106
A. Sanz
the formation of HB’s with the surrounding molecules. Given that whole body reorientation is the main contribution to the dielectric response of supercooled ethanol,
the dynamics for SCL and PC is essentially the same, only differing in the time
scale which is slightly slower for the rotator phase due to the spatial restrictions
imposed by the positional order [40, 47]. During the phase transition from PC into
the monoclinic crystal state, the mobility of the former experiences dramatic changes,
in particular at the initial stages of crystallization, in contrast to other low molecular weight glass formers where the location and broadening of the main dielectric
process remain nearly unchanged in the course of crystallization [48, 49]. Based
on the results presented above, it is postulated that a dynamic transition from PC to
SCL-like configuration through a deep reorganization of the HB network should take
place along the pathway followed by the ethanol molecules as they diffuse from the
mother phase and finally attach to the monoclinic crystalline front [31].
3 Structural Ordering in Short-Chain Alcohols
by Standard Dielectric Spectroscopy Techniques
In this section, we will review several examples on the study of crystallization
kinetics in short-chain alcohols by standard dielectric spectroscopy methods. We will
mainly focus on isothermal processes. Different works have been published over the
past years with the main purpose of establishing correlations between structure and
dynamics during crystallization. To the author´s knowledge, dielectric spectroscopy
has been employed for monitoring the crystallization of materials such as n-butanol
[20], glycerol [49–51], salol [52], sorbitol [53], and pharmaceuticals compounds
such as Biclotymol [54]. In these examples, one finds mono- and poly-alcohols, that
is, systems showing a prominent Debye peak at low frequencies and others where
this relaxation is absent due to the cancellation of macro-dipoles, although the view
connecting solely the Debye relaxation in monohydroxy alcohols with the presence
of macro-dipoles has been recently challenged [55].
Hecksher and co-workers studied systematically the crystallization of n-butanol
at different temperatures with two different dielectric cells. In this work, the authors
invoked Maxwell–Wagner effects to explain the peak shift towards higher frequencies and the strong reduction in intensity of the dielectric loss observed in the
course of crystallization [20]. The depletion of the dielectric strength is especially
pronounced for the strong Debye relaxation during the early stages of the phase transition. Hecksher and co-workers proposed that under the framework of the MaxwellWagner model, the crystallization of n-butanol can be interpreted by a transition from
primarily growth of crystal spheres to growth of a crystal layer.
This picture was supported by describing the crystallization kinetics data in terms
of the JMAK model, suggesting a transition from higher dimensional growth to a
lower one [20] (Fig. 15).
A. Sanz
the formation of HB’s with the surrounding molecules. Given that whole body reorientation is the main contribution to the dielectric response of supercooled ethanol,
the dynamics for SCL and PC is essentially the same, only differing in the time
scale which is slightly slower for the rotator phase due to the spatial restrictions
imposed by the positional order [40, 47]. During the phase transition from PC into
the monoclinic crystal state, the mobility of the former experiences dramatic changes,
in particular at the initial stages of crystallization, in contrast to other low molecular weight glass formers where the location and broadening of the main dielectric
process remain nearly unchanged in the course of crystallization [48, 49]. Based
on the results presented above, it is postulated that a dynamic transition from PC to
SCL-like configuration through a deep reorganization of the HB network should take
place along the pathway followed by the ethanol molecules as they diffuse from the
mother phase and finally attach to the monoclinic crystalline front [31].
3 Structural Ordering in Short-Chain Alcohols
by Standard Dielectric Spectroscopy Techniques
In this section, we will review several examples on the study of crystallization
kinetics in short-chain alcohols by standard dielectric spectroscopy methods. We will
mainly focus on isothermal processes. Different works have been published over the
past years with the main purpose of establishing correlations between structure and
dynamics during crystallization. To the author´s knowledge, dielectric spectroscopy
has been employed for monitoring the crystallization of materials such as n-butanol
[20], glycerol [49–51], salol [52], sorbitol [53], and pharmaceuticals compounds
such as Biclotymol [54]. In these examples, one finds mono- and poly-alcohols, that
is, systems showing a prominent Debye peak at low frequencies and others where
this relaxation is absent due to the cancellation of macro-dipoles, although the view
connecting solely the Debye relaxation in monohydroxy alcohols with the presence
of macro-dipoles has been recently challenged [55].
Hecksher and co-workers studied systematically the crystallization of n-butanol
at different temperatures with two different dielectric cells. In this work, the authors
invoked Maxwell–Wagner effects to explain the peak shift towards higher frequencies and the strong reduction in intensity of the dielectric loss observed in the
course of crystallization [20]. The depletion of the dielectric strength is especially
pronounced for the strong Debye relaxation during the early stages of the phase transition. Hecksher and co-workers proposed that under the framework of the MaxwellWagner model, the crystallization of n-butanol can be interpreted by a transition from
primarily growth of crystal spheres to growth of a crystal layer.
This picture was supported by describing the crystallization kinetics data in terms
of the JMAK model, suggesting a transition from higher dimensional growth to a
lower one [20] (Fig. 15).