Hydrophilic/Lipophilic Balance
Thanks to economics, regulations, environmental aspects, non-flammability, low cost and
other factors, most aerosols are formulated with large amounts of water. Virtually all the
water is modified into emulsions. Despite the huge 6.25 million tons per year (2005) U.S.
sales market for synthetic detergents alone, our universities pay little or no attention to
these unique organic compounds. Consequently, surfactant synthesizers, formulators and scientists
in similar disciplines have to develop their knowledge of emulsifiers from other sources: books, journals,
supplier literature, co-workers and experimentation. Utilization is often more art than science.
There are many different kinds of surfactants. Table 1 lists a number of categories, albeit with
some subjectivity:
TABLE 1: CATEGORIES OF SURFACTANTS
Anionics: oil-in-water (o/w) 73%
Non-ionics: oil-in-water (o/w) and water-in-oil (w/o) 20%
Amphoterics: (Zwitterions) 1%
Cationics: oil-in-water (o/w) 6%
Polymeric (micelles)
Micro-emulsions
Anhydrous emulsions
Ringing gel emulsions (potassium hydroxide KOH or thixotropic)
Technical Discussion
Within the non-ionic surfactant category there are a relatively small
number of emulsifiers that form w/o emulsions. Their molecules are
generally lightly ethylene-oxidated and these -CH2-CH2-O- units confer
some water dispersability. For
example, lauryl alcohol is insoluble
in water, but if two ethylene oxide
groups are added to the molecule
(called POE2), it becomes soluble
with difficulty (Brij 72) and can
produce w/o emulsions. If more
ethylene oxide groups are added,
the surfactant becomes readily
water-soluble, forming the much
more common o/w emulsions.
These interesting w/o emulsions
were apparently discovered by
C.F. Boe in 1950 (U.S. Patent No.
2,524,590) as an extension of earlier
work by W.D. Bancroft (Bancroft’s Rule).
It was refined and expanded almost
simultaneously by W. Griffen and Paul
Becher in 1965. The Becher tabulation
of “Hydrophilic-Lipophilic Balance”
MONtfORt A. JOHNseN, PH.D.
technical editor
Those mysterious
“HLB” numbers
28 Spray January 2020
Bancroft’s rule states, “The phase in
which an emulsifier is more soluble
constitutes the continuous phase.” It was
named after Wilder Dwight Bancroft, an
American physical chemist, who proposed
the rule in the 1910s.
Emulsions: Theory & Practice
Practice
by Paul Becher was published
in 1965.