90
A. Sanz
Abbreviations
CNT
Classical nucleation theory
DS
Dielectric spectroscopy
G crys
Gibbs free energy of the crystalline state
G liq
Gibbs free energy of the liquid state
HB
Hydrogen bond
JMAK Johnson–Mehl–Avrami–Kolmogorov
MW
Maxwell–Wagner effect
ND
Neutron diffraction
PC
Plastic crystal
SCL
Supercooled liquid
T g
Glass transition temperature
1 Introduction
In this chapter, we will describe how dielectric spectroscopy techniques have
contributed for a better understanding of the structure-dynamics correlations in low
molecular weight alcohols. We will focus on the transformation of liquid short-chain
alcohols into solid phases with periodic order, with especial attention paid to crystallization processes. Unlike liquids, where molecular entities exhibit rotational and
translational mobility, in the true crystalline state, molecular motions are restricted
to small oscillations about fixed positions in a regular lattice.
According to thermodynamics, crystallization would be possible as soon as the
difference between the Gibbs free energy of the crystal G crys and that of the liquid
G liq is negative [1–3]. Let us remind that the Gibbs free energy combines the internal
energy of the system H and the entropy S, being the latter modulated by temperature
as follows:
G = H − T S
(1)
In most cases, ordering transitions get triggered by a reduction of the internal
energy at temperatures below the corresponding melting point, although entropydriven transitions into more ordered phases may also take place [4]. In these latter
cases, freezing processes proceed via a partial ordering of the molecular entities
while density remains almost constant. This is then understood to be produced by
some molecular rearrangements allowing the stabilization of the new phase due to an
increase in entropy instead of through a minimization of the internal energy. Freezing
transitions in short-chain alcohols corresponding to both scenarios will be reviewed
in the sections below.
The first step in crystallization requires overcoming the energy barrier of creating
tiny crystalline seeds by thermal random fluctuations. During this process, termed
A. Sanz
Abbreviations
CNT
Classical nucleation theory
DS
Dielectric spectroscopy
G crys
Gibbs free energy of the crystalline state
G liq
Gibbs free energy of the liquid state
HB
Hydrogen bond
JMAK Johnson–Mehl–Avrami–Kolmogorov
MW
Maxwell–Wagner effect
ND
Neutron diffraction
PC
Plastic crystal
SCL
Supercooled liquid
T g
Glass transition temperature
1 Introduction
In this chapter, we will describe how dielectric spectroscopy techniques have
contributed for a better understanding of the structure-dynamics correlations in low
molecular weight alcohols. We will focus on the transformation of liquid short-chain
alcohols into solid phases with periodic order, with especial attention paid to crystallization processes. Unlike liquids, where molecular entities exhibit rotational and
translational mobility, in the true crystalline state, molecular motions are restricted
to small oscillations about fixed positions in a regular lattice.
According to thermodynamics, crystallization would be possible as soon as the
difference between the Gibbs free energy of the crystal G crys and that of the liquid
G liq is negative [1–3]. Let us remind that the Gibbs free energy combines the internal
energy of the system H and the entropy S, being the latter modulated by temperature
as follows:
G = H − T S
(1)
In most cases, ordering transitions get triggered by a reduction of the internal
energy at temperatures below the corresponding melting point, although entropydriven transitions into more ordered phases may also take place [4]. In these latter
cases, freezing processes proceed via a partial ordering of the molecular entities
while density remains almost constant. This is then understood to be produced by
some molecular rearrangements allowing the stabilization of the new phase due to an
increase in entropy instead of through a minimization of the internal energy. Freezing
transitions in short-chain alcohols corresponding to both scenarios will be reviewed
in the sections below.
The first step in crystallization requires overcoming the energy barrier of creating
tiny crystalline seeds by thermal random fluctuations. During this process, termed