Specific ion effects on the micellization of aqueous mixed cationic/anionic surfactant systems with various counterions

The micellization of cationic gemini surfactants with various counterions, [C12H25(CH3)(2)N(CH2)(3)N(CH3)(2)C12H25]X-2, designated as 12-3-12(2+) 2X, with X = F, Cl, Br, Ac, and NO3-, and anionic conventional surfactants alkali dodecylsulfonates with various counterions C12H25 SO3Y, designated as Y(+)AS(-), with Y'= Na', K', Rb' in aqueous solutions has been investigated. The mixed micellization of some 12-3-12(2+) 2X(-)/Y(+)AS-/H2O systems has also been investigated. For the single ionic surfactants, the critical micelle concentration (cmc) decreases in the Hofmeister sequence of the counterions, strong formation ability of ion pairs between surfactant ions and the corresponding counterions leads to strong micelle self -assembly ability and small cmc. For the 12-3-12(2+) 2X(-)/Y(+)AS(-)/H2O systems, to our surprise, the cmc values of the 12-3-12(2+) 2NO(3)(-)/Na(+)AS(-)/H2O systems with different compositions are much larger than those of the other 12-3-12(2+) 2X(-)/Y(+)AS(-)/H2O systems with the same compositions. Significantly different from the single ionic surfactants, if there is weak formation ability of ion pairs between counterions X- and Y+, strong formation ability of ion pairs between surfactant ions and the corresponding counterions may increase the cmc for 12-3-12(2+) 2X(-)/Y(+)AS(-)/H2O system. The dissociated Margules model has been used to correlate the mixed cmc values and fits the data well. The model parameters A(12) and A(21) can be used as measure of synergism between 12-3-12(2+) 2X(-) and Y(+)AS(-), and they reflect the two -body and three-body attractive interactions, and the interaction differences due to the specific ion effect of counterions X- and Y. (C) 2016 Elsevier B.V. All rights reserved.