92
A. Sanz
effort has been made over the past decades, in particular by utilizing dielectric spectroscopy methods [18]. Alcohols are extremely active in dielectric spectroscopy given
the high dipole moment of hydroxyl groups, which constitutes an important benefit
when dielectrics is employed for the study of crystallization processes, in particular
during the late stages of the phase transition where a small fraction of mobile phase
remains surrounded by the growing crystals [19, 20].
One of the most relevant features in liquid monohydroxy alcohols is the presence
of a strong dielectric dispersion located at lower frequencies than that corresponding
to the universal structural relaxation observed in all kinds of liquids. Due to its
narrow shape, this strong peak is known as the Debye peak. There is a general
consensus on the connection between the Debye peak and the dynamic character of
the HB network. In Fig. 1 we show the dielectric loss as a function of frequency
for supercooled isopropanol at 130.5 K, where the most intense process located at
the low-frequency flank of the spectrum corresponds to the Debye peak. The Debye
peak does not possess the main features of the typical structural or α relaxation and
different models have been proposed for explaining its origin but commonly associated with the hydrogen bonding network dynamics [21–24]. For instance, according
to the wait-and-switch model, first a molecule of the network switch its position to
reorient itself but it must wait until a favourable condition for reorientation exists
in the network [14, 22, 23, 25, 26]. Under the framework of this model, the mean
relaxation time of the Debye peak is expected to depend inversely upon the number
density of available hydrogen bonding sites [27].
The formation of superstructures via HB’s is corroborated by the presence of a prepeak at wave-vector values below the main peak (intermolecular correlations) in the
static structure factor. These pre-peaks stem from the intermolecular aggregates via
hydroxyl groups interactions and it is a well-documented signature in monohydroxy
alcohols [28, 29] (Fig. 2).
In this chapter, we will pay special attention to the main results obtained by using
a peculiar experimental setup that allows one to perform simultaneously dielectric
Fig. 1 Imaginary part of the
dielectric susceptibility for
isopropanol at 130.5 K. Solid
black line is the total fit of the
experimental data (◯) using
a Debye, Cole–Davidson and
log-normal distribution
function for describing the
primary, alpha and beta
relaxations respectively
10
-1
10
1
10
3
10
5
10
7
10
9
10
-2
10
-1
10
0
10
1
ε
Frequency (Hz)
A. Sanz
effort has been made over the past decades, in particular by utilizing dielectric spectroscopy methods [18]. Alcohols are extremely active in dielectric spectroscopy given
the high dipole moment of hydroxyl groups, which constitutes an important benefit
when dielectrics is employed for the study of crystallization processes, in particular
during the late stages of the phase transition where a small fraction of mobile phase
remains surrounded by the growing crystals [19, 20].
One of the most relevant features in liquid monohydroxy alcohols is the presence
of a strong dielectric dispersion located at lower frequencies than that corresponding
to the universal structural relaxation observed in all kinds of liquids. Due to its
narrow shape, this strong peak is known as the Debye peak. There is a general
consensus on the connection between the Debye peak and the dynamic character of
the HB network. In Fig. 1 we show the dielectric loss as a function of frequency
for supercooled isopropanol at 130.5 K, where the most intense process located at
the low-frequency flank of the spectrum corresponds to the Debye peak. The Debye
peak does not possess the main features of the typical structural or α relaxation and
different models have been proposed for explaining its origin but commonly associated with the hydrogen bonding network dynamics [21–24]. For instance, according
to the wait-and-switch model, first a molecule of the network switch its position to
reorient itself but it must wait until a favourable condition for reorientation exists
in the network [14, 22, 23, 25, 26]. Under the framework of this model, the mean
relaxation time of the Debye peak is expected to depend inversely upon the number
density of available hydrogen bonding sites [27].
The formation of superstructures via HB’s is corroborated by the presence of a prepeak at wave-vector values below the main peak (intermolecular correlations) in the
static structure factor. These pre-peaks stem from the intermolecular aggregates via
hydroxyl groups interactions and it is a well-documented signature in monohydroxy
alcohols [28, 29] (Fig. 2).
In this chapter, we will pay special attention to the main results obtained by using
a peculiar experimental setup that allows one to perform simultaneously dielectric
Fig. 1 Imaginary part of the
dielectric susceptibility for
isopropanol at 130.5 K. Solid
black line is the total fit of the
experimental data (◯) using
a Debye, Cole–Davidson and
log-normal distribution
function for describing the
primary, alpha and beta
relaxations respectively
10
-1
10
1
10
3
10
5
10
7
10
9
10
-2
10
-1
10
0
10
1
ε
Frequency (Hz)