Tensiometric and rheological investigations of single and mixed systems consisting of cocamidopropyl betaine (CAPB) and sodium dodecyl benzene sulfonate (SDBS) in aqueous solutions

Abstract The surface tension (σ), critical micelle concentration (CMC), surface excess (Γ), minimum area occupied by a surfactant molecule (Amin), the viscosity and oscillatory rheological studies of aqueous solutions containing cocamidopropyl betaine (CAPB) and sodium dodecylbenzene sulfonate (SDBS) at molar fractions of 0.00, 0.25, 0.50, 0.75 and 1.00 and 25 °C were presented. CAPB and SDBS were not found to interact synergistically in water at any of the molar fractions studied. This is due to the fact that the critical micelle concentrations of these mixtures were higher than those predicted by Clint’s equation, indicating an antagonism that rarely occurs in mixed amphoteric/anionic surfactant systems. The minimum area occupied by a surfactant molecule (Amin) was reduced in CAPB/SDBS mixtures compared to unmixed surfactants. In contrast, the viscosity of the mixed CAPB/SDBS system increased significantly from 1.0 mPa s to a maximum of 36.0 Pa s at higher CAPB mole fractions (0.5–0.8). The rheograms obtained from the oscillation measurements of the viscous CAPB/SDBS solutions are characteristic of wormlike micelles (WLMs) according to the Maxwell model. The results of this surprising binary CAPB/SDBS surfactant mixing system are presented and discussed.