Finding the Right Blend
and Ed Kanegsberg
BFK Solutions LLC
In most mixtures, the components can be read- ily separated. If you mix water and table salt (sodium chloride) and then expose the water to air the water eventually evaporates, leaving the
salt behind. However, if you distill whiskey, there is
a limit to the purification. Even after repeated distillation, the limit is approximately 96 percent ethyl
alcohol; the rest is water. Distillation to higher purity
is not possible under typical distillation conditions.
Blended chemistries, both aqueous and solvent-based and whether or not they are relatively separable, are ubiquitous in critical cleaning applications,
and with good reason. Blended chemistries can
provide the desired cleaning performance. Blended
chemistries may expand the solvency range. This
may make it easier to remove adherent soils or complex soil mixtures. Blending an aggressive chemical
with a milder chemical can lessen materials compatibility issues. Components and parts with tight
spacing are often difficult to clean because the cleaning agent cannot readily access the soil. Surfactants
make water less “water-like” by reducing its surface
tension. This allows the cleaning solution to flow
into small gaps and holes. Adding solvents with
relatively high density, low viscosity, and low surface
tension can allow the mixture to more readily access
the soil to be removed. In some instances, the blend
may show a desirable chemical reaction with the soil
of interest. Non-additive or synergistic effects may
happen in blends, so that cleaning is even better than
might be expected based on the individual properties
of the chemicals.
Blending can provide important safety and
environmental benefits. Skillful blending can yield
relatively effective cleaning agents with lower
permitting, containment, and monitoring issues.
Blending a non-flammable solvent with a more
active low flashpoint solvent can produce a mixture
with no flashpoint. Minimizing volatile organic
compounds (VOCs) can help companies meet air
quality requirements. Worker exposure issues may
limit the use of certain high-performing chemicals.
In many (but not all) instances, blending can result
in a cleaning agent that is acceptable to regulatory
agencies and to company safety/environmental professionals.
What is an azeotrope?
Let’s return to distilling whiskey. The limiting alco-hol/water blend is an azeotrope. While this limit
may be vexing to those attempting to create 200
proof spirits, there are many instances where the
goal is to create an azeotrope.
An azeotrope is a blend, usually of two or three
constituents, where the boiling rates of the constituents are in proportion to the ratio of the composition
itself. The word azeotrope comes from a combination
of Greek words that mean “no change on boiling.”
The vast majority of blends are not azeotropes. In an
azeotrope, the vapor and the liquid maintain the same
relative composition. This can be a crucial property
for maintaining the stability of a blend over time and
with repeated use. For example, if the purpose of the
blend is to suppress flammability, the mixture will
remain non-flammable. With a non-azeotrope, if the
component that has no flashpoint evaporates more
rapidly than the other component(s), unfortunate
incendiary events could occur.
The vapor pressure of a single component liquid is
determined by the cohesive forces between the molecules of the liquid. The stronger these forces, the
more energy it takes for molecules to break free and
form a vapor, resulting in a lower vapor pressure
and a higher boiling temperature.
In a mixture of two or more liquids, the vapor
pressures of the components depend not only on
the cohesive forces between similar molecules, but
also on the adhesive forces between dissimilar molecules. If the cohesive and adhesive forces are the
same, then the vapor pressures of the constituents
obey Raoult’s Law, a liquid analog of the Ideal Gas
Law, which states that the vapor pressure of a mixture is a linear combination of the vapor pressures
of the constituents weighted by their relative percentage in the mixture.
However, cohesive forces may be greater or lesser than adhesive forces. When the cohesive forces
are greater than the adhesive forces, the mixture has
a non-linear positive deviation from Raoult’s Law,
meaning that the vapor pressures of each constituent is higher than it would be for the pure liquid
at the same temperature. The boiling point of the
mixture is lowered. The mixture is endothermic; it