The Left Brain responds:
Rinse off and leave-on products are formulated differently for a reason. Almost any oily material will provide conditioning when left on the hair. But a special kind of ingredient is required to ensure that conditioning agents will “stick” to hair during rinsing.
Two kinds of conditioning agents:
There are two tricks that chemists use to deposit ingredients on hair. One is called “dilution deposition” which works well for silicones and some oils. In this approach the ingredients “fall out” of solution when rinse water hits the product.
The other approach is called “charge deposition” which requires conditioners known as “quaternary ammonium” compounds or quats for short. Quats are have a positive charge so they are attracted to the negative sites on damaged hair. One issue with quats, though, is that they are typically chloride salts and can be irritating to your skin. Because of this potential irritation issue, some quats that are used in rinse off products should not be used in products that are left in contact with the skin.
Cetrimonium chloride is one such example: it can’t be used at more than 1% in a leave on product. Because the average consumer has no way of knowing which ingredients will be irritating (and at what concentrations the ingredients are used at) there is some risk in leaving a rinse out conditioner in your hair.
The Beauty Brains bottom line
If you choose to experiment with leaving rinse out products in your hair, be mindful that you may experience increased skin irritation.
Conditioners Formulated for Both:
If you're looking to save money by combining your leave-in and conditioner, or just want something thicker for your hair, don't leave it up to chance. There are special conditioners formulated to be used as both rinse outs and leave ins.
DevaCurl One Condition
DevaCare One Condition
Ouidad Curl Recovery Whipped Curls
Redken Curvaceous Conditioner