Why do some liquids form azeotropes and not others?

November 11, 2010 9:08 AM

I know *what* an azeotrope is, but I don't really know *why* an azeotrope happens. Please enlighten me.

I was talking to a friend about why one can't distil liquor beyond a certain point and I mentioned that ethanol and water form an azeotrope. Almost immediately after I said it I realized that wasn't really an explanation as much as just playing a semantic game.

My old p. chem. textbooks are unhelpful -- they just say that it happens and don't say why.

So: why do some liquids form azeotropes and not others? What properties of the components in the mixture make this happen?

posted by selenized to Science & Nature (7 answers total)

Well, the Wikipedia article has an explanation blaming it on attractive forces between the different kinds of molecules through deviation from Raoult's law. Does that make sense to your chemist self?
posted by themel at 9:28 AM on November 11, 2010

Best answer: There's no general guideline as to what will and will not form an azeotrope. Azeotrope formation is a nonideal behavior, so you need some pretty sophisticated models of chemical behavior to predict it.

Very briefly, azeotrope formation (and nonideal solution behavior in general) is causes by enthalpic interactions between species. For instance, hydrogen bonding between water and ethanol.
posted by mr_roboto at 9:30 AM on November 11, 2010

Best answer: As I understand it, and from what I can recall about colligative properties of solutions, it all boils down (heh) to intermolecular attraction. Raoult's law, which is used to determine the vapor pressure of a solution, depends on the forces of adhesion and cohesion being equal. That is, the particles of substance X in a solution are attracted to other particles of X as equally as they are attracted to particles of Y. When that balance changes and particles of X start being more attracted to other particles of X than they are to particles of Y, the solution is considered to be non-ideal. Raoult's Law no longer applies.

What happens then?
"When the deviation is great enough to cause a maximum or minimum in the vapor pressure versus composition function, it is a mathematical consequence that at that point, the vapor will have the same composition as the liquid, and so an azeotrope is the result."

Here's an example of graphs comparing vapor composition and liquid composition.
posted by lholladay at 9:32 AM on November 11, 2010

The book Soft Condensed Matter by Richard Jones contains an approachable and well written discussion of how the spinodal phase transition (which gives rise to azeotropes) arises from different molecular interaction energies.

Did you know that Saran wrap works because of quantum vacuum energy? Neither did I till I read this book.
posted by 7-7 at 10:49 AM on November 11, 2010 [1 favorite]

Response by poster: So... if it all comes down to a difference between attractive forces, why do some mixtures of immiscible liquids form azeotropes (e.g. Toluene and Water) and others don't?
posted by selenized at 11:38 AM on November 11, 2010

Response by poster: Wait a minute: I'm guessing the answer is that actually Toluene and Water are partially miscible and it is that region of partial miscibility that gives rise to the azeotrope in that system.

7-7: I stared at the wikipedia entry for spinodal phase transition and I can sort-of see how that would apply to eutectics but not azeotropes per se. Wouldn't microstructures formed by diffusion require a quiescent solution? I mean azeotropes still happen when the liquid is being mixed strongly. My primitive brain doesn't really parse the math bits though, so there is that.
posted by selenized at 11:46 AM on November 11, 2010

One way to think about it is that the energy required to move most polar molecules from the liquid phase to the gas phase is higher when compared to non-polar substances of similar mass because you have to break the polar interactions (hydrogen bonds, electrostatic interactions, etc) to move the molecule into the gas phase.

Some molecules can pair in such a way that all of the possible polar interactions are utilized in the pair - for example a pair that forms hydrogen bonds with each other and not the bulk solvent doesn't break any polar interactions getting into the gas phase.
posted by overhauser at 2:37 PM on November 11, 2010