Why should shampoos have a PH less than 7?
The hint is the composition of protein, which makes up hair, which is made up of amino acids with a pH of about 5.5. Washing your hair in neutral or alkaline solution causes a reaction in the protein coating which roughens up the surface and deposits salts on the rough surface, which can make it brittle. If the solution is slightly acid however, the surface of the hair remains smooth, and dirt flows off it more easily as well. Too acid a solution can burn the hair however, but a mild acid (for example vinegar) is fine, especially when washing out an alkaline. I use vinegar after working with cement, which neutralises it very well. Edit – I am not sure Melhar the Cosmetologist is correct here. Normally you achieve that effect by washing in hot water to open the cuticles and release any trapped dirt, and rinsing in cold to close them up again. An alkaline conditioner might well leave the surface of the hair rough and vulnerable to attack. It can though enable a coating, such as silicone
