Because of the trend towards “micellar” formulations, surfactant manufacturers often get requests to prove the existence of micelles and/or provide values for Critical Micelle Concentrations (CMC). Therefore, the CESIO Working Group “Test Methods of Surfactants” and the TEGEWA Working Group “Surface Active Substances” have conducted round robin tests using “technical” surfactants, i.e. surfactants having alkyl chain length distributions: A) fatty alcohol ethoxylates with 9EO and different alkyl chain lengths (C12-C18) and B) two alkylamidopropyl betaines (coco vs. C12). The methods employed were the standard procedures established in industry, e.g. Wilhelmy Plate, du Noüy Ring or Pendant Drop.
Two aspects have been the focus of this work: A) the effect of alkyl chain length variations on the CMC values, and B) the influence of the experimental procedures on the results of the surface tension measurements. There is indeed a significant influence of the experimental procedure on the surface tension values – especially for surfactants with broad alkyl chain distribution.
Since these differences are mostly below CMC, the values of the CMCs itself are somewhat consistent. However, giving more “precise” values than one significant digit does not make much sense. But this should be sufficient, since in practical applications one is always well above CMC.