No abstract

The thickening behaviors in aqueous solution of mixed surfactant mixtures containing an amino acid-based surfactant, sodium lauroyl sarcosinate (SLSar), and a nonionic surfactant, N-dodecylpyrrolidone (C12P) were investigated using the rheological methods of steady-shear and frequency sweep. The results varying the added C12P concentration and the mass ratio of SLSar and C12P indicate that the addition of C12P can promote the formation of wormlike micelles and hydrogels, resulting in viscosity enhancement in the mixed aqueous solution of SLSar and C12P. Viscosity maxima in SLSar/water/C12P systems are sensitive to pH, temperature, the concentration of added C12P, and the mass ratio of SLSar and C12P. In the high concentration of total surfactant (30-39 wt%), the enhancement of viscosity obviously occurs by adding 6 wt% C12P, whereas in the low concentration of total surfactant (15 wt%), the C12P must be added more than 9 wt% in 15 wt% mixtures of SLSar and C12P. Moreover, during the solution pH lowered from SLSar' natural pH of 7.2 to the mixture's pKa 5.2, the intermolecular forces between SLSar acid, SLSar anionic salt, and C12P gradually reduce the overall headgroups areas and contribute to the formation of wormlike micelles, hydrogels, and the thickening behaviors.