In actual lab testing we’ve haven’t been able to demonstrate much of an effect from vinegar. But since vinegar is an acid, in theory, there are three things that the low pH could be doing for your hair.
Three Ways That Apple Cider Vinegar May Help Hair
1. Tightening the cuticle.
If your hair is damaged and the cuticles are upraised, an acid rinse could be helping them to lay flatter and therefore improving shine and detangle-ability.
2. Boosting conditioner efficacy
Conditioners based on quaternary ammonium compounds work better at a lower pH because the stick to hair better. Maybe the vinegar is helping to “lock” your conditioner onto the hair.
3. Removing shampoo residue
If shampoo isn’t rinsed completely it can leave a dulling residue on hair. Vinegar may be helping to remove buildup and letting the natural hair shine through.
Again, these are only theories. We have no prove that ACV is really good for your hair. The general scientific consensus is that conditioner will do a much better job than any kind of vinegar rinse. (The same conclusion is true for baking soda.)