Structure, rheological and responsive properties of a new mixed viscoelastic surfactant system

Abstract

A new viscoelastic surfactant system based on a mixture of cationic (n-dodecylpyridinium chloride) and anionic (potassium oleate) surfactants was prepared. The effect of the concentration of cationic surfactant at constant concentration of anionic surfactant on the structure and rheological properties was investigated. A transition from spherical to cylindrical micelles was observed upon increasing content of cationic surfactant. SANS data revealed that the cylindrical micelles have an elliptical cross-section with a major axis close to double length of oleate tail and minor axis close to the double length of n-dodecyl tail suggesting that the both surfactants are segregated in some way within the micelles. The local segregation of surfactants was confirmed by 1H NMR NOESY data. The rheological data show that the viscosity goes through maximum with increasing content of cationic surfactant, which is due to the growth of micelles in length (before the maximum) and their branching (after the maximum). A maximum viscosity was found at a molar ratio between cationic and anionic surfactants equal to 0.3. The mixed surfactant system was shown to be responsive to hydrocarbons, since added n-decane induced a decrease of viscosity by 5 orders of magnitude as a result of transformation of long cylindrical micelles into ellipsoidal microemulsion droplets as was evidenced by SANS.