Hair masks and conditioners are formulated to have a specific pH. Shampoo’s normally have a more alkaline pH in order to open the cuticles. Hair conditioners and masks have a more acidic pH to close the cuticles. It is imperative to close the cuticle to retain moisture, prevent tangles and prevent static.
Manufacturers use a variety of acidic substances like citric acid, hyaluronic acids, etc. to reduce the pH of hair conditioners and masks. However, strong acids are very dehydrating. It is also very corrosive and will damage the protein and lipids in the structure of the hair. The extent of the damage will depend on:
- The strength of the acid; and/or
- The duration of the exposure.
Thus, the answer here lies in the instructions on the bottle. If the bottle says to rinse out after 2 minutes, then rinse it out after 2 minutes. The formulation is probably too acidic to be left on longer than the indicated time and could severely dry out and damage your hair if the hair is exposed to the conditioner or mask for longer. Only leave a product in, if it specifically declares that it is a leave-in product.