One very grey area is the pre-wash treatment of hair (or prepoo meaning pre-shampoo for the uninitiated). I have seen a whole string of concoctions to prepare hair but I will narrow it down to two main groups
1. Oil applied to hair prior to wetting
2. Conditioner (or a conditoner/oil mix) applied to hair prior to washing
So, should you go for an oil or a conditoner or a conditioner oil mix? Well let us see what you get with each!
Role 1: I have previously talked hair damage caused by hair expanding when wet then contracting again when dry (hygral fatigue). Certain oils like coconut oil and olive oil are known to penetrate slightly under the cuticle and help reduce the expansion of hair when wet (J Cosmet Sci, pp 169-184, 2001).
Role 2: Shampoos work by lifting oil off from the surface of hair (see diagrams here). However for some people this leaves the hair feeling stripped. A prepoo treatment with oil is essentially creating greasy hair. Greasy hair usually means that one shampoo application will still leave some oil behind. This little bit of oil allows hair to not feel 'stripped'.
Role 3: Hair treated with coconut oil as a pre-wash has been shown to be less susceptible to breakage after washing. (J Cosmet Sci, pp 327-339, 1999)
Words of caution for Oil Prepoo - Not all oils can prevent or reduce hygral fatigue. Oils require several hours to penetrate the hair. Some people have naturally greasy hair and will not like the feel of extra oil on hair.
2. Conditioner mix
Role: Conditioners have a high component of water (usually over 50%). Applying a conditioner or a conditioner oil mix to hair prior to shampoo is therefore unlikely to be as effective in reducing hygral fatigue compared to an oil on its own. However there is no research on this at the moment.
The principle role of conditioner applied to hair prior to washing is to create a greasy layer similar to role 2 above.