Thickening mechanisms for an amino acid-derived surfactant composition

Wormlike micelles in mixed surfactant mixture containing an amino acid-derived anionic surfactant, sodium lauroyl sarcosinate (SLSar), and a zwitterionic surfactant, cocoamidopropyl hydroxysultaine (CAHS) were investigated. The total surfactant concentration was maintained at 15% wt/v, while the ratio of SLSar and CAHS was varied and the pH was lowered from 7.5 to 4.5. Steady-shear rheology, potentiometric titration, cross-polarized microscopy, zeta potential and cryo-TEM were used to obtain zero-shear viscosity and analyze the structure-rheology relationship. As pH dropped, viscosity increased to a maximum followed by a viscosity decrease in four out of five tested systems. The synergistic effect was strongest in the SLSar 6 %:CAHS 9 % composition or an equivalent molar ratio of 1:1 between the two surfactants, for which the viscosity maximum was 9.6 Pa.s at pH 4.8. The viscosity peak of each system occurred in the pH range 4.8–5.0, close to the system’s experimental pKa. The intermolecular forces between the SLSar acid form, SLSar anionic form and CAHS reduced the charge repulsion and contributed to the thickening behavior of these systems.