Handbook of Solvents - George Wypych - ChemTech - Ventech!

2.3 Basic physical and chemical properties 55
Dielectric constant
35
30
25
20
15
10
5
0
20 40 60 80 100 120 140
electric field and from the separation of charges in apolar molecules. This orientation causes
polarization of the molecules and a drop in electric field strength. Dielectric constant data
may be used in many ways. In particular, it is the factor which permits the evaluation of
electrostatic hazards. The rate of charge decay is a product of dielectric constant and resistivity.
In solvent research, dielectric constant has a special place as a parameter characteristic
of solvent polarity. The dielectric constant, ε, is used to calculate dipole moment, μ:
ε
μ =
ε
− 1
− 2 VM alcohols
aliphatic CH
esters
ketones
Molecular weight, Daltons
Figure 2.3.18. Dielectric constant vs. molecular
weight of selected solvents.
Dielectric constant
0
1.3 1.35 1.4 1.45 1.5
[2.3.10]
where:
VM molar volume
The product of dipole moment and dielectric constant is called the electrostatic factor and it
is a means of classifying solvents according to their polarity.
Figure 2.3.18 shows that the dielectric constant correlates with molecular weight. It is
only with aliphatic hydrocarbons that the dielectric constant increases slightly as the molecular
weight increases. The dielectric constant of alcohols, esters, and ketones decreases as
their molecular weight increases, but only alcohols and ketones have the same relationship.
The dielectric constants of esters are well below those of alcohols and ketones.
The dielectric constant also correlates with refractive index. In the case of aliphatic
hydrocarbons, the dielectric constant increases slightly as refractive index increases. Both
aromatic and aliphatic hydrocarbons have dielectric constants which follow the relation-
2
ship: ε≈nD. The dielectric constants of alcohols, esters and ketones decrease as the refractive
constants increase but only alcohols and ketones form a similar relationship. The
dielectric constants of ketones poorly correlate with their refractive indices.
35
30
25
20
15
10
5
Refractive index
alcohols
aliphatic CH
esters
ketones
Figure 2.3.19. Dielectric constant of selected solvents
vs. their refractive index.