Montfort A. Johnsen
Emulsion Stability
Thanks to the California Air Resources Board (CARB), as well as economics, water is
now incorporated in almost three billion U.S. aerosol products each year, usually as
one phase of an emulsion. An even greater number of pump-action, squeeze tube and
other chemical specialty products also contain emulsions. It is usually essential that
these emulsions “stay together” during the service life of the product. All emulsions are metastable—
that is, they possess thermodynamic energy and seek to approach ground level by phase
separation. This tendency must be carefully addressed by formulators.
Thousands of chemists and technicians are currently hard at work to improve or create emulsion
products. Emulsion technology is wide-ranging and often complex. Sadly, it is rarely taught
in universities. Product development chemists must educate themselves. Textbooks are virtually
non-existent and other information sources are often biased, academic or simply hard to find.
One treatise defines an emulsion as “a liquid dispersed in a liquid.” While this covers a majority
of emulsions, one of the more important exceptions are some furniture polishes that contain
carnauba wax, beeswax, montan wax and synthetic waxes. Unusual emulsions can include those
where the dispersed phase is mercury, gallium, low melting gallium alloy and even nano-silver.
This latter item has long-lasting disinfectant/deodorant property and contains metal particles in
the range of 0.005 micron. It is therefore outside the classical definition of an emulsion and may
even illustrate the mythical ninth colloidal system: “solid dispersed in liquid.”
Technical Editor
An increasing number of aerosol products depend on emulsion stability—never to separate into their
continuous and dispersed phases. A major example is the bag-on-valve (BOV) aerosol. The tri-laminate
bag is essentially 100% full of emulsion, leaving no head space. If the emulsion separates upon aging,
shaking the BOV dispenser will have little or no effect on reconstituting the original emulsion. To stabilize
these emulsions, thickening polymers are often added to the continuous (usually water) phase.
Structure of emulsions
There are several ways to divide emulsions into categories. The most common is oil-in-water (o/w) and
water-in-oil (w/o). The w/o is much less common but can be seen in non-foaming air fresheners that are
now mostly produced outside the U.S., and thus not subject to oppressive volatile organic compound
(VOC) limits.
24 Spray January 2021
Furniture waxes—
such as Goddards
Cabinet Makers
Wax containing bees
wax—are exceptions
to the definition of
an emulsion as “a
liquid dispersed in a
liquid.”
The tri-laminate bag of a BOV dispenser is usually completely full of emulsion with no head space. Thickening
polymers are often added to stabilize emulsions, as shaking the BOV will have no effect on reconstituting an
emulsion that has separated with age. Photo: Signature Filling Co.