Dear Donat,

·         Sodium chloride introduces sodium cat ions and chloride anions to the system. These counter ions attach to the polar head of surfactant molecules, reducing their overall charge and stabilizing the system. This effectively lowers the critical micellar concentration, making the solution thicker.

·         Sodium chloride  increases the ionic strength of the bulk phase resulting in the formation of larger micelles and thus having a positive influence on the viscosity.

Thanks, Salman; but do you know about the kinetics of the process? Is it posible to observe viscosity progressively increasing during several days?

Dear Victoria,

The kinetics should be very quick in systems with surfactants such as SLES and co. Salts are producing the elongation of spherical micelle in cylindrical one  (when they are formed with anionic surfactants) and it results in an increase of the viscosity of the system. I never observe it, but if you are using very large molecules, I guess the kinetics of elongation of micelles could be slowed down.

Hope it can help,

Thanks to all, especially to Jeremie for his clarifying answer.

From my experience workin on salt and some anionic surfactant systems, the kinetics is fast. The increase in viscosity will happen within few min or max few hrs. 

to clarify, we do not speak about viscosity increase of emulsion, it is viscosity increase in a surfactant solution. If you speak about changes in an emulsion (with dispersed oil or water phase) kinetics maybe different then.

Thanks Abhinav and Titus for your informative contributions.