Second CMC in surfactant micellar systems

• Published in: Current opinion in colloid & interface science 2016-04, Vol.22, p.46-50
• Main Author: Magnus Bergström, L.
• Format: Article
• Language: English
• Subjects: Critical micelle concentration; Growth behavior; Micelles; Second CMC; Surfactants
• ISSN: 1359-0294; 1879-0399; 1879-0399
• DOI: 10.1016/j.cocis.2016.02.008

Experimental reports of surfactant systems displaying a second critical micelle concentration (second CMC) have been surveyed. It turns out that surfactant micelles usually show a growth behavior with some typical features. (i) Micelles grow weakly at low surfactant concentrations but may switch to a much stronger growth behavior at higher concentrations. The second CMC is defined as the point of transition from weakly to strongly growing micelles. (ii) Micelles are found to be non-spherically shaped below the second CMC. (iii) At the second CMC micelles are found to be much smaller, with aggregation numbers typically 100–200, than expected for flexible micelles. (iv) Micelles of intermediate size are present in a narrow concentration regime close to the second CMC. (v) Micelles grow much stronger above the second CMC than expected from a sphere-to-rod transition. The conventional spherocylindrical micelle model predicts a smooth growth behavior that contradicts the appearance of a second CMC. Modifying the model by means of including swollen end caps neither account for the presence of micelles with intermediate size, nor the strong growth behavior above the second CMC. Taking into account micelle flexibility is not consistent with the rather low micelle aggregation numbers observed at the second CMC. On the other hand, a recently proposed alternative theoretical approach, the general micelle model, have been demonstrated to take into account basically all features that are typical of experimentally observed micellar growth behaviors. [Display omitted] •Surfactant systems that exhibit a second CMC are surveyed.•Experimental observations are summarized in some typical features of surfactant systems demonstrating a second CMC.•Typical experimental features are inconsistent with a conventional sphere-to-rod transition.•Taking into account swollen end caps or micelle flexibility cannot eliminate the discrepancy between theory and experiments.•The general micelle model predicts all typical features shown by surfactant micelles with a second CMC.