First of all, I want to thank everyone for their valuable feedback on the blog. It really does help us tailor content for your specific formulation needs. Please keep that feedback coming, either to [email protected] or to me at [email protected]
Missing Details
Our salt-free series generated interesting feedback, and I also realized that there was an important point that I glossed over quickly regarding how to thicken these formulations. Using anionic surfactants as primary surfactants is deeply entrenched in the industry, and for good reasons. They tend to have relatively compact head groups and long aliphatic tails (the classic tadpole), making them outstanding foamers and detergents. They also have a very high potential to form wormlike micelles which are needed for surfactant thickening.
There’s one thing that stands in the way of anionic surfactants readily forming wormlike micelles at low concentration: since all the surfactant heads are the same charge, they repel each other and the surfactant monomers cannot pack closely enough to facilitate the transition to wormlike micelles. There are two methods for fixing this fundamental property:
Electrolyte Addition
Adding electrolytes, usually in the form of Sodium Chloride (though advanced formulations may incorporate alternative salts) reduces the electrostatic repulsion between anionic head groups and facilitates the transition to wormlike micelles and highly viscous solutions at relatively low concentrations. A traditional surfactant like Sodium Lauryl Sulfate will generate substantial viscosity at 10% active matter or less with sufficient addition of electrolytes. But the amount tends to be quite high relative to the amount of surfactant and the aesthetics of SLS by itself are not conducive to a product with a high degree of consumer delight.
Secondary Surfactants
Another way to facilitate the transition to wormlike micelles at relatively low surfactant concentration is through the addition of a wide variety of secondary surfactants. Their efficacy is variable but the effect is the same; they reduce electrostatic repulsion between anionic head groups and ensure viscous solutions are achievable. Most commonly the secondary surfactant system is a combination of amphoteric and nonionic chemistries, allowing for extremely high efficiency and outstanding aesthetics.
Specific effects of secondary surfactants can vary from subtly shifting the salt curve toward lower salt addition or dramatically increasing the overall viscosity performance, widening or narrowing the salt curve, and dramatically shifting the salt curve. Specific formulation goals will help dictate which secondaries to use and how much. There are few hard and fast rules here and it’s mostly left to the formulator’s discretion how they want to approach this.
The Surfactant Triad
In a forthcoming webinar I’ll be discussing in more detail what I call The Surfactant Triad, a practically ideal combination of anionic, amphoteric, and nonionic chemistries that become the basis almost all personal care cleansing formulations. In “Low-No Salt Shampoos Part 3” I mentioned the use of nonionics as key additives for those systems. This is due to their potent ability to shift the salt curve so that no additional salt is needed to achieved the desired aesthetics. While rare, there are commercial examples of products that have been streamlined down to simply an anionic and nonionic surfactant together to achieve good aesthetics for body wash. The most commonly used nonionics are alkanolamides like Cocamide MIPA (Cola®Mid CFPA) or Cocamide DIPA (Cola®Liquid DC-5) but I have helped formulators use more unique ingredients like Glyceryl Laurate (Colonial Monolaurin) and Polyglyceryl-3 Oleate (Cola®Mulse 3GO) as more potent and mutli-functional additives. Additionally, we have explored non-traditional amphoterics in depth via two posters and a webinar as potent viscosity boosters that can also quite dramatically shift the salt curve toward low or no salt addition.
Conclusion
I don’t expect the number of formulator requests for improved foam and higher viscosity in “sulfate-free” systems to decrease any time soon. Adding “salt-free” as a claim on top creates a new level of difficulty and challenges that few formulators are prepared to handle. I hope this provides some additional insight into how to tackle these challenges. As always, we eagerly anticipate your feedback.
