The influence of inorganic salts on the rheological properties of 1,3-propanediyl bis(dodecyl dimethylammonium bromide) and sodium dodecylsulfonate aqueous mixed system

The rheological properties of isotropic 12-3-12/AS/H2O mixed systems in the absence and in the presence of salt have been investigated. The addition of AS into 12-3-12 aqueous solution or vice versa, or the addition of salt into isotropic 12-3-12/AS/H2O mixed systems, leads to changes in microstructure, phase behavior, shear viscosity η, and rheological behavior. In the process of gradually adding AS (or salt) to 12-3-12 aqueous solution, the microstructural transitions are as follows: coexistence of spherical micelles and rodlike micelles → somewhat elongation of rodlike micelles → coexistence of rodlike micelles and vesicles → vesicles; and the rheological behaviors change as follows: antithixotropic and shear thickening → complex thixotropic → shear thinning and thixotropic. η increases initially to a peak, then decreases, and finally the further addition of AS or salt results in aqueous two-phase separation. The origin and the small value (∼510 mPa s) of the η peak for isotropic 12-3-12/AS/H2O mixed system with excess 12-3-12 are linked with the above microstructural transitions. The η peak becomes lower with the addition of salt. The salt effect on the η peak is related to the matching of water affinities between the counterions and ionic head groups of surfactant. The isotropic 12-3-12/AS/H2O mixed systems with excess AS are Newtonian fluids or near Newtonian fluids with η ≈ 0.75 mPa s under the shear rate of 1500 s−1 due to the coexistence of spherical micelles and short rodlike micelles. The addition of salt changes these Newtonian or near Newtonian fluids to antithixotropic fluids, or complex thixotropic fluids, or thixotropic fluids, and leads to somewhat increase of η (usually less than 25 mPa s). Two-parameter power-law is used to model the above mentioned fluids. Most of the fluids excepting a few shear thinning fluids located near the phase boundary between isotropic single-phase region and adjacent aqueous two-phase region obey the power-law model.