W. Stephen Tait, Ph.D.
Chief Science Officer & Principal Consultant,
Pair O Docs Professionals, LLC
Corrosion Corner
Emulsions & Spray
Package Corrosion
Hello, Everyone. There are times when one or more formula
ingredients are not soluble in the rest of the formula. In
these instances, the insoluble ingredients are dissolved in
another solvent before mixing with the main formula.
However, what happens when the solvent for the insoluble
formula ingredient(s) is also insoluble in the main formula solvent?
Enter the emulsion!
What is an emulsion?
An emulsion is a type of colloid formed from liquid droplets dispersed
in a continuous liquid matrix. The dispersed droplets can have an
average size of 1–1,000 nanometers (nm)—size is referred to as an
average because there is typically a distribution of droplet sizes.
Spray packaging dispenses
liquid droplets into air; this
type of colloid is referred
to as an aerosol. Aerosol
droplet sizes from consumer
packaged goods range from
approximately 4–10 microns
(micro-meters) and medical
aerosol droplet sizes are
typically smaller than four
microns. In other words,
aerosol sprays are colloids but
not emulsions.
Probably one of the most
familiar emulsions is oil-invinegar
salad dressing. This
emulsion is formed by vigorously
shaking a mixture of
oil and vinegar to produce a
translucent emulsion that immediately
separates (breaks)
into oil and vinegar when the
Oil-and-vinegar emulsions immediately
separate and are not stable.
shaking is stopped. In other words, oil-in-vinegar emulsions are not
stable.
Milk is another common opaque emulsion that consists of water
and small water-insoluble liquid-fat droplets. Milk also separates into
two or more translucent and opaque phases over time (referred to as
cream phases). However, milk also has natural surfactants/emulsifiers
that stabilize the emulsion for a significantly longer time than
the oil-in-vinegar emulsion.
Hence stable emulsions are formed by dispersing one insoluble
liquid into another insoluble liquid with a surfactant (emulsifier).
Please note that stable is a relevant term, because all emulsions eventually
break (separate) into their separate insoluble components.
Types of emulsions
Emulsions are generically classified as either oil out—water droplets
in an oil matrix, or as water out—oil droplets in an aqueous matrix.
Emulsions can also be characterized by the average droplet size,
concentration or by the surfactant orientation on the droplet
surfaces. For example, microemulsions and nano-emulsions
have average dispersed droplet sizes below 100nm. They often
have a translucent, slightly blue color and a beam of light travels
through the micro/nano-emulsions without being refracted by
the droplets (Tyndall effect).
The dispersed phase in a hyper emulsion is above 74% of the
overall formula. Hyper emulsions are typically opaque and are
sometimes used to extinguish fires.
Emulsions can also be liquid crystals. In this situation,
dispersed droplets are encapsulated by a crystalline sheath of
surfactant molecules around the droplets.
Different surfactants for different emulsions
Surfactants are believed to stabilize dispersed droplets by either
forming a protective complex liquid barrier around each droplet
or by forming a charged surface around the droplets. Both barriers
and charged surfaces prevent droplets from contacting other
droplets and coalescing into larger droplets.
Consequently, different types of surfactants form and stabilize
different types of emulsions. Indeed, there are so many different
types of surfactants that I’m not even going to attempt to provide
an overview of which surfactants are used for each emulsion type.
In some instances, a critical surfactant concentration is needed
to form and stabilize an emulsion. Surfactant concentration can
also affect emulsion droplet size and droplet concentration. For
example, more surfactant is needed to form and stabilize a nano-
emulsion than a microemulsion.
Emulsions corrosivity & sprays
Are emulsions corrosive toward spray package materials? The
short answer is it depends! Whether or not corrosion will degrade
the package materials is a function of the:
• Type of package materials
• Chemical composition of your formula
26 Spray March 2021
Chart: Thoughtco.com