Theories for evaluating mixed surfactant systems are systematically reviewed, paying special attention to several points concerning the mixed surfactant systems, including interaction of counterions with the micelles, composition of mixed monolayers and mixed micelles, activity coefcients of the different surfactants in the mixed systems, and synergism observed in micellization as well as adsorbed lm formation upon mixing. These theories discuss the effect of the different mixed systems with a variety of surfactants (charged surfactants, e.g., cationic, anionic, zwitterionic; and uncharged surfactants, e.g., nonionic surfactants). The studied mixed system is composed of the cationic surfactant dodecyl dimethyl phenyl ammonium bromide (C12DMPAB) and nonionic surfactant decyl dimethyl phosphine oxide (C10DMPO) surfactants denoted as 1 and 2, respectively. It is shown that the composition of mixed micelles (X1) equilibrated with singly dispersed surfactant species in bulk solution phase and also the composition of adsorbed lm phase (Z2). Almost all combinations are discussed in terms of the respective interaction parameters, βσ, βm, where βσ and βm are the interaction parameters of the components in the adsorbed mixed monolayer and in mixed micelles, respectively. In addition, surface excess concentration (Γmax), molecular area (Amin), and minimum surface Gibbs energy ( )Gmin

s for evaluation of synergism are discussed.