New catanionic mixtures of dodecyldimethylammonium bromide/sodium dodecylbenzenesulphonate/water: I. Surface properties of dispersed particles

Spontaneous formation of micelles and vesicles in aqueous solutions of sodium dodecylbenzenesulphonate (SDBS), didodecyldimethylammonium bromide (DDAB) and their mixtures, SDBS/DDAB/H2O, was investigated using conductivity, tensiometry, potentiometry and electrophoresis. Conductometric measurements indicated a single micellar process in pure SDBS solutions, as well as in SDBS solutions with small amounts of DDAB, while aqueous DDAB solutions with or without SDBS showed two transitions. The first transition was associated with the critical vesicle concentration, while the second was attributed to the growing processes of vesicular associates from unilamellar to multilamellar vesicles. Critical parameters of the association of pure surfactants (SDBS and DDAB) were: cmc = (2.0 ± 0.2) × 10−3 mol dm−3 and cvc1 = (7.8 ± 0.6) × 10−5 mol dm−3, cvc2 = (3.6−6.6) × 10−4 mol dm−3, respectively. In the SDBS/DDAB/H2O mixtures, slightly lower critical values, i.e. cmc = (1.8 ± 0.6) × 10−3 mol dm−3 and cmc = (1.2 ± 0.1) × 10−3 mol dm−3, were obtained by conductometry and tensiometry, respectively. The degree of counterion dissociation from the micelles, α = (0.239 ± 0.006), was calculated using an appropriate model. The maximum SDBS surface excess, Γmax = (2.41 ± 0.55) × 10−6 mol m−2, related to the minimum area per molecule Amin = (0.689 ± 0.13) nm−2, is in agreement with the value obtained by quantum mechanical calculations, Amin = 0.69 nm−2.