air and water stable ionic liquids
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Air and water stable ionic liquids in physical chemistry
Frank Endres* and Sherif Zein El Abedinw
Received 12th January 2006, Accepted 24th February 2006
First published as an Advance Article on the web 17th March 2006
DOI: 10.1039/b600519p
Ionic liquids are defined today as liquids which solely consist of cations and anions and which by
definition must have a melting point of 100 1C or below. Originating from electrochemistry in
AlCl3 based liquids an enormous progress was made during the recent 10 years to synthesize ionic
liquids that can be handled under ambient conditions, and today about 300 ionic liquids are
already commercially available. Whereas the main interest is still focussed on organic and
technical chemistry, various aspects of physical chemistry in ionic liquids are discussed now in
literature. In this review article we give a short overview on physicochemical aspects of ionic
liquids, such as physical properties of ionic liquids, nanoparticles, nanotubes, batteries,
spectroscopy, thermodynamics and catalysis of/in ionic liquids. The focus is set on air and water
stable ionic liquids as they will presumably dominate various fields of chemistry in future.
Preface
When one of us (FE) gave a lecture with the title In situSTM
investigations of metal electrodeposition in room temperature
molten salts on a Bunsenkolloquium in 1999 in Germany,
one person from the audience asked how an STM can be
operated in a molten salt at high temperatures. This question
was unexpected at that time and the speaker (FE) answered as
diplomatically as possible that these salts are liquid at room
temperature, as mentioned in the title of the lecture. Never-
theless, this question made clear that such liquids were hardly
known at that time in electrochemistrynot too surprising
if one takes into account a worldwide output of maybe 50
papers per year in 1999 in the field of room temperature
molten salts/ionic liquids. The lecture was commented as very
interesting but unusual and a few people in the audience
expressed an opinion that these liquids will never be employed
in any technical process for the forthcoming 100 years. In the
Molten Salt Community (maybe 2030 groups worldwide)
on the other hand, these room temperature molten salts
were regarded as uncommon and as a curiosity for a while,
maybe because they need more chemistry than simple metal
halides. The experience of many colleagues working with these
liquids showed that the expression molten salt has always
been associated with high temperature, as we also had to
learn. It was about in the middle of the 1990s when it was
decided in the community to replace the term room tempera-
ture molten salt by ionic liquid, and an ionic liquid is
defined today as a liquid consisting solely of cations and
anions with a melting point of 100 1C and below. Although
any high temperature molten salt is an ionic liquid, too, this
novel term for the room temperature liquids clearly made a
distinction, and we ourselves were never asked again how
an in situ STM can be operated in molten salts at high
temperatures.
As we will show below, the output of papers with the
expression ionic liquid started to increase about 2000, and
in the following years even technical processes were intro-
duced. The most famous one might be the BASIL-process
from BASF (biphasic acid scavenging utilizing ionic liquids)
where the side product of an organic reaction is an easy to
process ionic liquid instead of a less favourable solid in the
conventional process. Fortunately, it took only a few years
since 1999 until the first commercial process was introduced.
In 2005 there were more than 1500 peer reviewed papers
containing the expression ionic liquid or ionic liquids,
and from 1995 to 2005 we found more than 4300 papers.
About 30% of these papers deal with any aspect of physical
chemistry. When we got the invitation to write this review
article we had to make a selection, and we are well aware
that other authors would probably have selected different
topics. As we focus on several aspects of interface electro-
chemistry in our own research field we summarize more or
less completely the state-of-the-art of nano-electrochemistry
and electrodeposition in air and water stable ionic liquids.
Furthermore, we introduce the physical properties of ionic
liquids, nanoparticles, nanotubes, batteries, spectroscopy,
thermodynamics and catalysis of/in ionic liquids. We focus
on air and water stable ionic liquids, as in our opinion they will
dominate various fields of chemistry in the futurenot at all
surprising if one takes into account that theoretically 1018
different ionic liquids are possible. The AlCl3-based ionic
liquids, with which the research began seriously in the 1980s,
will rather survive in electrochemistry, e.g. for the electrode-
position of aluminium and its alloys. We hope that with
our review article an inexperienced reader will get a starting
point to find his own way in the physical chemistry of ionic
liquids.
Faculty of Natural and Materials Sciences, Clausthal University ofTechnology, Robert-Koch-Str. 42, 38678 Clausthal-Zellerfeld,Germany. E-mail: [email protected]; Fax: 0049-5323-722460w Permanent address: Electrochemistry and Corrosion Laboratory,National Research Centre, Dokki, Cairo, Egypt.
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1. Introduction
1.1 A brief history
The early history of ionic liquids began in 1914 when the first
report of a room temperature molten salt was reported by
Walden.1 He reported the physical properties of ethylammo-
nium nitrate, [C2H5NH3] NO3, which has a melting point of
12 1C, formed by the reaction of ethylamine with concentrated
nitric acid. Then, Hurley and Weir2 stated that a room
temperature ionic liquid could be prepared by mixing and
warming 1-ethylpyridinium chloride with aluminum chloride.
In 1970s and 1980s, Osteryoung et al.3,4 and Hussey et al.57
carried out extensive research on organic chloridealuminium
chloride ambient temperature ionic liquids and the first major
review of room temperature ionic liquids was written by
Hussey.8 The ionic liquids based on AlCl3 can be regarded
as the first generation of ionic liquids.
The hygroscopic nature of AlCl3 based ionic liquids has
delayed the progress in their use in many applications since
they must be prepared and handled under inert gas atmo-
sphere. Thus, the synthesis of air and water stable ionic
liquids, which are considered as the second generation of
ionic liquids, attracted further interest in the use of ionic
liquids in various fields. In 1992, Wilkes and Zaworotko9
reported the first air and moisture stable ionic liquids based
on 1-ethyl-3-methylimidazolium cation with either tetrafluoro-
borate or hexafluorophosphate as anions. Unlike the chloro-
aluminate ionic liquids, these ionic liquids could be prepared
and safely stored outside of an inert atmosphere. Generally,
these ionic liquids are water insensitive, however, the exposure
to moisture for a long time can cause some changes in their
physical and chemical properties. From our experience, we
have found using in situ scanning tunneling microscopy that
the undried ionic liquid [BMIm] PF6 attacks the gold
substrate, and its aggressiveness increases with the increase
in water content. This is due to the formation of HF as a result
of decomposition of the ionic liquid in presence of water.
Therefore, ionic liquids based on more hydrophobic anions
such as tri-fluoromethanesulfonate (CF3SO
3 ), bis-(trifluoro-
methanesulfonyl) imide [(CF3SO2)2N] and tris-(trifluoro-
methanesulfonyl) methide [(CF3SO2)3C] have been
developed.1012 These ionic liquids have received extensive
attention not only because of their low reactivity with water
but also because of their large electrochemical windows.
Usually these ionic liquids can be well dried the water contents
below 1 ppm under vacuum at temperatures between 100
and 150 1C.
The histogram of Fig. 1 shows the increase of the number of
publications on ionic liquids during the last decade up to now.
As seen, the average number of publications in the last decade
is about 40 papers per year while in 2004 about 1000 papers
and in 2005 about 1500 papers were published. This reflects
the increased interest in ionic liquids in general.
Beside Osteryoung, Wilkes, Hussey and Seddon who are
pioneers in the field of ionic liquids, there are several scientists,
e.g. Rogers, Welton, Wasserscheid, MacFarlane, Ohno, End-
res, Davis, Jr, Abbott, and others, who entered this field
having a strong impact in introducing the ionic liquids in
many applications.
Rogers is one of the highly cited authors in the field of ionic
liquids. He focuses on the synthesis and characterization of
environmentally friendly ionic liquids as green solvents. He
measured and published physicochemical properties data for
many ionic liquids with the aim of providing data to start
evaluating the use of ionic liquids in a variety of processes.
Also, he works on the development of new materials from
cellulose utilizing ionic liquids.
Welton has published many papers dealing with the appli-
cations of ionic liquids as solvents for synthesis and catalysis.
He focuses on how the ionic liquids interact with solute species
to affect their reactivity and he works on replacing environ-
mentally damaging solvents with more benign alternatives. He
is also the author of one of the most cited papers13 which was
cited 1719 times up to November 2005.
Wasserscheid is an active member of the ionic liquid com-
munity and focuses on the preparation and characterization of
ionic liquids for use in the biphasic catalysis. For example, he
could show that the use of hexafluorophosphate ionic liquids
allows selective, biphasic oligomerization of ethylene to 1-
olefins. Together with Welton, he edited a very important
book entitled Ionic Liquids in Synthesis which presents the
synthesis and physicochemical properties of ionic liquids as
well as their use in catalysis, polymerization, and organic and
inorganic synthesis.14
MacFarlane works on the synthesis of new air and water
stable ionic liquids with the purpose of employing such ionic
liquids as indicators for sensing and displaying an environ-
mental parameter such as humidity. This process is controlled
by the colour change of the ionic liquids where they are
synthesized with either a coloured cation or anion, so that
the ionic liquids themselves are sensors. Also, he has published
many papers on the use of ionic liquids in electropolymeriza-
tion and in batteries.
Fig. 1 Publications containing the phrase ionic liquid or ionic
liquids in the title; abstract and key words; determined by ISI web
of science; as a function of time.
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Ohno concentrates his work on the synthesis of a series of
polymerizable ionic liquids and their polymerization to pre-
pare a new class of ion conductive polymers. For example, he
prepared polymer electrolytes with high ionic conductivity and
good elasticity by mixing nitrite rubber (poly(acrylonitrile-co-
butadiene) rubber) with the ionic liquid N-ethylimidazolium
bis(trifluoromethanesulfonyl)imide. Quite recently, he edited a
book entitled Electrochemical aspects of ionic liquids which
introduces some basic and advanced studies on ionic liquids in
the field of electrochemistry.15
Davis, Jr introduced the concept of task-specific ionic
liquids (TSILs) in the field of ionic liquids. TSILs are ionic
liquids in which a functional group is incorporated enabling
the liquid to behave not only as a reaction medium but also as
a reagent or catalyst in some reactions or processes.
Abbott has recently developed a range of ionic compounds,
which are fluid at room temperature. These ionic liquids are
based on simple precursors such as choline chloride (vitamin
B4) which is cheap and produced on a multitonne scale and
hence these ionic liquids/deep eutectic solvents can be applied
to large scale processes for the first time. Using these liquids, a
number of applications are now under development such as
electrodeposition of metals, electropolishing and ore processing.
We ourselves (Endres and Zein El Abedin) started about 10
years ago to study nanoscale processes at the interface elec-
trode/ionic liquid using in situ (electrochemical) scanning
tunneling microscopy (in situ-STM). We could show for the
first time that Ge, Si, Se, Ta and Al can be electrodeposited in
high quality in air and water stable ionic liquids. Presumably
many more elements and compounds can be made electro-
chemically. Some recent results of the nanoscale electrodepo-
sition in water and air stable ionic liquids will be presented.
2. Physical properties of ILs
2.1. Conductivity
Ionic liquids have reasonably good ionic conductivities com-
pared with those of organic solvents/electrolyte systems (up to
B10 mS cm1).16 At elevated temperatures of e.g. 200 1C a
conductivity of 0.1 O1 cm1 can be achieved for some
systems. However, at room temperature their conductivities
are usually lower than those of concentrated aqueous electro-
lytes. Based on the fact that ionic liquids are composed solely
of ions, it would be expected that ionic liquids have high
conductivities. This is not the case since the conductivity of
any solution depends not only on the number of charge
carriers but also on their mobility. The large constituent ions
of ionic liquids reduce the ion mobility which, in turn, leads to
lower conductivities. Furthermore, ion pair formation and/or
ion aggregation lead to reduced conductivity. The conductiv-
ity of ionic liquids is inversely linked to their viscosity. Hence,
ionic liquids of higher viscosity exhibit lower conductivity.
Increasing the temperature increases conductivity and lowers
viscosity.
2.2. Viscosity
Generally, ionic liquids are more viscous than common mole-
cular solvents and their viscosities are ranging from 10 mPa s
to about 500 mPa s at room temperature. The viscosities of
some popular air and water stable ionic liquids at room
temperature are: 312 mPa s for [BMIm]PF6;17 154 mPa s for
[BMIm]BF4;18 52 mPa s for [BMIm]TF2N;
10 85 mPa s for
[BMP]TF2N.12 The viscosity of ionic liquids is determined by
van der Waals forces and hydrogen bonding. Electrostatic
forces may also play an important role. Alkyl chain lengthen-
ing in the cation leads to an increase in viscosity.10 This is due
to stronger van der Waals forces between cations leading to
increase in the energy required for molecular motion. Also, the
ability of anions to form hydrogen bonding has a pronounced
effect on viscosity. The fluorinated anions such as BF4 and
PF6 form viscous ionic liquids due to the formation of
hydrogen bonding.19 In general, all ionic liquids show a
significant decrease in viscosity as the temperature increases
(see,e.g., ref. 20).
2.3. Density
Ionic liquids in general are denser than water with values
ranging from 1 to 1.6 g cm3 and their densities decrease with
increase in the length of the alkyl chain in the cation. 21 For
example, in ionic liquids composed of substituted imidazolium
cations and CF3SO3 anion the density decreases from 1.39 g
cm3 for [EMIm]1 to 1.33 g cm3 for [EEIm]1, to 1.29 g cm3
for [BMIm]1 and to 1.27 g cm3 for [BEIm]1.22 The densities
of ionic liquids are also affected by the identity of anions. For
example, the densities of 1-butyl-3-methylimidazolium type
ionic liquids with different anions, such as BF4, PF6, TFA and
Tf2N are 1.12 g cm3,23 1.21 g cm3,10 1.36 g cm3 23 and 1.43
g cm3,10 respectively. The order of increasing density for
ionic liquids composed of a single cation is: [CH3SO3]
E
[BF4]
o [CF3CO2]
o [CF3SO3]
o [C3F7CO2]
o
[(CF3SO2)2N].22
2.4. Melting point
As a class, ionic liquids have been defined to have melting
points below 100 1C and most of them are liquid at room
temperature. Both cations and anions contribute to the low
meting points of ionic liquids. The increase in anion size leads
to a decrease in melting point.24 For example, the melting
points of 1-ethyl-3-methylimidazolium type ionic liquids with
different anions, such as [BF4] and [Tf2N]
are 15 1C25 and
3 1C,10 respectively. Cations size and symmetry make an
important impact on the melting points of ionic liquids. Large
cations and increased asymmetric substitution results in a
melting point reduction.26
2.5. Thermal stability
Ionic liquids can be thermally stable up to temperatures of
450 1C. The thermal stability of ionic liquids is limited by the
strength of their heteroatomcarbon and their heteroatom
hydrogen bonds, respectively.24 Wilkes et al.27 reported that
the ionic liquids 1-ethyl-3-methyl-imidazolium tetrafluoro-
borate, 1-butyl-3-methyl-imdazolium tetrafluoroborate and
1,2-dimethyl-3-propyl imidazolium bis(trifluorosulfonyl)imide
are stable up to temperatures of 445, 423 and 457 1C,
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respectively. Our experiences shows that such high tempera-
tures are only tolerated by most liquids for a short time. Long
time exposure to such high temperatures inevitably leads to
decomposition. Most of the ionic liquids have extremely low
vapour pressures, which allows to remove water by simple
heating under vacuum. Water contents below 1 ppm are quite
easy to achieve with most of the liquids.
2.6. Electrochemical window
The electrochemical window is an important property and
plays a key role in using ionic liquids in electrodeposition of
metals and semiconductors. By definition, the electrochemical
window is the electrochemical potential range over which the
electrolyte is neither reduced nor oxidized at an electrode. This
value determines the electrochemical stability of solvents. As
known, the electrodeposition of elements and compounds in
water is limited by its low electrochemical window of only
about 1.2 V. On the contrary, ionic liquids have significantly
larger electrochemical windows,e.g., 4.15 V for [BMIm]PF6at
a platinum electrode,28 4.10 V for [BMIm]BF428 and 5.5 V for
[BMP]Tf2N at a glassy carbon electrode.12 In general, the wide
electrochemical windows of ionic liquids have opened the door
to electrodeposit metals and semiconductors at room tempera-
ture which were formerly obtained only from high temperature
molten salts. For example, Al, Mg, Si, Ge, and rare earth
elements can be obtained from room temperature ionic li-
quids. The thermal stability of ionic liquids allows to electro-
deposit Ta, Nb, V, Se and presumably many other ones at
elevated temperature.
3. Electrosynthesis in air and water stable ionic
liquids
In this section we will report on the use of some popular air
and water stable ionic liquids such as, ZnCl2/[EMIm] Cl,
[EMIm] BF4, [BMIm] BF4, [BMIm] PF6, [BMP] Tf2N,
[BMIm] Tf2N and choline chlorideMCl in electrodeposition
of metals and semiconductors in the bulk phase. Furthermore
we will introduce nanoscale processes at the interface elec-
trode/ionic liquid as well as in electropolymerization. We focus
solely on the novel air and water stable liquids, as in our
opinion they will be of significant interest for several aspects of
electrochemistry.
3.1. Electrodeposition of metals and alloys
Katayama et al.29 have reported that a room temperature
ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate
([EMIm]BF4) is applicable as an alternative electroplating
bath for silver. The ionic liquid [EMIm]BF4 is superior to the
chloroaluminate systems since the electrodeposition of silver
can be performed without the risk of aluminium codeposition.
Electrodeposition of silver in the ionic liquids 1-butyl-
3-methylimidazolium tetrafluoroborate ([BMIm]BF4) and
[BMIm]PF6 was also reported in ref. 30. It was furthermore
stated that Cd,31 Cu32 and Sb33 can be electrodeposited in a
mixture of 1-ethyl-3-methyl imidazolium tetrafluoroborate
([EMIm]BF4) and [EMIm]Cl. Recently, Sun et al. have de-
monstrated that compound semiconductors such as indium
antimonide (InSb)34 and cadmium telluride (CdTe)35 can be
electrodeposited in the Lewis basic 1-ethyl-3-methylimidazo-
lium tetrafluoroborate ionic liquid [BMIm]BF4. InSb is a
IIIV compound semiconductor and CdTe is a IIVI semi-
conductor, both are widely used in many fields such as
electronic devices and solar cells.
It was stated in ref. 36 that titanium can be electrodeposited
in thin layers of maybe 5 nm at room temperature in the ionic
liquid 1-butyl-3-methylimidazolium bis (trifluoromethylsulfo-
nyl) imide [BMIm] Tf2N. With all refractory metals the
challenge in depositing micrometer thick solely metallic layers
is to avoid the growth of non-stoichiometric subhalides.
It has been shown that ionic liquids can be formed
by the combination of zinc chloride with pyridinium-,37
dimethylethylphenyl-ammonium-, 38 1-ethyl-3-methylimidazo-
lium chloride [EMIm]Cl and 1-butyl-3-methylimidazolium
chloride [BMIm]Cl.3941 These ionic liquids are quite easy to
prepare and do not decompose in the presence of water and
air. It was reported42 that the potential limits for a basic 1 : 3
ZnCl2[EMIm]Cl ionic liquid corresponds to the cathodic
reduction of [EMIm]1 and anodic oxidation of Cl, giving
an electrochemical window of approximately 3.0 V. For acidic
ionic liquids that have a ZnCl2[EMIm]Cl molar ratio higher
than 0.5 : 1, the negative potential limit is due to the deposition
of metallic zinc, and the positive potential limit is due to the
oxidation of the chlorozincate complexes. As a result of this
fact, the electrodeposition of Zn and its alloys is possible in
the Lewis acidic liquids. It was shown that Lewis acidic
ZnCl2[EMIm]Cl (in which the molar percentage of ZnCl2 is
higher than 33 mol%) are potentially useful for the electro-
deposition of zinc and zinc containing alloys.4345 Huang and
Sun have reported that PtZn alloy,46 iron and ZnFe alloy,47
tin and SnZn alloy,48 cadmium and CdZn alloy49 can
be electrodeposited in Lewis acidic ZnCl2[EMIm]Cl ionic
liquids.
Abbott et al.50 have reported the synthesis and character-
ization of new moisture stable, Lewis acidic ionic liquids/deep
eutectic solvents made from metal chlorides and quaternary
ammonium salts which are commercially available. They have
shown that mixtures of choline (2-hydroxyethyltrimethylam-
monium) chloride [(H3C)3NC2H4OH)Cl] and MCl2(M = Zn,
Sn) give conducting and viscous liquids at or around room
temperature. These liquids are easy to prepare, they are water
and air insensitive and their low costs enable their use in large
scale applications. Furthermore, they have reported51 that a
dark green, viscous liquid can be formed by mixing choline
chloride with chromium(III) chloride hexahydrate and the
physical properties of this liquid are characteristic of an ionic
liquid. The eutectic composition is found to be 1 : 2 choline
chloride/chromium chloride. From this ionic liquid chromium
can be electrodeposited efficiently to yield a crack-free depos-
it.51 Addition of LiCl to the choline chloride/CrCl3 6H2O
mixture was found to allow the deposition of nanocrystalline
black chromium films.52 The use of this ionic liquid might offer
an environmentally friendly process for electrodeposition of
chromium instead of the currently used chromic acid based
baths.
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3.2. Electrodeposition on the nanoscale
Almost 10 years ago we started for the first time with in situ
STM studies on electrochemical phase formation in ionic
liquids. On the one hand, there was no knowledge on the
local processes of phase formation in ionic liquids at all; on the
other handdue to wide electrochemical windowsthese
systems give access to elements that cannot be obtained in
aqueous solutions, such as Al, Ge, Si, Ta and many more.Especially in the rapidly growing field of nanotechnology
where semiconductor nanostructures will play an important
role, we see a great chance for electrodeposition of nano-
structures in ionic liquids. For this purpose the electrochemical
processes and the factors that influence the deposition and the
stability of the structures have to be understood on the
nanometer scale.
3.2.1. Germanium. Germanium is an elemental semicon-
ductor with an indirect band gap of 0.67 eV at room tempera-
ture in the microcrystalline phase. Furthermore, quite in
contrast to metals, its crystal structure is determined by the
tetrahedral symmetry of the Ge atoms so that the diamond
structure is thermodynamically the most stable one. Germa-
nium can hardly be obtained in aqueous solutions as its
deposition in water is always accompanied by hydrogen
evolution. In contrast to the microcrystalline element nano-
crystalline Ge rather seems to be a direct semiconductor, 53 and
it is regarded today as a promising candidate for infrared
sensors. However, almost all studies on the production or
characterization of germanium nanoclusters or quantum dots
were performed up to now under ultrahigh vacuum conditions
e.g. by molecular beam epitaxy. For any technological appli-
cation such demanding experimental conditions are a bit
disadvantageous. Thus, our motivation was to find a way
how to make (nanocrystalline) germanium by electrochemical
means. In situ STM and in situ tunnelling spectroscopy are
valuable tools for analyzing the growing structures on a
nanometer scale.
Fig. 2 shows the typical cyclic voltammogram of high purity
and water free [BMIm]PF6saturated with GeCl4. For a better
comparison we have calibrated the processes vs. the germa-
nium overpotential deposition that we observed in this system.
As seen, we observe two main reduction peaks below the open
circuit potential (OCP) and several oxidation peaks for elec-
trode potentials above the OCP. The reduction peak at 500
mVvs. Ge corresponds to the reduction of Ge(IV) to Ge(II), the
rising cathodic current at 0 V vs. Ge is correlated with the
electrodeposition of elemental germanium that can even be
seen with the naked eye as a black deposit formed on the
electrode surface. The oxidation peak at 1000 mV is clearly
correlated with Ge electrooxidation whereas the peaks above
1500 mV are also observed if the CV is cycled between
1000 and 3000 mV vs. Ge. These redox processes are
correlated with the electrooxidation of the gold substrate.
Fig. 2 shows furthermore a series of STM pictures where,
together with the STM scan (from top to bottom) a cyclic
voltammogram was run on Au(111) from GeCl4 saturated in
[BMIm]PF6 with a scan rate of 10 mV s1. Fig. 2a shows a
typical Au(111) surface at 1200 mVvs. Ge. It is characterized
by 250 pm high gold terraces and some gold islands. The
electrode potential at the top of the STM picture of Fig. 2b is
1000 mV vs. Ge, it is 0 V at the bottom: it is quite evident
that islands grow on the gold surface at potentials positive
from the bulk deposition. Thus the deposition of Ge on
Fig. 2 Cyclic voltammogram and a set of STM pictures recorded simultaneously on Au(111) in the ionic liquid 1-butyl-3-methylimidazolium-
hexafluorophosphate, saturated with GeCl4.
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Au(111) begins in the underpotential deposition regime. As we
have pointed out in more detail in ref. 54, the underpotential
deposition of Ge starts at the steps of the terraces at 1000
mV, then 150 pm high Ge islands start to be deposited at 950
mV and at about 750 mV vs. Ge we observe 250 pm high
islands, presumably the result of alloying between Au and Ge.
Still in the UPD regime a completely closed monolayer forms
on the gold surface, as evidenced in Fig. 2b. The STM picture
of Fig. 2c (taken within a potential range from 0 to 1000 mV,
from top to bottom) shows the formation of a rough, thin Ge
layer on gold surface. The thin Ge layer we obtain under these
conditions can be ascribed to the relatively high potential scan
rate which does not provide enough time for a massive bulk
deposition. In the reverse scan, the rough Ge layer is stable in
the potential range between 1000 and 0 mV, as revealed in
the STM picture of Fig. 2d. In the potential range between 0
and1000 mV, the Ge layer redissolves producing holes in the
gold surface, which can be seen from a closer look at the STM
picture of Fig. 2e (arrows in Fig. 2e). This is typical for surface
alloying between deposit and substrate. On further potential
scan (from 1200 to 2200 mV) gold oxidation occurs which
starts first at the steps as can be clearly seen in the STM picture
of Fig. 2f.
The STM pictures of Fig. 3 show the formation of triangularly-
shaped Ge islands in [BMIm]PF6 saturated with GeBr4 as a
source of germanium on Au(111). As seen in Fig. 3a, at a potential
of200 mVvs. Ge a rough and coherent layer of Ge is observed.
In the upper left quarter of the picture a triangularly shaped island
is imaged, its height is between 0.6 and 1 nm and does not seem to
be complete. If the electrode potential is reduced to 0 V, islands
of about 5080 nm in diameter and with heights of up to 1.5 nm
have formed and they start growing vertically very slowly, Fig. 3b.
Upon further reducing the electrode potential the islands grow
both vertically and laterally and finally merge.
Fig. 3 (a) UPD covered Au at200 mVvs. Ge. The layer is coherent but rough. (b) Islands with heights of up to 1.5 nm form at 0 mV vs. Ge.
(c) Height profile of the triangular islands.
Fig. 4 In situ IUtunneling spectra of an approximately 200 nm thick
Ge layer and gold substrate.
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The current/voltage tunneling spectrum of an approxi-
mately 200 nm thick layer obtained at a potential of250
mVvs. Ge is presented in Fig. 4. As can be seen in the original
paper55 the surface is rough on the nanometer scale and some
individual islands rise above the surface with heights of some
nanometres. The I/U tunneling spectrum on such a Ge film
shows a band gap of 0.7 0.1 eV, in good agreement with the
band gap of 0.67 eV for intrinsic microcrystalline bulk Ge at
300 K. In contrast to water, ionic liquids have due to their
wide electrochemical window the benefit that no electroche-
mical side reactions like e.g. hydrogen evolution occur. Thus
the band gap can be reliably measured in situ under electro-
chemical conditionsa challenge in aqueous solutions.
Not only Ge layers can be obtained but also Ge nanoclus-
ters can be made by adjusting the experimental parameters. In
ref. 56 we could show that with GeCl4concentrations of about
5 103 mol l1 narrowly distributed nanoclusters can be
electrodeposited on Au(111).In situcurrentvoltage tunneling
spectroscopy on 10 nm thick clusters has clearly shown a band
gap of 0.7 0.1 eV, and there seems to be a metal to
semiconductor transition with increasing layer thickness.
3.2.2. Silicon. Silicon is one of the most important semi-
conductors as it is the basis of any computer chip. There were
several approaches in the past to electrodeposit silicon in
organic solvents.5759 However, the authors report on a dis-
turbing effect by water that can hardly be avoided in organic
solvents. Furthermore, there were studies on the electrodepo-
sition of silicon in high temperature molten salts.60 It was
reported by Katayama et al.61 that silicon can also be electro-
deposited in a low temperature molten salt. In this study the
authors employed 1-ethyl-3-methylimidazolium hexafluorosi-
licate, and at 90 1C they could deposit a thin layer of silicon.
However, this film reacted with water to form SiO 2 so that
evidence whether the deposited silicon species was elemental or
even semiconducting is missing. Recently, we have shown that
silicon can be well electrodeposited on the nanoscale in the
room temperature ionic liquid 1-butyl-1-methylpyrrolidinium
bis(trifluoromethylsulfonyl)imide saturated with SiCl4.62 This
liquid exhibits on highly oriented pyrolytic graphite (HOPG)
an electrochemical window of 4 V, which is limited in the
anodic regime by the degradation of HOPG, in the cathodic
regime by the irreversible reduction of the organic cation,
Fig. 5.
If the SiCl4 saturated ionic liquid is investigated, a strong
reduction current sets in at an electrode potential which is 600
mV positive from the cathodic decomposition limit of the
liquid on HOPG. After having passed the lower switching
potential the anodic scan crosses the cathodic one at 2000
mV vs . Fc/Fc1 which is typical for nucleation. Approaching
an electrode potential of 400 mV vs. Fc/Fc1 a strong
oxidation current starts which is in part correlated to the SiCl4reduction process beginning at 1600 mV vs. Fc/ Fc1 and in
part correlated to HOPG oxidation as with SiCl4in the liquid
a similar oxidation behaviour is observed if the scan is started
from the open circuit potential towards positive potentials.
Fig. 6 shows a high-resolution SEM picture of an electro-
deposited silicon layer on gold substrate. As seen, the deposit
contains small crystallites with sizes of around 50 nm. Often
the deposit keeps its dark appearance even under air. The
EDX analysis gave as a result only gold from the substrate and
silicon, but no detectable chlorine. This proves that obviously
elemental silicon was electrodeposited which is subject to some
oxidation under environmental conditions.
Fig. 7a shows the STM picture of an about 100 nm thick
silicon layer that was electrodeposited at 1600 mV vs. Fc/
Fc1, probed under potential control with the in situ STM. Its
surface is smooth on the nanometer scale. Fig. 7b shows an in
situ current/voltage tunneling spectrum of HOPG (curve 1)
and of the 100 nm thick silicon layer (curve 2). The spectra are
all over the surface of the same quality. Whereas the tunneling
spectrum of HOPG isas expectedmetallic, for the silicon
deposit a typical band gap is observed. An evaluation of the
Fig. 5 (1) Electrochemical window of [BMP]Tf2N on HOPG with the
ferrocene/ferrocinium couple. (2) Cyclic voltammogram of SiCl4saturated in the same ionic liquid. Scan rate each: 10 mV s1.
Fig. 6 SEM micrograph of electrodeposited silicon, made potentios-
tatically at 2.7 V vs. Fc/Fc1.
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band gap gives a value of 1.0 0.2 eV. This value is quite
similar to the value that we observed for hydrogen terminated
n-doped Si(111) in an ionic liquid.63 The value of microcrystal-
line silicon in the bulk phase at room temperature is 1.1 eV. Inthe light of these results, it can be concluded that elemental,
intrinsic semiconducting silicon was electrodeposited from the
employed ionic liquid.
3.2.3. Tantalum. Tantalum has unique properties that
make it useful for many applications, from electronics to
mechanical and chemical systems. Many efforts have been
done to develop an electroplating process for the electrodepo-
sition of Ta. High temperature molten salts were found to be
efficient baths for the electrodeposition of refractory metals.
To the best of our knowledge, until now no successful
attempts have been made for Ta electrodeposition at room
temperature or even at low temperature in ionic liquids. We
present here the first results of tantalum electrodeposition in
the air and water stable ionic liquid 1-butyl-1-methyl-pyrroli-
dinium bis(trifluoromethylsulfonyl) imide.
Fig. 8 shows the cyclic voltammogram of ([BMP]Tf2N)
containing 0.5 M TaF5 on Au(111) at room temperature. As
shown, two reduction processes are recorded in the forward
scan. The first one starts at a potential of0.5 V with a peak at
0.75 V, it might be correlated to the electrolytic reduction of
Ta(V) to Ta(III). The second process starts at a potential of
1.5 V and is accompanied by the formation of a black
deposit on the electrode surface. This can be attributed to
the reduction of Ta(III) to Ta metal simultaneously with the
formation of insoluble tantalum compounds. The anodic peak
recorded on the backward scan is due to the dissolution of the
electrodeposit which, however, is not completely reversible. At
E > 1.5 V the anodic current increases as a result of gold
dissolution. The deposit obtained only loosely adheres to the
surface and it can easily be removed by washing with acetone.
We also performed the electrodeposition of Ta at different
temperatures of up to 200 1C. It was found that the mechanical
quality and the adherence of the electrodeposits improve at
200 1C. Moreover, the quality and the adherence of the
electrodeposit were found to be improved upon addition of
LiF to the electrolyte.64 The SEM micrograph of the Ta
electrodeposit (Fig. 9a) made potentiostatically at 1.8 V in
([BMP]Tf2N) containing 0.25 M TaF5 and 0.25 M LiF on Pt
electrode at 200 1C for 1 h shows a smooth, coherent and
dense layer. XRD patterns of the electrodeposit clearly show
the characteristic patterns of crystalline tantalum, Fig. 9b.
In situ STM measurements under potentiostatic conditions
can give valuable information on the electrodeposition of Ta
in the employed ionic liquid ([BMP]Tf2N). The STM picture
of Fig. 10a shows a typical surface of gold on mica substrate
(Au(111)) in the ionic liquid ([BMP]Tf2N) containing 0.5 M
TaF5 at open circuit potential. As seen, the surface is char-
acterized by terraces with average step heights of about 250
pm, typical for Au(111). By applying a potential of1.25 V
(vs. Pt) the nature of the surface changes, as seen in the STM
picture of Fig. 10b. A rough layer of Ta forms rapidly and
some triangularly shaped islands with heights of several
nanometers grow above the deposited layer. With ongoing
time, these islands grow vertically and laterally and finally
merge together to a thick layer.
The 3-D STM picture of Fig. 11a shows the topography of
the electrodeposit, with a thickness of about 300 nm. In order
to investigate if the in situ deposit is metallic or not, current/
voltage tunneling spectroscopy was performed. A typical in
situ tunneling spectrum of the 300 nm thick layer of the
electrodeposit at different positions is shown in Fig. 11b.
Fig. 7 (a)In situSTM picture of an about 100 nm thick film (600 nm
200 nm). (b) In situ current/voltage tunneling spectra of HOPG
(curve 1) and of the silicon electrodeposit (curve 2) on HOPG.
Fig. 8 Cyclic voltammogram of 0.5 M TaF5 in ([BMP]Tf2N) on
Au(111) at room temperature. Scan rate 10 mV s1.
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TheIUspectrum clearly exhibits metallic behaviour with an
exponential-like rise of the current revealing that the electro-
deposited layer might be elemental Ta. Together with the in
situmeasurements we can conclude that the reduction of TaF5in ([BMP]Tf2N) leads to an at least 500 nm thick layer of
metallic tantalum.
3.3. Electrosynthesis of conducting polymers
Conducting polymers have attracted considerable attention as
new materials for the development of numerous electrochemi-
cal devices such as batteries, supercapacitors, sensors, electro-
chromic devices, electrochemical actuators and light emitting
diodes.65 These polymers can either be prepared by chemical
or by electrochemical polymerization. The electrochemical
synthesis offers some advantages, such as the generation of
polymers in the doped state, the easy control of the film
thickness. Furthermore, electropolymerization is an easy and
rapid method.
Recently attention has been directed to the potential benefits
of using ionic liquids as solvents for the electrochemical
synthesis of conducting polymers. Sekiguchi et al. reported
the polymerisation of pyrrole, thiophene and aniline66,67 in
1-ethyl-3-methylimidazolium trifluoromethanesulfonate. Mac-
Farlane and co-workers used the ionic liquids 1-butyl-3-
methylimidazolium hexafluorophosphate, 1-ethyl-3-methyl-
imidazolium bis(trifluoromethanesulfonyl) imide and 1-butyl-
1-methylpyrrolidinium bis(trifluoromethanesulfonyl) imide,
both as the growth medium and as an electrolyte for the
electrochemical cycling of polypyrrole films. The polymer films
grown in the ionic liquids show higher conductivity and better
mechanical behaviour than those prepared in conventional
solvents.68
The synthesis of poly(3-(4-fluorophenyl)thiophene) in the
ionic liquids 1-ethyl-2,3-dimethylimidazolium bis(trifluoro-
methylsulfonyl) imide and 1,3-diethyl-5-methylimidazolium
bis(trifluoromethylsulfonyl) imide was reported.69 Also, there
are some recent studies on the synthesis of poly(3-(4-fluoro-
phenyl)thiophene) in ionic liquids.7073 MacFarlane and
Fig. 9 (a) SEM micrograph of the electrodeposit formed potentios-
tatically on Pt in ([BMP]Tf2N) containing 0.25 M TaF5and 0.25 LiF
at a potential of1.8 V for 1 h at 200 1C. (b) XRD patterns of the
deposited layer obtained potentiostatically on Pt in ([BMP]Tf2N)containing 0.25 M TaF5 and 0.25 LiF at a potential of1.8 V for
1 h at 200 1C.
Fig. 10 (a)In situ STM picture of Au(111) in ([BMP]Tf2N) contain-
ing 0.5 M TaF5at the open circuit potential (0.2 V). (b)In situSTM
picture of the electrodeposit obtained at a potential of1.25 V.
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co-workers reported the electropolymerization of thiophene,
bithiophene and terthiophene using the ionic liquids 1-ethyl-3-
methylimidazolium bis(trifluoromethylsulfonyl) imide and 1-
butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl) imide
as the growth medium and supporting electrolyte.74 They
reported also the synthesis of poly(3,4-ethylenedioxy) thio-
phene in the same ionic liquids.75 Quite recently we reported
the synthesis and characterization of poly(para)phenylene inthe ionic liquid 1-hexyl-3-methylimidazolium tris(pentafluo-
roethyl) trifluorophosphate.76,77
4. Synthesis of colloidal nanoparticles
There are some studies available in the literature on the
synthesis of stable crystalline nanoparticles in ionic liquids
which are now emerging as an important class of catalysts for
various reactions. Metal nanoparticles have unique electronic
properties, chemical reactivity and potential applications due
to the quantum size effect which is derived from a dramatic
reduction of the number of free electrons in nanoparticles
smaller than 5 nm.
It was reported78 that very fine and stable nanoparticles of
Ir(0) and Ru(0) with 2.02.5 nm diameters can be synthesized
in the dry ionic liquid 1-butyl-3-methylimidazolium hexafluor-
ophosphate by chemical reduction. The presence of water
causes the partial decomposition of the ionic liquid with the
formation of phosphates, HF and metal fluorides. The isolated
nanoparticles can be redispersed in the ionic liquid, in acetone
or used in solventless conditions for the liquidliquid biphasic,
homogeneous or heterogeneous hydrogenation of arenes un-
der mild reaction conditions (75 1C and 4 atm).78 Moreover,
these catalytic systems can be recovered and reused several
times.
Stable, isolable Pt(0) nanoparticles of 23 nm diameter and
with a narrow size distribution can be easily obtained via
decomposition of Pt-organometallic precoursors, e.g.,
Pt2(dba)3 (dba = bis-dibenzylidene acetone), in 1-butyl-3-
methylimidazolium hexafluorophosphate ionic liquid.79 These
nanoparticles are recyclable catalytic systems for the solvent-
less or biphasic hydrogenation of alkenes and arenes under
mild reaction conditions. The catalytic activity of the Pt
nanoparticles is higher than that obtained for the classical
PtO2 catalyst under the same reaction conditions.79
Itohet al.80 reported the synthesis and functionalization of
gold nanoparticles modified with ionic liquids based on the
imidazolium cation. The obtained gold nanoparticles can be
used as exceptionally high extinction dyes for colourimetric
sensing of anions in water via particle aggregation process.80
Gold and platinum nanoparticles with diameters of 23.5 and
23.2 nm, respectively, can also be synthesized using novel
thiol-functionalized ionic liquids (TFILs).81 TFILs act as a
highly effective medium for the preparation and stablization of
gold and platinum nanoparticles, thus becoming highly dis-
persible in aqueous media.81
Zhou and Antonietti reported on a low temperature synth-
esis of crystalline TiO2 nanoparticles in ionic liquids.82 TiO2
nanoparticles of 23 nm diameter and with surface areas of
554 m2 g1 were obtained by stoichiometric hydrolysis of
titanium tetrachloride in 1-butyl-3-methylimidazolium tetra-
fluoroborate (water-poor conditions) at 80 1C.82 This material
is expected to have potential in solar energy conversion,
catalysis, and optoelectronic devices. The simplicity of the
preparation method reflects the advantage of the use of ionic
liquids since they facilitate direct synthesis of crystalline
species under ambient conditions.
Quite recently it was demonstrated that nanorods, hyper-
branched nanorods and nanoparticles with different CoPt
compositions can be synthesized in 1-butyl-3-methylimidazo-
lium bis(trifluoromethylsulfonyl)imide.83 To get more infor-
mation on the synthesis of functional nanoparticles and other
inorganic nanostructures we would like to refer to the mini-
review of Antonietti.84
5. Carbon nanotubes
Since the discovery of carbon nanotubes (CNT) in 1991 by S.
Iijima85 they have attracted considerable attention due to their
unique properties. Carbon nanotubes are long graphitic thin
Fig. 11 (a)In situ 3-D STMpicture of about 300 nm thick layer of the
electrodepsoit. (b) In situ IUtunneling spectrum of the electrodeposit.
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cylinders which, if simplified, can be regarded as a sheet of
graphite rolled into a cylinder. They can have a single cylind-
rical wall (SWNTs) or multiple walls (MWNTs), cylinders
inside the other cylinders. Carbon nanotubes are currently
being studied in an effort to understand their novel structural,
electronic, and mechanical properties and to explore their
huge potential for many applications in nanoelectronics,86
and as actuators87 and sensors.88 Electrodes composed of
carbon nanotubes have generated much interest because of
their high conductivity, large surface area89,90 and their ability
to facilitate catalytic processes.91
Recently, carbon nanotubes and ionic liquids have gener-
ated great interest in many areas. It was found that the use of
ionic liquids as electrolytes for electrochemical applications
involving carbon nanotube electrodes has proved possible and
advantageous.92 The good electrochemical behaviour of car-
bon nanotube electrodes in ionic liquids, coupled to the wider
electrochemical window and nonvolatility of these electrolytes,
suggests new approaches for the design of capacitors, batteries
and electromechanical actuators.92
It was shown that some ionic liquids exhibit unexpected
strong interactions with carbon nanotubes, forming ionic gels
after grinding together.93,94 Usuiet al.95 prepared ionic nano-
composite gel electrolytes by dispersing carbon nanotubes into
the ionic liquid [EMIm] Tf2N and assembled dye-sensitized
solar cells (DSCs) using these electrolytes. They found that the
energy conversion efficiency of DSCs prepared using such
ionic electrolytes improved.95 Wallace et al. reported the
mechanical properties of carbon nanotube electrodes in the
ionic liquid [BMIm] BF4.96 They found that the ionic liquid
interacts strongly with the carbon nanotubes affecting the
mechanical properties of the electrodes. It was also reported
that carbon nanotubes can be doped by ionic liquids. 97
6. Batteries
Lithium batteries are used widely in portable electronic devices
and electric vehicles. They show the highest energy density
among the applicable chemical and electrochemical energy
storage systems (up to 180 Wh kg1). It is necessary that
solvents for the electrolytes in Li-batteries are aprotic because
of the requirements of wide electrochemical windows up to the
cathodic limit of Li/Li1 potential. As known, the aprotic
organic solvents are usually volatile and flammable. Therefore,
the use of ionic liquids as electrolytes in Li-batteries is very
promising. Matsumotoet al.98,99 applied several kinds of ionic
liquids consisting of quaternary ammonium cation and imide
anions to the classical lithium cell and they found that the
ionic liquid 1-propyl-1-methylpiperidinium bis(trifluoro-
methylsulfonyl) imide is the most promising candidate as the
electrolyte base. Nakagawa et al.100 reported that the use of
the binary electrolyte [EMIm]BF4LiBF4 shows high thermal
stability and better electrochemical performance. Batteries
using the ionic liquid [EMIm]Tf2N containing LiTf2N show
better performance and low self-discharge.101 The self-dis-
charge of the cell after 2000 h is less than 5% per month,
which means that little corrosion and degradation of cell
components take place.101 MacFarlane and co-workers102
investigated the ionic liquid [BMP]Tf2N containing LiTf2N
for use as an electrolyte in Li-batteries. It was reported that the
ionic liquid [EMIm] Tf2N shows a good electrolyte perfor-
mance in Liair batteries.103 A new ionic mixture composed of
LiTf2N and acetamide was prepared and characterized as an
electrolyte for Li batteries, too.104 The LiTf2N/acetamide
mixture is liquid at room temperature between the molar
ratios of 1 : 2 and 1 : 6, and it can be suggested for potential
applications as lithium battery electrolytes.104
7. Spectroscopy
Many papers were published on the spectroscopy of ionic
liquids using several spectroscopic techniques, such as infrared
(IR), ultraviolet (UV), optical Kerr effect (OKE), ultraviolet
photoemission spectroscopy (UPS), mass spectroscopy (MS),
Raman, fluorescence, in order to study the molecular and
electronic structures, molecular dynamics and possible inter-
actions. Some available results from the literature will be
presented in this section.
7.1. IR and Raman spectroscopy
There are some IR and Raman spectroscopy studies in ionic
liquids. In these studies, information was provided in order to
understand at molecular level the general interactions that
exist in ionic liquids.
Talaty et al.105 measured IR and Raman spectra of a
series of 1-alkyl-3-methylimidazolium hexafluorophosphate
([C24Mim]PF6 ionic liquids and correlated the results with
those obtained from calculations. These ionic liquids have
common Raman CH stretching frequencies that may serve as
possible probes in studies of ionic liquid interactions. Hydro-
gen bonding interactions were observed between the fluorine
atoms of the PF6 anion and the C2 hydrogen on the imida-
zolium ring, and between PF6 anion and the H atoms on the
adjacent alkyl side chains.105
In situ Fourier transform infrared reflection absorption
spectroscopy (FT-IRAS) was utilized to study the molecular
structure of the electrified interphase between the ionic liquid
[EMIm]BF4and gold substrate.106 The feature in the FT-IRA
spectra suggested that [EMIm]1 is adsorbed at the interphase
and orients vertically with the molecular axis in the imidazo-
lium ring nearly parallel to the electrode surface in a potential
range of1.3 V to0.6 Vvs. Ag/Ag1.106
Tran et al.107 employed near-infrared spectroscopy (NIR)
technique for the noninvasive and in situ determination of
concentrations and structure of water absorbed by the ionic
liquids [BMIm]BF4
, [BMIm]PF6
and [BMIm]Tf2
N. It was
found that absorbed water interacts with the anions of the
ionic liquids; [BF4] provides the strongest interactions and
[PF6] the weakest. In 24 h, [BMIm]BF4 can absorb up to
0.320 M of water, whereas [BMIm]PF6 only absorbs 8.3
102 M of water107 at the same time. Furthermore, they
demonstrated that it is possible to use the NIR technique
not only to characterize aggregation of surfactants in ionic
liquids but also to determine kinetics and to identify products
of reactions in ionic liquids as well as in microreactors
provided by micelles in ionic liquids.108 NIR spectroscopy
technique was used for sensitive and direct determination
of critical micelle concentration (cmc) values of various
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nonionic surfactants in the ionic liquids [BMIm]PF6 and
[EMIm]Tf2N.108
Raman investigation of the ionic liquid 1-propyl-1-methyl-
pyrrolidinium bis(trifluoromethylsulfonyl) imide ([PMP]Tf2N)
and its 2/1 mixture with LiTf2N was reported.109 The results
showed that the [Tf2N] anions have only a very weak
interaction with the [PMP]1 cations, sterically shielded, but
strong coordination to the Li1 cations.109
7.2. UPS, UV and fluorescence spectroscopy
Ultraviolet photoemission spectroscopy (UPS) is one of the
powerful methods to probe the electronic structures of materi-
als. However, it is usually difficult to apply to liquid samples
due to their high vapour pressure. The usually extremely low
vapour pressure of ionic liquids gives rise to applying the UPS
technique to study electronic structures of ionic liquids, even
under ultra-high vacuum conditions. Yoshimura et al.110
studied the electronic structures of the ionic liquids
[BMIm]BF4, [BMIm]PF6 and [BMIm]Tf2N by UPS with syn-
chrotron radiation.110 They found that the top of the valence
states in the liquids is derived from the organic cation, althoughthe highest occupied molecular orbitals (HOMOs) of the
isolated anions are higher than that of the isolated cation. 110
The optical properties of [BMIm]PF6, [BMIm]BF4 and
[EMIm]BF4 were recently investigated.111,112 The results
showed that all imidazolium-based ionic liquids have signifi-
cant absorption in the entire UV region and a long absorption
tail that extends into the visible region. Furthermore, they all
exhibit a very interesting excitation wavelength dependent
fluorescence behaviour. Billard et al.113 demonstrated the
importance of the purity of the ionic liquid [BMIm]PF6 in
spectroscopic studies and showed that purification procedures
suppress the absorption in the range 250300 nm and beyond.
Fluorescence spectroscopy is a very useful technique toinvestigate molecular dynamics, molecular association, and
microstructure within organized media. Recently, fluorescence
techniques have been employed to characterize physicochem-
ical properties of ionic liquids. Using fluorescence technique,
Pandey and coworkers114116 reported that the physicochem-
ical properties of [BMIm]PF6 are altered by the addition of
cosolvents.
Alvaro et al.117 investigated the energy, hydrogen, and
electron transfer reactions within [BMIm]PF6. They observed
slow molecular diffusion and low oxygen solubility within this
relatively high viscosity IL, as well as an increase in the lifetime
of radical ions and the triplet excited state. During the
investigation of the possibility for cellulase catalyzed reactionsin ionic liquids, Rogerset al.118 studied enzyme stability within
1-butyl-3-methylimidazolium chloride using a fluorescence
techniques.
7.3. OKE spectroscopy
There are a few studies on the use of optical heterodyne-
detected Raman-induced Kerr effect spectroscopy (OHD-
RIKES) to probe experimentally the intermolecular and or-
ientation dynamics of ionic liquids. Quitevis and co-workers119
reported a study of the effect of the alkyl chain length on the
low frequency (0250 cm1) spectra for a homologous series of
the ionic liquids 1-alkyl-3-methylimidazolium bis(trifluoro-
methylsufonyl)imide, [CnMim]Tf2N, n = 2, 4, 5, 6, 8, 10.
The study of the temperature dependence of the low-frequency
spectrum of [C5MIm]Tf2N was also reported.120
Using OHD-RIKES, Giraudet al.121 investigated the ultra-
fast solvent dynamics of some ionic liquids, [BMMIm]Tf2N,
[BMIm]PF6, [BMIm]Tf2N, [BMIm]TfO and [OMIm]Tf2N, by
studying the effects of cation and anion substitution on the
low-frequency spectra. It was found in all five samples that the
signal is due to vibration of the imidazolium ring at three
frequencies around 30, 65, and 100 cm1 corresponding to
three local configurations of the anion with respect to the
cation.
7.4. Mass spectroscopy
Electrospray ionization mass spectroscopy (ESI-MS) was used
to detect both the cations and anions of the ionic liquids as
well as their solubility in water.122,123 It was found that in
addition to the main peaks of the parent ions, fragmentation
products are observed upon increasing the cone voltage,
whereas aggregates of the parent ions with one or more ionicliquid molecules are observed upon decreasing the cone vol-
tage. The main fragmentations of most studied ionic liquids
were due to the loss of butene molecule.123
Dyson et al.124 reported a dilution method for analyzing
ionic liquids and catalysts dissolved in ionic liquids by ESI-
MS. Jackson and Duckworth125 showed that the ionic liquids
could be analysed without dilution using ESI-MS. They also
demonstrated that ionic impurities or dissolved additives,
especially those that are solvent reactive, could be detected
overcoming the limitations of the dilution method.
Laser desorption/ionization (LDI) and matrix-assisted laser
desorption/ionization (MALDI) mass spectroscopy are both
methods which allow the investigation and characterization ofionic liquids. Tholey and co-workers126 used (LDI) and
(MALDI) mass spectroscopy to characterize five different
ionic liquids as well as studying the analysis of amino acids,
peptides and proteins dissolved in these ionic liquids. Li and
Gross127 tested some ionic liquids as MALDI matrices for
quantification of peptides and proteins. Armstrong et al.128
introduced a class of specially designed ionic liquids that are
capable of absorbing laser light and transferring protons to the
analyte as matrices for MALDI mass spectroscopy.
8. Thermodynamics
Up to now, a number of papers have been published on the
thermodynamic properties of ionic liquids. In this section we
present some available literature data on the thermodynamic
properties of ionic liquids.
Thermodynamic activity coefficients are a measure of the
deviation from ideal behaviour in liquid mixtures. Activity
coefficients at infinite dilution can be directly used for the
selection of solvents for extractive distillation, liquid extrac-
tion, solvent-aided crystallization, and even chemical reaction.
Activity coefficients at infinite dilution give a direct measure of
interactions between unlike molecules in the absence of
solutesolute interactions.
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Gas chromatography is widely used for determining ther-
modynamic properties of pure substances or solvent properties
of binary mixtures. Using inverse gas chromatography, Mute-
let and Jaubert129 determined the activity coefficients for 29
polar and non-polar compounds (alkanes, alkenes, alkynes,
cycloalkanes, aromatics, alcohols) in the two following ionic
liquids: 1-butyl-3-methylimidazolium octyl sulfate ([BMIm]1
[C8H17OSO3]) at 323.15, 333.15, 343.15 K, and 1-ethyl-3-
methylimidazolium tosylate ([EMIm]1[C7H7SO3]) at 323.15 K.
Heintz and co-workers130 determined the activity coeffi-
cients at infinite dilution gNi of the linear and branched C1
to C6 alcohols, acetone, acetonitrile, ethylacetate, alkylethers,
and chloromethane in the ionic liquid 4-methyl-N-butyl-
pyridinium tetrafluoroborate by gas chromatography using
the ionic liquid as the stationary phase. The partial molar
excess enthalpies at infinite dilution HE,Ni of the polar solutes
in the ionic liquid can be derived from the temperature
dependence of the limiting activity coefficients. According to
the GibbsHelmholtz equation, the value of HE,Ni can be
directly obtained from the slope of a straight line derived
from the following equation:
@ln g1
@1=T
H1
R
whereR is the gas constant.
Vapourliquid equilibria (VLE) of binary mixtures contain-
ing ethanol, propanol and benzene in the ionic liquids
[BMIm]Tf2N131 and [EMIm]Tf2N
132 were studied and the
activity coefficients of these solvents in the ionic liquids were
determined from VLE data.
The suitability of a solvent for separating mixtures of two
components is defined as selectivity and can be determined
using the equation:
S112 g11
g12
where SN12 is the selectivity, gN
1 and gN
2 are the activity
coefficients of components 1 and 2, respectively, in infinite
dilution in an ionic liquid.
The SN12 values obtained for different binary mixtures
indicated that the ionic liquid [EMIm]tosylate can play an
important role for separation of aromatics, chloroalkanes and
alcohols from alkanes.129 To get more information on the
thermodynamics of non-aqueous mixtures containing ionic
liquids we refer to a recently published review article. 133
9. Catalysis
Nowadays, ionic liquids are widely used in catalysis not only
as solvents or reaction media but also as catalysts, or catalyst
activators. As there are a number of excellent reviews134137 on
the application of ionic liquids in catalysis and biocatalysis, we
give here only a few examples of the use of some air and water
stable ionic liquids in catalysis.
MacFarlaneet al.138 reported that dicyanamide based ionic
liquids, [BMIm][dca] and [EMIm][dca], act as active base
catalysts in the acetylation of alcohols. A suspension of
palladium nanoparticles can be formed by reducing a solution
of palladium acetate in the ionic liquid [BMIm]PF6 with H2.
This recyclable catalytic system was used for the hydrogena-
tion of alkenes,139 see section 4.
Welton and co-workers140 demonstrated the possibility of
using a thermally controlled ionic liquid N-octyl-3-methylimi-
dazolium tetrafluoroborate [C8C1Im]BF4)water biphasic or
homogeneous system for hydrogenation of but-2-yne-1,4-diol.
The ionic liquid [C8C1Im]BF4is immiscible with water at room
temperature, but fully miscible at the reaction temperature of
80 1C. When the system is cooled to room temperature, it
separates into two phases and the product is removed with the
water phase and the catalyst remains in the ionic liquid. Favre
et al.141 reported that a wide range of ionic liquids based on
imidazolium and pyrrolidinium cations and weakly coordinat-
ing anions (such as BF4 , PF
6 , Tf2N, TfO) proved to be
efficient solvents for the biphasic rhodium catalyzed hydro-
formylation of 1-hexene.
Several aromatic aldehydes were oxidised in the ionic liquid
[BMIm]PF6 using the catalyst Ni(acac)2 (acac = acetylaceto-
nate) as the oxidant.142 The catalyst and ionic liquid could be
recycled after extraction of the carboxylic acid product. The
same catalytic system, ionic liquid and oxidant, was also used
for the oxidation of ethylbenzene forming ethylbenzene hy-
droperoxide.143 Namboodiri et al.144 reported the oxidation of
styrene to acetophenone ,the Wacker oxidation, in the ionic
liquids [BMIm]BF4and [BMIm]PF6using PdCl2as a catalyst.
Seddon and Stark145 reported the oxidation of benzyl alcohol
to benzaldehyde in imidazolium based ionic liquids by oxygen
and a palladium acetate catalyst source.
Conclusion
In this review article we have tried to give an overview on the
importance of ionic liquids in physical chemistry and wesummarized literature until the end of 2005. Whereas ionic
liquids were regarded as relatively new until about 2000 the
situation has changed dramatically throughout the recent 3
years. In 2005 there were about 1500 peer reviewed papers on
ionic liquids. The still rising interest in ionic liquids in various
fields of chemistry will surely lead to a rising output of papers,
stimulating further studies. It can be expected that ionic
liquids develop to a main stream in various fields of chemistry
and physical chemistry in the near future. We ourselves are
very curious to see the future developments in this field and we
are looking forward to many more papers dealing with these
fascinating liquids.
List of some abbreviations
Abbreviation Name
[BEIm] 1-Butyl-3-ethylimidazolium
[BMIm]BF4 1-Butyl-3-methylimidazolium
tetrafluoroborate
[BMIm]PF6 1-Butyl-3-methylimidazolium
hexafluorophoshate
[BMIm]Tf2N 1-Butyl-3-methylimidazolium
bis(trifluoromethylsulfonyl) imide
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