The traditional view that oil droplets in o/w emulsions are spherical is true only if
they do not touch or influence each other. Close-packed oil spheres can occupy only
73% of an emulsion, but a few o/w types show 99% oil droplets in a 1% continuous
phase. In such cases, the oil particles are warped into cubes or “bricks” with slightly
rounder corners and arranged in layers. The interstitial forces confer stability to the
emulsion.
An interesting product of this type is the “quick-start” charcoal briquette igniter.
The dispersed phase is 96% and contains odorless mineral spirits, about 8% propane
and 1% surfactants. The 4% continuous phase is water. A spiral of very stable white
foam is laid on a bed of briquettes, which are ignited by touching a flame to the
emulsion. The heavy foam deposit quickly “melts,” igniting the charcoal. A study of
retained products showed that the foam survived over 16 years at room temperature.
It is difficult to identify the particle size of aerosol o/w emulsions. Most have a
preponderance of 2.5 to 5.0 micron diameter droplets by weight. However, they have
been outnumbered 10-to-1 or even 100-to-1 by particles down to 0.005 micron. Oversized
particles can also be seen. In the absence of colored ingredients, these emulsions
are either milky-white or sometimes gray-white.
Emulsion destabilization
Emulsions are subject to two disabling influences: creaming and coalescence. Most
experts consider coalescence as the irreversible end result of an emulsion breakdown.
Shake before using: some shave creams will produce a runny foam
if not shaken before use.
January 2021 SPRAY 25
Studies have shown that
foam from “quick-start”
charcoal briquette igniters
can survive over 16 years
at room temperature.
It involves the contact and joining of two dispersion
particles to form a larger one. The morphing
process may take minutes or months. Eventually, all
the particles will combine as a single liquid phase.
It may float or sink, depending on how its density
compares with that of the continuous phase.
Coalescence is often preceded by flocculation
or creaming. This is where the dispersed particles
gather together, yet do not merge. The initial emulsion
can sometimes be reconstituted by shaking
the dispenser. For example, some shave creams will
produce a relatively runny, dense foam with larger
bubbles if not shaken before use. As the name
suggests, creaming is a concentration of disperse
particle at the top of an emulsion.
Despite the opacity of many emulsions, creaming
and flocculation can usually be determined by
preparing the emulsion in a heavy glass, pressureresistant
tube, allowing it to age and then slowly tilting
it. The tube wall can usually expose these problems.
As a rule, if creaming or flocculation is in evidence,
it is prudent to add stabilizing ingredients to the emulsion. Creaming, or flocculation, poses immediate
problems and may be a harbinger of a complete emulsion breakdown.
Emulsion stabilization
Probably the character and vigor of the film on dispersed particles is the most important factor in stability
work. A number of substances improve emulsion stability by enhancing the repulsion between dispersed
particles. A second approach has been to add viscosifying agents. Finely powdered inorganics may also be
utilized. They include zinc oxide, calcium carbonate and titanium dioxide. Research involving the electric
double layer is an increasingly useful predictive tool.
Stabilizers often have a thickening effect on emulsions. For example, stearyl alcohol, polyoxyethylene
palmityl ether and cetyl alcohol have been used to stabilize shave creams. They not only stiffen the aerosol
emulsions, but also enhance the viscosity of resulting foams. Trimethyl vinylammonium palmitate is not
only a stabilizer, but produces extremely long-lasting foams.
Various thickening additives have been tested for BOVs and other aerosols where permanent stability
is needed. Sodium carboxy methyl cellulose is useful. Others are polyoxy WSR-3000 gellant and PEG-910
gellant; it is said these are able to stabilize the emulsion for more than three years. Lastly, there is sodium
methyl cellulose (low viscosity).