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

Abstract

Electrophoretic light-scattering measurements and potentiometric titrations were carried out on aqueous mixtures of dodecyldimethylamine oxide and sodium dodecyl sulfate. The electrophoretic mobility and the surface charge density of the micelles were always negative, ranging from −2.5 to −3.1×10⁻⁴ cm²V⁻¹s⁻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 atX=[SDS]/([SDS]+[C₁₂DAO])=1, while that of Cl⁻ decreased from 0.25 atX=0 to zero atX>0.4.