When I first got into the cosmetic industry I worked on hair care products, specifically, shampoos. One of the things that I found fascinating about shampoos was that if you added salt to them they would get thicker. I later found that if you added too much they get thinner. It turns out there is a range of salt concentrations at which a shampoo formula will get thicker or thinner. We call this the Salt Curve and it is an important thing to know because it can help save many production batches.
Salt Curve and Shampoos
To understand why salt affects the viscosity of a shampoo (or body wash) you have to first realize that shampoos are mixtures of surfactants that arrange themselves in tiny structures called micelles. The viscosity of the shampoo solution depends on the size and packing structure of these micelles.