Trialkylsulfonium dicyanamides - A new family of ionic liquids with very low viscosities

Article abstract of DOI:10.1039/b510736a

Trialkylsulfonium dicyanamides show surprisingly low viscosities down to -20 °C and are therefore highly interesting liquid materials for separation processes and electrolyte applications at low temperatures. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b510736a

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Trialkylsulfonium dicyanamides - a new family of ionic liquids with very
low viscosities{
Dirk Gerhard,a Samim Cenk Alpaslan,a Heiner Jakob Gores,b Marc Uerdingenc and Peter Wasserscheid*a
Received (in Cambridge, UK) 28th July 2005, Accepted 19th August 2005
First published as an Advance Article on the web 14th September 2005
DOI: 10.1039/b510736a
synthesized and characterized includes some bis(trifluoromethyl-
sulfonyl)imide,12 bromide13 and haloaluminate13 salts. A particu-
larly low viscosity and high conductivity has been reported for
trimethylsulfonium bromide/hydrobromic mixtures (8.3 mPa*s/
56 mS*cm21 at 25 uC),14 but again the system is very difficult to
handle due to its high corrosiveness.
Trialkylsulfonium dicyanamides show surprisingly low visco-
sities down to 220 uC and are therefore highly interesting liquid
materials for separation processes and electrolyte applications
at low temperatures.
Ionic liquids (ILs) have gained wide popularity in recent years
because of their unique properties.1 They have been used in a
broad variety of catalytic,2 engineering3 and electrochemical
applications (like dye-sensitized-solar-cells4 or lithium-ion bat-
teries5). However, one of the strongest barriers to many
applications of ionic liquids is their relatively high viscosity
compared to organic solvents.6 High viscosity results in low
diffusion coefficients and thus leads to slow mass transfer in ionic
liquid/fluid multiphase systems. Furthermore, electric conductiv-
ities and viscosities are linked in a proportional fashion in many
systems with high viscosities leading to low conductivities.
Therefore, considerable research effort has been dedicated to the
development of new, low viscous ionic liquids in recent years. So
far most of this research has been focussed on the use of
imidazolium based ionic liquids. Namely systems including the
1-ethyl-3-methylimidazolium (EMIM) ions proved to be of high
interest. Combinations of this cation with tetrachloroaluminate
(18 mPa*s/23.0 mS*cm21 at 25 uC),7 tricyanomethanide
(18 mPa*s/18 mS*cm21 at 25 uC),8 dicyanamide (21 mPa*s/
27 mS*cm21 at 25 uC)9 and thiocyanate (21 mPa*s/21 mS*cm21 at
25 uC)10 ions were amongst the lowest viscous ionic liquids
reported so far. To the best of our knowledge the combination of
the [EMIM] cation and fluoride/hydrofluoric acid (4.85 mPa*s/
100 mS*cm21 at 25 uC)11 results in the lowest viscous ionic liquid
systems known to date. However, this system is not well defined
and extremely difficult to handle: Therefore, it is very unlikely that
this system will find practical applications.
In the present publication we present for the first time a new
family of ionic liquids with surprisingly low viscosities and high
conductivities based on trialkylsulfonium dicyanamides.
These ionic liquids are easily obtained in high quality by
metathesis of the corresponding trialkylsulfonium iodides and
silver dicyanamide (see Scheme 1). The precursors were prepared
following the synthetic approach of Paulsson et al.15 by a reaction
of equimolar amounts of dialkylsulfide and alkyl iodide in dry
acetone at ambient conditions.
Scheme 1 Synthesis of trialkylsulfonium dicyanamides.
Silver dicyanamide was precipitated by mixing aqueous
solutions of silver nitrate and sodium dicyanamide (1 : 1 molar
ratio). All ionic liquids were obtained as colourless or faint yellow
liquids. Details of the synthesis are given in the ESI{.
All new ionic liquids described in this paper were characterized
1
by H and 13C-NMR as well as by ESI-MS. The water content
was determined by Karl–Fischer titration (for more details
see ESI{).
The viscosities of all ionic liquids were measured in the range
from 220 uC to 80 uC. The conductivities were recorded at 20 uC.
All substances show the typical decrease of viscosity with
increasing temperature, with [Et3S][N(CN)2] showing at 80 uC the
lowest viscosity of all systems under investigation (5.7 mPa*s). All
results of the viscosity measurements are summarized in Table 1.
All trialkylsulfonium dicyanamides prepared show low visco-
sities at room temperature, ranging from 20.9 mPa*s for
[Et3S][N(CN)2] to 60.0 mPa*s for [MeBu2S][N(CN)2].
Remarkably, the viscosities decrease in the order [R9Bu2S] .
[R9Pr2S] . [R9Me2S] . [R9Et2S] for the methylated (R9 5 Me)
and the ethylated (R9 5 Et) species. Furthermore the viscosities of
ionic liquids with R9 5 Et are always slightly lower than those
of the corresponding methyl-substituted compounds. To the best
of our knowledge some of the new ionic liquids show the lowest
viscosities at 220 uC that have been reported so far for binary
ionic liquid systems. In order to do a proper comparison at this
Furthermore, N,N-dialkylpyrolidinium dicyanamide salts have
been reported to form low viscous ionic liquids as dicyanamide
salts (e.g. for methylpropyl pyrrolinium dicynamide a viscosity of
45 mPa*s at 25 uC has been published).9
So far only a few trialkylsulfonium based ionic liquids have been
described in the literature. The list of salts that have been
aLehrstuhl fu¨r Chemische Reaktionstechnik, Universita¨t Erlangen-
Nu¨rnberg, Egerlandstr. 3, Erlangen, D-91058, Germany.
E-mail: Wasserscheid@crt.cbi.uni-erlangen.de;
Fax: +49 (0)9131 8527421; Tel: +49 (0)9131 8527420;
bInstitut fu¨r Physikalische und Theoretische Chemie, Universita¨t
Regensburg, Regensburg, D-93040, Germany
cSolvent Innovation GmbH, Ko¨ln, Nattermannallee 1, Ko¨ln, D-50829,
Germany
{ Electronic supplementary information (ESI) available: Experimental
details. See http://dx.doi.org/10.1039/b510736a
5080 | Chem. Commun., 2005, 5080–5082
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Table 1 Properties of trialkylsulfonium dicyanamide ionic liquids
H2O
/ppm
g220 uC
mPa*s
/
g0 uC
mPa*s
/
g20 uC
mPa*s
/
g40 uC
mPa*s
/
g60 uC
mPa*s
/
g80 uC
mPa*s
/
x20 uC
/
Tdec
uCb
/
[R2R9S] [N(CN)2]
mS*cm21
a
Me3S
250.7
582.4
388.4
139.2
484.7
271.3
161.4
136.8
76.2
47.9
70.7
169.0
55.7
43.3
68.6
143.0
27.2
22.9
29.5
60.0
25.3
20.9
29.4
51.7
17.1
13.2
15.7
27.8
14.1
12.1
15.6
24.8
10.5
8.7
9.7
15.4
9.1
7.9
9.6
7.2
6.2
6.6
9.7
6.4
5.7
6.5
8.8
20.8
26.8
15.0
5.9
22.1
22.4
14.7
6.9
183
176
178
178
182
180
176
179
MeEt2S
MePr2S
MeBu2S
EtMe2S
Et3S
EtPr2S
EtBu2S
a
102.0
185.0
509.0
120.0
94.9
173.0
494.0
14.0
b
Solid at 220 uC. TGA-onset.
temperature we also determined the viscosity data of the well
known [EMIM] systems [EMIM][N(CN)2] (122mPa*s at 355 ppm
water content), [EMIM][SCN] (110 mPa*s at 131 ppm water
content) and [EMIM][(CF3SO2)2N] (231 mPa*s at 21 ppm water
content). The comparison of these values with the low temperature
viscosity data of the trialkylsulfonium dicyanmides clearly reveals
the high attractiveness of the new ionic liquids for low temperature
applications.
with a constant argon flow influences the thermal stability, because
volatile decomposition products are constantly removed from the
equilibrium. As shown in Fig. 1, trimethylsulfonium dicyanamide
showed good thermal stability over 12 h at 50 uC, but lost about
5% of its weight in the 12 h experiment at 100 uC. At more elevated
temperature the decomposition process was much faster.
Remarkably, the TGA measurements at 150 uC indicate a
maximum weight loss of ca. 43%. For the trialkylsulfonium
dicyanamides under investigation this finding correlates to the
formation of volatile dimethylsulfide and a non-volatile methyldi-
cyanamide species over time with the latter remaining in the
sample chamber.
The new ILs show moderate to high conductivities up to
26.8 mS*cm21 for [MeEt2S][N(CN)2] at 25 uC. The conductivity
depends on the chain length of the alkyl groups attached to the
sulfur. For R9 5 Me and R9 5 Et the conductivity also follows
the order [R9Bu2S] . [R9Pr2S] . [R9Me2S] . [R9Et2S], indicating
the expected correlation between viscosity and conductivity.
Therefore the highest conductivities are obtained for
The absence of the imidazolium moiety in the new ionic liquids
prompted us to study the UV/Vis spectra of these new melts. Ionic
liquids with no absorbance of light down to the UV range may be
of interest for some optical applications. Indeed, trimethylsulfo-
nium dicyanamide dissolved in water shows the same absorbance
spectra than an aqueous solution of sodium dicyanamide,
indicating that the cation itself is UV/Vis inactive. However, for
the same ionic liquid in neat form (measured in a 2 mm cuvette) a
significant absorbance occurred at 350 nm with a fairly long
absorption tail even beyond 400 nm. The spectra are given in the
ESI{. Trimethylsulfonium dicyanamide is soluble in water,
acetonitrile and acetone. Miscibility gaps are found with toluene,
diethylether and—very remarkably—also with dichloromethane
and chloroform.
[MeEt2S][N(CN)2] (26.8 mS*cm121
)
and [Et3S][N(CN)2]
(22.4 mS*cm21). It is noteworthy that the ionic liquid with the
lowest viscosity in this study did not show the highest conductivity.
This effect cannot be attributed to impurities as water or traces of
halides should change both the conductivity and the viscosity.16
All new ionic liquids were also studied by DSC and TGA
analysis. However, using DSC only the melting points of three of
the substances were obtained. The melting point of trimethylsul-
fonium dicyanamide was determined to be at 21 uC, whereas
ethyldimethylsulfonium dicyanamide melts at 228 uC and
ethyldipropylsulfonium dicyanamide melts at 234 uC. All other
systems did not show a proper melting point down to 2140 uC.
Moreover, all systems except [Me3S][N(CN2)] were liquids
at 220 uC.
In conclusion, our results provide strong evidence that
trialkylsulfonium dicyanamides are a very promising family of
new ionic liquid materials especially for low temperature
Since ionic liquids have no apparent vapour pressure, their
upper temperature limit is determined by their thermal decom-
position temperature. For the new trialkylsufonium dicyanamides
these values are given as TGA onset data in Table 1. In
comparison to typical imidazolium based ionic liquids the
trialkylsulfonium dicyanamides show significant lower tempera-
tures of decomposition (for comparison: the temperature of
decomposition for [EMIM][N(CN)2] has been found to be
240 uC9). All ionic liquids studied here decompose in a very
narrow range between 176 to 183 uC. From this fact, we conclude
that the process of thermal degradation is influenced very little by
the alkyl substitution pattern at the ionic liquid’s cation.
Furthermore, long-term thermal stability of trimethylsulfonium
dicyanamide was determined by isothermal TGA experiments in
an argon purged system over 12 h. As expected, it was found that
the long-term stability in the open system is significantly lower
than the TGA onset data. Of course, purging the TGA chamber
Fig. 1 Long-term stability of trimethylsulfonium dicyanamide.
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viscosity which may help to overcome traditional problems of slow
mass transport and low electrical conductivity found in the
traditional, more viscous ionic liquids at these low temperatures.
We could further show that the thermal stability for applications at
temperatures below 50 uC is excellent and an operating
temperature range of up to 100 uC may be realistic in closed
systems. We anticipate that these new ionic liquids will find
applications in the area of low temperature separation
technologies (e.g., gas separation) and electrochemical devices
such as dye-sensitized-solar-cells.
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