Chapter 6
Melting of Molecular Crystals
6.1 Melting in Reality
Before proceeding to analysis and understanding, it is necessary to see the melting
behavior in the real world. Many molecular crystals of small molecules melt into
isotropic liquid in a single step along with the vertical and central arrow in Fig. 6.1.
Although crystals of both rare gas elements and benzene belong to this group, there
exists a significant difference in between. While the molecule of any rare gas, the
atom in reality, only has the translational degrees of freedom, the molecule of benzene has the rotational degrees of freedom additionally. In this sense, two types of
structural order are lost during the melting of benzene crystal simultaneously. This
understanding naturally leads to the expectation that two types of melting may occur
separately and successively. Then, depending on the order which takes place first,
two types of partially molten phases are expected during the process of an ordered
crystal to the isotropic liquid phase. Such a phase appearing in the course of melting
is generally called a mesophase.
Representatives of mesophases are known as plastic crystal and liquid crystal. The
plastic crystal is a crystalline phase in the sense that the molecular centers of gravity
locate periodically. However, molecular orientations around them should be utterly
disordered because of the melting of the orientational degrees of freedom: the degree
of the completeness of disordering is assessed through entropy, as discussed later. In
reality, many substances exhibit various crystalline phases with a moderate disorder
in molecular orientations. Such phases are regarded as not a plastic crystalline phase
but simply a disordered phase. On the other hand, the liquid crystal expected here
should be an anisotropic fluid because it appears as a result of the melting of the
translational degrees of freedom. This type of liquid crystalline phase is called the
nematic phase (see Sect. 7.1.2). In contrast to the case of plastic crystals, the term,
liquid crystalline phase, covers not only the nematic phase with complete disorder
concerning the translational degrees of freedom but also many other phases with the
partial disorder.
© The Editor(s) (if applicable) and The Author(s), under exclusive license
to Springer Nature Singapore Pte Ltd. 2020
K. Saito, Chemical Physics of Molecular Condensed Matter,
Lecture Notes in Chemistry 104,
https://doi.org/10.1007/978-981-15-9023-8_6
119
Melting of Molecular Crystals
6.1 Melting in Reality
Before proceeding to analysis and understanding, it is necessary to see the melting
behavior in the real world. Many molecular crystals of small molecules melt into
isotropic liquid in a single step along with the vertical and central arrow in Fig. 6.1.
Although crystals of both rare gas elements and benzene belong to this group, there
exists a significant difference in between. While the molecule of any rare gas, the
atom in reality, only has the translational degrees of freedom, the molecule of benzene has the rotational degrees of freedom additionally. In this sense, two types of
structural order are lost during the melting of benzene crystal simultaneously. This
understanding naturally leads to the expectation that two types of melting may occur
separately and successively. Then, depending on the order which takes place first,
two types of partially molten phases are expected during the process of an ordered
crystal to the isotropic liquid phase. Such a phase appearing in the course of melting
is generally called a mesophase.
Representatives of mesophases are known as plastic crystal and liquid crystal. The
plastic crystal is a crystalline phase in the sense that the molecular centers of gravity
locate periodically. However, molecular orientations around them should be utterly
disordered because of the melting of the orientational degrees of freedom: the degree
of the completeness of disordering is assessed through entropy, as discussed later. In
reality, many substances exhibit various crystalline phases with a moderate disorder
in molecular orientations. Such phases are regarded as not a plastic crystalline phase
but simply a disordered phase. On the other hand, the liquid crystal expected here
should be an anisotropic fluid because it appears as a result of the melting of the
translational degrees of freedom. This type of liquid crystalline phase is called the
nematic phase (see Sect. 7.1.2). In contrast to the case of plastic crystals, the term,
liquid crystalline phase, covers not only the nematic phase with complete disorder
concerning the translational degrees of freedom but also many other phases with the
partial disorder.
© The Editor(s) (if applicable) and The Author(s), under exclusive license
to Springer Nature Singapore Pte Ltd. 2020
K. Saito, Chemical Physics of Molecular Condensed Matter,
Lecture Notes in Chemistry 104,
https://doi.org/10.1007/978-981-15-9023-8_6
119