Kind of, sometimes, but I don’t recommend it. Soap is technically a surfactant, but for the purposes of this FAQ I’ll be referring to saponified fats as “soap” and products like Caprylyl/Capryl Glucoside, Sodium Cocoyl Isethionate (SCI), Cocamidopropyl Betaine, etc. as “surfactants” (also known as syndets aka synthetic detergents).
Surfactants are a lot more versatile than soap, so it’s always going to be a less-than-ideal swap. Both soap and surfactants lather/foam and cleanse, but that’s roughly where the universal similarities stop. Using one for the other will result in a different end product, so if you make the swap, be prepared for the changes.
One of the biggest reasons I use surfactants over soap is that surfactants can be acidic, while soap cannot. Mildly acidic products are gentler on the skin than products with a basic (above 7) pH. They help keep our moisture barrier intact and firing on all cylinders. Read this to learn more. So, at a bare minimum, if you are using soap instead of surfactants your product will be basic, and that is harder on the skin and hair. If you suffer from dry hands I highly recommend switching to washing your hands with a mildly acidic syndet product rather than soap—this simple change has made a massive improvement in the state of the skin on my hands!
There are also many different surfactants available, with different charges (anionic, cationic, and non-ionic), and these surfactants will have different strengths and weaknesses. Caprylyl/Capryl Glucoside, for instance, is very gentle as it is non-ionic, but it’s also a great solubilizer, so I’ll often use it in products like foaming hand and face washes to solubilize an essential oil without incorporating an additional solubilizer. Compared to Caprylyl/Capryl Glucoside, soap is basic (instead of acidic—harder on the skin), anionic (harder on the skin), and may require an additional solubilizer as soap isn’t as good of a solubilizer is Caprylyl/Capryl Glucoside.
Liquid castile soap can sort of work as a stand-in for liquid surfactants, but again, I don’t recommend it. A finished castile soap product (like Dr. Bronner’s) will already be diluted with water, so using it in place of an undiluted liquid surfactant product will result in a much weaker product. A concentrated castile soap paste would be a slightly better option, but it will still be very basic and I don’t recommend it.
In a solid syndet bar the product is usually comprised of 50%+ solid surfactants like Sodium Cocoyl Isethionate (SCI), Sodium Lauryl Sulfoacetate (SLSa), and Sodium Coco Sulfate (SCS). I really do not recommend using solid soap in place of solid surfactants in products like these.
An additional consideration is preservation: it is not uncommon for preservatives to require an acidic pH to function, and with soap being basic this may compromise your preservative system. While pure soap does not require preservatives it’s hard to guarantee products made with some soap will be stable, so that is something else you would need to monitor if you made that change.
As with anything, you can certainly try it, but I don’t recommend this particular switch. The end result will be less gentle on skin and hair due to the higher pH, and you may also compromise efficacy and other “jobs” like solubilizing.
Posted in: Substitutions