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.