Chemistry Reference
In-Depth Information
TABLE 9.4. HLB Numbers for Typically Encountered
Oil Phases
Oil Phase
Nominal HLB
Lauric acid
16
Oleic acid
17
Cetyl alcohol
15
Decyl alcohol
14
Benzene
15
Castor oil
14
Kerosene
14
Soybean oil
13
Lanolin
12
Carnauba wax
12
Paraffin wax
10
Beeswax
9
Clearly, the process of selecting the best surfactant(s) for the preparation of an
emulsion has been greatly simplified by the development of the generally empirical,
semiquantitative approaches exemplified by the HLB and PIT methods described
above. Unfortunately, each method has its limitations and cannot eliminate the need
for some amount of trial-and-error experimentation. As our fundamental under-
standing of the complex phenomena occurring at oil-water interfaces improves,
and the effects of additives and environmental factors on those phenomena become
more clear, it may become possible for a single, comprehensive theory of emulsion
formation and stabilization to lead to a single, quantitative scheme for the selection
of the proper surfactant system.
9.7.4. Effects of Additives on the ''
Effective
''
HLB of Surfactants
The exact mechanisms by which various additives affect the effective HLBs of
surfactants are not fully understood. For nonionic POE surfactants, in which hydra-
tion of the POE chain is the primary solubilizing mechanism in aqueous solution,
the extent of chain hydration has seldom been found to be increased by the addition
of materials that ''salt in'' the surfactants. That conclusion is based on the obser-
vation that the viscosity of the solutions is not significantly affected, indicating
that the hydrodynamic radius of the molecules is not increased by increased hydra-
tion. In fact, the actions of such additives are in all probability related to their
effects on the structure of the solvent, altering the thermodynamics of solvent-
solute interactions.
A proposed relationship between the HLB and the heat of hydration Q
h
of a
surfactant is given by
HLB
¼
0
:
42 Q
h
þ
7
:
5
ð
9
:
20
Þ
where Q
h
is given in calories per gram. From this equation, it seems that the addi-
tion of materials that increase the heat of hydration, such as sodium thiocyanate,
