Critical demicellization concentration

Mysels [109] introduced the concept of a critical demicellization concentration (cdc). Below cmc, essentially no micelles are present above cmc, most of the surfactant has associated to form micelles (Fig. 7.12). When micelles in mixtures of surfactants have a limited solubility, a second cmc may exist [68]. As the surfactant concentration increases, the monomers of one surfactant attain their cmc and form micelles, whereas the other surfactant is still in its monomeric state. At a higher concentration, the second surfactant reaches its cmc and forms micelles. Above this cmc, two kinds of micelles are present. As the surfactant concentration increases further, one kind of micelle may disappear at a concentration denoted as cdc. 

Lake [117] used kinetic dialysis to examine the existence of cdc. The cdc of the sodium decanoate-sodium perfluorooctanoate mixed-micelle system was found to occur at a specific surfactant concentration and mole fraction. Ben Ghoulam et al. [118] determined the demixing diagram of the Neos Ftergent (a branched alkylbenzenesulfonate by surface tensiometry) and measured second critical micelle concentrations. However, a critical demicellization concentration was not observed. 

The existence of two types of micelle depends also on the surfactant concentrations. As the total surfactant concentration is increased, one type of micelle disappears due to the larger solubilization capacity and growth of the other type of mixed micelle [76]. Above the critical demicellization temperature (cdc), only one kind of mixed micelle exists below cdc, two micellar species can coexist [112]. 

An example where these characteristic changes are easily observable, are titration experiments with micellar solutions of surfactants which are titrated into the vessel filled with pure water. Initially, dilution of the micellar solution leads to complete demicellization. When the concentration of monomers in the cell approaches the critical micellar concentration cmc the heat effect connected with the transfer of surfactants from the micelle to the aqueous solution disappears and the heat of reaction approaches zero [118-123]. 

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