When you formulate an anionic surfactant based cleansing formula, you don’t normally have to include a separate thickening system. This is because salt will thicken surfactant systems. This is a pretty handy phenomena as it reduces the cost of your formula and can give you better control over the final viscosity. However, to get that control you need to do a salt curve analysis. Here’s what you do.
Salt Curve Analysis
The fundamental idea behind a salt curve analysis is that the concentration of salt predictably affects the viscosity of the system. Therefore, you can create a plot of the salt concentration versus the viscosity which can then be used to determine the final salt concentration of your formulation. It can also be used to adjust a batch if the viscosity is too low. Here is what you do.
1. Make a large batch of your formulation but do not add any salt. Record the initial viscosity.
2. Pour off samples (100g ) of your batch and create different salt concentrations for each. Use the following salt concentrations. (0.2%, 0.4%, 0.6%, 0.8%, 1%, 1.2%, 1.4%, 1.6%, 1.8%, 2%) You may want to go higher in salt concentration.
3. Remove any air from the samples, let them equillibrate at 25C and take viscosity measurements.
4. Plot the concentration of salt versus viscosity on a graph.
5. Choose the most desired viscosity level and set that as the salt concentration of your formulation.
6. Create a new batch of the formulation using the new salt curve determined salt concentration.
7. Measure viscosity and compare to predicted salt curve level.
Using the salt curve
You should have no problem creating a salt curve for your system. Since it is dependent on the composition of the entire formula, you should do this for any new formula. Even if you simply change the fragrance it could have a significant impact on the salt curve.
For most systems, the salt curve will max out at 2% and viscosity will actually start to decrease. When you are formulating you’ll want to set your salt concentration on the left side of the salt curve. That way you will have some room to add salt if the viscosity is too low. If you set the salt concentration at the peak of the salt curve, your compounder might put in too much and you will have no way to adjust the viscosity except blending with a new batch.
To use the salt curve in production, just look up the viscosity of the final batch, estimate where it is on the salt curve, then add just enough salt to the batch to thicken it up. You should try this in the lab first as you want to make sure it works.
And for a more detailed and complicated discussion of what is going on with the salt curve, see this excellent article by Kevin Penfield.