Odd observation about azeotropes possibly leading to a new phenomenological law?

I've been mucking around with phase calculations for azeotropes using the UNIFAC additive model for activity coefficients in free software called ChemSep Lite.

I started considering the Txy phase diagrams of azeotropic mixtures that have lower boiling points than either compound A and B (trivial case).

Now I expanded this to ternary systems (three component), seeing if I could find a compound C that shows a ternary azeotrope, that is to say, some combination of A, B, C that have still lower an azeotropic boiling point.

This is analogous to ternary eutectics and higher, where having many solids mixed together lowers the eutectic still lower.

EXCEPT, in the case of liquid-vapour azeotropes, I could never find a component C that does this. In all cases, I could find lower boiling azeotropes by introducing a C, but in all cases, either A or B would have to go to zero in the composition for the lowest possible boiling azeotrope to be found.

Put another way, on the 3-d surface plots of the boiling point and composition of A, B, C, the lowest azeotropic minimum would always be sitting on one of the sides and not somewhere internally on the surface.

I was surprised by this because for solid-liquid phase equilibria, one can find ternary combinations that lower the eutectic point still lower.

Have I stumbled upon a new law concerning azeotropes or is this simply an artifact of UNIFAC or chance (i.e. exceptions exist which I missed).

For those who love maths, maybe there is a proof of this. Obviously if someone knows of an exception... then this is not true, so please do tell and I can then check it in the program to see if this is an artifact of using UNIFAC (doubtful, but nevertheless).

EDIT: Oh and no, I don't particularly love maths

[Edited on 7-12-2014 by deltaH]

Thanks forgottenpassword, it's doubtful though as ChemSep is used heavily in chemical engineering circles and has been extensively evaluated.

I'm been giving this some thought oven eating a McFlurry (spam?) just now

[Rubbish?]

Let's consider a specific case I was playing around with. Acetaldehyde (b.p. = 20C) is a highly polar molecule. As such, I suspected it's polarity results in strong acetaldehyde-acetaldehyde electrostatic interactions with the effect of raising it's boiling point higher than what it would be without those interactions. So going with that train of though, I'll introduced an alkane to break the interactions and hopefully form an azeotrope of lower boiling point than either. This worked and some trial and error showed isopentane (b.p. = 28C) to be the most effective, resulting in an azeotrope of a little over 12C according to the UNIFAC model.

The reason I think that searching for a third component to depress the acetaldehyde-isopentane azeotrope even further failed because as per my rationalisation of why it formed in the first place, you needed something to break the acetaldehyde-acetaldehyde interactions and this could be done with a strongly non-polar component. While it works with other alkanes as well, isopentane worked best, that is to say, anything that breaks those interactions simply does it better or not, so would negate the third component.

There is no rationalisation I can think of that makes, say a n-pentane for example, better at breaking the polarity of acetaldehyde that would not be best when used on its own (as is the case with isopentane). Anyhow, I'm not so sure I explained that well enough, but hopefully that makes sense.

Okay, that is a lot of blah and yet still not a proof [/Rubish?]

****
Oh wow, Magpie, now all this thinking was for nothing
Thanks, I'll try to plug those in...

Nice McFlurry though lol

[Edited on 7-12-2014 by deltaH]

Thanks Chemosynthesis for the link and I did not know that (ban on comparison). If I ever were to publish it, then I think I would need to do it preferably from the experimental data as opposed to models... while there's nothing wrong with models and in fact, as always, I am impressed with how well UNIFAC works for even complex systems, I just think it will be easier to skirt all these doubts about accuracy or software bugs.

Also, this is still pretty much just a phenomenological observation about phase behaviour. I suppose I compare data on as many known ternary systems and show that they all have either just a saddle point higher up, or liquid-liquid region on the one side with a slightly lower point internally, it becomes compelling enough... well until the day a four component lowest point azeotrope is found

By the way, UNIFAC is also perfectly capable of describing liquid-liquid phase equilibrium, it's just that ChemSep Lite doesn't allow the option of a VLL calculation on its phase diagrams. The calculation simply entails equating the two liquid fugacities also using UNIFAC.

Though if I'm not mistaken, UNIFAC does not describe liquid-liquid equilibrium very accurately, instead, something like a NRTL model works much better, but that is not predictive and requires parameter fitting to experimental data AFAIK.

[Edited on 7-12-2014 by deltaH]

You're welcome. I will be pleased if the link is helpful to you, but I am curious... is this really a phenomenonogical observation? QSPR doesn't work that way, and the link I posted makes it very clear they are using models extrapolated from curve fits that don't necessarily take into account outliers, and have an accepted average error for their chosen training dataset.

I won't believe it until I see it, and even if you do publish, I'll probably grumble criticisms unless and until I am forced to accept it .

I have published it already here on SM and you grumbled already

Anyhow, like I said, just an observation of what seems to be a trend, probably not a law just yet. But maybe now that it's been stated, people will try to disprove it and hey presto... science at work!