Physicochemical Behavior of Mixed Nonionic‐Ionic Surfactants in Water and Aqueous Salt Solutions

Abstract

Cloud point, viscosity, and ultrasonic velocity of aqueous solutions of nonionic surfactants (Triton X‐114 and Triton X‐100) in the presence of ionic surfactants (anionic: sodium alkyl sulfate and cationic: alkytrimethylammonium bromides) are reported. The addition of ionic surfactant (at concentrations much less than CMC) to the salt‐free solution of nonionic surfactants increases the cloud point of nonionic surfactants. Such cloud point increase was dependant on the type and hydrophobic tail of surfactants and has been explained in terms of mixed micelles formation. However, in the presence of NaCl (≥0.3 M), the cloud point curve shows a deep minimum as a function of anionic surfactant concentrations. Such minima were not observed in lower concentrations of NaCl in the case of anionic surfactants and in the presence of cationic surfactants. It was seen that the stronger effect was obtained for anionic surfactant. The relative viscosity of this system increased markedly with the variation of the concentration of NaCl, which may be due to shape transition of the micelles. The relative sound velocity results are found to be in agreement with the viscosity results.