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December 21, 2018

What is an Azeotrope?

The basic definition of an azeotrope is a mixture of two or more liquid compounds with constant boiling point and composition throughout distillation.  That definition doesn’t tell us why azeotropes are useful or how they work. To do this, we’ll start by exploring the difference between positive and negative azeotropes:

What are Positive and Negative Azeotropes?

In order to learn about positive and negative azeotropic behavior, let’s play a drinking game.  Well, put more precisely, let’s start with some trivia that involves a drink: Water boils at 100oC and Ethanol boils at 78.4oC.  If you take the strongest moonshine your Auntie Beth ever distilled in a backwoods bathtub (let’s assume that is 95.63% ethanol and 4.37% water) your mixture will have a boiling point of:

A) 98.4oC
B) 78.2oC

Two of these answers make sense: Options A (98.4oC) and C (79.7oC) are between the boiling points of Ethanol (78.4oC) and water (100oC).  Azeotropes, however, don’t seem to make sense.  If you guessed option B, which is below the boiling points of the moonshine’s constituents, you are correct!  

Your Aunt Beth’s moonshine is what we call a “positive azeotrope,” wherein the boiling point of the azeotrope is lower than the boiling points of the constituents.  What’s more, a positive azeotrope will usually have a lower boiling point than any other ratio of its constituents, so if Aunt Beth makes a weaker or stronger moonshine than the 95.63% ethanol, the boiling point will be higher than 78.2oC.  A negative azeotrope, on the other hand, has a higher boiling point than any other ratio of its constituents.

But, Why are They Called Negative and Positive Azeotropes?

It’s a strange naming scheme: positive azeotropes have a lower boiling point than any other ratio of their constituents, and negative azeotropes have a higher boiling point than their constituents.  It seems like it should be flipped, right?  To understand the nomenclature, it’s important to know a little bit about Raoult’s Law.  

Raoult’s Law and Azeotropes

Raoult’s Law is basically this: the individual components of a solution should have the same attraction to each other as they do for themselves and this should be reflected in the vapor pressure as well.  So, let’s say we have a solution of liquid A and liquid B. Raoult’s law describes the behavior of an ideal solution, so if liquid A and liquid B are just as attracted to each other as they are to themselves, and the total vapor pressure can be predicted by the individual vapor pressures of liquid A and B.

Now that we have a basic understanding of Raoult’s Law, we can examine the “positive/negative” nomenclature.  Let’s say that the molecules of liquid A have a stronger attraction to each other than they do to molecules in liquid B, this would mean that the liquids will escape more readily from the liquid phase and become vapor.  As a result, the total vapor pressure will be higher than predicted by an ideal solution that follows Raoult’s Law. This higher vapor pressure would be referred to as a positive deviation from Raoult’s Law.  If, conversely, the constituents have a higher affinity for each other than they do for themselves, i.e. liquid A is pulled tightly to liquid B, the liquids will be more difficult to pull apart from each other and the vapor pressure will be lower than expected by Raoult’s Law, which is a negative deviation from Raoult’s Law.  

So, your Aunt Beth’s moonshine is a positive azeotrope because ethanol and water have the least affinity for each other at the very specific percentage of 95.63% ethanol and will therefore escape the liquid phase and become vapor at a lower temperature than expected by Raoult’s Law, which is a positive deviation (a higher vapor pressure than expected) from Raoult’s Law.

How do We Use Azeotropes?

One key use of azeotropes is in flammability suppression for cleaning applications.  Let’s say we have a cleaning liquid that has almost all the properties one would desire for a vapor degreasing application (to learn more about vapor degreasing, read Why Boeing Approved FluoSolv® AP for use as a Vapor Degreaser), it has a low boiling point and is able to remove the toughest greases and oils and leave a residue-free surface.  The only downside of this dream solvent is its high flammability.

Now, let’s say we found a second solvent that is able to suppress the flammability of the flammable cleaning solvent, but they do not form an azeotrope.  As you heat the solution in a vapor degreaser, one solvent will boil out of the solution faster than the other. If, for instance, the non-flammable solvent boils out of the solution, the solution will change in percent composition while it is in use.  If the percent of non-flammable solvent drops so low in the solution that it is no longer suppressing the flammability, the solution has changed from non-hazardous to flammable in the workplace, which could be catastrophic.

This is why azeotropes are so useful.  As they boil, all constituents boil into vapor at the same rate, so the physical characteristics of the liquid remain constant throughout use.  This is why NuGenTec® has spent countless research and development hours developing the FluoSolv® family of azeotropic performance solvents that outperform legacy solvents in cleaning ability, workplace safety, and ecological impact.

The FluoSolv® family of products includes safe and powerful alternatives for nPB replacement (1-Bromoproprane or n-propyl bromide), TCE replacement (trichloroethylene), and AK-225 replacement (HCFC-225).

Features & Benefits of FluoSolv® Products

FluoSolv® products are effective, non-toxic, and non-flammable solutions for vapor degreasing, ultrasonic vapor degreasing, and other demanding applications. Features and benefits of these unique products include:
  • Non-flammable
  • Non-toxic
  • Chemically stable; will not acidify
  • High Kb value for excellent solvency in tough-to-clean residues
  • Low GWP (global warming potential)
  • High AEL (allowable exposure limit) thanks to low toxicity
  • No ceiling on instantaneous exposure

NuGenTec®: Redefining Chemistry

If you are seeking healthier, more environmentally responsible, and less costly alternatives for your industrial processes, contact NuGenTec today.  To learn more about FluoSolv® and azeotropes, click here.