Solvent evaporation

[willferral]

If multiple solvents are mixed together with different evaporation rates will they evaporate together all at the same rate or separately?   For example if Butyl Acetate has a evaporation rate of 1.  and Acetone has a evaporation rate of 6.1 and ethanol at 1.7 and they are mixed will they evaporate at a uniform rate or will the acetone evaporate first?

 

If I add a slow drying solvent as a retarder such as Diacetone Alcohol with a .12 evaporation rate will it slow down the evaporation of all the solvents or will they evaporate at the same rate and just whatever amount of Diacetone Alcohol I added will be left behind?

This is assuming all the solvents are miscible and mix together well.  Thanks for any help.

Edited March 21 by willferral
wrong word was used

[chenbeier]

The miscibility of liquids and the evaporation rate are different things. If you mix the three solvents they will dissolve directly.

The compound of the highest vapor pressure will evaporate faster as the other ones.

[swansont]

 

19 minutes ago, willferral said:

If multiple solvents are mixed together with different evaporation rates will they dissolve together all at the same rate or separately?   For example if Butyl Acetate has a evaporation rate of 1.  and Acetone has a evaporation rate of 6.1 and ethanol at 1.7 and they are mixed will they dissolve at a uniform rate or will the acetone dissolve first?

Dissolve?

[willferral]

sorry I meant evaporate not dissolve

7 minutes ago, swansont said:

 

Dissolve?

 

[exchemist]

57 minutes ago, willferral said:

If multiple solvents are mixed together with different evaporation rates will they evaporate together all at the same rate or separately?   For example if Butyl Acetate has a evaporation rate of 1.  and Acetone has a evaporation rate of 6.1 and ethanol at 1.7 and they are mixed will they evaporate at a uniform rate or will the acetone evaporate first?

 

If I add a slow drying solvent as a retarder such as Diacetone Alcohol with a .12 evaporation rate will it slow down the evaporation of all the solvents or will they evaporate at the same rate and just whatever amount of Diacetone Alcohol I added will be left behind?

This is assuming all the solvents are miscible and mix together well.  Thanks for any help.

If they are miscible, i.e. you don't get a layer of one of them on the surface that prevents the materials beneath evaporating, then to a first approximation they will each evaporate at a rate given by Raoult's Law. This states that the vapour pressure of each component will be proportional to its mole fraction in the mixture. For example if a component comprises 1/3 of the molecules in the mixture, it will contribute a vapour pressure 1/3 that of the pure substance. 

There can be deviations from Raoult's Law when the molecules of 2 substances have more affinity for one another than for molecules of their own sort, or conversely when they have stronger affinity for their own sort than for the other.

Here is a link to a fuller explanation: https://en.wikipedia.org/wiki/Raoult's_law. The 2 graphs give you first what happens in an ideal case and then what deviations from Raoult's Law can do. 

But in all cases the principle will be that of contributing a vapour pressure (which translates to an evaporation rate) more or less in proportion to how much of the mixture each components represents. N.B. this is on a molar basis (not mass or volume), as it depends of how many of the molecules in the evaporating surface layer are of each substance. 

So if you add a miscible slow-evaporating ingredient it will only slow down the evaporation of the other substances according to proportion of them it replaces in the mixture. (The limiting case is adding salt to water. The vapour pressure of salt is negligible. Dissolving salt reduces the vapour pressure of water and thereby elevates the boiling point. But it can only do so to a limited extent because you can't dissolve that much salt in water, so the maximum mole fraction has a limit.)

If however it is a immiscible liquid of lower density, it will float as a layer on the surface and prevent what is beneath from evaporating.     

  

[swansont]

I think it would also depend on the vapor pressure of each; if the vapor reaches the equivalent of 100% humidity then you won’t get more evaporation.

[willferral]

31 minutes ago, exchemist said:

If they are miscible, i.e. you don't get a layer of one of them on the surface that prevents the materials beneath evaporating, then to a first approximation they will each evaporate at a rate given by Raoult's Law. This states that the vapour pressure of each component will be proportional to its mole fraction in the mixture. For example if a component comprises 1/3 of the molecules in the mixture, it will contribute a vapour pressure 1/3 that of the pure substance. 

There can be deviations from Raoult's Law when the molecules of 2 substances have more affinity for one another than for molecules of their own sort, or conversely when they have stronger affinity for their own sort than for the other.

Here is a link to a fuller explanation: https://en.wikipedia.org/wiki/Raoult's_law. The 2 graphs give you first what happens in an ideal case and then what deviations from Raoult's Law can do. 

But in all cases the principle will be that of contributing a vapour pressure (which translates to an evaporation rate) more or less in proportion to how much of the mixture each components represents. N.B. this is on a molar basis (not mass or volume), as it depends of how many of the molecules in the evaporating surface layer are of each substance. 

So if you add a miscible slow-evaporating ingredient it will only slow down the evaporation of the other substances according to proportion of them it replaces in the mixture. (The limiting case is adding salt to water. The vapour pressure of salt is negligible. Dissolving salt reduces the vapour pressure of water and thereby elevates the boiling point. But it can only do so to a limited extent because you can't dissolve that much salt in water, so the maximum mole fraction has a limit.)

If however it is a immiscible liquid of lower density, it will float as a layer on the surface and prevent what is beneath from evaporating.     

  

Thanks.. that cleared it up for me.