Understanding – Salt Curves
The formulation of shampoos and other aqueous surfactant containing systems in a complicate undertaking. This is because each ingredient interacts with the other ingredients to make complex self-assembling units. These interactions affect viscosity, foam, rheology and aesthetics of the formulation and often commercial success or failure of a formulation. This article will address the effect of salt, presence of amides, and anionic / amphoteric on basic surfactant systems.
The so-called salt curve is well documented in the literature, whereby the viscosity of a surfactant solution initially increases with the addition of salt but then decreases with further increases in salinity (1).
Perry Romanowski explains: ”The salt curve is the concept of adding salt - an electrolyte - to a surfactant mix to make it thicker. It does this in two ways. The first - the electrolyte increases the size of the micelles in the surfactants, so the viscosity increases. The second - the electrolytes compete with the surfactants for water, so as we add more salt, we fool the product into thinking we’ve increased the concentration of the surfactants, which will increase viscosity. Ideally you’ll use between 1% and 3% salt in your creation - remember to add it slowly so you don’t end up on the wrong side of that curve!” (2) When this happens adding water can increase viscosity, shifting the curve to the left.
The factors that control viscosity and rheology of a formulation work on the alteration of the size of the micelles. These factors include (1):
- Concentration o ... ...