This minimum is due to small amounts of surface active impurities. The classic example is for SDS where the minimum is due to dodecanol created by hydrolysis splitting off the sulphate group.

Thanks for the reply. According to these, after a "purification" of the surfactant should the minimum disappear? Has anyone ever reported such a result?

Adam Juhasz

The experiment and the theory of this phenomenon were explained by

A.I. Rusanov.

https://www.researchgate.net/profile/Anatoly-Rusanov/research

If we use a surfactant for research, we recrystallize the surfactant to the absence of a minimum on the surface tension isotherm near the CMC.

Yuri Mirgorod, it would be nice if you would give a specific reference, instead of the full reference list. Prof. Rusanov has 264 published articles.

Here is a paper discussing the problem for non-ionic surfactants:

Article On the measurement of critical micelle concentrations of pur...

The problem is a very old one, for SDS it seems to go back to Elworthy and Mysels in 1966.

for the Triton-X what do you want to purify - it is a mixture of homologous t-Octylphenol-polyethyleneglycol ethers. In more detail - surfactants (dissolved) distribute between liquid phase and surface, where the more surface homologes are adsorbed preferentially. As soon as first micelles form, these also distribute into the micelles (are 'extracted from the interface') and their percentage at the interface approaches the composition in the bulk. Remarkably, there is a broad minimum also for highly ethoxylated ones, where content of strongly surface active homologes is very low. This should have a different mechanism.

Alan Parker

I present a link to the monograph by A.I. Rusanov.

Rusanov A.I. Micelle formation in solutions of surfactants. St. Petersburg. Chemistry, 1992 .-- 280 p.

The robot only gave me this page after recommending my answer today. It is necessary to tell the robot the name and surname of the consultant.