Dielectric relaxation, viscosity and freezing

It is shown that polar liquids may be divided into two classes according to whether or not the rotation of the dipoles is prevented by solidification. For liquids belonging to the first class, and having rigid molecules, an equation similar to Debye’s can be used to relate the dielectric relaxation time and the viscosity. This equation does not involve the ‘molecular radius’ which has always made the interpretation of Debye’s equation uncertain, and it can, in consequence, be used to determine from the experimental results how the ratio of the microscopic to the macroscopic relaxation time ( ז/ז ') depends on the static dielectric con­stant. The theory has been applied to a number of organic liquids of rigid molecular struc­ture, and indicates that the dependence of ז/ז ' on the dielectric constant is best expressed by Powles’s equation ז/ז ' = (2∊ 0 + ∊ ∞ )/3∊ 0 . For liquids belonging to the second class no simple relation between dielectric relaxation time and viscosity can be expected, but it may be possible to relate the relaxation time in some way to the transition which occurs in the solid state, in which the freedom of dipole rotation is lost.