Request for references for the pH of haircare products, esp shampoo
I recently got into a discussion with a (non-cosmetic) chemist about some things I’d previously thought were generally accepted facts regarding haircare formulation, specifically shampoo. That chemist challenged a lot of my knowledge, and I thought I’d get some more insights from a few pros, if anyone’s willing!
I’ve read numerous places that hair’s natural pH is ~ 4.5–5.5, and that most hair products (unless you’re trying to lift the cuticle for some reason) should be in this range, including shampoo. I’ve seen peer-reviewed articles that support this, but the chemist I was talking to pointed out that most of the articles I’d referenced were weak research (here’s an example), and once they did that, I realized most of my references for hair and formulation pH don’t seem to have any real studies attached.
I’ve spent some time hunting, but I’m having trouble finding studies that verify hair’s pH (which… since hair is not a liquid, I’m unclear how that works?) and the ideal pH of haircare formulations. I feel like this research must exist since I’ve seen it referenced often in books, articles, etc., but I can’t find it. Can anyone help point me toward studies about ideal pH for haircare products, and the pH of hair itself?