6
M. B. Shiflett et al.
history of ionic liquids by Freemantle in his tutorial book entitled An Introduction to
Ionic Liquids and released in 2010 also provides key dates in ionic liquid research
[4].
1.2 Ionic Liquid Generations
The classification of ionic liquids into different “generations” along a timeline is an
established concept; however, there is no single criterion to apply except a general
assessment on structure, properties, and potential applications.
During the 1960s and 1970s, chloroaluminate molten salts were believed to have
a low melting temperature and to form a eutectic medium that could improve the
overall performance in thermal batteries [22]. Further research and the employment of high-performance computers resulted in an emphasis on the dialkylimidazolium cations for their larger electrochemical window. The proper handling of
these substances, however, was quite sophisticated because they required completely
water-free conditions.
That is why a new generation (the second generation, but some researchers put
the first generation here [5, 25]) became focused on ionic liquids composed of the
air- and water-stable alkyl-substituted imidazolium cations with weakly coordinating anions, such as tetrafluoroborate [BF 4 ]
− and hexafluorophosphate [PF 6 ]
− [26].
Due to limited stability, these anions were later often replaced by the more stable
tris(pentafluoroethyl) trifluorophosphate “FAP,” triflate [CF 3 SO 3 ]
− or [OTf]
− , and
particularly bis(trifluoromethanesulfonyl)amide [(CF 3 SO 2 ) 2 N]
− or [NTf 2 ]
− , as well
as by the halide-free anions methylsulfate [C 1 SO 4 ]
− , acetate [C 1 COO]
− or [Ace]
− ,
thiocyanate [SCN]
− , and others. Typical cations were, among others, (substituted)
ammonium, imidazolium, pyridinium, or pyrrolidinium.
Initially, ionic liquids were considered to be “green solvents” because of their
very low volatility. Not surprisingly, the question soon arose whether these substances
were really “green solvents.” Studies on toxicity, persistence, and degradability were
conducted [27, 28]. Ionic liquid research began to target ions with proven low toxicity while retaining the desired (and established) material properties. The methods
of synthesis were also scrutinized to avoid toxic educts, to improve yields, and to
minimize impurities. At the turn of the millennium, choline- and lactate-based ionic
liquids were developed [29]. A new feature characterizing the third generation of
ionic liquids was customized biological properties [25, 30]. Rogers and co-workers
used the term “The Third Evolution of Ionic Liquids” in their paper on ionic liquids
featuring active pharmaceutical ingredients [25]. The term “Ioliomics” has now been
coined [31], and it is certain that the next step will focus on sustainability.
To sum up the evolution of ionic liquids in a single statement, unique tunable
physical properties characterize generation one (or two by others), targeted chemical
properties combined with chosen physical properties characterize generation two,
and ultimately targeted biological properties while keeping the others characterize
M. B. Shiflett et al.
history of ionic liquids by Freemantle in his tutorial book entitled An Introduction to
Ionic Liquids and released in 2010 also provides key dates in ionic liquid research
[4].
1.2 Ionic Liquid Generations
The classification of ionic liquids into different “generations” along a timeline is an
established concept; however, there is no single criterion to apply except a general
assessment on structure, properties, and potential applications.
During the 1960s and 1970s, chloroaluminate molten salts were believed to have
a low melting temperature and to form a eutectic medium that could improve the
overall performance in thermal batteries [22]. Further research and the employment of high-performance computers resulted in an emphasis on the dialkylimidazolium cations for their larger electrochemical window. The proper handling of
these substances, however, was quite sophisticated because they required completely
water-free conditions.
That is why a new generation (the second generation, but some researchers put
the first generation here [5, 25]) became focused on ionic liquids composed of the
air- and water-stable alkyl-substituted imidazolium cations with weakly coordinating anions, such as tetrafluoroborate [BF 4 ]
− and hexafluorophosphate [PF 6 ]
− [26].
Due to limited stability, these anions were later often replaced by the more stable
tris(pentafluoroethyl) trifluorophosphate “FAP,” triflate [CF 3 SO 3 ]
− or [OTf]
− , and
particularly bis(trifluoromethanesulfonyl)amide [(CF 3 SO 2 ) 2 N]
− or [NTf 2 ]
− , as well
as by the halide-free anions methylsulfate [C 1 SO 4 ]
− , acetate [C 1 COO]
− or [Ace]
− ,
thiocyanate [SCN]
− , and others. Typical cations were, among others, (substituted)
ammonium, imidazolium, pyridinium, or pyrrolidinium.
Initially, ionic liquids were considered to be “green solvents” because of their
very low volatility. Not surprisingly, the question soon arose whether these substances
were really “green solvents.” Studies on toxicity, persistence, and degradability were
conducted [27, 28]. Ionic liquid research began to target ions with proven low toxicity while retaining the desired (and established) material properties. The methods
of synthesis were also scrutinized to avoid toxic educts, to improve yields, and to
minimize impurities. At the turn of the millennium, choline- and lactate-based ionic
liquids were developed [29]. A new feature characterizing the third generation of
ionic liquids was customized biological properties [25, 30]. Rogers and co-workers
used the term “The Third Evolution of Ionic Liquids” in their paper on ionic liquids
featuring active pharmaceutical ingredients [25]. The term “Ioliomics” has now been
coined [31], and it is certain that the next step will focus on sustainability.
To sum up the evolution of ionic liquids in a single statement, unique tunable
physical properties characterize generation one (or two by others), targeted chemical
properties combined with chosen physical properties characterize generation two,
and ultimately targeted biological properties while keeping the others characterize