3.3 Ionic Liquids
63
combination appropriately, for example, we can potentially tune properties such as
miscibility to water or oils, and the solvability of specific chemicals.
Central issues about the ionic liquids have focussed on its characterization together
with potential applications as designer liquids. That is, how different they are from
non-ionic simple liquids or aqueous solutions of electrolytes. The structural studies
have indicated that the ionic liquids generally have a local structural order with a
characteristic length of the nm scale [18–20]. The order originates in the segregation between polar and nonpolar regions, the latter of which mostly consists of alkyl
groups. Depending on the length of the alkyl group, therefore, the morphology of the
nonpolar region changes from isolated domains to flat layers through the randomly
connected network [21], as in lyotropic liquid crystals (see Sect. 7.4). In the clarification of the segregated structure, molecular simulations have played a dominant role
[18, 20, 21], because of the difficulty in characterizing molecular liquids described
in Sect. 3.1.1.
One of the central questions concerning the ionic liquids is the origin of the low
melting point. The difficulty in crystallization often mentioned cannot serve as its
reason because the difficulty only affects the kinetics. Namely, the difficulty explains
the ease of obtaining liquid-quenched glasses (see Chap. 8). Instead, we must be
aware that the temperature of the first-order transition is the temperature where the
Gibbs energies of two phases coincide. We cannot reach the correct answer if we
consider only one phase, i.e., the liquid in this case. Predicting the properties of
one phase is more accessible than that of the melting temperature irrespective of its
strategy, based on either the physical mechanism or statistical learnings [22]. There
is also a claim [23] the liquid phase properties are mostly normal, whereas those of
the crystal may be strange. Keeping this in mind, we describe a few factors to lower
potentially the melting point.
First, we discuss the possible change in the interionic interaction energy. The
ionic liquids contain molecular ion(s), which is bulkier than simple inorganic ions.
Bulkiness itself pushes ionic centers apart, resulting in a reduction in the interionic
interactions. Further, molecular ions are more polarizable and screen effectively the
interionic interaction, which is electrostatic. In some cases, the ionic charge delocalizes over an aromatic moiety. The delocalization plausibly alters the interaction
energies. Next, we see the entropic aspect. Remember a thermodynamic relation,
T fus = Δ fus H/Δ fus S. Since the difference in density is insignificant between the
crystal and liquid, the interionic distance also remains similar. The change in interionic interaction energy can stay similar, accordingly. On the other hand, entropy
can increase through intermolecular disordering. If the structure of molecular ions
is appropriate, the melting temperature becomes low. Indeed, many ionic liquids
contain molecular cations that have an alkyl group(s), potential roles of which we
will discuss in more detail in Chap. 9. The nano-segregation described in a preceding
paragraph does not have a positive but negative effect on low melting points because
the segregation generally reduces the entropy of liquid through structure formation.
It is noteworthy here that the possible origins described above are generally hard to
assess directly and quantitively through only experiments.
63
combination appropriately, for example, we can potentially tune properties such as
miscibility to water or oils, and the solvability of specific chemicals.
Central issues about the ionic liquids have focussed on its characterization together
with potential applications as designer liquids. That is, how different they are from
non-ionic simple liquids or aqueous solutions of electrolytes. The structural studies
have indicated that the ionic liquids generally have a local structural order with a
characteristic length of the nm scale [18–20]. The order originates in the segregation between polar and nonpolar regions, the latter of which mostly consists of alkyl
groups. Depending on the length of the alkyl group, therefore, the morphology of the
nonpolar region changes from isolated domains to flat layers through the randomly
connected network [21], as in lyotropic liquid crystals (see Sect. 7.4). In the clarification of the segregated structure, molecular simulations have played a dominant role
[18, 20, 21], because of the difficulty in characterizing molecular liquids described
in Sect. 3.1.1.
One of the central questions concerning the ionic liquids is the origin of the low
melting point. The difficulty in crystallization often mentioned cannot serve as its
reason because the difficulty only affects the kinetics. Namely, the difficulty explains
the ease of obtaining liquid-quenched glasses (see Chap. 8). Instead, we must be
aware that the temperature of the first-order transition is the temperature where the
Gibbs energies of two phases coincide. We cannot reach the correct answer if we
consider only one phase, i.e., the liquid in this case. Predicting the properties of
one phase is more accessible than that of the melting temperature irrespective of its
strategy, based on either the physical mechanism or statistical learnings [22]. There
is also a claim [23] the liquid phase properties are mostly normal, whereas those of
the crystal may be strange. Keeping this in mind, we describe a few factors to lower
potentially the melting point.
First, we discuss the possible change in the interionic interaction energy. The
ionic liquids contain molecular ion(s), which is bulkier than simple inorganic ions.
Bulkiness itself pushes ionic centers apart, resulting in a reduction in the interionic
interactions. Further, molecular ions are more polarizable and screen effectively the
interionic interaction, which is electrostatic. In some cases, the ionic charge delocalizes over an aromatic moiety. The delocalization plausibly alters the interaction
energies. Next, we see the entropic aspect. Remember a thermodynamic relation,
T fus = Δ fus H/Δ fus S. Since the difference in density is insignificant between the
crystal and liquid, the interionic distance also remains similar. The change in interionic interaction energy can stay similar, accordingly. On the other hand, entropy
can increase through intermolecular disordering. If the structure of molecular ions
is appropriate, the melting temperature becomes low. Indeed, many ionic liquids
contain molecular cations that have an alkyl group(s), potential roles of which we
will discuss in more detail in Chap. 9. The nano-segregation described in a preceding
paragraph does not have a positive but negative effect on low melting points because
the segregation generally reduces the entropy of liquid through structure formation.
It is noteworthy here that the possible origins described above are generally hard to
assess directly and quantitively through only experiments.