Ordering Transitions in Short-Chain Alcohols
99
Fig. 7 Schematic
representation of the Gibbs
free energy for different
structural phases in
condensed ethanol. Adapted
with permission from Talón
et al. [38]. Copyright (1998)
by the American Physical
Society
This metastable phase presents translational order, in which the centre of mass of
the molecular species remains fixed at the nodes of a crystalline lattice, but it lacks
rotational order. It is important to note that the rotational mobility in orientationally
disordered crystals gets frozen at a characteristic T g , transforming into the so-called
orientational glass with random molecular orientations. As indicated in Fig. 7, both
the supercooled liquid and plastic crystals phases form their respective glasses at the
same temperature and very close densities [35]. The formation of these rotationally
disordered crystals, commonly named plastic crystals, is fairly common in organic
substances with small steric hindrance to molecular reorientation and some degree
of intrinsic disorder [36, 37]. An increase of the rotational mobility (increase of
rotational entropy) provides the necessary stabilization of these metastable phases.
Figure 8 presents the corresponding neutron diffractograms for the supercooled
liquid, plastic crystal and stable monoclinic phase. A rigorous neutron and X-ray
diffraction study by Bermejo et al. [39], revealed that the structure of plastic crystal
ethanol corresponds to a cubic lattice (BCC), with two independent molecules per
unit cell.
A precise experimental protocol must be followed in order to explore the formation
of such structural phases. Liquid ethanol at room temperature must be cooled rapidly
(cooling rate > 6 K/min) to temperatures below T g , with the purpose of avoiding
crystallization on cooling. In this particular case, the sample was quenched to 75 K.
If supercooled liquid (SCL) ethanol is annealed between 105 and 110 K, the plastic
crystal (PC) is formed. Finally, the plastic crystal phase tends to crystallize into a
fully-ordered monoclinic phase at temperatures above 115 K.
Before we start discussing the main results obtained using the ND-DS technique,
the transformation from SCL into the PC phase was previously investigated by standard real-time dielectric spectroscopy [40]. In this work, Benkhof and co-authors
already pointed out that PC ethanol forms in a narrow temperature window around
99
Fig. 7 Schematic
representation of the Gibbs
free energy for different
structural phases in
condensed ethanol. Adapted
with permission from Talón
et al. [38]. Copyright (1998)
by the American Physical
Society
This metastable phase presents translational order, in which the centre of mass of
the molecular species remains fixed at the nodes of a crystalline lattice, but it lacks
rotational order. It is important to note that the rotational mobility in orientationally
disordered crystals gets frozen at a characteristic T g , transforming into the so-called
orientational glass with random molecular orientations. As indicated in Fig. 7, both
the supercooled liquid and plastic crystals phases form their respective glasses at the
same temperature and very close densities [35]. The formation of these rotationally
disordered crystals, commonly named plastic crystals, is fairly common in organic
substances with small steric hindrance to molecular reorientation and some degree
of intrinsic disorder [36, 37]. An increase of the rotational mobility (increase of
rotational entropy) provides the necessary stabilization of these metastable phases.
Figure 8 presents the corresponding neutron diffractograms for the supercooled
liquid, plastic crystal and stable monoclinic phase. A rigorous neutron and X-ray
diffraction study by Bermejo et al. [39], revealed that the structure of plastic crystal
ethanol corresponds to a cubic lattice (BCC), with two independent molecules per
unit cell.
A precise experimental protocol must be followed in order to explore the formation
of such structural phases. Liquid ethanol at room temperature must be cooled rapidly
(cooling rate > 6 K/min) to temperatures below T g , with the purpose of avoiding
crystallization on cooling. In this particular case, the sample was quenched to 75 K.
If supercooled liquid (SCL) ethanol is annealed between 105 and 110 K, the plastic
crystal (PC) is formed. Finally, the plastic crystal phase tends to crystallize into a
fully-ordered monoclinic phase at temperatures above 115 K.
Before we start discussing the main results obtained using the ND-DS technique,
the transformation from SCL into the PC phase was previously investigated by standard real-time dielectric spectroscopy [40]. In this work, Benkhof and co-authors
already pointed out that PC ethanol forms in a narrow temperature window around