Optimum formulation conditions for cationic surfactants via rheo-titration in turbulent regime

Recently a new, time saving, approach to the determination of the composition at which a microemulsion is balanced has been developed using the so called HLD-titration. The key of the method is the observation that in correspondence of the balanced state where the microemulsion coexist with excess oil and water (Winsor III phase equilibrium), the level reached by the three-phasic system under stirring has a consistent maximum allowing a fast and low-cost readout of the balanced state that permits the evaluation of the surfactants characteristic parameters (s). To better understand how the formation of a balanced microemulsion is related to rheology, here a titration experimental approach was adopted under turbulent flow meanwhile fluid friction. A mixture of equal volumes of brine and oil and didodecyldimethylammonium bromide (DDAB) is titrated with dodecyltrimethylammonium bromide (LTAB), in the Couette cell of a rheometer under continuous rotation, promoting the transition from a Winsor II (w/o microemulsion coexisting with brine) to a Winsor III phase equilibrium. The turbulent apparent viscosity was measured after each addition of the LTAB. We demonstrate that the turbulent apparent viscosity attains a minimum at the Winsor III phase equilibrium. Furthermore, we have investigated the microstructural evolution of the microemulsions found at the different DDAB/LTAB ratios that reproduces the HLD-titration by means of small-angle X-ray scattering (SAXS) and diffusion nuclear magnetic resonance (NMR) and rheo- small-angle light scattering (rheo-SALS). The anisotropy index, measured by rheo-SALS, increases upon increasing the shear rate suggesting the ability of oil and water domains, in the balanced state, to elongate along the streamlines.