The inspiration for this question is over on cooking.stackexchange, asking more about actual measurements for commonly consumed liquids, but I'm interested more generally as well.
What determines the behavior of surface tension for solutions and mixtures with respect to concentration? Of course, I expect that the answer depends on the liquid, since different liquids have different causes of cohesive forces. I would be interested both in quantitative answers (very approximate/general, probably) and qualitative ones.
Additionally, what is the dependence of surface tension on temperature? Does that have any interaction with dependence on solution/mixture concentration?
(I know this question borders on chemistry, but there's no chemistry stackexchange, and besides, I'm sure at least some surface tension properties admit explanations from statistical mechanics!)
Best Answer
About the surface tension of mixtures, this is quite complex, and the answer is certainly not straight. However, it looks like the surface tension of the mixture of two fluids is always in between the surface tension of the pure fluids somehow (so it is simpler than azeotropes for the boiling temperatures of mixtures). The dependency of the surface tension on the concentration may be linear, but usually is parabolic. You can find some graphs of surface tension as a fonction of the concentration of the mixture in this paper. About the temperature dependence, as far as I know, surface tension always decreases with temperature. It is due to the fact that higher thermal fluctuations give smaller cohesion.