What it does
The EU's CosIng database lists four functions for sodium chloride: bulking, fragrance, oral care and viscosity controlling.1 Viscosity controlling means increasing or decreasing the thickness of a product.2
In shampoo, thickeners such as salt are added to increase viscosity and have no effect on how well the shampoo cleans.3 A review of shampoo ingredients gives the reason they are there at all: consumers feel that a thick shampoo works better than a thin one.4 The same is true of conditioners, where salt appears among the bodying agents and thickeners alongside fatty alcohols, waxes and gums.3
How salt thickens a shampoo
The effect comes from the surfactants, not from the salt itself. When salt is added to a solution of charged surfactants, it screens the charges on their head groups. The surfactant molecules then pack into longer structures, changing from spherical micelles to long "wormlike" ones and finally to branched networks, and those entangled networks are what make the liquid thick. The result is a viscosity peak: a study modelling this process reproduced the position and height of the maximum seen in measured salt curves of sodium laureth sulfate, and showed that added fragrance oils change the curve as well.5
A simulation study of Sodium Laureth Sulfate found that more salt favours longer micelles, and that adding Cocamidopropyl Betaine, a common partner in shampoo, stabilises wormlike micelles at a lower salt concentration.6 Because the curve has a maximum, more salt is not always thicker: past the peak, viscosity falls again.5
Sulfate-free formulas behave differently
Salt does not thicken every surfactant system. In a study of sulfate-free cleansers built from an olefin sulfonate and a sultaine, salt had no effect on the viscosity of the sultaine alone, raised the viscosity of the olefin sulfonate at 2% salt, and lowered the viscosity of the mixture of the two as more was added.7 What sulfate-free shampoo means and shampoo surfactant families cover the cleansers involved.
What official sources say
Sodium chloride has no entry in any annex of the EU cosmetics regulation, so no maximum concentration or condition of use applies to it there8, and CosIng lists no restriction for it.1
How it appears on a label
As Sodium Chloride.1 Sea salt has a name of its own, Maris Sal, which CosIng describes as naturally occurring inorganic salts derived from sea water and lists as skin conditioning.9 Because of the salt curve, a higher place on the list does not mean a thicker product, and the list cannot tell you how thick the formula is. The guide to reading a shampoo ingredient list shows how thickeners fit among the other ingredients, and the shampoo ingredient checker sorts your own list by job.