Chapter 3
Molecular Liquids
3.1 Liquid Structure
Even when gasses consist of not spherical atoms but anisotropic molecules, they
can mostly be treated as ideal ones that obey the ideal gas law, pV = n RT . It is,
however, noted that their extensive properties certainly depend on not only masses of
molecules but also their shapes.
1 The situation is much more complicated in liquids
due to their shape and interactions among them. Indeed, the presence of interparticle
interaction and resulting local structure is the primary difference between gas and
liquid.
3.1.1 Scattering from Molecular Liquid
The intensity of the scattered radiation (such as neutron and electron beams, or X-ray)
from a sample can generally be given as
I (q) ∝
V
ρ(r) exp(i q · r)dV
2
(3.1)
with the density of scatterers, ρ(r). If the liquid consists of only one atomic species,
the I (q) can be decomposed into the product of the squared atomic form factor f (q)
and the so-called structure factor S(q) as
I (q) = f (q)
2 S(q).
(3.2)
1 For example, the heat capacity of a classical ideal gas (perfect gas) is 3R/2, 5R/2, and 3R (R, the
gas constant) for those of monatomic, linear (e.g., diatomic), and non-linear molecules, respectively,
while ignoring internal vibrational degree(s) of freedom.
© 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_3
53