As per this study, products with a PH above 3.67 may increase tangling and damage to the hair shaft and and a PH above 5.5 may cause scalp irritation.

Below is the PH of several natural alternatives for hair.

 

Lemon Juice: 2.0

Apple cider Vinegar: 2.5

Amla: 3.0

Shikakai: 5

Soapnut (Reehta): 5

Kaolin Clay: 6

Rhassoul Clay: 7.2

Heilerde clay: 8.5

Sugar soap: 8.5

Bentonite Clay: 8.7

Castile soap: 8.9

This scientist shows with a microscope how well different natural cleansers actually clean the hair.