Ask A Scientist© Chemistry Archive

name        Aura
status      student
age         15

Question -  Does surface tension effect evaporation?  How?
Do oil and soap evaporate?
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Aura,

The surface tension of a liquid is a measure of how well the molecules in the
liquid are able to hold on to each other. Thus, surface tension does affect
evaporation rate.

The rather strong mutual attraction of water molecules makes for its rather
large surface tension. Soaps and detergents weaken water's surface tension,
thus allowing water and the cleaning agent to penetrate into the pores of
soiled materials. Go here for more information ==>

http://146.139.100.40/webpages/askasci/gen99/gen99821.htm

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Will oil and soap evaporate?

If we define evaporation as a process in which molecules in the liquid phase
move away from the liquid at the liquid-air interface and enter the gas
phase, I suppose that one could say that almost anything will evaporate if
given enough time. Why? It's because we can't be absolutely certain that not
a single molecule of the liquid couldn't/wouldn't ever spontaneously jump off
its surface. However, in practical terms, things like oil (and soap) are not
very prone to evaporate. Still, something like a liquid soap left exposed to
the air might get gummy from evaporative loss of water in the soap. Bar soap,
on the other hand, will likely just sit there and dry out rather than
evaporate.

Regards,
ProfHoff
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The surface tension of a pure liquid essentially has no effect on
evaporation. However, surface active chemicals that have an excess
concentration at the surface of water, for example stearyl alcohol, form a
molecular layer on the surface and retard evaporation.

In general oils and soaps have a high molecular weight and are not volatile.

Vince Calder
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