Human hair has a natural pH (called its isoelectric point) of around 4.5. Any product with a pH higher than that is therefore alkaline with respect to the hair, which makes it inherently more drying and damaging to the cuticle and the fatty acid layers on the surface of and within the cuticle. Many of the surfactants used for shampoos are stable only in a narrow pH range—often between 5-7. For this reason, the majority of shampoos are formulated to be around 6 on the pH scale. It is possible to obtain a gentler product by formulating it to have a pH of around 4.5 - 5.5, closer to the natural state of hair (pH = 4-4.5), but it must contain surfactants that are stable at that pH. These types of products will usually have citric acid or some other mild acid near the end of the ingredients list. You can order pH test strips if you want to check out some of the products in your bathroom yourself.
Some ingredients to look for when choosing a gentle or mild shampoo for your hair are:
- cocamidopropyl betaine and other betaine surfactants
- carboxylate surfactants
- sodium lauroyl lactylate, sodium caproyl lactylate
- sodium laurylglucosides hydroxypropyl sulfonate
- cocoglucosides hydroxypropyltrimonium chloride
- sodium cocoamphopropionate, sodium cocoyl isethionate, disodium laureth sulfosuccinate
- nonionic polymeric surfactants
- lauryl-glucoside sodium maleate crosspolymer, lauryl-glucoside sodium succinate crosspolymer, decyl-glucoside sodium maleate crosspolymer
In conclusion, it is important to remember that a shampoo is comprised of a number of ingredients meant to work together to achieve the goal of clean, manageable hair. The label does not reveal the entire story, but it can allow us to glean important clues as to whether a particular shampoo might be more or less inclined to strip our curls of too much moisture. A single ingredient (or lack thereof) is not sufficient to predict the performance of a particular product, and the list of mild surfactants is in no way comprehensive. It is important to consider the total ingredient list, where one should be looking for not only certain types of cleansing agents, but also various combinations of different types of surfactants, polymers, oils, and other conditioning agents (humectants, vitamins, etc.). Physical properties such as pH also play a role in the mildness of a shampoo.
So, when selecting a new product, consider all of these factors in your evaluation. As always, if you find something that works for you and that you like, consider that to be your most valuable bit of scientific data and keep using it, even if the "science" says it might be harmful. In contrast, if the "science" says that a product should be good and gentle (i.e. "sulfate-free"), but your hair responds poorly to it, listen to that information as well! Only you know what you want most from your curls.