Hair is immune to both acids and alkalis. Every hair has tiny scales, like snake shingles or scales. Acid lets the scales lie flat, leaving the hair smooth and shiny. On the other hand, Alkali makes the scales stand up and makes your hair feel rough and look sluggish.
If you use soap to wash your naturally curly hair in hard water, then the combination of the scales of your hair standing up and the scum that forms soap makes your hair a terrible mess. And, when your hair is all messy and rough, all the soap cannot be rinsed out, which makes it look bad.
The primary problem with washing hair with soap is that a chemical reaction can occur, forming an insoluble material in water. That is what causes you to see the “soap scum” lining bathrooms and sinks. On your hair, this same kind of “picture” can develop, leaving hair feeling dirty and weighed. This “film” is not a problem as Synthetic detergents don’t react like this.
Another cons of using soap for washing hair is that they are generally solid at room temperature, and the number of ingredients you can add to them is limited. Synthetic detergents are liquids that offer much more versatility to cosmetic chemists when formulating with them.
Of course you can wash your hair with soap, but if you’re used to liquid shampoos you probably won’t be pleased with the results. Soap will leave hair feeling dry and waxy, and does not have all of the liquid detergents’ benefits. However, if clean hair is your only target then soap can be an option.
Know the problem with soap use is in the bath, not the soap. Only solve the hard water problem and soap is good to use.
There is no doubt that soap is better for your hair, and if you compensate for the hard water, you can use it. All you need to test is that the soap is made by cold pressing that ensures that the vitamins A & E contained in the natural oils do not get lost during the heating cycle. Such Vitamins are essential for your skin and hair nourishment. Besides when cold presses saponify soap, it leaves Glycerin in the soap, which stops the hair from drying out. Many soaps are made through the cycle of lye, and should not be used on the hair.