Electrokinetic properties of mixed solutions of dodecyldimethylamine oxide and sodium dodecyl sulfate: Specific adsorption effects of small ions

Electrophoretic light-scattering measurements and potentiometric titrations were carried out on aqueous mixtures of dodecyl-dimethylamine oxide and sodium dodecyl sulfate. The electrophoretic mobility and the surface charge density of the micelles were always negative, ranging from -2.5 to -3.1 x 10(-4) cm(2)V(-1) s(-1), and -0.033 to -0.045 cm(-2), respectively, for all surfactant mixing ratios, indicating the specific adsorption of Cl-, in addition to Na+, on micelles. The solution pH, as well as the aggregation number previously reported, displayed maxima at intermediate surfactant mole fractions, that is, the non-ideal behavior. The fractional adsorption of Na+ per surfactant molecule in the micelles increased gradually with mixing fraction up to 0.82 at X = [SDS]/([SDS] + [C(12)DAO]) = 1, while that of Cl- decreased from 0.25 at X = 0 to zero at X > 0.4.