Rheology of Aqueous Solutions Containing SLES, CAPB, and Microemulsion: Influence of Cosurfactant and Salt

Aqueous solutions containing sodium lauryl ether sulphate (SLES) and cocamidopropyl Betaine (CAPB) with a constant total surfactant concentration (10 wt%) but different volume mixing ratios were prepared. A remarkable increase in the solutions’ viscosity at a volume mixing ratio of 34 v/v% SLES to 66 v/v% CAPB was observed. By increasing the volume ratio of SLES (r-value) in the binary mixture, the viscosity of 10 wt% SLES, which is close to that of water, increases to a maximum, but then drops down drastically at high r values (>45 v/v%). The maximum viscosity (Vmax) is 2.8 Pas, whereas the minimum is 2 mPas. The rheological behavior at Vmax has the remarkable feature of a simple Maxwell fluid over a large frequency range with one relaxation time. By adding a microemulsion (Plantasil Micro), the value of Vmax decreases extremely, whereas adding cosurfactants like isodecyltrietheleneoxide (IT3) or salts, like calcium chloride, leads to a pronounced increase in the value of Vmax. At 0.76 wt% CaCl2, Vmax is 140 Pas and 150 Pas in the presence of 1 wt% IT3. By increasing the SLES volume mixing ratio in the solution the maximum viscosity is shifted to higher CaCl2 amounts. The increasing viscosity can be explained by a transformation of the micelle shape from spherical to rod-like. Whereas adding cosurfactants or salts leads to an increased entanglement between the rod-like micelles. Consequently, higher microemulsion amounts can be added before reaching the minimum viscosity.