This series is brought to you by virtue of my developing a sensitivity to the shampoo I'd used for about 9 years. I "DIY" my shampoo, so let's learn together. This isn't so much a lesson in "the science of detergents." This is going to be related to the way they work in a product and how that translates into your experience. "Surfactant" is a technical term for detergent and I tend to use them interchangeably. Here, I'll call them detergents.
Sulfate-free shampoos have some advantages and disadvantages. There are many characteristics of ingredients that contribute to the experience of using a product, and our perception of the product. This goes beyond the simple question of "whether sulfates are harsh or not."
The first thing I want to mention is concentration and foam. They are very much related. For every positive, there is a negative. That's life, right!? 😀
In order to have a nice foam, a product that lathers up right away, bubble size isn't too large nor too small and dense, one often needs a higher concentration of the sulfate-free detergents than traditional sulfate detergents (Sodium lauryl sulfate, Sodium laureth sulfate, Ammonium lauryl sulfate, Sodium laureth sulfate).
The only sulfate-free detergent that lathers well for me alone at low(ish) concentrations are the Glucosides (Decyl glucoside, Lauryl glucoside). And while the glucoside-detergent molecules are too large to penetrate hair and skin and therefore mild to skin, they can still lead to dryness because they de-grease (form micelles) at relative low concentrations - thus the foam!. That makes it a mild, but still potentially oil-stripping detergent. Oh, the irony! You'll often see this detergent combined with others for it's foam-boosting power, and for added mildness to skin. (Seriously, don't avoid this ingredient, it was my favorite for my picky skin for years. It's an excellent ingredient for mild shampoos).
Other sulfate-free detergents foam less on their own, but help thicken a product or add mildness to skin (such as Cocamidopropyl betaine, Sodium sulfosuccinate, Sodium cocoyl isethionate).
Product formulators need to do more detergent-combining to find a product that will achieve a nice sensory experience - because nobody likes wimpy foam that disappears immediately. But we also want it to rinse out quickly and easily. With sulfate detergents, detergent-combining tends to be more straight-forward. Add 1 detergent for mildness, maybe another for foaming and you're done.
If you're creating a "boutique" shampoo for a specific audience, you have more flexibility in the end product. So you can use the (more expensive) sulfate-free detergents because we assume you're going to charge more, and also to manage people's expectations as part of the "brand." But if you're going for a broader market, you need to meet a wide variety of expectations. That probably means more detergent.
Detergent concentration has a lot to do with irritancy and oil-removal. A higher concentration of mild detergents can sometimes be as irritating or as oil-stripping as a lesser concentration of sulfate detergents. A lot depends on other ingredients in the formula.
Take-home message: Combining detergents is the key to great shampoos!
Up next: More topics in sulfate-free shampoo:
Thickening, conditioning, skin irritation, hard water interactions, mildness-creation for hair and skin, that tangly squeaky-clean feeling.