ELECTROCHEMICAL ASPECTS OF IONIC LIQUIDS ppt

ELECTROCHEMICAL
ASPECTS OF IONIC
LIQUIDS
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ELECTROCHEMICAL
ASPECTS OF IONIC
LIQUIDS
Second Edition
Edited by
HIROYUKI OHNO
A JOHN WILEY & SONS, INC., PUBLICATION
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Copyright © 2011 by John Wiley & Sons, Inc. All rights reserved
Published by John Wiley & Sons, Inc., Hoboken, New Jersey
Published simultaneously in Canada
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Library of Congress Cataloging-in-Publication Data:
Electrochemical aspects of ionic liquids / edited by Hiroyuki Ohno.
p. cm.
Includes bibliographical references and index.
ISBN 978-0-470-64781-3 (cloth)
1. Ionic solutions. 2. Electrochemistry. 3. Polymerization. I. Ohno, Hiroyuki, 1953–
QD562.I65E38 2011
541′.372–dc22
2010034796
Printed in Singapore.
10 9 8 7 6 5 4 3 2 1
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PREFACE TO THE SECOND EDITION ix
PREFACE TO THE FIRST EDITION xi
ACKNOWLEDGMENTS FOR THE SECOND EDITION xiii
CONTRIBUTORS xv
1 Importance and Possibility of Ionic Liquids 1
Hiroyuki Ohno
2 Physical Chemistry of Ionic Liquids: Inorganic and Organic
as Well as Protic and Aprotic 5
C. A. Angell, W. Xu, M. Yoshizawa-Fujita, A. Hayashi, J P. Belieres,
P. Lucas., M. Videa, Z F. Zhao, K. Ueno, Y. Ansari, J. Thomson, and
D. Gervasio

PART I BASIC ELECTROCHEMISTRY 33
3 General Techniques 35
Yasushi Katayama
4 Electrochemical Windows of Room-Temperature Ionic
Liquids (RTILs) 43
Hajime Matsumoto
CONTENTS
v
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vi CONTENTS
5 Diffusion in Ionic Liquids and Correlation with Ionic
Transport Behavior 65
Md. Abu Bin Hasan Susan, Akihiro Noda, and Masayoshi Watanabe
6 Ionic Conductivity 87
Hiroyuki Ohno, Masahiro Yoshizawa-Fujita, and Tomonobu Mizumo
7 Optical Waveguide Spectroscopy 95
Hiroyuki Ohno and Kyoko Fujita
8 Electrolytic Reactions 101
Toshio Fuchigami and Shinsuke Inagi
9 Electrodeposition of Metals in Ionic Liquids 129
Yasushi Katayama
PART II BIOELECTROCHEMISTRY 157
10 Enzymatic Reactions 159
Noritaka Iwai and Tomoya Kitazume
11 Molecular Self-assembly in Ionic Liquids 169
Nobuo Kimizuka and Takuya Nakashima
12 Solubilization of Biomaterials into Ionic Liquids 183
Kyoko Fujita, Yukinobu Fukaya, and Hiroyuki Ohno
13 Redox Reaction of Proteins 193
Kyoko Fujita and Hiroyuki Ohno

PART III IONIC DEVICES 203
14 Li Batteries 205
Hikari Sakaebe and Hajime Matsumoto
15 Photoelectrochemical Cells 221
Hajime Matsumoto
16 Fuel Cells 235
Masahiro Yoshizawa-Fujita and Hiroyuki Ohno
17 Double-Layer Capacitors 243
Makoto Ue
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CONTENTS vii
18 Actuators 271
Kinji Asaka
PART IV FUNCTIONAL DESIGN 279
19 Novel Fluoroanion Salts 281
Rika Hagiwara and Kazuhiko Matsumoto
20 Neutralized Amines 293
Hiroyuki Ohno
21 Zwitterionic Liquids 301
Masahiro Yoshizawa-Fujita, Asako Narita, and Hiroyuki Ohno
22 Alkali Metal Ionic Liquids 317
Wataru Ogihara, Masahiro Yoshizawa-Fujita, and Hiroyuki Ohno
23 Polyether/Salt Hybrids 325
Tomonobu Mizumo and Hiroyuki Ohno
24 Electric Conductivity and Magnetic Ionic Liquids 337
Gunzi Saito
PART V IONIC LIQUIDS IN ORDERED
STRUCTURES 347
25 Ion Conduction in Organic Ionic Plastic Crystals 349
Maria Forsyth, Jennifer M. Pringle, and Douglas R. MacFarlane

26 Liquid Crystalline Ionic Liquids 375
Takashi Kato and Masafumi Yoshio
PART VI GEL-TYPE POLYMER ELECTROLYTES 393
27 Ionic Liquid Gels 395
Kenji Hanabusa
28 Zwitterionic Liquid/Polymer Gels 403
Masahiro Yoshizawa-Fujita and Hiroyuki Ohno
29 Ionic Liquidized DNA 409
Naomi Nishimura and Hiroyuki Ohno
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viii CONTENTS
PART VII POLYMERIZED IONIC LIQUIDS 417
30 Ion Conductive Polymers 419
Hiroyuki Ohno and Masahiro Yoshizawa-Fujita
31 Amphoteric Polymers 433
Hiroyuki Ohno, Masahiro Yoshizawa-Fujita, and Wataru Ogihara
32 Polymer Brushes 441
Masahiro Yoshizawa-Fujita and Hiroyuki Ohno
PART VIII CONCLUSION 457
33 Future Prospects 459
Hiroyuki Ohno
APPENDIX: STRUCTURES OF ZWITTERIONS 463
INDEX 465
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PREFACE TO THE SECOND EDITION
ix
The fi rst edition of this book was published in 2005 as the fi rst book on the
basic study and application of the ionic liquids for electrochemical aspects. At
this time, there is increasing interest in ionic liquids as an electrolyte solution
substituent. In particular, interests are focused on the safety of the organic ion

conductive liquids. Despite the safety of ionic liquids, there is still hesitation
in using these ionic liquids as an electrolyte solution. This might be caused by
two major reasons, one is cost, and the other is the great possibility of the
development of better ionic liquids. The former is actually important for indus-
try, but it should also be a matter of demand. Larger demand lowers the price.
The second reason is a bit serious, because there is always the possibility of
fi nding or developing new and better ionic liquids. There should be a kind of
hesitation in deciding on the industrial use of current ionic liquids, because no
one can deny that there is the possibility that better ones will emerge. In any
case, it should be most important to develop ionic liquids having suffi cient
properties for practical use. Understanding of the latest in ionic liquid science
is important to provide motivation for researchers to use them.
In the second edition, we considerably updated the content to catch up with
the fast changes in ionic liquid science. Also, interesting new chapters have
been added. In every chapter, we tried to add the latest information while
keeping the number of pages as low as possible. It will be one of our great
pleasures if readers fi nd some interesting point regarding ionic liquid science
that aids in their research.
H IROYUKI OHNO
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PREFACE TO THE FIRST EDITION
xi
This book introduces some basic and advanced studies on ionic liquids in the
electrochemical fi eld. Although ionic liquids are known by only a few scientists
and engineers, their applications ’ potential in future technologies is unlimited.
There are already many reports of basic and applied studies of ionic liquids
as reaction solvents, but the reaction solvent is not the only brilliant future of
the ionic liquids. Electrochemistry has become a big fi eld covering several key
ideas such as energy, environment, nanotechnology, and analysis. It is hoped
that the contributions on ionic liquids in this book will open other areas

of study as well as to inspire future aspects in the electrochemical fi eld.
The applications of ionic liquids in this book have been narrowed to the
latest results of electrochemistry. For this reason only the results on room -
temperature ionic liquids are presented, and not on high - temperature melts.
The reader of this book should have some basic knowledge of electrochem-
istry. Those who are engaged in work or study of electrochemistry will get to
know the great advantages of using ionic liquids. Some readers may fi nd the
functionally designed ionic liquids to be helpful in developing novel materials
not only in electrochemistry but also in other scientifi c fi elds. This book covers
a wide range of subjects involving electrochemistry. Subjects such as the solu-
bilization of biomolecules may not seem to be necessary for electrochemistry
concerning ionic liquids, but some readers will recognize the signifi cance of
solubility control of functional molecules in ionic liquids even in an electro-
chemical fi eld. Many more examples and topics on ionic liquids as solvents
have been summarized and published elsewhere, and the interested reader will
benefi t from studying the references that are provided at the end of each
chapter.
Hiroyuki Ohno
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ACKNOWLEDGMENTS FOR THE
SECOND EDITION
xiii
First of all, I would like to express my sincere thanks to all the contributors
for the second edition. All authors kindly agreed to reuse their chapters
and made an effort to put the latest information in every chapter. A new
chapter has been added in the second edition for better reviewing in
electrochemistry.
Next an acknowledgment should be given to Dr. Naomi Nishimura of the
Department of Biotechnology, Tokyo University of Agriculture and Technology.
Naomi worked hard to help me to edit manuscripts. She was so systematic

that there were no serious problems in the editing of the manuscript. Without
her energetic contribution, this book would not be published by the due date.
Finally I would like to thank Dr. Arza Seidel of John Wiley and Sons, Inc.
for her kind support and encouragement.
H iroyuki O hno
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CONTRIBUTORS
xv
C. Austen Angell, Department of Chemistry and Biochemistry, Arizona State
University
Younes Ansari, Department of Chemistry and Biochemistry, Arizona State
University
Kinji Asaka, National Institute of Advanced Industrial Science and
Technology (AIST)
Jean - Philippe Belieres, Department of Chemistry and Biochemistry, Arizona
State University
Maria Forsyth, Department of Materials Engineering, Monash University
Toshio Fuchigami, Department of Electronic Chemistry, Tokyo Institute of
Technology
Kyoko Fujita, Department of Biotechnology, Tokyo University of Agriculture
and Technology
Masahiro Yoshizawa - Fujita, Department of Materials and Life Sciences,
Sophia University
Yukinobu Fukaya, Department of Biotechnology, Tokyo University of
Agriculture and Technology
Dominic Gervasio, Center for Applied Nanobioscience in the BioDesign
Institute and School of Materials, Arizona State University
Rika Hagiwara, Department of Fundamental Energy Science, Kyoto
University
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xvi CONTRIBUTORS
Kenji Hanabusa, Graduate School of Science and Technology, Shinshu
University
Akitoshi Hayashi, Department of Applied Chemistry, Graduate School of
Engineering, Osaka Prefecture University
Shinsuke Inagi, Department of Electronic Chemistry, Tokyo Institute of
Technology
Noritaka Iwai, Department of Bioengineering, Tokyo Institute of Technology
Yasushi Katayama, Department of Applied Chemistry, Faculty of Science and
Technology, Keio University
Takashi Kato, Department of Chemistry and Biotechnology, School of
Engineering, The University of Tokyo
Nobuo Kimizuka, Department of Chemistry and Biochemistry, Graduate
School of Engineering, Kyushu University
Tomoya Kitazume, Department of Bioengineering, Tokyo Institute of
Technology
Pierre Lucas, Department of Chemistry and Biochemistry, Arizona State
University
Douglas R. MacFarlane, School of Chemistry, Monash University
Hajime Matsumoto, Research Institute for Ubiquitous Energy Devices,
National Institute of Advanced Industrial Science and Technology (AIST)
Kazuhiko Matsumoto, Graduate School of Energy Science, Kyoto University
Tomonobu Mizumo, Department of Applied Chemistry, Hiroshima University
Takuya Nakashima, Graduate School of Materials Science, Nara Institute of
Science and Technology
Asako Narita, Department of Polymer Chemistry, Graduate School of
Engineering, Kyoto University
Naomi Nishimura, Department of Biotechnology, Tokyo University of
Agriculture and Technology
Akihiro Noda, Honda R & D Co., Ltd.

Wataru Ogihara, Nissan Motor Co., Ltd.
Hiroyuki Ohno, Department of Biotechnology, Tokyo University of
Agriculture and Technology
Jennifer M. Pringle, Department of Materials Engineering and School of
Chemistry, Monash University
Gunzi Saito, Research Institute, Meijo University
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CONTRIBUTORS xvii
Hikari Sakaebe, Research Institute for Ubiquitous Energy Devices, National
Institute of Advanced Industrial Science and Technology (AIST)
Md. Abu Bin Hasan Susan, Department of Chemistry, University of Dhaka
Jeffery Thomson, Center for Applied Nanobioscience in the BioDesign
Institute and School of Materials, Arizona State University
Makoto Ue, Fellow, Mitsubishi Chemical Corporation
Kazuhide Ueno, Department of Chemistry and Biochemistry, Arizona State
University
Marcelo Videa, Department of Chemistry and Biochemistry, Arizona State
University
Masayoshi Watanabe, Department of Chemistry and Biotechnology,
Yokohama National University
Xu Wu, Department of Chemistry and Biochemistry, Arizona State University
Masafumi Yoshio, Department of Chemistry and Biotechnology, School of
Engineering, The University of Tokyo
Zuofeng Zhao, Department of Chemistry and Biochemistry, Arizona State
University
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1
1
IMPORTANCE AND POSSIBILITY
OF IONIC LIQUIDS

Hiroyuki Ohno
1.1 IONIC LIQUIDS
Ionic liquids are salts with a very low melting temperature. Ionic liquids have
been of great interest recently because of their unusual properties as liquids.
Because these unique properties of ionic liquids have been mentioned in a
few other books, we will not repeat them here but will summarize them in
Table 1.1 . Note that these are entirely different properties from those of ordi-
nary molecular liquids. Also, every ionic liquid does not always show these
properties. For electrochemical usage, the most important properties should
be both nonvolatility and high ion conductivity. These are essentially the prop-
erties of advanced (and safe) electrolyte solutions that are critical to energy
devices put in outdoor use.
Safety is a more important issue than performance these days, and it has
been taken into account in the materials developed for practical use. Thus,
more developments in ionic liquids are expected in the future. The nonvolatile
electrolyte solution will change the shape and performance of electronic and
ionic devices. These devices will become safer and have longer operational
lives. They are composed of organic ions, and these organic compounds have
unlimited structural variations because of the easy preparation of many dif-
ferent components. So there are unlimited possibilities open to the new fi eld
of ionic liquids. The most compelling idea is that ionic liquids are “ designable ”
or “ fi ne - tunable. ” Therefore, we can easily expect explosive developments in
fi elds using these remarkable materials.
Electrochemical Aspects of Ionic Liquids, Second Edition. Edited by Hiroyuki Ohno.
© 2011 John Wiley & Sons, Inc. Published 2011 by John Wiley & Sons, Inc.
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2 IMPORTANCE AND POSSIBILITY OF IONIC LIQUIDS
1.2 IMPORTANCE OF IONIC LIQUIDS
Ionic liquids are salts that melt at ambient temperature. The principles of
physical chemistry involved in the great difference between solution proper-

ties of molecular solvents and molten salts have already been introduced and
summarized in several books. Thousands of papers have already been pub-
lished on their outstanding characteristics and effectiveness for a variety of
fi elds. Thus, as mentioned, in this book, we take the most important point that
these salts are composed of organic ions and explore the unlimited possibility
of creating extraordinary materials using molten salts.
Because ionic liquids are composed of only ions, they usually show very
high ionic conductivity, nonvolatility, and fl ame retardancy. The organic liquids
with both high ionic conductivity and fl ame retardancy are practical materials
for use in electrochemistry. At the same time, the fl ame retardancy based on
nonvolatility inherent in ion conductive liquids opens new possibilities in other
fi elds as well. Because most energy devices can accidentally explode or ignite,
for motor vehicles there is plenty of incentive to seek safe materials. Ionic
liquids are being developed for energy devices. It is therefore important to
have an understanding of the basic properties of these interesting materials.
The ionic liquids are multipurpose materials, so there should be considerable
(and unexpected) applications. In this book we, however, will not venture into
too many other areas. Our concern will be to assess the possible uses of ionic
liquids in electrochemistry and allied research areas.
1.3 POTENTIAL OF IONIC LIQUIDS
At present, most interest in ionic liquids is centered on the design of new
solvents. Although the development of “ new solvents ” has led the develop-
ment of possible applications for ionic liquids, there is more potential for
development of electrochemical applications.
Electrochemistry basically needs two materials: electroconductive materials
and ion conductive materials. Ionic liquids open the possibility of improving
TABLE 1.1. Basic and Possible Characteristics of Organic Ionic Liquids
Low melting point
•
Treated as liquid at ambient temperature

•
Wide usable temperature range
Nonvolatility
•
Thermal stability

•
Flame retardancy
Composed by ions
•
High ion density

•
High ion conductivity
Organic ions
•
Various kinds of salts

•
Designable

•
Unlimited combinations
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POTENTIAL OF IONIC LIQUIDS 3
ion conductive materials. The aqueous salt solution is one of the best electro-
lyte solutions for electrochemical studies. However, because water is volatile,
it is impossible to use this at a wide temperature range or on a very small scale.
Many other organic polar solvents have been used instead of water to prepare

electrolyte solutions. They, however, have more or less the same drawback,
depending on the characteristics. The material known to be a nonvolatile ion
conductor is the polymer electrolyte. Polymers do not vaporize but decompose
at higher temperatures; the vapor pressure at ambient temperature is zero.
Polymer electrolytes are considered a top class of electrolytes except for the
one drawback: relatively low ionic conductivity.
One remarkable propertie of ionic liquids is the proton conduction at a
temperature higher than 100 ° C. Water - based proton conductors cannot be
operated at such a high temperature because of vaporization of water. As
mentioned in a later chapter, proton - conductive ionic liquids are the most
expected materials.
Some literature has included statements that the ionic liquids are thermally
stable and never decompose. This kind of statement has led to a misunder-
standing that the ionic liquids are never vaporized and are stable even when
on fi re. Are the ionic liquids indestructable? The answer is “ no. ” However,
although inorganic salts are entirely stable, the thermal stability of organic
salts depends largely on their structure. Because ionic liquids are organic
compounds, their degradation begins at the weakest covalent bond by heating.
Nevertheless, ionic liquids are stable enough at temperatures of 200 ° C to
300 ° C. This upper limit is high enough for ordinary use.
Does it need more energy or cost to decompose ionic liquids after fi nishing
their role? It is not diffi cult to design novel ionic liquids that can be decom-
posed at a certain temperature or by a certain trigger. It also is possible to
design unique catalysts (or catalytic systems) that can decompose target ionic
liquids. Some catalysts such as metal oxides or metal complexes have the
potential to become excellent catalysts for the decomposition of certain ionic
liquids under mild conditions. The post - treatment technologies of ionic liquids
should therefore be developed along with the work on the design of ionic
liquids.
At the present time there has been little progress in this area. Although

post - treatment technologies are beyond the scope of this book, we do attempt
to give ideas on the various future developments in ionic liquid technologies
as well as in electrochemistry. This book is dedicated to introducing, analyzing,
and discussing ionic liquids as nonvolatile and highly ion conductive electro-
lyte solutions. The astute reader will fi nd the future prospects for ionic liquids
between the lines in all chapters of this book.

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2
5
PHYSICAL CHEMISTRY OF
IONIC LIQUIDS: INORGANIC
AND ORGANIC AS WELL AS
PROTIC AND APROTIC
C. A. Angell , W. Xu , M. Yoshizawa - Fujita , A. Hayashi ,
J. - P. Belieres , P. Lucas , M. Videa , Z. - F. Zhao , K. Ueno ,
Y. Ansari , J. Thomson , and D. Gervasio
2.1 CLASSES OF IONIC LIQUIDS
Ionic liquids in their high - temperature manifestations (liquid oxides, silicates,
and salts) have been studied for a long time, using sophisticated methods, and
much of the physics is understood. By contrast, the low - temperature ionic
liquid (IL) fi eld ( < 100 ° C ILs), the subject of the present volume, is still under
development. The many interesting studies on transport and thermodynamic
properties of the < 100 ° C ILs have focused mainly on characterizing new
systems for potential applications [1 – 5] . The task of placing this behavior
within the wider phenomenology of liquid and amorphous solid electrolytes
as well as in the context of the liquid state in general still has a long way to
go. In this chapter, we review the current state of knowledge of physical prop-
erties of ionic liquids in an attempt to place them within this larger picture.
We make an effort to emphasize the special status of the protic subclass of

ionic liquids because these offer a degree of freedom not encountered in other
branches of the solvent - free liquid state.
The fi rst requirement of an ionic liquid is that, contrary to experience with
most liquids consisting of ions, it must have a melting point that is not much
Electrochemical Aspects of Ionic Liquids, Second Edition. Edited by Hiroyuki Ohno.
© 2011 John Wiley & Sons, Inc. Published 2011 by John Wiley & Sons, Inc.
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6 PHYSICAL CHEMISTRY OF IONIC LIQUIDS
higher than room temperature. The limit commonly suggested is 100 ° C [1b] .
Given the cohesive energy of ionic liquids (about which more will be said later
on), ambient melting requires that the melting point occur at a temperature
not too much higher than the glass transition temperature, T
g
, which provides
the natural base for liquid - like behavior. Ionic liquids nearly all melt within
the range that we call the “ low - temperature regime ” of liquid behavior [6,7] .
This means that in most cases, they will supercool readily and will exhibit
“ super - Arrhenius ” transport behavior near and below ambient temperature —
as is nearly always reported.
Such liquids come in different classes. The most heavily researched class is
the aprotic organic cation class [1 – 4,8 – 15] . In this cation class, the low melting
point is a consequence of the problem of effi ciently packing large, irregular
organic cations with small inorganic anions. More on this class is given in
Section 2.3 .
A second class [16] is one that may enjoy increased interest in the future
because of the presence of one of its members in the fi rst industrial IL process
[1b] , because of the new fi nding that its members can have aqueous solution -
like conductivities [17] and can serve as novel electrolytes for fuel cells [18] ,
and fi nally, because of the evidence that these liquids, in hydrated form, can
be used as tunable solvents for biomolecules, on which stability against aggre-

gation and hydrolysis may be provided under the right tuning [19] . This class
is closely related to the fi rst but differs in that the cation has been formed by
transfer of a proton from a Br ø nsted acid to a Br ø nsted base. The process is
reversible if the free energy of proton transfer is not too large. When the gap
across which the proton must jump to reform the original molecular liquid is
small, the liquid will have a low conductivity and a high vapor pressure. These
properties are not of great interest in an ionic liquid, although the liquid may
be fl uid. If the gap is large, as in the case of ammonium nitrate, 87 kJ/mol (from
data for HNO
3
+ N H
3
→ N H
4
NO
3
), then the proton will remain largely on the
cation, and for many purposes, the system is a molten salt. If the acid is a strong
acid like trifl ic acid, HSO
3
CF
3,
or a superacid like HTFSI [16] , then the transfer
of the proton will be energetic, and the original acid will not be regenerated
on heating before the organic cation decomposes. Such liquids will not be
easily distinguishable in properties from the conventional aprotic salts in
which some alkyl group, rather than a proton, has been transferred to the basic
site. This is particularly true of the protic ionic liquids (PILs) recently reported
by Luo et al. [20] using superbases as proton acceptors. The stability of these
systems has been characterized in terms of the relation between the boiling

point elevation (or excess boiling point) over the linear (or average value) of
the components [21] , and the excess was shown to be a linear function of the
difference in pK
a
values determined in aqueous solutions.
This relation is shown in Figure 2.1 . It seems to be free of exceptions when
the base is a simple amine nitrogen. The protic ionic liquids as a class [16 – 18]
are considerably more fl uid than the aprotic ionic liquids [17] , most likely
because of their generally reduced ionicity (see Section 2.6 ).
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CLASSES OF IONIC LIQUIDS 7
The third and distinct class of ionic liquid is the one that consists entirely
of inorganic entities. These are formed mostly as a consequence of the mis-
match of large anions like tetrachloroaluminate or iodide with small cations
like Li
+
. The eutectic in the system LiAlCl
4
– LiAlI
4
system, for instance, lies at
65 ° C, and the liquid is highly fl uid — more fl uid than most aprotic ionic liquids.
The phase diagram is shown in Figure 2.2 [22] . Also in this group is the more
viscous system containing silver and alkali halides [23] , which exhibits an
ambient temperature electrical conductivity, 10
− 1.4
S/cm, that is higher than that
of any aprotic ionic liquid because of the highly decoupled state of the silver
ions. Here, a protic subclass also may be important; in fact the fi rst “ ionic
liquids ” made were probably of this type, namely mixtures of ammonium salts

like the NH
4
SCN and NH
4
NO
3
eutectic recently shown to provide interesting
Figure 2.1. Correlation of the excess boiling point (determined at the 1 : 1 composition)
with the difference in aqueous solution pK
a
values for the component Br ø nsted acids
and bases of the respective ionic liquids Δ pK
a
. The Δ T
b
value is determined as the dif-
ference between the measured boiling point and the value, at 1 : 1, of the linear con-
nection between pure acid and pure base boiling points. Note the large excess boiling
points extrapolated for the ionic liquids formed from the superacid HTf (open
triangles). These values could not be determined experimentally because of prior
decomposition. (Notation: EA = ethylammonium, PA = propylammonium, α Pic =
α - picolinium = 2 - methylpyridinium = 2MPy, FA = formate, TFAc = trifl uoroacetate,
Tf = trifl ate = trifl uoromethanesulfonate) (from Yoshizawa et al. [21] ). Data for the
three protic nitrates of [17] (ethylammonium nitrate, dimethylammonium nitrate, and
methylammonium nitrate) fi t precisely on this diagram.
0
50
100
150
200

250
300
350
400
450
500
0102030
Δ
pK
a
ΔT
b
/ K
1:1 (mol) Base-Acid ILs
Estimated value
α
Pic-FA
α
Pic-TFAc
PA-FA
PA-TFAc
α
Pic-Tf
PA-Tf
EA-FA
c02.indd 7c02.indd 7 1/20/2011 10:26:43 AM1/20/2011 10:26:43 AM
8 PHYSICAL CHEMISTRY OF IONIC LIQUIDS
fuel cell electrolytes [24] . Hydrazinium nitrate is known to melt at 80 ° C [25] .
Waiting to be created here is a subclass in which the cations are derived from
inorganic molecular entities, for instance, (PNCl

2
)
3
or (SN)
4
. A protic salt of
the latter is on record, but its melting point is unknown. We have obtained a
sharply melting compound (PNCl
2
)
3
. HTFSI, from the equimolar melt of the
components [26] , but its conductivity is too low to class it as an ionic liquid
(i.e., it is of low “ ionicity ” [27 – 31] ). The nitrogen atoms of PNCl
2
are known
to be only weakly basic, so the salt, if formed with a stronger acid (e.g., HSbF
6
),
although of higher ionicity, also would be of high “ acidity ” [32] ; in fact, it would
be a member of the class of superacidic ionic liquids [33] that we will describe
subsequently.
A fourth class may be considered, although it contains nonionic entities.
This is the liquid state of various ionic solvates. In these systems, molecules
usually thought of as solvent molecules are bound tightly to high fi eld cations
and have no solvent function. Such “ molten solvates ” have low vapor pressures
at ambient temperatures and only boil at temperatures near 200 ° C; for
instance, LiZnBr
4
· 3 H

2
O, has T
b
= 190 ° C, whereas T
g
= − 120 ° C [34] .
To provide a better perspective on ionic liquids, we fi rst make some obser-
vations on inorganic salts and the factors that make it possible to observe them
as ionic liquids below 100 ° C.
Figure 2.2. Phase diagram of the system LiAlCl
4
+ LiAlI
4
(from Lucas et al. [22] )
showing ionic liquid domain (T
l
< 100 ° C) in the middiagram.
LiAlCl
Temperature (°C)
0
0
50
100
150
200
250
65
20 40 60 80 100
Mol% LiAlI
4

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LOW-TEMPERATURE LIQUID BEHAVIOR OF IONIC MELTS 9
2.2 LOW - TEMPERATURE LIQUID BEHAVIOR OF IONIC MELTS
Most inorganic salts, when they melt, are found to fl ow and conduct electricity
according to a simple Arrhenius law at all temperatures down to their melting
points. For instance, unless measurements of high precision are used, the alkali
halides seem to obey the Arrhenius equation, even down to the deep eutectic
temperatures of their mixtures with other salts. LiCl and KCl form a eutectic
mixture with a freezing point of 351 ° C, approximately 300 K below either pure
salt freezing point; yet the viscosity of the melt barely departs from Arrhenius
behavior before freezing.
To see the viscosity behavior of the highly super - Arrhenius type typical of
almost any individual room - temperature molten salt (RTMS), it is necessary
to avoid alkali halides altogether and examine salts that cannot form such
symmetrical crystal lattices. For instance, alkali nitrates, like KNO
3
, do not
occupy much more volume in the liquid state than KI, but they melt at much
lower temperatures. However, even KNO
3
exhibits an Arrhenius - type viscos-
ity temperature dependence according to any but the most precise measure-
ments. It is only with deep eutectics like those for the ternary systems
LiNO
3
– NaNO
3
– KNO
3
( T

E
= 143 ° C) and LiNO
3
– NaNO
2
– KNO
3
( T
E
= 125 ° C)
that one starts to observe clear deviations from the Arrhenius law [6a,7] . This
stands in clear contrast with the behavior of the ILs (RTMS) or molten
hydrates.
With all < 100 ° C ILs, deviations from the Arrhenius law, considerably in
excess of those noted for the ternary LiNaK nitrate eutectic, are found well
above their melting points. There is no need, with such ILs, to invoke eutectic
mixtures to extend the stable liquid range to observe the “ low - temperature
domain ” behavior. The low - temperature domain typically is found in the tem-
perature range at T < 2 T
0
[6] , where T
0
is the theoretical low - temperature limit
to the liquid state based on the Kauzmann extrapolation [35] . At T
0
, extrapola-
tions of experimental data suggest that the excess entropy of the liquid (excess
over that of the crystal) would vanish and that the time scale for fl uid fl ow
would diverge [7,35] . The practical low - temperature limit to the liquid state
given by the glass temperature, T

g
, is considerably higher than the theoretical
T
0
, by an amount that depends on the liquid fragility. Typically T
g
/ T
0
= 1.2 – 1.3,
unless the liquid is “ strong ” [35b] . So the low - temperature domain is entered
at T
g
/ T ≈ 0.53 – 0.66. For Arrhenius behavior, which represents the “ strong ”
liquid limit of behavior is rarely observed, T
0
= OK and T
g
/ T
0
= ∞ . The upper
end of this range, T
g
/ T = 0.66, is the number usually associated with the ratio
of T
g
/ T
m
for glassformers (the “ 2/3 rule ” ), although we have argued elsewhere
[36] that this is not a rule but a tautology.
The behavior of ionic liquids is familiar to workers experienced with molten

hydrates. With molten hydrates, the cation size is increased effectively by the
shell of water molecules shielding the central cation from its anionic neighbors
such that the cation acquires a size not unlike those of cations in the typical
IL. We show a selection of viscosity data for normal molten salts, molten
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10 PHYSICAL CHEMISTRY OF IONIC LIQUIDS
hydrates, and ionic liquids in Figure 2.2 , using a scaled Arrhenius plot to bring
a wide range of data together on a single plot. A blurred distinction between
“ normal ” and “ low - temperature ” domain behavior can be made by putting a
vertical line at about T
g
/ T = 0.625.
The deviations from Arrhenius behavior observed in the low - temperature
regime of Figure 2.3 in most cases are well accounted for by the three -
parameter Vogel – Fulcher – Tammann (VFT) equation [37] in the modifi ed
form:

ηη
00 0
=−exp( [ ])DT T/
(2.1)
where η
0
, D , and T
0
are constants, and T
0
, the vanishing mobility temperature,
lies below the glass transition temperature if a suitable range of data ( > 3 orders
of magnitude) are included in the data fi tting. The different curvatures in

Figure 2.3 then are reproduced by variations in the parameter D of equation
(2.1) [6b] .
Figure 2.3. T
g
- scaled Arrhenius plot showing data for molten salts ZnCl
2
and calcium
potassium nitrate (CKN), with data for the calcium nitrate hydrate (CaNO
3
· 8 H
2
O)
and the tetrafl uoroborates of quaternary ammonium (MOMNM
2
E, M = methyl,
E = ethyl) and 1 - n - butyl - 3 - methyl - imidazolium (BMI) cations, and the bis - oxalatoborate
(BOB) of the latter cation, in relation to other liquids of varying fragility (from Xu
et al. [15] ).
1.00.90.80.70.60.50.40.30.20.10.0
-5
-3
-1
1
3
5
7
9
11
13
o-terphenyl (247)

GeO2 (810)
Ca(NO3)2 . 4H20
SiO2
ZnCl2 (380)
CKN (338)
propanol (98)
LiCl 5.8H2O
Ca(NO3)2.8H2O
CaAl2SiO8 (1134)
NaAlSi3O8 (1095)
MOMNMe2EBF4
BMIBF4
BMIBOB
Tg/T
log(viscosity/poise)
strong
intermediate
fragile
FRAGILE
IONIC
LIQUIDS
(PRESENT
WORK)
STRONG
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LOW-TEMPERATURE LIQUID BEHAVIOR OF IONIC MELTS 11
Figure 2.3 displays behavior that is almost universal to < 100 ° C ILs. Note
that, at their normal melting points, these liquids are almost always in the
“ low - temperature region ” of liquid behavior (defi ned by easily recognized
“ super - Arrhenius ” transport behavior) — that is, they melt within the same

range that is generally diffi cult to access for uni - univalent inorganic molten
salt systems and their mixtures. To understand how this can occur, it is helpful
to give some consideration to the factors that decide at which temperature a
given substance will melt (which we do in the next section).
Figure 2.4 , however, shows behavior for some inorganic ILs [22] , which
although superfi cially similar to that of Figure 2.3 (super - Arrhenius), is almost
never found in the organic cation ILs, either protic or aprotic, or in the molten
hydrates. The distinction lies in the value of the conductivity near the glass
temperature T
g
. The conductivity of the low - melting eutectic of Figure 2.4 can
be measured easily at temperatures near and below T
g
if the melt is cooled
rapidly to avoid crystallization. What is interesting is that the conductivity at
T
g
is approximately nine orders of magnitude higher than that of the typical
IL measured at its glass temperature (which requires special equipment). The
explanation for this dramatic difference lies in the ability of the small cation,
Li
+
, to slip through the crevices in the vitrifi ed structure and to establish a
Figure 2.4. Arrhenius plots for conductivity of lithium haloaluminate or pseudohalo-
aluminate melts, showing super - Arrhenius conductivity that remains high at the glass
temperature because of decoupling of Li
+
motion from its surroundings, in the “ low -
temperature regime. ” The break in the upper curve is a result of crystallization of the
supercooled liquid during reheating. Note the change from curvilinear to Arrhenius

behavior as T falls below T
g
and the structure becomes fi xed (from Lucas et al. [22] ).
100°C25°C
20
–7
–6
–5
–4
–3
Log σ (S.cm
–1
)
–2
–1
25
Tg
30 35 40 45 50
10000/T (K)
LiAlCl
4
-LiAlI
4
eutectic
LiAl(SCN,Cl,I)
4
melt
Tg
c02.indd 11c02.indd 11 1/20/2011 10:26:43 AM1/20/2011 10:26:43 AM
12 PHYSICAL CHEMISTRY OF IONIC LIQUIDS

solid - state ionic conductivity. These are known as “ decoupled ” systems, and
they are much sought - after for solid - state ionic devices. The only equivalent
in IL phenomenology of this behavior is found in some protic salts of proton-
ated anions like
HPO
24
−
in which the proton motion apparently can be some-
what decoupled from the structural relaxation [38] . However, in these systems,
the absolute conductivity is relatively low. A high degree of decoupling of
protons, together with a high conductivity as in the Grotthus mechanism of
aqueous solutions, is an urgent goal of research on protic ionic liquids. We
discuss tests of this sort of behavior later on.
2.3 MELTING POINTS AND THE LATTICE ENERGY
What is implied by the observations in the preceding section is that the crystal
lattices of substances of the IL type, like salt hydrate (and salt solvate
crystals in general), become thermodynamically unstable with respect to their
liquid phases ( G
liq
< G
crys
) at low temperatures, relative to their cohesive
energies.
The simplest explanation that can be given for this circumstance focuses on
the diffi culty that the more complex and size - mismatched ions characteristic
of IL and inorganic salt hydrates fi nd in packing closely. This causes the cohe-
sive energy (zero in the gas phase) to be fi xed at a value less negative than
would be obtained if the centers of attraction could be packed together more
closely. The lattice energies refl ecting this end up being smaller numerically
(i.e., less negative on a scale starting at zero in the reference gas phase). The

idea is illustrated in Figure 2.5 , which shows the Gibbs free energies of several
crystalline forms of the same fi ctitious substance, along with that of the liquid
phase, over a range of temperatures. The crystals are supposed to be noncon-
vertible except to the liquid and are supposed to differ only in their lattice
energies (lattice energy, E
L
= G at T = 0 K ) .
Figure 2.5 shows that the lowest melting point must belong to the crystal
phase with the lowest lattice energy. This can be verifi ed using data for isomers
of the same substance (e.g., xylene in which all three isomers o - , m - , and p -
have essentially the same viscosity and boiling point) for which adequate
thermodynamic data are available. We have shown elsewhere [39] how the
difference in melting points of approximately 70 K between m - and p - isomers
can be traced to a difference in lattice energy of ∼ 1 kcal/mol. Of course, the
polymorph with the lowest melting point will be the one with the highest
viscosity at the melting point and the one whose melting point will fall within,
or closest to, the “ low - temperature region ” of the liquid state [40] . Most
< 100 ° C ILs therefore are characterized by low lattice energies that approach
that of the glass formed by supercooling of the liquid to the vitreous state.
(A method of quantifying the lattice energies for crystalline forms of
ILs of known structure has been described recently by Izgorodino and
MacFarlane [41] .)
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ELECTRICAL CONDUCTIVITY AND LOW VAPOR PRESSURE 13
This conclusion is consistent with the usual strategy for making ILs. When
the salt of a given organic cation does not melt at a temperature low enough
to qualify as a < 100 ° C IL, the problem usually is rectifi ed by the addition of
a side - group or by the replacement of an existing side - group with a larger one
or one that breaks a previous symmetry [1 – 5,42,43] . What has been done is, of
course, to interfere with the possibility of achieving low energies by effi cient

packing in three - dimensional (3D) order. This improves the competitive status
of the disordered state; that is, it lowers the T / T
g
value at the melting point.
2.4 RELATION BETWEEN ELECTRICAL CONDUCTIVITY AND
LOW VAPOR PRESSURE
To vaporize an aprotic ionic liquid, it is necessary to do work against electrical
forces to remove an ion pair from the bulk of the liquid into the vacuum of
space. If the ionic liquid were to consist mainly of ion pairs, then this work
would be the same as in a strongly dipolar liquid. Fortunately, it is much larger.
It is larger because much of the stabilization energy of an ionic liquid is gained
by the formation of a quasilattice that is almost as effi cient in minimizing the
Figure 2.5. Gibbs free energies for a system in which there are multiple nonintercon-
vertable crystalline phases, each with a different lattice energy ( E
L
= G, at T = 0 K) but
all with the same entropy. All yield a liquid with the same free energy. Arrows show
access to the different crystalline phases from the liquid. The crystal with the lowest
melting point must be the one with the lowest lattice energy; it will be the one that
melts to a liquid with the highest viscosity and the one that therefore is the least likely
to crystallize during cooling. When crystallization does not occur, the glassy state does.
glass
liquid
T
melt
XtI
1
XtI
2
XtI

3
XtI
4
XtI
5
gibbs energy, G
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