remain above 50 1C (Table S1, ESI,† and no 10
Table 1). This could indicate that the melting temperature of
these DILs is affected similarly whether it is the linker or the
side-chain that is modified. The melting points of the DILs are
reduced further if ether, rather than aliphatic, side chains
are present, e.g. [(C3Oim)2C1][NTf2]2 versus [(C4im)2C1][NTf2]2
(no 11 vs. 10 in Table 1). The ether chains increase the conforma-
tional flexibility of the side-chain to a greater extent than the
presence of butyl chains. [(C3Oim)2C1][NTf2]2 was liquid at
room temperature and remained fluid after rigorous drying and
prolonged storage. To the best of our knowledge this is the lowest
molecular weight aprotic room temperature DIL.
The other DILs and monovalent ILs explored were all liquid
at 20 1C with the exception of [C10C1C1C1biim][NTf2]2 (no 7,
Table 1) which has a melting point above 60 1C. Its singly
charged counterpart, [C10C1C1biim][NTf2], is a liquid at room
temperature. However, it can be seen that many factors influence
the melting temperature of these ionic liquids, of which charge
is only one. For example, [C10C1C1C1biim][NTf2]2 has a considerably
higher melting point than [C10imC1imC1][NTf2]2, probably due to
the greater rigidity and charge delocalisation of the biimidazolium
moiety, whereas [(C4im)2C1][NTf2]2 has a similar melting point to
[C10C1C1C1biim][NTf2]2, suggesting perhaps that the effect of the
Outside of these three DIL motifs, ILs containing monovalent
cations either with or without ether groups were also prepared
(no 1–5 Table 1, 1st row Fig. 2). These were quaternised using the
conventional two-step route using an alkyl halide substitution
followed by an aqueous ion exchange step (see ESI†).
Physical properties of the pure ionic liquids
ILs typically do not show clear structure–property relationships symmetry of the cation on the melting point is of a similar order as
for their melting points, owing to the many factors that govern that of the presence of the methylene linker.
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Fig. 3 Molar volume (Vm) of the synthesised [NTf2]-based ILs as a function
of temperature (T). Cation abbreviations and structures are listed in Table 1,
symbols and corresponding structures are clarified in Fig. 2. Lines repre-
sent a second order polynomial interpolation of the data (Table S3, ESI†);
fitting parameters are listed in Table S4 (ESI†).
Fig. 3 shows that, as expected, the molar volumes of all pure
ILs increase with temperature over the investigated temperature
range. The data was derived from density measurements and
interpolated using a second order polynomial, with regression
parameters listed in Table S4 (ESI†). For the majority of studied
ILs a first order (linear) fit yielded minor, but non-random
residuals (Fig. S4, ESI†). The derivative of the interpolations is
very similar for all ILs. Hence, the introduction of a divalent
cation does not seem to have a pronounced effect on the thermal
expansion within the studied temperature range.
Previous studies have shown that the molar volume of an IL
is additive with that of its ions.38,43 This allows for the description
and prediction of the molar volume using group contribution
methods. Moreover, it was found that the additivity held for
sub-groups of ions for which the side-chain was extended or
modified.38,43 Following this approach, the molar volumes of
the ILs highlighted in Fig. 2 were predicted using eqn (1). The
corresponding group coefficients and the number of groups per
Fig. 4 (a) Molar volume (Vm) as function of molar mass (Mw) at 353.15 K
calculated from the experimental density data (exp) and described/
predicted using the group contribution method (GCM). Cation charge
(divalent/monovalent) and the nature of the imidazolium side-chain
(aliphatic/ether) are distinguished. (b) Relative difference between the
experimental and calculated molar volume using the GCM (DeGxCpMVm) at
different temperatures.
IL can be found in Tables S5 and S6 (ESI†), respectively. Fig. 4a significantly higher uncertainty than the data presented in this
and b show the resulting molar volumes and the relative work. Despite these uncertainties, a clear negative deviation
difference between experiment and prediction. The presence of from the GCM-predicted molar volumes can be observed for
ether groups on the alkyl chain systematically increases the these DILs as well. Hence, the increased charge density of the
density (and decreases Vm) of the ILs, regardless of whether cations under study is not the sole cause for the break-down of
divalent or monovalent. This effect has been reported the additivity.
previously32,43 and has been attributed to the improved packing
of ether chains due to their greater flexibility.
This is emphasized when a new group, [(C0imC1)2]2+: a non-
functionalised divalent cation consisting of two imidazolium
The additivity of the molar volume is valid for the monovalent rings linked by a single carbon atom, is introduced to correct
ILs, with maximum relative deviations in the order of 1%, typical for the over-estimation. The parameters for this group were
for these kinds of models.38 For the DILs, however, the molar obtained using the molar volumes of [C10imC1imC1][NTf2]2
volume is over-estimated when two methylimidazolium (Table S3, ESI†). The resulting predictions were surprisingly
([C0imC1]+) groups are used to describe the cationic charge accurate for the structurally different [C10C1C1C1biim][NTf2]2
centres. This is also the case when predicting molar volumes and [((C3Oim)2C1][NTf2]2 (Fig. S5C and D, ESI†). However,
for a reported32 series with increasing linker size m (Fig. S5A and [(C7O3im)C1imC1][NTf2]2 which is of similar size and symmetry
B, ESI†) with R1 = R1 = C1 and m = 3–12, following the as [C10imC1imC1][NTf2]2 showed significant systematic negative
nomenclature of Fig. 1A. The densities of this series were deviations. The new group performed well for DILs reported
measured using a volumetric flask, and therefore reflect a elsewhere that featured a larger linker chain, despite the
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increased distance between the imidazolium rings (Fig. S5, temperature (323.15 K). This suggests that the closer proximity
ESI†).
of the two charges has the expected effect of significantly
The differences between the predictions based on the original increasing the viscosity of these ILs.
parametrisation and the actual molar volumes are in the order of
Ether functionalisation of the sidechain resulted in a
a single CH2 group for most DILs (Fig. S5E and F, ESI†). decrease in viscosity for the monovalent ILs, as shown by
This indicates that the differences are likely to originate from comparing [(C7O3)C1im][NTf2] with [C10C1im][NTf2]. This effect
the side-chains rather than the imidazolium groups. One main has been reported previously.48 However, for the corresponding
difference between the monovalent alkylimidazolium cations DILs, [C10imC1imC1][NTf2]2 and [(C7O3)imC1imC1][NTf2]2, the
used for the original parametrisation and the divalent cations opposite effect is observed and the viscosity of the aliphatic-
studied here is the nature of the alkyl chain linker between the based DIL is lower than that of the ether functionalised DIL.
imidazolium rings. When the imidazolium cations are linked, Although many factors can influence the dynamics of ILs, the
the alkyl chain linkers are not capped by a CH3 group, unlike the higher proportion of anions to cations in the DIL (2 to 1) seems
terminal alkyl chains. Whether this is the main cause for the to overcome the fluidifying effect of ether chains, common in
observed discrepancies needs to be confirmed by studying more monovalent ILs.48 The more concentrated charges increase
ILs, but GCMs for non-ILs showed that the CH3 contribution Coulombic interactions that often strongly affect the viscosity.
is nearly 60% larger than that of CH2.44 Tail end-groups are As a result, interactions between ether chains and imidazolium
therefore best considered as separate moieties. However, this rings49 could be forced, thus additionally increasing the viscosity.
would require the current parameters and groups reported in the However, it is difficult to ascertain the generality of these findings
open literature for the IL-GCM to be revisited, which is outside as only one ether/aliphatic DIL combination was examined.
the scope of this work.
The thermal stability of the prepared ILs was examined by
The viscosities of the simple ILs were measured over various thermogravimetric analysis (TGA) and characterised by Tonset
,
temperature ranges, limited by the melting temperature of the the lowest temperature where mass loss can be observed
IL. They were interpolated using the Vogel–Fu¨lcher–Tamman– (see Fig. S2, ESI†). The major noticeable difference between
Hesse equation:45–47
the studied DILs and monovalent ILs that can be observed
(Table 2 and Fig. S1, ESI†), is that the DILs are prone to degrade
at lower temperatures. This is in stark contrast to reports
claiming that DILs are inherently more thermally stable than
similar monovalent ILs.30,31 This can clearly be seen from the
B
ln Z ¼ A þ
(8)
T ꢀ T0
where Z is the dynamic viscosity, and A, B and T0 are fitting
parameters. The results are presented in Fig. 5 and indicate
that the DILs are uniformly more viscous than structurally
similar monovalent ILs. The viscosities of DILs with closely
positioned headgroups are greater than for those that contain a
longer alkyl spacer between the cationic groups. The viscosity of
[(C1im)2C9][NTf2]2, for instance, is 443 mPa s at 303.15 K32
while that of [C10imC1imC1][NTf2]2 is 1202 mPa s at a higher
Tonset values (Table 2) decreasing in the order [C10C1im]+
4
[C10imC1imC1]2+ 4 [C10C1C1C1biim]2+. From these Tonset values,
it appears these DILs decompose at temperatures approximately
40–60 1C lower than their related monovalent counterparts
(Table 2). These Tonset values remain appreciable for organic
compounds.
The rationale for the reduced Tonset of these DILs as compared
to the more thermally stable DILs lies in their structure. Geminal
divalent cations, i.e. those with m = 1 (Fig. 1A) and those with
directly linked charged centres, are likely to display an increase in
electrophilicity and/or a-hydrogen acidity, due to the decreased
separation of the charged centres. These factors have been found
to play a role in IL degradation pathways, and would account for
Table 2 Tonset values for the ILs examined
IL
Cation charge
Tonseta/1C
[C2C1im][NTf2]
[C10C1im][NTf2]
1+
1+
1+
1+
1+
1+
2+
2+
2+
2+
2+
450
441
435
383
369
368
329
400
341
411
387
[(C3O)C1im][NTf2]
[(C5O2)C1im][NTf2]
[(C7O3)C1im][NTf2]
[C10C1C1biim][NTf2]
[C10C1C1C1biim][NTf2]2
[C10imC1imC1][NTf2]2
[(C7O3)imC1imC1][NTf2]2
[(C4im)2C1][NTf2]2
[((C3O)im)2C1][NTf2]2
Fig. 5 Viscosity (Z) of the synthesised [NTf2]-based ILs as a function of
temperature (T). Cation abbreviations and structures are listed in Table 1,
symbols and corresponding structures are clarified in Fig. 2. Lines repre-
sent an interpolation of the data using eqn (8); data and fitting parameters
are listed in Tables S10 and S11 (ESI†), respectively.
a
Determined from step tangent analysis with a heating rate of
10 1C minꢀ1
.
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the observed differences in thermal stability.30 The more stable features. Moreover, it will give additional insight into the
DILs have m 4 3, and hence the charge centres were separated by relative importance of the divalency of the cation and the
a much greater distance, reducing the electrophilicity of the cation. side-chain functionality. In order to understand how these ILs
The observed increase in thermal stability of these might also arise mix, we explored their thermodynamic properties of mixing.
from their substantially increased viscosity, through an effect on Firstly, the excess molar volume (VE) was calculated from the
the onset temperatures. The results obtained here indicate that measured densities of the simple ILs and their mixtures,
any viscosity effects are offset by the increased electrophilicity of elucidating the free volume changes when the salts are mixed.
cations that contain charge centres in close proximity.
Secondly, the enthalpy of mixing, or molar excess enthalpy (HE),
As might be expected, ILs with heteroatom-rich functional was measured, providing direct experimental information
groups degrade more easily than their unfunctionalised counter- about the energetic changes linked to the formation of these
parts. This is the case for monovalent and divalent ether- mixtures.
functionalised ILs where the Tonset observed for [(C7O3)C1im][NTf2]
The experimental partial molar enthalpies of mixing
and [(C7O3)imC1imC1][NTf2]2 were lower than for [C10C1im][NTf2] extrapolated towards infinite dilution (HEi ,N) can be regarded
and [C10imC1imC1][NTf2]2, respectively. At high temperatures as a direct measurement of the interactions between the two
these functional groups could behave as nucleophiles or bases, components of the mixture.13 These experimental data are
which accelerate the rate of IL decomposition. Patil et al. proposed essential to better understand the resulting properties from
that these C–O bonds break after an intramolecular oxygen mixing these complex salts. Liquid phase data alone, however,
attack on b-carbon atoms in DILs with ether spacers.50 This do not allow the calculation of the excess Gibbs energy (GE).
transformation should be even more sterically accessible within The description of GE would permit a full comprehension of
mono-substituted polyethylene glycol chains. The monovalent how these salts mix. Usually, vapour–liquid equilibrium
[(C7O3)C1im][NTf2] shows a multi-step decomposition pattern measurements are used to access GE, which is practically
(Fig. S1A, ESI†). Indeed, repeated degradation experiments for this unfeasible at temperatures close to ambient for mixtures of
IL at a lower heating rate show the first distinct mass loss to be ILs, due their remarkably low vapour pressures.
approximately 13%, comparable with the expected 15% for a loss
As shown in Fig. 6 and 7, the studied IL mixtures have VE
of ethylene glycol monomethyl ether (Fig. S3 in ESI†). This is in values close to zero, although they show significant trends. The
good agreement with the fact that non-functionalised ILs such as magnitude of VE is considerably lower than the molar volume
[C2C1im][NTf2] have significantly higher Tonset, as the [NTf2]ꢀ of the pure salts (Fig. 3 and Table S7, ESI†), which is in line with
anion itself is remarkably stable and non-nucleophilic.
other IL mixtures studied in the literature.51
The studied ILs exhibited some general trends upon mixing.
When ILs with the same side-chain functionalisation (ether–
Physical properties of ionic liquid mixtures
Mixing the original ILs prepared herein allows the tuning of ether and aliphatic–aliphatic) were mixed, a nearly ideal mixture
their physical properties without losing their structural in terms of the molar volume or a mixture with negative
Fig. 6 Excess molar volume (VE, left) and excess molar enthalpy of mixing (HE, right) as a function of composition (x2, mole fraction of component 2) for
the studied mixtures of ionic liquids (ILs) at 313.15 K. A visual overview of these mixtures—including symbols and line styles—is given in Fig. 7. Solid
symbols and solid/dash-dotted lines correspond to mixtures containing a divalent and monovalent cation. Open symbols and dashed lines correspond to
mixtures containing monovalent cations only. The red horizontal line at 0 represents ideal mixing behaviour. The symbols in the right panel were added
for readability purposes and indicate the compositions at which the calorimetric injections were made. They do not correspond to raw data, as HE can
only be obtained indirectly using isothermal titration calorimetry. See ESI,† Tables S7 and S8 for raw data and fitting parameters.
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When the length of the ether side-chain of the monovalent
IL2 is increased (R = C3O - C5O2 - C7O3), both VE and HE
become increasingly negative. Hence, an increasing mismatch
in the ether chain-length leads to a (slightly) more negative VE
and HE, corresponding to a volume contraction and energetically
more favourable mixing. For IL mixtures with a common anion
negative HE is exceptional,13 as is a negative HE for side-chains
with similar functionality. The few other mixtures that are known
to have a negative HE are 1-butyl-2,3-dimethylimidazolium tetra-
fluoroborate with 1-ethyl-3-methylimidazolium tetrafluoroborate
as well as acetate based ILs mixed with phosphonium and
imidazolium cations.8
When the longest ether side-chain is substituted by an
aliphatic side-chain of equivalent length ([(C7O3)C1im]+
-
[C10C1im]+), the mixture with [(C3Oim)2C1]2+ shows positive VE
and HE. This is in line with the behaviour of molecular binary
n-alkane–ether mixtures that always show thermodynamically
unfavourable mixing energies and volume expansion upon
mixing.52
Five binary IL mixtures contained sets of cations with
differently functionalised side-chains. Their excess properties
were compared to those of molecular n-alkane–polyether mixtures
at equimolar composition (Fig. 8).52 IL mixtures comprised of
monovalent cations with different short side-chains mixed almost
ideally (+0.04 kJ molꢀ1 and +0.05 cm3 molꢀ1 for HE and VE,
respectively) and were therefore not included in Fig. 8. The order
of magnitude of the IL mixture excess properties is similar to
that of n-alkane–ethyl ether mixtures, although it seems that the
ionic nature of the ILs leads to slightly suppressed excess proper-
ties as compared to those of the molecular mixtures. It is therefore
notable that IL mixtures containing divalent ether-functionalised
Fig. 7 Partial molar excess enthalpy at infinite dilution (HEi ,N), the excess
enthalpy of mixing at equimolar composition (HE0.5), and the excess molar
volume at equimolar composition (VE0.5) of the mixture for the studied binary
mixtures of ILs with common [NTf2]-anions. The symbols, colours, and line styles
correspond to Fig. 6, a table-like representation is available in the ESI† (Table S9).
deviations from the ideal molar volume was formed (VE r 0). cations yielded larger volume expansions (VE) and energetically
This corresponds to a mixture molar volume proportional to that less favourable mixing (HE) than mixtures with monovalent
of the pure components’ or to a contraction of the molar volume cations, even when their side-chain was equivalent.
upon mixing, respectively. On the other hand, IL mixtures with
Divalent cations with high charge density, such as the ones
different functionalities (ether-aliphatic and vice-versa) showed studied herein, show enhanced excess properties when compared
quasi-ideal behaviour or positive deviations from ideality in with those of mixtures of monovalent ions with similar side-
terms of the molar volume, corresponding to an expansion upon chains. For example, VE becomes more negative and HE more
mixing (VE Z 0). The most extreme VE values in each direction positive when increasing the cation charge of IL1 for IL mixtures
(ꢀ0.19 cm3 molꢀ1, and +1.15 cm3 molꢀ1) were found for where both cations have aliphatic side-chains ([C10C1C1biim]+–
the mixtures where
([C10imC1imC1]2+) was mixed with a monovalent IL with the
a
DIL with an aliphatic side-chain [C10C1im]+ vs. [C10imC1imC1]2+–[C10C1im]+).
The quasi-ideality of the [C10C1C1biim]+–[C10C1im]+ mixture
same aliphatic side-chain ([C10C1im]+); and where an ether shows that this is not an effect of the cationic head group size.
functionalised DIL ([(C3Oim)2C1]2+) was mixed with a monovalent Mixing the divalent [C10imC1imC1]2+ with its monovalent
IL with an aliphatic side-chain ([C10C1im]+), respectively. These equivalent [C10C1im]+ yields small, but significant, excess
represent relatively small VE values, nonetheless several trends can properties (Fig. 7). For mixtures containing divalent ILs, the
be observed when comparing mixtures with a common IL.
excess properties may be due to the increased local association
The impact of side-chain functionality on IL mixing is of anions with 2 [NTf2]ꢀ anions required per divalent cation
highlighted by mixtures where [(C3Oim)2C1]2+ is combined with compared to one for the monovalent species. The interactions
a series of monovalent ILs (6th row Fig. 7, circles in Fig. 6). between this additional anion and the side-chain may be
When this DIL which is symmetrically functionalised with responsible for the enhanced excess properties, rather than
short ether side-chains is mixed with a monovalent cation with (or in addition to) an increased charge density. To determine
the same side-chain, the mixture behaves as quasi-ideal. Both which mechanism is responsible for the observed effects,
VE and HE are close to zero. This shows that the presence of a additional IL mixtures could be explored. Possible systems of
divalent cation in a binary IL mixture does not necessarily lead interest are mixtures where both ILs are divalent, or mixtures
to deviations from ideal mixing.
containing symmetrically functionalised monovalent ILs such
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[C10imC1imC1]+–[C10C1im]+ mixture, which shows opposite
signs for VE and HE.
In addition to the HE at equimolar composition, partial
excess enthalpies at infinite dilution (HEi ,N) are reported in
Fig. 7. For three of the studied mixtures there is a significant
difference between the HEi ,N of each IL, meaning that the
energetics of mixing are different on each side of the equimolar
composition. In particular, the addition of a limiting amount of
[C10C1im]+ to the other IL, namely those based on
[((C3O)im)2C1]2+, [(C7O3)imC1imC1]2+, or [(C3O)C1im]+ cations
and to a lesser extent [(C7O3)C1im]+, is less favourable enthal-
pically than if a limiting addition is done the other way around.
This behaviour is a sign of large perturbations of the structure
and molecular interactions of the ether functionalised ILs by
[C10C1im]+.
Finally, it should be noted that all values in Fig. 6 were
interpolated using a zero-order Redlich–Kister function, yielding
symmetrical curves. It is clear, however, that the excess volume of
the [(C3Oim)2C1]2+–[C10C1im]+ mixture is asymmetric with com-
position. Hence, a small quantity of the monovalent cation
disrupts the packing of the DIL, leading to a stronger expansion
than in the reverse case. Higher-order Redlich–Kister expansions
would be able to accurately describe this phenomenon. We
specifically chose not to do so, because most systems were not
evaluated at a sufficient number of different compositions (owing
to limited sample quantities) to allow such accurate interpola-
tion. Including higher-order descriptions only for mixtures for
which three or more compositions were studied could lead to the
false conclusion that the mixtures with fewer data exhibit a
symmetrical behaviour of VE around equimolar composition.
Fig. 8 The excess enthalpy of mixing at equimolar composition (H0E.5
top), and the excess molar volume at equimolar composition (V0E.5
bottom) for n-alkane–polyether mixtures52 (half-filled symbols) and stu-
died IL mixtures (filled and open symbols). nC corresponds to the number
of carbon atoms in the n-alkane or the alkyl side-chain of IL2
[C10C1im][NTf2], respectively. Structural formulas of the ethers and IL
cations are given for clarity, lines are a guide to the eye.
,
,
Conclusions
We have prepared several novel DILs based on imidazolium
cations and on the [NTf2]ꢀ anion, namely [((C3O)im)2C1][NTf2]2,
[(C7O3)imC1imC1][NTf2]2, [C10C1C1C1biim][NTf]2 and [C10imC1imC1]
[NTf2]2. The cations of these DILs possess formal charges in
closer proximity than similar DILs that have been previously
reported. Many changes in the physical properties of these
as [(C3O)(C3O)im][NTf2] as well as ILs with longer linker lengths DILs, as compared to their monovalent counterparts, were
to distinguish the effect of charge density from formal charge. consistent with those that have been observed for other DILs,
From Fig. 7 and 8 it can be observed that a mismatch in side- such as a larger molar volumes and higher viscosities. However,
chain length has a stronger effect on the excess property the more localised positive charge of these DILs led to reduced
magnitude than a mismatch in side-chain functionality. The thermal stability as compared to their monovalent equivalents,
short-chained mixture [(C3O)C1im]+–[C2C1im]+ behaves nearly rather than the increase in thermal stability that has been
ideally, while the short–long system [(C3O)C1im]+–[C10C1im]+ observed for other classes of DILs. This reduction in thermal
shows significant (positive) deviations from ideality. This trend stability was attributed to increased electrophilicity when both
is corroborated by the mixture [(C7O3)C1im]+–[C10C1im]+, which formally charged groups are directly bonded or separated only
shows smaller deviations for a longer but comparable length by a single atom linker.
ether side-chain. The same trend has been found for the VE of
The effect of oligoether side-chains was compared with
molecular n-alkane–ethyl ether mixtures52 (Fig. 8). However, the aliphatic side-chains in the cations. Oligoether containing ILs
trend for the HE of IL mixtures is opposite to that of the were found to be generally denser and less thermally stable
molecular mixtures, showing that the ionic nature of ILs does compared to their aliphatic counterparts. The viscosity of
affect the resulting mixing behaviour. It also confirms that VE monovalent ILs with oligoether side-chains was lower than
and HE are decoupled. This is better illustrated by the the corresponding ILs with aliphatic side-chains. The converse
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was true for DILs and the viscosity was found to increase for Acknowledgements
DILs with oligoether sidechains. This suggests that for DILs,
terminal aliphatic chains rather than oligoether chains may
lead to relatively lower viscosities.
EB acknowledges UK Engineering and Physical Sciences
Research Council for funding of doctoral studies, MCG and
LP thank the Institute of Chemistry at Clermont-Ferrand for the
use of the densimeter and the viscometer. MCG and AvdB
acknowledge the financial support of the project IDEXLYON
of the University of Lyon (ANR-16-IDEX-0005). TW acknowledges
an ERC Advanced Investigator Grant for funding.
Upon mixing the studied ILs, the sign of the excess properties
is determined by the functionality of the side-chains. Both VE and
HE are negative for mixtures of ILs that both have an ether-
functionalised side-chain, and positive for mixtures of ILs where
one contains ether side-chains and the other alkyl side-chains. The
extent to which these mixtures showed deviations from ideality is
governed by the mismatch in side-chain length, with DILs signifi-
cantly enhancing this effect. The exact mechanism behind the
differences observed between divalent–monovalent mixtures and
solely monovalent-based mixtures is yet unknown and probably
difficult to explain using simple descriptors. It should be kept in
mind, however, that both the molar volume and the excess
enthalpy of mixing are relatively low. This is especially true for
the excess volume, which is typically two to four orders of
magnitude lower than the molar volume of the mixture.
Collectively, these results suggest some potential design
principles when developing DILs or their mixtures for various
applications. When thermal stability is desired, it appears that
the length of the spacer between cations should be greater than
one carbon atom, despite the corresponding reduction in cation
charge density. For DILs, the use of terminal aliphatic chains
rather than oligoether chains appears to be a more successful
route to reducing their viscosity. Since mixing DILs with less
viscous monovalent ILs with the same side-chains yield virtually
ideal mixtures, this could be an interesting route to predictably
lowering the viscosity of these ILs similarly to asymmetric DILs
with linkers longer than a single atom.23 Alternatively, combin-
ing mismatched sidechains of different functionality appears to
be a route to generating slightly positive excess molar volumes,
which often indicate an increase in free volume within the
mixture. This effect is nevertheless difficult to predict using
group contribution methods as they are not yet accurate enough
for DILs. The origin of this inaccuracy remains to be determined,
but can probably be solved by a re-parametrisation that explicitly
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There are no conflicts to declare.
4634 | Phys. Chem. Chem. Phys., 2021, 23, 4624ꢀ4635
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