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.
Since shampoos are typically made from anionic surfactants the outer surface has a specific charge density. This will affect the way that the micelles can pack together. A higher charge density will cause the micelles to repel and result in a thinner solution.
The sodium ions from the salt lower the charge density of the micelle surface. This makes them more able to pack closer together and creates a thicker solution. This is why salt thickening is really only affective with anionic-based or anionic/nonionic-based shampoos.
Micelles and salt
While salt can help increase the viscosity of a shampoo system, it works differently in different formulas. Some systems will be highly tolerant of salt while others get thin as water with just a small increase in salt concentration. This is because the size of the micelles is dependent on a number of factors including
- Concentration of surfactants
- Type of surfactants
- Ratio of surfactants
- Charge density
The way salt reduces the charge density is that it pushes the dissociation equilibrium of the surfactant to the left. As an example, consider this dissociation equation for Sodium Lauryl Sulfate.
C12SO4Na = C12SO4- + Na+
More sodium ions push the equilibrium to the associated state.
This drops the micelle charge density and the size increases. This in turn causes more micelle agglomeration which can ultimately lead to lammellar structures which can form a gel.
Creating a salt curve
When formulating an anionic cleansing system, it is useful to create a salt curve so when your manufacturing people present you with a product that is too thin, you’ll know exactly how much salt they should add to get the right viscosity. Here’s how you do it.
1. Make a 500 g batch of shampoo (or body wash) and leave out the salt.
2. Split the batch into ten 50g samples.
3. Add salt levels in the following increments. (0.2%, 0.4%, 0.6%, 0.8%, 1%, 1.2%, 1.4%, 1.6%, 1.8%, 2%)
4. Record viscosity & plot viscosity versus concentration.
Most systems should not tolerate much more than 2% salt levels. Based on the results you can refine your salt curve to figure out even tighter % levels.
Each formula type you make should have a salt curve like this so you can tell Production how much they’ll need to add to adjust the viscosity. You might also consider doing the same thing with the fragrance as it can affect the viscosity in a similar way.
We’ll save that answer for another time.