Does NaCl reduce the surface tension of water? And why?
For example soap does it, and I was wondering if NaCl does the same.
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3$\begingroup$ The surface tension of NaCl solution is greater than that of water. $\endgroup$– aventurinCommented Apr 16, 2016 at 9:59
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1 Answer
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Gibbs adsorption isotherm tells us that (roughly speaking) the decrease in surface tension is proportional to the surface excess concentration of the solute.
Positive surface excess means that the solute tends to concentrate on the surface rather than stay in the solution. The surface concentration has no physical limit and can go arbitrarily high. In soap and other surfactants, it is hundreds of times higher than the concentration in the solution, therefore their effect on surface tension is negative and very strong.
Negative surface excess means that the compound prefers to stay in the solution rather than on the surface. It has a physical limit, though: the concentration can't be lower than 0. Therefore the effect of such compounds on surface tension is positive, but small (not noticeable with naked eye). This is the case with $\ce{NaCl}$ and pretty much all "normal" inorganic salts.
answered Apr 16, 2016 at 15:24