Blends have proven useful in improving cleaning efficacy while at the same time reducing concerns of worker exposure, environmental damage or flammability. However, in most situations, the constituents of a blend volatilize or boil at different rates; the blend changes over time. This means that cleaning performance, materials compatibility, and flammability characteristics are likely to change. How do we get the properties of a blend while achieving consistency? One solution that is particularly valuable for vapor degreasing is to use an azeotrope.
What is 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 the Greek words ‘zein’, which means ‘boil’ and ‘tropos’, which means ‘turning’ or change. The prefix ‘a’ creates a negative, so the combination means ‘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 stability makes azeotropes valuable to assemblers and components manufacturers for critical product cleaning.
Why azeotropes are useful
Azeotropic mixtures tend to remain stable in consistency; this means that the bath will not require as much monitoring. It means that the cleaning properties are less likely to shift. The solvency properties and the materials compatibility aspects are less likely to shift.
An azeotropic mixture can simplify a process. For example, cyclohexane and isopropyl alcohol form an azeotrope, albeit one with a low flashpoint. It has found utility because you have the non-polar aspects of cyclohexane and the relatively polar (water-like) aspects of isopropyl alcohol in a single process step. This step can include both liquid and vapor-phase cleaning. Of course, a low-flashpoint system is needed to use this blend.
Flammability of a chemical can sometimes be suppressed by blending with a non-flammable chemical. For example, a number of fluorine-based solvents that are not flammable but are very mild solvents can be combined with trans-1,2-dichloroethylene to make a flashpoint-inerted blend. If the mixture is an azeotrope, the non-flammability characteristic of the mixture will remain as the liquid boils. However, if the liquid is not an azeotrope, one component will boil off faster, leaving either the remaining mixture or the vapor rich in flammable components.
Defeating Raoul’s Law
Azeotropes can occur when vapor pressures of mixtures of two or more liquids deviate from Raoul’s Law. Raoul’s Law is a liquid analog of the Ideal Gas Law and states that the vapor pressure of a mixture is a linear combination of the vapor pressures of the constituents weighted by their relative percentage of the mixture. This relationship is followed when thecohesive forces between similar molecules is the same as the adhesive forces between dissimilar molecules. However, for most liquids, cohesive forces may be greater or lesser than adhesive forces. When this occurs, there might be a condition under which the molecules of the different constituents combine to make a “super-blend” that may have properties that differ from those of the individual constituents.
When the cohesive forces are greater than the adhesive forces, the mixture has a positive deviation from Raoul’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 negative deviation from Raoul’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. The Chemistry 101 admonition to “add acid to water”, comes from the exothermic nature of acid dilution.
For some, but not all, mixtures, the deviations from Raoul’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 (Fig 1). 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. 
Fig. 1 Vapor pressures for mixtures of liquids
(By Karlhahn at en.wikipedia [Public domain], from Wikimedia Commons)
Near azeotropes, azeotropic range
Not all liquid blends are azeotropes. Only if the deviation from Raoul’s Law is large enough for there to be a maximum or minimum, and then only if the mixture composition is at the maximum or minimum, will the mixture be azeotropic.
Some chemical blends are touted as “near azeotropes” or “azeotrope like.” They do not have a zero slope condition that defines an azeotrope. We encourage manufacturers use a true azeotrope with similar cleaning efficacy if available and practical. Even under working conditions, non-azeotropic mixtures can change with time, requiring much more monitoring to maintain proper consistency.
For any azeotropic mixture, the conditions will be maintained only over a finite range of temperatures. So a liquid that is an azeotrope at its boiling point may not be one at cooler temperatures. So conditions must be considered (e.g. sealed containers) to assure that the azeotropic ratios do not change during storage between uses.
Non-azeotropes as cleaning agents
Not all cleaning agents are azeotropes; and not all of them need to be azeotropes. Where the blend, typically a blend of organic solvents, is used immediately and then replaced; or when it is dispensed from a sealed container such as in aerosol applications, in-use variability is not an issue. There are azeotropes containing water; one notable one consists of alcohol and water. However, most formulated aqueous cleaning agents are complex mixtures of organic chemicals, various salts, and water; but they are not sold for any azeotropic properties.
Smarter cleaning
Fortunately, many solvents with cleaning ability deviate sufficiently from Raoul’s Law to have an azeotropic range. Incorporating azeotropic blends can help you achieve less complex, more controllable, more cost-effective cleaning; this is important in maintaining a competitive advantage.
Get educated about all your solvent blends. All blends, including azeotropic blends, are mixtures. Mixtures do not behave exactly single-molecule solvents. Understand the benefits and limitations of both azeotropic and non-azeotropic blends and you will achieve and maintain reliable critical cleaning processes.
