Whiskey still at the
Old Jameson Distillery
in Dublin, Ireland.
Image: Barbara and Ed
When the cohesive forces are smaller than the
adhesive forces, the mixture has a non-linear negative
deviation from Raoult’s Law, meaning that the vapor
pressures of each constituent is lower than it would
be for the pure liquid at the same temperature. The
boiling point of the mixture is raised. The mixture
is exothermic; it releases heat and warms when the
components mix. An example is nitric acid and water.
Thus, the Chemistry 101 admonition to “add acid to
water, not water to acid,” comes from the exothermic
nature of acid dilution. A memorable version of this
rule comes with a Boston accent, “Do as you otta, add
acid to watta.”
For some, but not all, mixtures, the deviations
from Raoult’s Law are large enough that there is a
maximum (for positive deviations) or minimum
(for negative deviations). In these cases, the boiling
point of the mixture is lower (for positive deviations)
or higher (for negative deviations) than the boiling
points of any of the constituents. At these maxima or
minima, the slope or derivative of the curve is zero
and the mixture is azeotropic. The most important
attribute for an azeotropic mixture from a critical
cleaning standpoint is that the vapor has the same
constituent ratios as the liquid, so that as the liquid
vaporizes or boils, it does not change in constituency.
An azeotrope is invaluable where the goal is to maintain the consistency of the mixture.
Not all liquid blends are azeotropes. Only if the deviation from Raoult’s Law is large enough for there to be a
maximum or minimum, and then only if the mixture
composition is at that maximum or minimum, will the
mixture be azeotropic.
Some chemical blends are referred to as “near azeotropes” or “azeotrope like.” If these mixtures do not have
the zero slope condition that defines an azeotrope, they
can have the same problems as non-azeotropic mixtures. Problems include changes in composition, process
inconsistency, and development of flammability.
Even azeotropic mixtures have limits. Azeotropic
conditions occur only over a finite temperature range.
A liquid that is an azeotrope at its boiling point may
not be one at cooler temperatures. Appropriate storage
conditions (e.g. sealed containers) are essential to assure
that the azeotropic ratios are maintained between uses.
Changes in composition could also occur during warm-up or cool-down or if the boiling point changes (e.g. in
a vacuum or reduced pressure contained system).
Don’t forget the soil that is removed during cleaning.
Soil becomes part of the mixture and has the potential
Search for new azeotropes
to break the azeotrope through changes in the balance
of cohesive and adhesive forces. As has always been the
case, process monitoring is required whether your pure
solvent is prone to hydrolysis or your azeotropic blend is
prone to break with a particular soil.
There is a need for additional effective cleaning options.
Given worker safety and environmental issues, the
number of effective single molecule cleaning agents are
relatively few; new molecules are difficult to develop.
At the same time, there are cleaning applications where
many mixtures, especially those that require rinsing or
displacement, are not desirable. New azeotropes are one
The U.S. Department of Defense, through the
Strategic Environmental Research and Development
Program (SERDP), is addressing this problem. A
SERDP project is underway to develop azeotropic
solvent blends that are effective for cleaning, are not
classed as Hazardous Air Pollutants, and are low
VOC. The study is headed by Dr. Darren Williams,
Associate Professor of Physical Chemistry at Sam
Houston State University, Huntsville, Texas; we are
participants. The study should be useful for many
high-value parts and components, as well as for military applications. Results will be published.
The authors thank Dr. Darren Williams for contributing
to and reviewing this article.
Barbara Kanegsberg and Ed Kanegsberg (the Cleaning
Lady and the Rocket Scientist) are experienced consultants
and educators in critical and precision cleaning, surface
preparation, and contamination control. Their diverse projects include medical device manufacturing, microelectronics,
optics, and aerospace. email@example.com