May 28, 2020, 09:14:06 PM
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Topic: Using Vapor liquid equilibrium versus temperature of solvent mixtures  (Read 297 times)

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Offline gibbw

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Hello all,

So my issue is that I am attempting to control the molar composition of a gaseous atmosphere above a mixture of two solvents.  The idea is to have a small enclosed space 1 square foot that has a small heating "cup" that I can put a binary mixture of solvents in.  I would control the molar composition of the gas using the initial liquid molar ratio and heat. 
So I have two issues: 
First, the problem of mixed solvents and deviation from Raoults Law is usually talked about in terms of distillation with one of the solvents are above it's boiling point.  I don't necessarily want to go above the boiling point but would still need to account for the non-ideallity of the mixture.  So am I fundamentally wrong about this, I.E. does the non-ideality only happen in the regime of distillation?  It seems to me that the interactions that govern this still exist under boiling point.  How can I predict the gas molar composition at temperatures below boiling?
Second, I have been looking for resources that have these Temperature-composition plots (such as http://www.ddbst.com/ddb-vle.html), however, obviously these show the point at which one solvent is at or above boiling. Does anyone know of any resources or data bases that have information below b.p.?

I feel that this is a fundamental misunderstanding on my part and most likely conflating two or more ideas.  I just don't know what I am conflating.

Any help is greatly appreciated!!

 

Offline Enthalpy

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Re: Using Vapor liquid equilibrium versus temperature of solvent mixtures
« Reply #1 on: January 20, 2020, 02:30:33 PM »
Hi gibbw,

if liquids interact, the mixture's vapour pressure doesn't result simply from individual vapour pressures and mix ratio. This applies below the boiling points too. Boiling points aren't specific to the evaporating substances anyway, they depend on the atmospheric pressure.

To make a prediction, you need assumptions, like "the two liquids act like their proportions". I'd be wary of that especially with solvents.

Maybe a good chemist could tell "rather yes" or "rather no" just be looking at the two liquids, but I don't expect a quantitative answer resulting from general a theory. Case-by-case experimentation.

Offline gibbw

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Re: Using Vapor liquid equilibrium versus temperature of solvent mixtures
« Reply #2 on: January 23, 2020, 07:00:51 PM »
Thanks Enthalpy for your time and answer.

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