Sebum is steadily replenished by your scalp over the day through those sebaceous glands attached to your hair follicle. The amount of sebum produced varies from person to person, but usually reaches its maximum at puberty, and is affected by the seasons, the temperature, and hormonal variations.
Sebum is really sticky and can cement soil and other particles to your hair and scalp. Although brushing, combing, and contact with your pillow will redistribute the sebum, you can't remove it entirely by physical means because it's so sticky! And it means that the oilier your hair, the more stuff will get attached to it, like dirt particles, smoke, bits from rain water, skin flakes, and so on. It can oxidize like any other fatty acid, leading to that dirty hair smell you get when you haven't washed your hair in a while. (Click here for a post on rancidity of oils for more information.)
Sebum can be your friend! It is good at sealing the hair shaft and protecting it from the outside world and it does make your hair temporarily glossy. (Some hair types - like African hair - love sebum and actually see an increase in shine and less breakage when their hair is oiled!) But you do need to remove it because it really isn't that attractive to have seriously greasy hair.
Interestingly enough, the spread of sebum through your hair is increased when you blow dry it. So if you're a particularly greasy haired girl, leaving your hair to dry naturally can buy you more time between washings!
Proteinaceous matter from the desquamation of your scalp and your sweat can build up in your hair as well. And as I mentioned above, sebum attracts environmental pollutants, so washing your hair on a regular basis removes those pollutants, soils, and sebum to leave your hair fresh and clean.
How does shampoo remove these things from our hair? We know that surfactants lower surface tension (for a quick review, click here) but they're also effective at deflocculating soil and dirt clumps in our hair. (Deflocculating means "to disperse an agglomerate into fine particles and form a colloid" - in other words, to disperse a clump of something into finer particles. You might remember flocculation from the epic lotion fail post - this means for finer particles to clump together!) The shampoo keeps the fine particles in suspension so they can be washed away and not go back onto your hair or scalp.
Detergents work in a few different ways...
- "Roll-up mechanism": Causes a rapid detachment of oils from your hair, scalp, body, and/or clothes, which are displaced by the surfactant.
- Micellular solubilization mechanism: The soils are solubilized into the micelles and washed away (this is dependent upon micelle concentration). It's all about displacing the oils in your hair and scalp with the detergent solution.
- Dispersion and emulsification: Soil particles are emulsified into the solution. Sebum might actually help this process.
- remove sebum and soil from our hair and scalp
- remove residue of styling products
- leave hair in good condition after rinsing, meaning it can be combed or brushed when wet or dry
- deposit lovely things like panthenol, conditioning agents, and so on
- frequent use shampoo
- basic shampoo
- conditioning shampoo
- 2-in-1 shampoo
- anti-dandruff shampoo (we won't be making these)
- medicated shampoo (we won't be making these)
- baby shampoo
So join me tomorrow as we take a look at how to formulate a basic shampoo!