The rheological characteristics for the mixtures of cationic surfactant and anionic–nonionic surfactants: the role of ethylene oxide moieties

This study systematically reports the rheological behaviour and mechanism for mixtures of cationic surfactant cetyltrimethyl ammonium bromide (CTAB) and anionic–nonionic carboxylate surfactants (NPEC-n). The effects of molar ratio, total concentration, salinity, shearing time, temperature, and ethylene oxide (EO) moieties on the microstructures of the mixtures were investigated in detail using rheometry, freeze-fracture transmission electron microscopy (FF-TEM), cryo-transmission electron microscopy (Cryo-TEM), etc. The results indicate that the conformations of the EO moieties concern the head-group areas and steric hindrance, which affect the arrangement of the surfactant molecules. The aggregates with diverse morphologies endow the solutions with different rheological behaviours. Except for the CTAB/NPEC-10 system, the CTAB/NPEC-5 system and CTAB/NPEC-7 system show viscoelastic behaviour under some conditions and their highest viscosities appear at the molar ratio of 76 : 24 and 40 : 60, respectively. The transition temperature of the mixture appears at 35 C, accompanied with a sharp decrease in the viscosity. The salt thickening and shear-resistant properties of the mixtures have also been discussed, indicating good salt-resistance and shear-resistance of the mixtures.