The usual average particle size for aerosol emulsions (including non-ionic w/o types) is about 1.0
to 1.8 microns. This degree of dispersion produces a dull, milky appearance. By homogenization,
the micelles can be reduced to the 0.1 to 1.0 micron range, with the color turning blue-white and
the product becoming more stable. Upon aging, some flocculation may occur, which is a reversible
loose clustering of the dispersed phase. While rare, both flocculation and the more deadly irreversible
coagulation have to be considered by aerosol formulators. The use of a homogenizer can make
these effects even more unlikely.
Aerosol water-based pesticides and air fresheners were invented about 1960, using HLB technology.
While U.S regulators have now decimated this approach, using volatile organic compound
(VOC) limits, they were in vogue for almost 50 years, saving marketers millions of dollars. Formulas
that utilize hydrocarbon/CFC-152a propellants are still being utilized. With about 25% to 30%
propellant, they deliver insecticide and air freshening sprays that are very effective.
The preferred formulas for furniture polishes also use HLB systems to deliver the product in
essentially foam-free distributions. Several other household and automotive care aerosols also use
this technology. Emulsion science is still developing, with a long way to go.
While many low HLB (w/o) emulsifiers are lightly ethoxylated R-OH organics, such as higher
alcohols, glycols, glyceryls, sorbitols and so forth, chemists have other options. The ethylene oxide
groups can be replaced with propylene oxide groups for slightly reduced foaming tendencies in the
upper w/o HLB 6–8 range.
Other options include using oleamid DEA (diethanolamides, such as Schercomid SO-A),
sucrose distearate (such as Crodesta F10 or F50) and various hydroxylated non-ionic surfactants
of about HLB >7. In some formulations, blends may be more effective than trying to use a single
surfactant. If the HLB numbers are known, the HLB of the blend can be readily calculated as
their weighted average. For example, the HLB of a blend of 75% surfactant A (HLB = 4) and 25%
surfactant B (HLB = 10) will be 5.5 for surfactants A+B in the above ratio.
HLB numbers have been published for over a thousand surfactants and other substances. Most
values are outside of the approximate HLB range of HLB = 3–7. Non-ionic surfactants are available
where the molecule contains 10 to more than 100 ethylene oxide moieties. With rare exceptions,
they always produce oil-in-water (o/w) emulsions. Anionic surfactants have HLB values generally in
the 20–35 range. This means that very small levels of added anionic surfactant will drive water-inoil
(w/o) emulsions into the oil-in-water (o/w) state. Cationic surfactants behave similarly when
present at somewhat higher levels.
Over the years, there has been speculation concerning the possibility of “duplex” HLB emulsions.
They would be characterized as w/o/w or o/w/o. They might explain the presence of tiny
foam particles in some w/o aerosol sprays. However, Sanders, et.al. did not find any evidence for
such structures using microscopic and other experimental techniques.
30 Spray January 2020
The popularity of “water-based,” w/o air
fresheners and insecticides has been
considerably diminished in the U.S. due to
the impact of VOC/MIR regulations developed
by the California Air Resources Board that
have had the effect of arbitrarily reducing the
percentage of hydrocarbon propellants to the degree where
the spray quality has been adversely affected…