There are three ways to thicken a surfactant based creation - increase the concentration of the surfactants, make the micelles larger, or create a gel. (There's a brief summary about this topic in this post...)
You can increase the surfactant very easily by adding more surfactants and removing the water. But, when you increase the surfactant concentrations above the suggested usage rates, you are asking for irritation, plus you're wasting supplies.
The other way to increase the surfactant concentration is to use the salt curve.
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.
When we add salt to the mixture, there is a distinct curve (pictured to the left). As we gradually increase the salt, it will thicken nicely and stay that way. But if we add too much salt, we eventually start to thin it out back to the watery state. This is one of the reasons we add it slowly - as you can see, the difference between 3% and 3.5% is huge! This is due to the imbalance of charges between the various ingredients in your creation.
When we see a surfactant can help thicken another - like cocamidopropyl betaine or SMC taurate - this is thanks to the increase of electrolytes in the mix. When we see something like cocamide DEA increase viscosity, we have fooled the system into thinking we've increased the concentration of the surfactants, and it thickens.
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! The down side of using salt is that it can make your creations cloudy, so you'll want to bottle those in a non-clear container (like I should have done with this bottle of cocamide DEA thickened bubble bath!) or just accept it!
Join me tomorrow for more fun with thickeners - increasing the micelle size!