In most mixtures, the components can be readily 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.
Why blend?
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 alcohol/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.
Raoult’s Law
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 absorbs heat and cools when the components mix. An example is ethanol and water.
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.
Limitations
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 warmup 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 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.
Search for new azeotropes
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 potential solution.
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 protected]
This article appeared in the September/October 2015 issue of Controlled Environments.
