Air and Water Stable Ionic Liquids

8/11/2019 Air and Water Stable Ionic Liquids

1/16

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.

This journal is c the Owner Societies 2006 Phys. Chem. Chem. Phys., 2006, 8, 21012116 | 2101

INVITED ARTICLE www.rsc.org/pccp | Physical Chemistry Chemical Physics

View Article Online / Journal Homepage / Table of Contents for this issue
http://pubs.rsc.org/en/journals/journal/CP?issueid=CP008018http://pubs.rsc.org/en/journals/journal/CPhttp://dx.doi.org/10.1039/B600519P


2/16

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.

2102 | Phys. Chem. Chem. Phys., 2006, 8, 21012116 This journal is c the Owner Societies 2006

View Article Online
http://dx.doi.org/10.1039/B600519P


3/16

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,


View Article Online


4/16

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.


View Article Online


5/16

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.


View Article Online


6/16

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.


View Article Online


7/16

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.


View Article Online


8/16

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.


View Article Online


9/16

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.


View Article Online


10/16

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.


View Article Online


11/16

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


View Article Online


12/16

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.


View Article Online


13/16

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


View Article Online


14/16


15/16

59 J. Gobet and H. Tannenberger,J. Electrochem. Soc., 1988, 135,109.

60 T. Matsuda, S. Nakamura, K. Ide, K. Nyudo, S. J. Yae and Y.Nakato,Chem. Lett., 1996, 7, 569.

61 Y. Katayama, M. Yokomizo, T. Miura and T. Kishi, Electro-chemistry, 2001, 69, 834.

62 S. Zein El Abedin, N. Boressinko and F. Endres, Electrochem.Commun., 2004, 6, 510.

63 W. Freyland, C. A. Zell, S. Zein El Abedin and F. Endres,Electrochim. Acta, 2003, 48, 3053.

64 S. Zein El Abedin, H. K. Farag, E. M. Moustafa, U. Welz-Biermann and F. Endres, Phys. Chem. Chem. Phys., 2005, 7,2333.

65 Handbook of Conducting Polymers, ed. T. A. Skotheim, R. L.Elsenbaumerand J. R. Reynolds, Marcel DekkerInc., New York,2nd edn., 1998.

66 K. Sekiguchi, M. Atobe and T. Fuchigami,J. Electroanal. Chem.,2003, 557, 1.

67 K. Sekiguchi, M. Atobe and T. Fuchigami, Electrochem. Com-mun., 2002, 4, 881.

68 J. M. Pringle, J. Efthimiadis, P. C. Howlett, J. Efthimiadis, D. R.MacFarlane and A. B. Chaplin et al ., Polymer, 2004, 45,1447.

69 E. Naudin, H. A. Ho, S. Branchaud, L. Breau and D. Belanger,J.Phys. Chem. B, 2002, 106, 10585.

70 H. Randriamahazaka, C. Plesse, D. Teyssie and C. Chevrot,

Electrochem. Commun., 2003, 5, 613.71 H. Randriamahazaka, C. Plesse, D. Teyssie and C. Chevrot,

Electrochem. Commun., 2004, 6, 299.72 P. Damlin, C. Kvarnstro m and A. Ivaska, J. Electroanal. Chem.,

2004, 570, 113.73 P. Danielsson, J. Bobacka and A. Ivaska,J. Solid State Electro-

chem., 2004, 8, 809.74 J. M. Pringle, M. Forsyth, D. R. MacFarlane, K. Wagner, S. B.

Hall and D. L. Officer, Polymer, 2005, 46, 2047.75 K. Wagner, J. M. Pringle, S. B. Hall, M. Forsyth, D. R.

MacFarlane and D. L. Officer, Synth. Met., 2005, 153, 257.76 S. Zein El Abedin, N. Borissenko and F. Endres, Electrochem.

Commun., 2004, 6, 422.77 O. Schneider, A. Bund, A. Ispas, N. Borissenko, S. Zein El

Abedin and F. Endres, J. Phys. Chem. B, 2005, 109, 7159.78 G. S. Fonseca, A. P. Umpierre, P. F. P. Fichtner, S. R. Teixera

and J. Dupont, Chem.- Eur. J., 2003, 9, 3263.

79 C. W. Scheeren, G. Machado, J. Dupont, P. F. P. Fichtner and S.R. Teixera, Inorg. Chem., 2003, 42, 4738.

80 H. Itoh, K. Naka and Y. Chujo, J. Am. Chem. Soc., 2004, 1 26,3026.

81 K.-S. Kim, D. Demberelnyamba and H. Lee,Langmuir, 2004,20,556.

82 Y. Zhou and M. Antonietti,J. Am. Chem. Soc., 2003,125, 14960.83 Y. Wang and H. Yang, J. Am. Chem. Soc., 2005, 127, 5316.84 M. Antonietti, D. Kuang, B. Smarsly and Y. Zhou, Angew.

Chem., Int. Ed., 2004, 43, 4988.85 S. Iijima,Nature, 1991, 354, 56.86 P. G. Collins, M. S. Arnold and P. Avouris, Science, 2001, 292,

706.87 R. H. Baughman, C. T. Cui, A. A. Zakhidov, Z. Iqbal, J. N.

Batisci, G. M. Spinks, G. G. Wallace, A. Mazzddi, D. De Rossi,A. G. Rinzler, O. Jaschinski, S. Roth and M. Kertesz, Science,

1999, 284, 1340.88 J. Kong, N. R. Franklin, C. W. Zhou, M. G. Chopline, S. Peng,K. J. Cho and H. J. Dai, Science, 2000, 287, 622.

89 A. Srivastava, O. N. Srivastava, S. Talapatra, R. Vajtai and P. M.Ajayan,Nat. Mater., 2004, 3, 610.

90 A. Anson, J. Jagiello, J. B. Parra, M. L. Sanjuan, A. M. Benito,W. K. Maser and M. T. Martinez, J. Phys. Chem. B, 2004, 108,15820.

91 K. Gong, Y. Dong, S. Xiong, Y. Chen and L. Mao, Biosens.Bioelectron., 2004, 20, 253.

92 J. N. Barisci, G. G. Wallace, D. R. MacFarlane and R. H.Baughman, Electrochem. Commun., 2004, 6, 22.

93 T. Fukushima, A. Kosaka, Y. Ishimura, T. Yamamoto, T.Takigawa, N. Ishii and T. Aida, Science, 2003, 300, 2072.

94 F. Zhao, X. Wu, M. Wang, Y. Liu, L. Gao and S. Dong, Anal.Chem., 2004, 76, 4960.

95 H. Usui, H. Matsui, N. Tanabe and S. Yanagida, J. Photochem.Photobiol. A: Chem., 2004, 164, 97.

96 P. G. Whitten, G. M. Spinks and G. G. Wallace, Carbon, 2005,43, 1891.

97 L. Kavan and L. Dunsch,ChemPhysChem, 2003, 4, 944.98 H. Sakaebe and H. Matsumoto, Electrochem. Commun., 2003,5,

594.99 H. Sakaebe, H. Matsumoto and K. Tatsumi, J. Power Sources,

2005, 146, 693.100 H. Nakagawa, S. Izuchi, K. Kuwana, T. Nukuda and Aihara,J.

Electrochem. Soc., 2003, 150, A695.101 B. Garcia, S. Lavallee, G. Perron, C. Michot and M. Armand,

Electrochim. Acta, 2004, 49, 4583.102 P. C. Howlett, D. R. MacFarlane and A. F. Hollenkamp,

Electrochem. Solid-State Lett., 2004, 7, A97.103 T. Kuboki, T. Okuyama, T. Ohsaki and N. Takami, J. Power

Sources, 2005, 146, 766.104 Y. Hu, H. Li, X. Huang and L. Chen, Electrochem. Commun.,

2004, 6, 28.105 E. R. Talaty, S. Raja, V. J. Storhaug, A. Do lle and W. R. Carper,

J. Phys. Chem. B, 2004, 108, 13177.106 N. Nanbu, Y. Sasaki and F. Kitamura, Electrochem. Commun.,

2003, 5, 383.107 C. D. Tran, S. H. D. Lacerda and D. Oliveira, Appl. Spectrosc.,

2003, 57, 152.108 C. D. Tran and S. Yu, J. Colloid Interface Sci., 2005, 283, 613.

109 M. Castriota, T. Caruso, R. G. Agostino, E. Cazzanelli, W. A.Henderson and S. Passerini, J. Phys. Chem. A, 2005, 109, 92.

110 D. Yoshimura, T. Yokoyama, T. Nishi, H. Ishii, R. Ozawa, H.Hamaguchi and K. Seki, J. Elect. Spectrosc. Related Phenom.,2005, 144147, 319.

111 A. Paul, P. K. Mandal and A. Samanta,J. Phys. Chem. B, 2005,109, 9148.

112 A. Paul, P. K. Mandal and A. Samanta,Chem. Phys. Lett., 2005,402, 375.

113 I. Billard, G. Moutiers, A. Labet, A. El Azzi, C. Gaillard, C.Mariet and K. Lutzenkirchen, Inorg. Chem., 2003, 42, 1726.

114 K. A. Fletcher and S. Pandey, Appl. Spectrosc., 2002, 56,266.

115 K. A. Fletcher and S. Pandey, J. Phys. Chem. B, 2003, 107,13532.

116 K. A. Fletcher, S. N. Baker, G. A. Baker and S. Pandey,New J.Chem., 2003, 27, 1706.

117 M. Alvaro, B. Ferrer, H. Garcia and M. Narayana, Chem. Phys.Lett., 2002, 362, 435.

118 M. B. Turner, S. K. Spear, J. G. Huddleston, J. D. Holbrey andR. D. Rogers, Green Chem., 2003, 5, 443.

119 B. R. Hyun, S. V. Dzyuba, R. A. Bartsch and E. L. Quitevis, J.Phys. Chem. A, 2002, 106, 7579.

120 J. R. Rajian, S. Li, R. A. Bartsch and E. L. Quitevis,Chem. Phys.Lett., 2004, 393, 372.

121 G. Giraud, C. M. Gordon, I. R. Dunkin and K. Wynne,J. Chem.Phys., 2003, 119, 464.

122 Z. B. Alfassi, R. E. Huie, B. L. Milman and P. Neta, Anal.Bioanal. Chem., 2003, 377, 159.

123 B. L. Milman and Z. B. Alfassi, Eur. J. Mass Spectrom., 2005, 11, 35.124 P. J. Dyson, J. S. McIndoe and D. Zhao, Chem. Commun., 2003,

508.125 G. P. Jackson and D. C. Duckworth, Chem. Commun., 2004,

522.126 M. Z. Moghaddam, R. Kru ger, E. Heinzle and A. Tholey, J.Mass Spectrom., 2004, 39, 1494.

127 Y. L. Li and M. L. Gross,J. Am. Soc. Mass Spectrom., 2004,15,1833.

128 D. W. Armstrong, L.-K. Zhang, L. He and M. L. Gross, Anal.Chem., 2001, 73, 3679.

129 F. Mutelet and J.-N. Jaubert,J. Chromatogr., A, (in press).130 A. Heintz, D. V. Kulikov and S. P. Verevkin,J. Chem. Thermo-

dyn., 2002, 34, 1341.131 S. P. Verevkin, J. Safarov, E. Bich, E. Hassel and A. Heintz,Fluid

Phase Equilib., 2005, 236, 222.132 S. P. Verevkin, T. V. Vasiltsova, E. Bich and A. Heintz, Fluid

Phase Equilib., 2004, 218, 165.133 A. Heintz,J. Chem. Thermodyn., 2005, 37, 525.134 T. Welton,Coord. Chem. Rev., 2004, 148, 2459.


View Article Online


16/16

135 Z. Yang and W. Pan, Enzyme Microb. Technol., 2005, 37, 19.136 D. Zhao, M. Wu, Y. Kou and E. Min, Catal. Today, 2002, 7 4,

157.137 P. Wasserscheid and W. Keim,Angew. Chem., Int. Ed., 2000,39,

3772.138 S. A. Forsyth, D. R. MacFarlane, R. J. Thompson and M. von

Ltzstein, Chem. Commun., 2002, 714.139 J. Huang, T. Jiang, B. Han, H. Gao, Y. Chang, G. Zhao and W.

Wu, Chem. Commun., 2003, 1654.

140 P. J. Dyson,D. J. Ellis and T.Welton, Can. J. Chem., 2001, 79, 705.141 F. Favre, H. Olivier-Bourbigou, D. Commereuc and L. Saussine,

Chem. Commun., 2001, 1360.142 J. Howarth, Tetrahedron Lett., 2000, 41, 6627.143 R. Alca ntara, L. Canoira, P. Guilherme-Joao and P. Pe rez-

Mendo, Appl. Catal., A, 2001, 218, 269.144 V. V. Namboodiri, R. S. Varma, E. Sahle-Demessie and U. R.

Pillai, Green Chem., 2002, 4, 170.145 K. R. Seddon and A. Stark,Green Chem., 2002, 4, 119.

View Article Online

Documents

Air and Water Stable Ionic Liquids