Synthesis, physico-chemical properties and toxicity data of new hydrophobic ionic liquids
containing dimethylpyridinium and trimethylpyridinium cations
Nicolas Papaiconomou1,†, Julien Estager1,††, Pierre Bauduin2, Corine Bas3, Sophie Roche4,
Sylvie Viboud5, Micheline Draye1
1 : LCME (EA 1651), Polytech’Savoie, Université de Savoie, 73376, Bourget-du-Lac Cedex,
France
2 : ICSM, UMR 5257 (CEA/CNRS/UM2/ENSCM) BP 17171, 30207 Bagnols sur Céze
Cedex, France
3 : LEM, UMR CNRS 5041, Université de Savoie, Technolac, Bâtiment IUT, 73376 Bourgetdu-Lac Cedex, France
4 : Laboratoire d’Electrochimie de Matériaux, UMR 7555, Université Paul Verlaine, 1 bd
Arago, 57078 Metz Cedex 3, France
5 : CARRTEL, UMR42 (UDS/INRA), Université de Savoie, Technolac 73376, Bourget-duLac Cedex, France
† : To whom correspondence should be addressed. E-mail: npapa@univ-savoie.fr
†† : Present address The QUILL Centre, The Queen's University of Belfast, David Keir
Building, Stranmillis Road, Belfast BT95AG, UK
Abstract
New hydrophobic ionic liquids containing cations 1-butyl-2,3-dimethylpyridinium
[B2M3MPYR],
1-butyl-3,5-dimethylpyridinium
[B3M5MPYR],
1-butyl-2-methyl-5-
ethylpyridinium [B2M5EPYR], 1-butyl-2,3,5-trimethylpyridinium [B2M3M5MPYR], 1octyl-2,3-dimethylpyridinium
[O3M5MPYR],
[O2M3MPYR],
1-octyl-2-methyl-5-ethylpyridinium
1-octyl-3,5-dimethylpyridinium
[O2M5EPYR],
1-octyl-2,3,5-
trimethylpyridinium [O2M3M5MPYR] and anions bis(trifluoromethylsulfonyl)imide [NTf2],
trifluoromethylsulfonate [TfO], dicyanamide [N(CN)2], thiocyanate [SCN] or iodide anions
have been synthesized and their physico-chemial properties studied. Thermal properties,
solubility in water and water content, UV-Vis spectra, densities, refractive indices,
electrochemical windows and toxicity on Vibrio fischeri have been measured. These ionic
liquids, mostly liquid at room-temperature, exhibit high refractive indices and increased
hydrophobicity. Despite high toxicity values towards Vibrio fischeri, ionic liquids containing
octyl chains and trialkylpyridinium or tetraalkylpyridinium represent good alternatives to
extracting solvents.
Introduction
Intensively studied since over a decade, ionic liquids are a new class of molten salts that
exhibit melting points below 100 °C. They are usually composed with linear or cyclic
ammonium or phosphonium cations. Amongst them, imidazolium and pyridinium cations
have been the most common cations reported in the literature.
1-14
Fewer reports dealt with
ionic liquids containing pyrrolidinium, piperidinium or morpholinium cations15-22 and other
cations such as phosphonium23-28 or sulfonium. 29,30
It is known that an ionic liquid exhibits extremely low vapor pressure31, high thermal
stability and relatively low flammability. Therefore, ionic liquids are often referred to as
promising solvent replacements to volatile organic compounds (VOC). Besides, physicochemical properties such as melting point, density, viscosity, solubility in solvents,
conductivity or solvent properties, are easily tunable by simply changing the nature of the
cation or anion. Considering the number of potential combinations between cations and
anions yielding ionic liquids, much work still needs to be done in order to extend existing
ionic liquid database and to gain better insight into the physico-chemical properties of such
compounds.
Due to their wide variety of properties, ionic liquids have been applied in the past years in
most fields of chemistry and chemical engineering, including organic synthesis and catalysis,
32-42
biochemistry,43-48 electrochemistry,20,49-54 extraction55-81 and industrial processes.82
For the specific application of metal extraction from wastewater, ionic liquids with low
melting points, moderate to low viscosity and, most of all, solubility in water as low as
possible are required.
Extraction of heavy metals with hydrophobic ionic liquids has been reported in numerous
works.55,
56, 58-64, 66-68, 70, 71, 73, 75-78, 81
Most reports, however, used ionic liquids containing
imidazolium cations that exhibit high solubility in water, typically around 10 000 ppm.83
Besides, distribution coefficients for metal ions between water and ionic liquid phases were
found to be low, typically below 1, for “generic” ionic liquids containing a simple alkyl chain
appended to the cation.71, 75
Use of so-called task-specific ionic liquids (TSIL), incorporating functional groups
appended either to the cation alkyl chain or anion, have also been carried out.70, 71, 76, 77 By
doing so, various ionic liquids containing functional groups such as nitrile, sulphide, sulfur or
urea were found to yield high extraction capabilities and high selectivities towards metallic
ions such as mercury,71, 77 cadmium,77, 84 americium,70 copper, palladium or silver ions.71, 77
In order to investigate potential new ionic liquids candidates for extraction application,
synthesis and properties of ionic liquids containing cations 1-octylpyridinium [OPYR], 1octyl-2-methyl pyridinium [2MOPYR], or 1-octyl-4-methyl pyridinium [4MOPYR] were
recently reported.8 These ionic liquids were synthesized starting from water-soluble pyridine
or picoline reactants. In this work, the properties of ionic liquids based on pyridinium cations
are investigated further. The objective of this work is to provide ionic liquids with increased
hydrophobicity and reduced hygroscopicity compared to ionic liquids containing imidazolium
cations. Therefore, are reported here synthesis and physico-chemical properties of new
hydrophobic ionic liquids based on reactants exhibiting limited solubility in water: 2,3dimethylpyridine, 3,5-dimethylpyridine, 2-methyl-5-ethylpyridine or 2,3,5-trimethylpyridine.
Ionic liquids reported here contain cations 1-butyl-2,3-dimethylpyridinium [B2M3MPYR],
1-butyl-3,5-dimethylpyridinium [B3M5MPYR], 1-butyl-2-methyl-5-ethylpyridinium
[B2M5EPYR], 1-octyl-2,3,5-trimethylpyridinium [O2M3M5MPYR], 1-octyl-2,3dimethylpyridinium [O2M3MPYR], 1-octyl-3,5-dimethylpyridinium [O3M5MPYR], 1-octyl2,3,5-trimethylpyridinium [O2M3M5MPYR] or 1-octyl-2-methyl-5-ethylpyridinium
[O2M5EPYR], and anions iodide [I], tetrafluoroborate [BF4], trifluoromethanesulfonate
[TfO], dicyanamide [N(CN)2], thiocyanate [SCN] or bis(trifluoromethane sulfonate)imide
[NTf2].
Experimental section
Synthesis. Synthesis of ionic liquids have been performed according to previously reported
procedures. Briefly, bromoalcane or iodoalcane was mixed with 2,3-dimethylpyridine, 3,5dimethylpyridine, 2-methyl,5-ethylpyridine or 2,3,5- trimethylpyridine in ethylacetate or
acetonitrile for 2 days under reflux, to synthesize corresponding bromide or iodide ionic
liquids. Bromide or iodide anion was then replaced by mixing sodium or potassium salts in
water, acetonitrile or acetone, thus yielding ionic liquids containing [BF4], [TfO], [N(CN)2],
[SCN] or [NTf2] anion. Purity was checked with NMR, IR and halide content was checked
with silver nitrate tests. NMR 1H and 13C were recorded at 300 and 75.57 MHz respectively,
on a Brücker spectrometer (Houston, Texas, USA). IR spectra were recorded on a Nicolet 380
FTIR spectrometer from Thermo electron corporation. Details are collected in the supporting
information section. Figure 1 shows structure and abbreviations of all ions used in this study.
Melting points and glass-transition temperatures. Melting points and glass-transition
temperatures were measured with a Thermal Analysis DSC 2920 Modulated DSC apparatus.
Samples were prepared by adding a small amount of ionic liquids, typically 6 mg in an
aluminium pan. Samples were then heated in the DSC apparatus at 125 °C for 10 minutes to
remove all traces of water. Samples were afterwards cooled down to -100 °C. Heating and
cooling curves were then recorded at a rate of 10 °C/min. Melting points, crystallization
temperature and glass-transition temperatures were then measured from the second heating
curve. Results are given in °C with a precision of ± 1 °C.
Thermo-gravimetric analysis.
Prior to measurements, sample was dried up over night under vacuum. Samples were then
vacuumed and flushed with argon three times in the apparatus. A thermal analyzer, TG 209
F1 Iris, from Netzsch was used for the TG analysis. The experiments were performed under
helium atmosphere at a heating rate of 10°C/min, from 30°C to 800°C. Decomposition
temperatures corresponding to 1 and 5 % weight loss have been measured. Results are given ±
1 °C.
Additionally, isothermal thermo-gravimetric experiments were carried out on a home-made
apparatus, consisting of a micro-balance from Metler Toledo connected to a computer and a
high-temperature oven monitored with the help of an electronic temperature controller
Eurotherm. Typically, 100 mg of sample was dried under vacuum over night and put into an
alumina pan. Then, sample was heated up to 225 °C at a rate of 3 °C / min under air and kept
at 225 °C for 5 hours.
Solubility in water. Solubilities in water were measured according to previously reported
protocols.8 An aqueous solution saturated with an ionic liquid was first prepared by mixing an
amount of ionic liquid and water in a tube so as to obtain two phases. Calibration solutions
were then prepared by adding known amounts of ionic liquids in water. UV-Vis spectra of
calibration and IL-saturated aqueous solutions were then recorded between 200 and 500 nm
using a Cary 50 spectrophotometer from Varian and 2 mm thick quartz cell. For all ionic
liquids, two absorption bands were obtained corresponding to the absorption of
trialkylpyridinium and tetraalkylpyridinium cations. Absorption intensities at the maximum
absorption wavelengths (max) were then recorded. Saturation concentration of an ionic liquid
in water was obtained with the help of the Beer-Lambert’s law.
Experiments were performed in triplicates. Results are given in milligrams of ionic liquid
per kilo of solution (ppm), with a precision of ±5 %.
Densities. Densities of ionic liquids at 20 °C equilibrated with air were measured by
weighing an ionic liquid in a 1 mL volumetric flask. Results are given in g / mL with a
precision of ± 0.01.
Water-content. Water-content was measured for air-saturated ionic liquids with the help of
a Metrohm coulometric Karl-Fisher 831KF titration apparatus. Prior to measurements, ionic
liquids have been left five weeks in contact with air. Water-content was then measured by
introducing few milligrams of an ionic liquid in the apparatus with the help of a needle.
Experiments were performed three times. Results are given in milligrams of water per kilo
of ILs (ppm) with a precision of ±7 %.
Refractive index. Refractive indices were measured with an Automatic digital
refractometer, Atago RX-7000α. Prior to measurements, ionic liquids were dried under
vacuum for 12 hours. Measurements were carried out in triplicates at 20 ± 0.2 °C by using a
Peltier unit to control the temperature. Values are given ± 0.0005.
Electrochemical window. Electrochemical measurements were performed using a PJT24-1
potentiometer monitored by a PIL101T from Tacussel. A glass carbon working electrode, a
platinum auxiliary electrode and a Ag/AgCl pseudo-reference wire were used. Measurements
were performed with an 1 mL ionic liquid sample. Ionic liquid, stirring bar and electrodes
were put into an electrochemical cell and set under vacuum for one hour prior to
measurement. Then, ionic liquid was set under a continuous argon flow during measurements.
All electrochemical windows were performed in duplicates. Cathodic and anodic potentials
correspond to a current density of 50 µA.cm-2. Values are given ± 50 mV.
Toxicity bioassays. Ionic liquids toxicity was assessed using a bioluminescent bacteria
Vibrio fischeri. Light emission is related to the bacterial metabolism and its inhibition is an
indicator of the level of toxicity. This method has been widely used in environmental
ecotoxicology assays and standardized.85 The strain Vibrio fischeri (formerly Photobacterium
phosphoreum) was purchased from DSMZ GmbH (DMS 2167). Bacteria were cultivated in
an optimized NY liquid growth medium.86 Inoculums of bacterial suspension are mixed with
different concentrations of ionic liquid. Toxicity is expressed as EC50_15min (effective
concentration) that is the concentration at which a 50% reduction of the light emission is
observed after an exposure time of 15 min. EC50 values were then calculated on the basis of
the best-fit linear regression models assessing the concentration–effect relationship.
Results and discussion.
Glass-transition temperatures, crystallization temperatures and melting points. Glasstransition temperature (tg), crystallization temperatures (tcc) and melting points (tm) have been
measured for all ionic liquids synthesized here. Results are collected in Table 1. Figure 2
shows DSC curves obtained for selected ionic liquids.
As shown in figure 2, four types DSC thermograms are observed. a) only a glass-transition
temperature, and no melting nor crystallization points are observed upon heating. b) a glasstransition temperature and a melting point are observed upon heating, corresponding to most
room-temperature ionic liquids. c) a glass-transition temperature, a crystallization temperature
and a melting point are obtained upon heating. d) only a melting point, usually observed for
high-melting point? ionic liquids, such as ionic liquids containing halogenous, triflate or large
size anions, for instance. Details will be discussed below.
Most ionic liquids studied here exhibit glass-transition temperatures. tg values range from 31 °C for [O2M3M5MPYR][I] to -81 °C for [B2M3MPYR][NTf2]. Because the spherical
shape of iodide anion increases symmetry and ordering in ionic liquids, ionic liquids
containing this type of anion exhibit highest glass-transition temperatures measured here. In
agreement with previous observations concerning ionic liquids containing [NTf2] anions,
glass-transition temperatures are low.1,
7, 8
Values vary between -72 and -81 °C for
[O2M3M5MPYR][NTf2] and [B2M3MPYR][NTf2] respectively. For ionic liquids containing
dicyanamide anions, glass-transition temperatures are close to previously reported values.
[B2M3MPYR][N(CN)2] and [O3M5MPYR][N(CN)2]
respectively.
Values
of
-69,
-79
and
-80
°C
exhibit values of -72 and -80 °C,
were
previously
reported
for
[4MBPYR][N(CN)2], [4MOPYR][N(CN)2] and [OMIM][N(CN)2].8 Most ionic liquids
containing butyl chains exhibited lower tg values than those containing octyl chains. This is
probably due to the fact that when a shorter chain is present, electrostatic interactions are
stronger between ions. Additionally, lateral packing and van der Waals forces increases with
the alkyl chain length, giving rise to an increase in the cohesive energy of the IL.(le
polymorphisme n’est pas observé experiementalement, on peut juste comparer, mais il vaut
mieux ne pas en parler). Changing from a methyl to an ethyl group appended to the cation
such as from [O2M3M5MPYR] to [???] (je ne vois pas dans le tableau d’equivalent de
[O2M3M5MPYR] avec un ethyl? Par exemple [O2E3M5MPYR], pour vraiement comparer il
faudrait une même isomérie de position?), increases short-range interaction and further
increases glass-transition temperatures.
In this study, seven ionic liquids out of thirteen exhibiting melting points also exhibited
crystallization points (tcc). The apparition of crystallization point is favored by the presence of
multiple methyl or ethyl groups appended on the pyridinium ring. The apparition of such
thermal transitions happened with all types of cations and anions used here, and no clear trend
in terms of influence of cation or anion structure on crystallization point can be established.
As expected, ionic liquids containing iodide anions such as [O3M5MPYR][I] or
[O2M5EPYR][I] are solid at room-temperature. Despite the presence of three methyl
substituents on the pyridinium cation [O2M3M5MPYR][I] is liquid at room temperature,
though very viscous. Previous works had reported that addition and the position of methyl
substituents around imidazolium or pyridinium cations yielded in many cases ionic liquids
exhibiting higher melting points than for ionic liquids containing no alkyl substituents. 1,7,8
Because [NTf2] anion presents a highly delocalized charge and a rod-like shape, all ionic
liquids containing butyl chains and [NTf2] anions exhibited glass-transition temperatures, and
no melting point, whatever the alkyl chain or the cation ring used.
As reported previously,8 cation symmetry has a significant influence on melting points of
ionic liquids containing [TfO] anions. Here, [B2M3MPYR][TfO] exhibits a melting point
approximately 70 °C lower than that for [B3M5MPYR][TfO]. This is even more clear for
[O2M3MPYR][TfO]
which
exhibit
a
glass-transition
temperature
only,
whereas
[O3M5MPYR][TfO] exhibits a melting point of 76 °C.
Except for [O2M5EPYR][N(CN)2], all ionic liquids containing [N(CN)2] anion were found
to be liquid at room-temperature. This is due to the high delocalization of charge and the rodlike shape of the anion. Here again, cation symmetry influences melting points.
[B3M5MPYR][N(CN)2] exhibits a melting point of -1 °C, close to that for
[4MBPYR][N(CN)2] (-5 °C).12 As expected, when longer octyl chains are used, only glasstransition temperatures and no melting points are observed.
Similar to dicyanamide anion, thiocyanate is a small anion, with an important electron
delocalization between atoms. As expected, the longer the alkyl chain, the lower the meltingpoint. Besides, more symmetrical cations such as in [O3M5MPYR][SCN] compared to
[O2M3MPYR][SCN] yield ionic liquids with higher melting points. Unlike ionic liquids
containing
[TfO]
anions,
addition
of
an
extra
methyl
group
such
as
in
[O2M3M5MPYR][SCN] increases melting points.
Thermo-gravimetric analysis. Decomposition temperatures at 1 and 5 % weight loss,
denoted as t1% and t5%, have been measured for all ionic liquids reported here. Results are
shown in table 1, and plot of thermal degradation of selected ionic liquids are shown in
figures 3 and 4.
In agreement with previous observations, degradation of ionic liquids is mostly influenced
by the anion nature. For any cation studied here, decomposition of ionic liquids occurs in the
following anion order:
[I] < [SCN] < [N(CN)2] < [NTf2] ≤ [TfO] < [BF4]
Ionic liquids containing iodide anion exhibit lowest degradation temperatures with a t1%
ranging from 188 to 200 °C. In contrast, the highest degradation temperature is obtained for
[O3M5MPYR][BF4]. Due to the high thermal stability of [TfO] and [NTf2] anions, ionic
liquids containing these anions exhibit high t1% values ranging from 260 to 300 °C. Due to the
presence of cyanide groups in both thiocyanate and dicyanamide anions, thermal stabilities
obtained for ionic liquids containing [N(CN)2] or [SCN] anion are close and relatively low.
Ionic liquids containing thiocyanate anions exhibit t1% ranging from 220 to 240°C, and from
205 to 230°C for [N(CN)2]-containing ionic liquids, which is in agreement with previous
results obtained for 1-octyl-4-methylpyridinium dicyanamide (228 °C).87 As illustrated by fig.
4, the thermal degradation of ionic liquids containing [N(CN)2] anions occurs in a two-step
process.
Interestingly, (t5% - t1%) values appear to be mostly influenced by the nature of the anion.
For ionic liquids containing [SCN] anions, (t5% - t1%) values is of about 15-20°C. For ionic
liquids containing [N(CN)2]anions, (t5% - t1%) is 26 °C. Less thermally stable ionic liquids are
also those that decompose faster. For ionic liquids containing [TfO] and [NTf2] anions, (t5% t1%) values are higher, typically ranging from 20 to 45°C. Despite lower t1% values, ionic
liquids containing [NTf2] anion appear to decompose less rapidly than those for ionic liquids
containing [TfO] anions. These observations are clearly represented by a plot of t5% as a
function of t1% shown in figure 4. Ionic liquids are clearly separated in five groups according
to the nature of the anion.
The influence of the cation on thermal stability is not so clear. For the same anion, values
for t1% are not easily correlated with the cation structure. Nevertheless, except for
[O3M5MPYR][TfO], ionic liquids containing [O3M5MPYR] exhibits a 5°C decomposition
temperature higher than that obtained for other ionic liquids containing the same
anion(jcomprends pas cette phrase? lequel par rapport auquel?). The temperature variation
between t1% and t5% appears to be the largest when cations [B3M5MPYR] or [O3M5MPYR]
are used. For instance, (t5% - t1%) for [B3M5MPYR][NTf2] and [B3M5MPYR][TfO] are 38
and 36 °C, respectively. This type of cations therefore appears to yield most thermally stable
ionic liquids.
For ionic liquids based on [O2M5EPYR] cations, thermal stability is approximately 20 °C
lower than that obtained for ionic liquids containing the same anion(s’ils sont basés sur
[O2M5EPYR] et qu’ils contiennent le meme anion, alors…idem que la remarque plus haut,
c’est pas clair sur ce point) excepted for the iodide anion. Thus, the ethyl substituent appears
to be the weak link of these molecules.
Additionally, isothermal thermogravimetric analysis was performed by exposing ionic
liquids to a temperature of 225 °C for five hours in air. Ionic liquids [B3M5MPYR][TfO],
[B3M5MPYR][NTf2],
[O3M5MPYR][TfO]
and
[O3M5MPYR][NTf2]
were
tested.
Surprisingly, ionic liquids containing [NTf2] anion went under some decomposition, losing
approximately 5 % of weight after 5 hours spent under 225 °C, and turning from a clear to a
dark color. Ionic liquids containing [TfO], on the other hand, did not exhibit any color change,
and no significant weight loss was observed. These results are surprising, considering the
similar thermal stability obtained from decomposition temperatures. This tends to show that
ionic liquids containing [TfO] anions studied here are the most stable ionic liquids to be used
at high temperatures over long periods of time. (est-ce que c’est classique ce genre d’etude de
stabilités thermique isotherme en function du temps? Si non ca vaut le coup de point this out!
Ici ou dans la conclu, Car souvent on dit que c’est stable point , pour des applications ca peut
être très important)
Densities and refractive indexes. Densities and refractive indexes have been measured for
various ionic liquids exhibiting melting points below 20 °C. Results are collected in Table 2.
In agreement with previous reports, ionic liquids with [NTf2] anions exhibit highest
densities.1,3,87 This is confirmed here with the highest density measured here for
[B3M5MPYR][NTf2] (c juste pour eviter la repition de “exhibit highest”. Densities for ionic
liquids containing butyl chains are approximately 0.10 g.cm-3 higher than those of ionic
liquids containing octyl chains. Besides, except for ionic liquids containing [NTf2] anions,
presence of two methyl groups increases density of ionic liquids compared to those obtained
for 1-butyl-4-mehtylpyridinium or 1-octyl-4-methylpyridinium homologues.87 As reported
previously
1,3
, ionic liquids containing triflate anion exhibit densities lower than that for
[NTf2] anion. Because of the absence of trifluoromethyl groups on the anion, ionic liquids
containing dicyanamide or thiocyanate anions exhibit low densities close to or below that of
water. Hydrophobic [O3M5MPYR][N(CN)2] or [O3M5MPYR][SCN], for instance, are
slightly less dense than water since each ionic liquid remains above water upon mixing.
Refractive indexes range from 1.4476 to 1.5776. Results show that for the same anion,
refractive indexes of ionic liquids measured here are higher than those reported elsewhere for
ionic liquids based on imidazolium or tetrahydrothiophenium cations.3,23 For instance,
refractive index for [O2M3M5MPYR][I] is higher than that for 1-methyl-4-butylimidazolium
iodide (1.572) reported elsewhere.3 Anions have the most significant influence on refractive
indices: the smaller the anion, the higher the refractive index. For the same cation, refractive
indices are in the following anion order: n(NTf2) < n(TfO) < n(N(CN)2) < n(SCN) < n(I).
High refractive indexes found for dicyanamide or thiocyanate anions are consistent with
previous results 9 and are related to a high mobility of electrons in the anion. Despite the fact
that ionic liquids containing pyridinium cations generally exhibit higher refractive indexes,
the influence of the cation is not clear. Increasing the alkyl chain length or adding methyl
groups on the pyridinium have no systematic influence on refractive indexes.
Solubility in water. Solubilities in water were measured by measuring the intensity at the
maximum absorption wavelength (max) of pyridinium-based ionic liquids in water. UV-Vis
spectra of several ionic liquids are plotted in Figure 5. Maximum absorption wavelengths and
molecular extinction coefficients for selected ionic liquids are collected in Table 3. For the
same cation, max values are equal whatever the anion used.
Unlike 1-methylimidazole, pyridine or picoline reactants,
the compounds 2,3-
dimethylpyridine, 3,5- dimethylpyridine and 2,3,5-trimethylpyridine that are fragments of the
ILs studied here(sinon ce qui est comparé) have limited solubility in water. Solubilities in
water of ionic liquids based on, 2,3-dimethylpyridine, 3,5-dimethylpyridine and 2,3,5dimethylpyridine are therefore expected to be lower than those previously reported for ionic
liquids based on pyridine or picoline compounds.
In agreement with previous reports showing that ionic liquids containing octylpyridinium
cation were hydrophobic,8 all ionic liquids containing octyl chains studied here formed two
phases when they are in contact with water. Except for those containing [NTf2] anions, ionic
liquids containing butyl chains are soluble in water. Results are all reported in table 4.
Here, ionic liquids containing iodide anion are found to be hydrophobic. To our best
knowledge this is the first example of such hydrophobic ionic liquids based on iodide reported
so far. Solubility measurements were carried out in dark vials to avoid reaction of iodide and
water. Solubilities in water obtained for these ionic liquids are high, but are similar to other
ionic liquids previously reported containing octylmethylimidazolium [OMIM] (pas encore
réferencé ici), [OPYR] or [O4MPYRpour reprendre ta notation ici?] cations and anions [TfO],
[N(CN)2] or even [BF4].8
Furthermore, because of the increased hydrophobicity of the cation, solubilities in water for
ionic liquids containing dicyanamide or triflate anions are lower than those obtained for their
[OPYR] and [O4MPYR] homologues. For instance, [OPYR][TfO] exhibits a solubility in
water of 15900 ppm13 compared to 9750 and 10860 ppm obtained for [O3M5MPYR][TfO]
and [O2M3M5MPYR][TfO]. Solubility in water for [O4MPYR][N(CN)2] has been reported
to be above 50000 ppm.13 Here, values of 20005 and 15200 ppm have been found for
[O3M5MPYR][N(CN)2] and [O2M5EPYR][N(CN)2] respectively. Ionic liquids containing
thiocyanate anion and octyl chain exhibit solubility in water of approximately 10000 ppm,
and are in the range of those obtained for triflate.
Among all ionic liquids studied here, ionic liquids containing NTf2 anions exhibit lowest
solubilities in water. Solubilities in water for such compounds range from 3390 to 1660 ppm
for butyl-based ionic liquids, and from 400 to 260 ppm for octyl-based ionic liquids. As
previously reported, changing from butyl to octyl chain yields a 10-fold decrease in solubility
of ionic liquid in water. [B2M3M5MPYR][NTf2] and [O2M5EPYR][NTf2] exhibit lowest
solubility in water respectively for butyl and octyl NTf2 containing ILs (suggestion!). This
shows that hydrophobicity increases with the number of alkyl substituents on the cation ring.
Furthermore, solubilities in water of ionic liquids studied here are approximately twice as low
as those obtained for 1-butyl-4-methylpyridinium homologues.8 Solubility in water for
[B4MPYR][NTf2] was previously reported to be 4800 ppm, compared to 1660 ppm obtained
for [B2M3M5MPYR][NTf2].
With longer alkyl chain length, however, results are not similar to those obtained for butylbased ionic liquids. When the ionic liquid is very hydrophobic due to the anion and a longer
alkyl chain, the influence of cation ring is less pronounced. Therefore, no significant
differences in water solubility were found between [OPYR][NTf2] (350 ppm) measured
elsewhere8 and [O3M5MPYR]+[NTf2]- (310 ppm), [O2M3MPYR][NTf2] (400 ppm) or
[O2M5EPYR][NTf2] (260 ppm).
It appears that using a cation ring with increased hydrophobicity to synthesize ionic liquids
has a significant influence on solubility in water of ionic liquids when used with hydrophilic
anions and with short alkyl chain length. For ionic liquids containing hydrophobic anion such
as [NTf2] anion, and long alkyl chain, however, influence of trialkylpyridinium or
tetraalkylpyridinium cation on solubility in water is rather small.
Water content. Water-contents of several ionic liquids synthesized in this work have been
measured with the help of a Karl-Fisher coulometric apparatus. Prior to measurements,
samples were left in contact with air five weeks under vigorous stirring. Results are collected
in table 4.
Water-content (est-ce qu’on peut parler d’hygroscopy?) of ionic liquids is primarily related
to the nature of the anion. For every cation studied here, the water-content follows the trend
w(NTf2) < w(TfO) < w(N(CN)2) < w(SCN). Ionic liquids containing [NTf2] anions exhibit
lowest water-contents. The lowest water-content is 913 ppm for [O3M5MPYR][NTf2]. As
expected, the presence of extra methyl groups appended on the cation ring or a longer alkyl
chain yields lower water-contents. [OPYR][NTf2] exhibited a water-content of 2635 ppm,
compared to 913, 1064 and 969 obtained for [O3M5MPYR][NTf2], [O2M3MPYR][NTf2] and
[O2M5EPYR][NTf2] respectively. Water-content for [O2M3MPYR][TfO] is higher than
those obtained for [NTf2] based ionic liquids, which is in agreement with previous results. 1,8
When more hydrophilic anions such as [SCN] or [N(CN)2] are used, water-contents are
high,
ranging
from
25705
to
32913
ppm
for
[O3M5MPYR][N(CN)2]
and
[O2M3MPYR][SCN], respectively. Interestingly, it appears that ionic liquids containing
[SCN] are more hygroscopic than their [N(CN)2] homologues, despite the fact that the latter
ionic liquids exhibit higher solubility in water.
Electrochemical properties. Electrochemical windows have been recorded for four ionic
liquids,
namely
[OPYR][NTf2],
[O2M3MPPYR][NTf2],
[O3M5MPYR][NTf2]
and
[O2M3M5MPYR][NTf2], respectively. Results are given in Table 5.
Previous reports revealed that ionic liquids containing saturated cation heterocycles such as
pyrrolidinium (PYRRO) or piperidinium (PIP) are subject to reduction at much more negative
potentials than ionic liquids containing unsaturated imidazolium heterocycles. For instance,
reduction potentials below 3 V vs. Ag/AgCl were reported for [OMPYRRO][NTf2] (1methyl-1-octylpyrrolidinium NTf2),21 [OMPIP][NTf2] (1-methyl-1-octylpiperidinium NTf2)21
and below 2 V for [OMIM][NTf2],1 respectively. Oxidation potentials, on the other hand,
were found to be mostly influenced by anion nature, whatever the cation used.
20, 22,52
. Very
few works reported electrochemical windows for pyridinium ionic liquids. The value of -1.0
V
vs.
Ag/AgCl
for
1-butyronitrilepyridinium
bis(trifluoromethylsulfonyl)imide
([BCNPYR][NTf2]) was reported, consistent with the fact that hydrogen present on the
pyridinium cation, which is aromatic, are easily reduced.9
Our results show that electrochemical window for [OPYR][NTf2] is similar to that reported
previously for [BCNPYR][NTf2]: reduction and oxidation occur at -1.0 and 2.6 V vs.
Ag/AgCl. As it was observed for imidazolium cations,52 when methyl groups are added on
pyridinium cations, reduction occurs at slightly lower potentials. Reduction potentials of -1.2,
-1.3 and -1.3 V vs. Ag/AgCl were obtained for [O2M3MPYR][NTf2], [O3M5MPYR][NTf2]
and [O2M3M5MPYR][NTf2] respectively. The presence of methyl groups appended on the
pyridinium cation appears to have only small influence on the electrochemical stability of the
cation, due to the number of unsaturated carbons bound to hydrogen that are still available for
reduction.
This study confirms that pyridinium-based ionic liquids are poor electrochemical solvents.
Nevertheless, their high oxidation stability and thermal stability can be useful in hightemperature electrochemical applications involving oxidative species, such as ruthenium
complexes in dye-sensitized solar cells (DSSC), for instance. (ref non ?)
Toxicity. Besides the physico-chemical properties, toxicity of several ionic liquids on a
marine bacteria, Vibrio Fischeri bacteria has been studied. In order to compare the influence
of cation and anion on the toxicity of ionic liquids, five water-soluble ionic liquids containing
bromide anion and four hydrophobic ionic liquids containing NTf2 anion have been tested.
Results are shown in Table 6.
As expected, toxicity increases with alkyl chain length. For instance, [O3M5MPYR][Br] is
more toxic than [B3M5MPYR][Br]. For the latter, toxicity value measured here is consistent
with previous publications (EC50 = (4.9±2) 10-4 mol.L-1).88 The position of methyl groups on
pyridinium cation has very limited influence on toxicity as observed for [O3M5MPYR][Br]
and [O2M3MPYR][Br], while adding an extra substituent leads to a significant increase in
toxicity as for [O2M3M5MPYR][Br]. Question au passage avec des chaines alkyl fluorées
qu’est-ce ca donne pour la toxicité ? pour les tensioactifs c’est pas du « tout » toxique et ca
permet d’augmenter fortement l’hydrophobie)
In agreement with previous reports,88 hydrophobic ionic liquids containing butyl chain and
[NTf2] anion synthesized here exhibit higher toxicity (lower DL50 values) than water-soluble
bromide homologues. Toxicity of [B3M5MPYR][NTF2] is one order of magnitude higher
than
that
obtained
for
[B3M5MPYR][Br].
Similarly,
[O3M5MPYR][NTf2]
and
[O3M5MPYR][Br] exhibit EC50 values of 2.27 10-6 and 1.33 10-5 mol.L-1, respectively.
On the contrary, ionic liquids containing [O2M3M5MPYR] cations exhibit similar
toxicities whether bromide or [NTf2] anions are used. [O2M3M5MPYR][Br] and
[O2M3M5MPYR][NTf2] exhibit DL50 values of 4.38 10-6 and 4.20 10-6 mol/L-1 respectively.
In this case, ionic liquid toxicity is determined by the most toxic element (i.e.,
[O2M3M5MPYR], a cation containing a long alkyl chain and many methyl substituents) of
the ionic liquid. This implies that toxic effect of cations and anions are not always additive.
Conclusion. New hydrophobic ionic liquids, starting from hydrophobic alkylpyridine
reactants have been synthesized. Some properties are similar and many others are?
significantly different to those obtained for ionic liquids containing imidazolium or
pyridinium cations. Most ionic liquids are liquid at room temperature, despite the increase in
methyl groups appended on the cation. Decomposition temperatures are similar to those
previously reported for ionic liquids containing pyridinium cations. Electrochemical windows
are narrow, similar to those obtained for other pyridinium homologues and significantly
narrower than those reported for ILs containing imidazolium cations. Densities are slightly
lower than those obtained for other pyridinium homologues. Water-contents are lower and
refractive indices significantly higher than those reported for other ionic liquids.
Most important, however, all ionic liquids containing an octyl chain, even those containing
an iodide anion, are immiscible(only very slightly soluble in?) with water. Except for ionic
liquids containing an octyl chain and [NTf2] anion, solubilities in water are lower than those
for their methylpyridinium homologues. These new ionic liquids might therefore provide new
ionic liquids with increased hydrophobicity for various processes such as purification
processes of atmospheric gases or the extraction of metal ions from wastewater. In the case of
liquid-liquid extraction processes, though, the increased toxicity related to the presence of
extra methyl groups, implies that an efficient removal of ionic liquids from water after the
extraction procedure must be carried out. (j’aurais dit l’inverse que ca serait un pb pour le
wasterwater management (ou l’eau traitée est rejetée ) plutot que pour le liq/liq extraction qui
est utilisé en système fermé…?)
Supporting Information Available: Experimental details with synthesis procedures, 1H
NMR,
13C
NMR, and IR spectra, are available free of charge via the Internet at
http://pubs.acs.org.
Table 1. Glass-transition temperature, tg, crystallization temperature, tcc, melting point, tm,
heat capacity change, Cp, enthalpy of fusion, Hm, thermal degradation temperature at 1 %
weight loss, t1% and thermal degradation temperature at 5 % weight loss, t5% of ionic liquids
studied in this work.
tg/°C
[O3M5MPYR][I]
[O2M5EPYR][I]
[O2M3M5MPYR][I]
[B3M5MPYR][NTf2]
[B2M3MPYR][NTf2]
[B2M5EPYR][NTf2]
[B2M3M5MPYR][NTf2]
[O3M5MPYR][NTf2]
[O2M3MPYR][NTf2]
[O2M5EPYR][NTf2]
[O2M3M5MPYR][NTf2]
[B3M5MPYR][TfO]
[B2M3MPYR ?][TfO]
[B2M3M5MPYR][TfO]
[O3M5MPYR][TfO]
[O2M3MPYR][TfO]
[O2M3M5MPYR][TfO]
[B3M5MPYR][N(CN)2]
[B2M3MPYR][N(CN)2]
[O3M5MPYR][N(CN)2]
[O2M5EPYR][N(CN)2]
[B3M5MPYR][SCN]
[O3M5MPYR][SCN]
[O2M3MPYR][SCN]
[O2M3M5MPYR][SCN]
[O3M5MPYR][BF4]
-41
-33
-31
-73
-81
-81
-74
-78
-70
-78
-72
-69
-63
-56
-80
-72
-78
-59
-57
-74
-65
-58
-
tcc/°C
-11
8
-30
-32
22
-10
-
tm/°C
Cp /
 Hm /
t1%/°C
t5%/°C
82
87
91
17
74
76
-1
53
13
-38
20
56
J·g-1·K-1
0.1153
0.4211
0.3195
0.2684
0.3358
0.3256
0.2875
0.3857
0.3049
0.3812
0.2669
0.3241
0.3927
0.4926
0.5793
0.4453
0.3772
0.1543
0.2265
0.5040
0.5226
-
kJ·mol-1
25.45
30.87
28.50
16.88
20.97
24.81
3.85
16.04
6.39
14.66
28.07
202
194
188
264
261
266
266
288
260
255
283
281
305
294
288
295
300
227
226
231
205
212
219
206
204
312
220
208
210
302
289
292
296
334
301
284
318
317
322
319
329
315
321
254
252
257
231
239
236
221
223
349
Table 2. Density, , and refractive index, nD, for selected room-temperature ionic liquids at
20 °C.
Ionic liquid
[4MBPYR][NTf2]
[B3M5MPYR][NTf2]
[B2M3MPYR][NTf2]
[B25MEPYR][NTf2]
[B2M3M5MPYR][NTf2]
[OPYR][NTf2]
/g·cm-3
a
1.35
1.39
1.37
1.37
1.37
1.31
nD
1.4559
1.4497
1.4552
1.4583
1.4476
[O3M5MPYR][NTf2]
[O2M3MPYR][NTf2]
[O25MEPYR][NTf2]
[O2M3M5MPYR][NTf2]
[B2M3MPYR][TfO]
[O2M3MPYR][TfO]
[B3M5MPYR][N(CN)2]
[B2M3MPYR][N(CN)2]
[O3M5MPYR][N(CN)2]
[O3M5MPYR][SCN]
[O2M3MPYR][SCN]
[O2M3M5MPYR][I]
1.28
1.29
1.28
1.28
1.18
1.00
1.00
1.02
-
1.4526
1.4571
1.4617
1.4748
1.4725
1.5278
1.5403
1.5196
1.5415
1.5489
1.5776
Table 3. Maximum absorption wavelengths and molecular extinction coefficients for selected
ionic liquids containing trialkylpyridinium or tetraalkylpyridinium cations in water.
Ionic liquid
[B3M5MPYR][NTf2]
[B2M3MPYR][NTf2]
[B25MEPYR][NTf2]
[B2M3M5MPYR][NTf2]
[O3M5MPYR][NTf2]
[O2M3MPYR][NTf2]
[O2M5EPYR][NTf2]
[O3M5MPYR][TfO]
[O2M3M5MPYR][TfO]
Mw/g.mol-1
444.42
444.42
458.44
458.44
500.53
500.53
514.55
369.45
383.48
/nm
/L·mol-1·cm-1
271
271
273
277
271
271
271
271
277
6771
4783
6036
6552
5192
5778
6554
5051
6599
Table 4. Solubility in water, w, and water-contents for several ionic liquids exposed to air at
16 ± 1 °C.
Ionic Liquid
[O2M5EPYR][I]
[O2M3M5MPYR][I]
[B3M5MPYR][NTf2]
[B2M3MPYR][NTf2]
[B2M5EPYR][NTf2]
[B2M3M5MPYR][NTf2]
[O3M5MPYR][NTf2]
[O2M3MPYR][NTf2]
[O2M5EPYR][NTf2]
[O2M3M5MPYR][NTf2]
[OPYR][NTf2]
[O2M3MPYR][TfO]
[O3M5MPYR][TfO]
[O2M3M5MPYR][TfO]
[O3M5MPYR][N(CN)2]
[O2M3MPYR][N(CN)2]
w/ppm a
23850
28650
3390
2780
2490
1660
310
400
260
14870
9750
10860
20005
-
Wat. Cont./ppm b
1688
1505
1174 d
913
1064
969 c,d
1350 e
2635
9371
27576
27655
[O2M5EPYR][N(CN)2]
15210
[O2M3M5MPYR][N(CN)2]
25705
[O3M5MPYR][SCN]
10530
31329
[O2M3MPYR][SCN]
9720
32913
[O2M3M5MPYR][SCN]
29418
a: Solubility in water is reported in mg / kg of solution. b: Water-content is expressed in mg
per liter of solution. Ionic liquids were exposed 5 weeks to air at 28 ± 1 % relative humidity,
unless specified. c: Value obtained for ionic liquid exposed 2 weeks to air at 29 ± 1 % relative
humidity. d: single data measured. e: Value obtained for ionic liquid exposed 1 week to air at
29 ± 1 % relative humidity.
Table 5. Reduction potential, Ered, oxidation potential, Eox, and electrochemical windows, E,
for several ionic liquids containing pyridinium cation. All data are recrded with an Ag/AgCl
pseudo-reference electrode, at 20 °C.
Ionic Liquid
Ered/V a
Eox/V a
E/V
[O2M3MPYR][NTf2]
-1.2
2.4
3.6
[O2M5EPYR][NTf2]
-1.2
2.6
3.8
[O2M3M5MPYR][NTf2]
-1.3
2.6
3.9
[O3M5MPYR][NTf2]
-1.3
2.6
3.9
[OPYR][NTf2]
-1.0
2.6
3.6
a: Potential versus Ag/AgCl pseudo-reference electrode obtained for a current density of
i=50 µA.cm-2.
Table 6. Toxicities, DL50, for several ionic liquids on Vibrio fisheri.
Ionic Liquid
[B3M5MPYR][Br]
[B2M3M5MPYR][Br]
[O3M5MPYR][Br]
[O2M3MPYR][Br]
[O2M3M5MPYR][Br]
[B3M5MPYR][NTf2]
[B2M3M5MPYR][NTf2]
[O3M5MPYR][NTf2]
[O2M3M5MPYR][NTf2]
EC50/mol·L-1
3.10 10-4
3.01 10-4
1.33 10-5
1.19 10-5
4.38 10-6
7.18 10-5
3.65 10 -5
2.27 10-6
4.20 10-6
log(EC50)
-3.51
-3.52
-4.88
-4.92
-5.36
-4.14
-4.44
-5.64
-5.38
Figure Captions
Figure 1. Cation and anion structures used in this study.
Figure 2. DSC curves for selected ionic liquids.
Figure 3. TGA curves for ionic liquids containing 1-octyl-3,5-dimethylpyridinium cations.
Figure 4. Plot of t5% as a function of t1% for ionic liquids studied here.
Figure 5. UV-Vis spectra for selected ionic liquids.
Figure 1. Cation and anion structures used in this study.
+
N
+
R
R = 4, 1-butyl-2,3-dimethyl-pyridinium [B2M3MPYR]
R = 8, 1-octyl-2,3-dimethyl-pyridinium [B2M3MPYR]
+
N
N
R
R = 4, 1-butyl-3,5-dimethyl-pyridinium [B3M5MPYR]
R = 8, 1-octyl-3,5-dimethyl-pyridinium [B3M5MPYR]
+
R
N
R
R = 4, 1-butyl-2,3,5-trimethyl-pyridinium [B2M3M5MPYR]R = 4, 1-butyl-2-methyl-5-ethyl-pyridinium [B2M5EPYR]
R = 8, 1-octyl-2,3,5-trimethyl-pyridinium [B2M3M5MPYR]R = 8, 1-octyl-2-methyl-5-ethyl-pyridinium [B2M5EPYR]
O
F
F
N
S
N–
O
S
O O
F
F
F
O
F
N
F 3C
F
–
S
S
O
C
N–
N–
[NTf 2] anion
N
N
O
[TFO] anion
[SCN] anion
F
B– F
F
[N(CN) 2] anion
[BF4] anion
Figure 2. DSC curves for selected ionic liquids.
0
0
b)
a)
-2
Heat flow (W / g)
Heat flow (W / g)
-1
-2
-3
-4
-5
-100
-4
-6
-8
-10
-80
-60
-40
-20
0
20
40
60
-80
-60
-40
t (°C)
-20
0
20
60
0
8
6
-2
C)
d)
Heat flow (mW / g)
4
Heat flow (W / g)
40
t (°C)
2
0
-2
-4
-4
-6
-8
-10
-6
-12
-8
-10
-100
-80
-60
-40
-20
t (°C)
0
20
40
60
-14
-100
-50
0
t (°C)
50
100
a): DSC curve for [O2M3MPYR][NTf2]. b): DSC curve for [O2M3M5MPYR][SCN]. c):
DSC curve for [B3M5MPYR][SCN]. d): DSC curve for [O3M5MPYR][BF4]
Mass Weigth Loss (%)
Figure 3. TGA curves for ionic liquids containing 1-octyl-3,5-dimethylpyridinium cations.
I
SCN
NCN2
TFO
BF4
NTF2
100
90
80
70
60
50
40
30
20
10
0
50
150
250
350
450
550
650
Temperature (°C)
Figure 4. Plot of t5% as a function of t1% for ionic liquids studied here.
360
340
320
300
t
5%
280
260
240
220
200
180
180
200
220
240
260
280
300
320
340
360
t 1%
(●): Ionic liquid containing iodide anion. (): Ionic liquids containing SCN anion. ( ):
Ionic liquid containing N(CN)2 anion. (▼): Ionic liquids containing TfO anion. (○): ionic
liquids containing NTf2 anion. (□): Ionic liquid containing BF4 anion.
Figure 5. UV-Vis spectra for selected ionic liquids.
2,0
Absorbance
1,5
1,0
0,5
0,0
200
220
240
260
Wavelength (nm)
280
300
(●●●): UV-Vis spectra for [O2M3MPYR][TfO]. max = 271 nm. (●▬●): UV-Vis spectra
for [O3M5MPYR][NTf2]. max = 271 nm. (▬ ▬): UV-Vis spectra for [O2M5EPYR][NTf2].
max = 273 nm. (▬▬): UV-Vis spectra for [O2M3M5MPYR][NTf2]. max = 277 nm.
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