Shampoo Science

SHAMPOO + SALT

by Mrs Helen Ellis BSc MTTS
(an experienced surfactants chemist and Past President of The Trichological Society)
addressing scientific and practical issues associated with shampoo science:

Question 1
Why does shampoo thicken on the addition of salt?

Answer
Salt thickens shampoo due to the ability of the sodium ions to lower the charge density on the outside of the micelles in the shampoo. This usually only applies to anionic species or anionic nonionic mixtures.
Question 2
What is the actual interaction between the salt and all other ingredients in the shampoo on the molecular scale?

Answer

The viscocity of a surfactant mixture is dependant upon the size of the micelles in the system. This is determined by several factors – concentration of surfactants, the type and ratio of species, temperature and charge density on the micelles.
In mixed micelles the arrangement of the molecules is limited to some extent by the charge density on the surface of the micelle. This can be reduced in several ways.

Addition of more nonionic surfactant so that the ratio in the micelles rises and the negatively charged heads of the anionic surfactants are held further apart.

Additon of a substance that reduces the charge density on the surface of the micelles.ie salt. The sodium ions of the salt affect the dissociation constant of the anionic surfactant in the formulation and the balance is forced back to the left of the dissociation equilibrium equation. C12SO4Na = C12SO4- + Na+ (sorry my e mail won’t allow me to add proper characters for this equation)

By doing this the charge density drops and the micelle size can increase. It also causes a rise in micelle aggregation number and the transition from spherical to cylindrical micelles. This transition also leads to a jump in the formulation viscocity as the threads can get knotted around each other. If this process continues a lammellar structure can be formed which is a gel. However it is very variable the response to salt addition in the system. If too much salt is added to a mixture then it can “crash”. This is a sudden transition from thick back to thin liquid. In some cases this can be reversible with the addition of water. But too much salt reduces the solubility of the surfactant so much (again the equilibrium is forced to the left) that it comes out of solution and precipitates.

Other cationic metals can be used, eg potassium, magnesium salts. Magnesium salts are twice as effective as they are 2+ species. But Ca2+ & Zn+ produce such insoluble salts that precipitation is instant. Magnesium can also be a bit problematic.

© 2002 – Helen Ellis BSc. MTTS

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