I saw a store soap that listed PK Acid as one of its ingredients. When I googled it I read it had emulsifying properties and it was used to add opacity. Has anyone used this and would like to share their results? Where can this be purchased?
It's considered a type of superfat, though it's not fat exactly.
The most common on ingredient lists is probably coconut acid. Palm kernel acid would be very similar. You might also see palm acid or even tallow acid.
These are free fatty acids created from breaking apart oil molecules from these various fats. The fat molecules, or triglycerides, can be split into three fatty acid molecules each and glycerin. In soap making the fatty acids combine with the alkali to form soap but they can also exist unsaponified as free acids, which can be liquids or waxy solids.
The solid free fatty acids act as skin conditioners and make the soap a little less alkaline -- basically they create a milder product since commercial soaps often don't contain unsaponified oil or glycerin like artisanal soap typically does.
As for buying it, you can see by googling that there are manufacturers, but I haven't seen the exact product available in small quantities. However, the individual fatty acids are easily obtained. The most useful are probably lauric acid, myristic acid and stearic acid. They can be saponified or used in other ways.