A comparison between three methods for determination of the critical micelle concenteration in ionic surfactants

Micelles are small colloidal particles, the micelle formation depends strongly on molecular structure (hydrophobic and hydrophilic groups). The concentration above which micelles form is called the critical micelle concentration (CMC). The value of the CMC can be determined by the change in the physicochemical properties of the surfactant solution as the surfactant concentration increases. In this work, CMC values of number of cationic surfactants (DETAB, DOTAB, TTAB, CTAB and IL ([C12mim]Br)( and anionic surfactants (LDS, NDS and SDS) determined by three methods(spectroscopic, conductometric and tensiometry). Fig. 1 shows the effect of increasing surfactant (NDS) concentration on the UV-absorption spectrum of aqueous BZA solutions of 0.07mM (as probe). Below the CMC no changes are observed in the presence of surfactant monomers; above the CMC the absorption band centered at λ = 312 nm (due to the enolic form) is augmented, typically [1].The advantages and disadvantages of each method are indicated and compared. Also, it is important to recognize that there may be systematic differences among methods for determining the CMC. The results obtained (CMC values) show a good consistency between two methods (conductometric and tensiometry). The electrical conductance method of ionic surfactant was carried out to determine the CMC, degree of dissociation of counter-ions (β) and standard Gibbs free energies of micellization ( 0 mic G ) values according to the Carpena et al. method[2].