Evaporation Rate and Surface Tension

[ImagineDragons]

Why does acetone evaporate faster than ethanol yet still has a higher surface tension? I just don't understand. I thought that the surface tension is correlated with the evaporation rate.

 

Also, since acetone does have a higher surface tension than ethanol, does that mean that a drop of acetone has a higher height and smaller width than a drop of ethanol?

[Sensei]

Acetone has boiling point 56 C. Ethanol has boiling point 78 C, 22 C more than acetone.

Less energy is needed to change state of matter of acetone to vapor than ethanol.

[Sriman Dutta]

You must know that surface tension is correlated with the intermolecular force of attraction or the force of cohesion. If a liquid has a high cohesive force, then each of its molecules will exert a greater pull on the other molecules. The result is that the drop of the liquid will form a spherical shape. This phenomenon is known as surface tension.

Mathematically, surface tension is the force exerted on the surface molecules per unit an imaginary line-length. T = F/l.

[John Cuthber]

You have both made valid points, but neither of you has answered the question.

Sensei has pointed out that it's easier to separate the molecules of acetone from the bulk liquid. The implication is that the forces holding the molecules together are stronger in alcohol.

Sriman Dutta has pointed out that surface tension is related to the forces between molecules; in general, if those forces are large you will get a high surface tension.

 

So you would expect alcohol to have a higher surface tension because the forces holding the molecules together are stronger ( as witnessed by the lower volatility).

 

But, as the OP points out: that's the wrong answer.

 

Alcohol has a lower surface tension than acetone.

 

I admit, I am not sure why, but just telling the OP what the heat of evaporation is, or quoting the nature of surface tension doesn't actually help.

 

Does anyone know why alcohol molecules stick together better than acetone when judged by volatility, but worse when judged by surface tension?

Part of the answer must be related to what substance the surface tension is measured with- they commonly use glass plates.

https://en.wikipedia.org/wiki/Wilhelmy_plate

If glass does a good job of sticking to alcohol (and it has a very polar surface, so that's plausible) then the measured surface tension of alcohol would be higher.

 

Anyway, to look at the second question

Also, since acetone does have a higher surface tension than ethanol, does that mean that a drop of acetone has a higher height and smaller width than a drop of ethanol?

The answer is that it would- if the densities were the same, and they are pretty similar. It also depends on the nature of the surface.

Edited October 30, 2016 by John Cuthber