Chemistry Reference
In-Depth Information
optimum emulsification and stability. Such rules of thumb, while having great prac-
tical utility, are less than satisfying on a scientific level. One would really like to
have a neat, quantitative formula for the design of complete emulsion systems. A
number of attempts have been made over the years to develop just such a quanti-
tative approach to surfactant selection, and a brief discussion of some such
approaches follows.
9.7.1. Hydrophile-Lipophile Balance (HLB)
It has been a long-term goal of surfactant chemists and formulators to devise a
quantitative way of correlating the chemical structure of surfactant molecules
with their surface activity through some quantitative relationship that would faci-
litate the choice of material for use in a given formulation. Perhaps the greatest suc-
cess along these lines has been achieved in the field of emulsions; therefore, it is
appropriate to discuss the subject in some detail in that context.
The first reasonably successful attempt to quantitatively correlate surfactant
structures with their effectiveness as emulsifiers was the hydrophile-lipophile
balance (HLB) system, in which the objective is to calculate a number that ''mea-
sures'' the emulsifying potential, in terms of emulsion quality and stability or the
so-called HLB, of a surfactant from its chemical structure, and to match that num-
ber with the corresponding HLB of the oil phase to be dispersed. The system
employs certain empirical formulas to calculate the HLB number, normally giving
answers within a range of 0-20 on some arbitrary scale. At the high end of the scale
lie hydrophilic surfactants, which possess high water solubility and generally act as
good solubilizing agents, detergents, and stabilizers for O/W emulsions; at the low
end are surfactants with low water solubility, which act as solubilizers of water in
oils and good W/O emulsion stabilizers. The effectiveness of a given surfactant
in stabilizing a particular emulsion system would then depend on the balance
between the HLBs of the surfactant and the oil phase involved.
For nonionic surfactants with polyoxyethylene solubilizing groups, the HLB was
calculated from the formula
mol% hydrophilic group
5
HLB
¼
ð
9
:
7
Þ
In such a scheme, an unsubstituted polyoxyethylene glycol would have an HLB of
20. HLB values for some typical nonionic surfactants are given in Table 9.1.
Surfactants based on polyhydric alcohol fatty acid esters such as glycerol mono-
stearate can be handled by the relationship
S
A
HLB
¼
20 1
ð
9
:
8
Þ
where S is the saponification number of the ester and A is the acid number of the
acid. A typical surfactant of this type, commercially known as Tween 20 [polyoxy-
ethylene (20) sorbitan monolaurate], with S
¼
45.5 and A
¼
276, would have an
