Page 279 - Introduction to Colloid and Surface Chemistry

268 Emulsions and foams
formation of this particular O/W emulsion by use of other emulsifier
systems; for example, the optimum proportions in a mixture of
sorbitan mono-oleate (HLB 4.3) and polyoxyethylene sorbitan
monopalmitate (HLB 15.6) should be approximately 30 per cent and
70 per cent, respectively. In commercial emulsion formulation, HLB
numbers are used advantageously in this way as an initial guide prior
to a certain amount of trial and error testing.

Table 10.1 HLB values

Applications Dispersibility in water

3-6 W/O emulsions 1-4 Nil
7-9 Wetting agents 3-6 Poor
8-15 O/W emulsions 6-8 Unstable milky dispersion
13-15 Detergent 8-10 Stable milky dispersion
15-18 Solubiliser 10-13 Translucent dispersion/solution
13- Clear solution

A major disadvantage of the HLB concept is that it makes no
allowance for temperature effects. With increasing temperature, the
hydration of lyophilic (particularly poly(ethylene oxide)) groups
decreases and the emulsifying agent becomes less hydrophilic- i.e. its
HLB decreases.
An alternative method for characterising emulsifying agents is in
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terms of a phase inversion temperature (PIT) . An emulsion
containing equal weights of oil and water phases and 3-5 per cent
emulsifier is heated and agitated, and the temperature at which the
emulsion inverts is noted; i.e. the emulsion is W/O above and O/W
below the PIT. Therefore, the PIT should be higher than the
proposed storage temperature for an O/W emulsion and lower than
the proposed storage temperature for a W/O emulsion.
O/W emulsions with a very small droplet size can be obtained if
prepared at only a few kelvins below the PIT. At this temperature the
emulsion will be unstable to coalescence, but subsequent cooling to
20 K or more below the PIT can enhance the stability of the emulsion
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while retaining the small average droplet size .