62
3 Molecular Liquids
from state to state: the weight per molecule of the cyclic trimer is less than that of
the cyclic tetramer because of its structural rigidity. The electric dipole moment of
cyclic aggregates would be small, even if the cancellation is incomplete.
This simple model was successfully applied to some alcohols [9, 15], of which
molecules have bulky substituents near the hydroxy group relevant for H-bond formation. In the case of DCHM [9], for example, the model coherently explains the
temperature dependence of the number of H-bonded molecules estimated by FT-IR
spectroscopy, a large hump in heat capacity (
c p dT ≈ ε HB ), and a small dielectric
constant at low temperatures and a broad hump in its temperature dependence. The
temperature-dependent breakage of H-bonds results in a hump in heat capacity. The
small dielectric constant at low temperature is due to the cancellation of electric
dipole moments resulting from cyclic aggregates as confirmed by quantum chemical calculations. Besides, the hump in its temperature dependence comes from the
destruction of cyclic aggregates upon heating. Furthermore, a similar model reasonably describes the temperature dependence of populations of protons in differently
H-bonded states in the case of 1-phenyl-1-cyclohexanol [15]. Note that plural experimental techniques concertedly contribute to the development of the understanding
of molecular association based on this simple statistical model.
3.3 Ionic Liquids
Ionic liquid originally refers to a class of liquids consisting of only ions. In a sense,
the ionic liquid is a molten (fused) salt. Due to stronger interaction between the
consisting particles than between neutral molecules, the vapor pressure is generally
much lower. Although typical inorganic salts such as NaCl exist in liquid form only
at high temperatures (T ≥ 801
◦ C), ionic liquids with low melting temperatures
have attracted much attention recently. Historically speaking, the first example of
such ionic liquids was in the literature as early as 1914 [16]. Nowadays, the term
“ionic liquid” mostly means those with the melting temperature lower than typically
100
◦ C. The “room temperature ionic liquid” more clearly specifies the compounds.
The literature related to the ionic liquid is so numerous that the readers should consult
specialized reviews [17] or books.
Due to their intrinsic nature, ionic liquids transmit electricity without supporting
electrolytes. As the potential window, which is the capable width in the electrical
potential for meaningful electrochemical measurements, is rather broad, some applications as conducting media are supposed. Besides, the negligible vapor pressure of
ionic liquids results in effective non-volatility and, consequently, non-flammability
because the combustion of matter continues as exothermic reactions of vapors. The
non-flammability is highly preferable in many applications. The ionic liquids are
much safer than those that consist of salt(s) and a flammable organic solvent.
Since combinations of a cation and anion exhibit broad flexibility while keeping
their natures as ionic liquids, the ionic liquid is a designer liquid. By modifying the
3 Molecular Liquids
from state to state: the weight per molecule of the cyclic trimer is less than that of
the cyclic tetramer because of its structural rigidity. The electric dipole moment of
cyclic aggregates would be small, even if the cancellation is incomplete.
This simple model was successfully applied to some alcohols [9, 15], of which
molecules have bulky substituents near the hydroxy group relevant for H-bond formation. In the case of DCHM [9], for example, the model coherently explains the
temperature dependence of the number of H-bonded molecules estimated by FT-IR
spectroscopy, a large hump in heat capacity (
c p dT ≈ ε HB ), and a small dielectric
constant at low temperatures and a broad hump in its temperature dependence. The
temperature-dependent breakage of H-bonds results in a hump in heat capacity. The
small dielectric constant at low temperature is due to the cancellation of electric
dipole moments resulting from cyclic aggregates as confirmed by quantum chemical calculations. Besides, the hump in its temperature dependence comes from the
destruction of cyclic aggregates upon heating. Furthermore, a similar model reasonably describes the temperature dependence of populations of protons in differently
H-bonded states in the case of 1-phenyl-1-cyclohexanol [15]. Note that plural experimental techniques concertedly contribute to the development of the understanding
of molecular association based on this simple statistical model.
3.3 Ionic Liquids
Ionic liquid originally refers to a class of liquids consisting of only ions. In a sense,
the ionic liquid is a molten (fused) salt. Due to stronger interaction between the
consisting particles than between neutral molecules, the vapor pressure is generally
much lower. Although typical inorganic salts such as NaCl exist in liquid form only
at high temperatures (T ≥ 801
◦ C), ionic liquids with low melting temperatures
have attracted much attention recently. Historically speaking, the first example of
such ionic liquids was in the literature as early as 1914 [16]. Nowadays, the term
“ionic liquid” mostly means those with the melting temperature lower than typically
100
◦ C. The “room temperature ionic liquid” more clearly specifies the compounds.
The literature related to the ionic liquid is so numerous that the readers should consult
specialized reviews [17] or books.
Due to their intrinsic nature, ionic liquids transmit electricity without supporting
electrolytes. As the potential window, which is the capable width in the electrical
potential for meaningful electrochemical measurements, is rather broad, some applications as conducting media are supposed. Besides, the negligible vapor pressure of
ionic liquids results in effective non-volatility and, consequently, non-flammability
because the combustion of matter continues as exothermic reactions of vapors. The
non-flammability is highly preferable in many applications. The ionic liquids are
much safer than those that consist of salt(s) and a flammable organic solvent.
Since combinations of a cation and anion exhibit broad flexibility while keeping
their natures as ionic liquids, the ionic liquid is a designer liquid. By modifying the