The basic chemistry of hair shampoos and conditioners is remarkably similar to that of laundry detergents and fabric softeners. The reason for this is that the type of cleaning they have to to do is pretty much the same.
For both hair and clothes the major contaminant is body fat. Clothes pick it up from contact with your skin, whereas hair gets it from your scalp.
The difference between the two is that whereas clothes may have other stains in them, from food and so on, with hair pretty much body fats are all there is. For this reason, shampoos are more simple formulations than laundry detergents.
There are three discrete types of shampoos/conditioners. A cleansing shampoo” is the older type where you shampoo first and then you condition; a conditioning shampoo” is something like Pert, where both shampoo and conditioner are present together, and a mild (or baby)shampoo” is one that is intended for use on sensitive skins (such as babies).
We will look at each separately.
5.9.1 Cleansing Shampoos
These are the simplest of the three. In essence, since it only needs to remove body fat from your hair, all that is needed in a shampoo is a (primary) alkaline (anionic) surfactant for cleaning (normally around 10 to 20%), a secondary surfactant (normally non-ionic or amphoteric) to produce foam, and a thickening agent to make it into a gel, and this is what you will find it in the cheaper shampoos that you buy from the discount stores.
Other components in the premium brands may be sunscreens, anti-dandruff agents, or various additives such as vitamins and herbs (along with accompanying claims about the wonders they can work for your hair).
Hand in hand with a cleansing shampoo is the need for a conditioner. This is essentially to neutralise the negative charge that the primary surfactant has placed on your hair. And it is this negative charge that causes unconditioned hair to feel coarse and get tangled. The itchy scalp that goes with unconditioned hair is simply due to the highly alkaline pH.
To allow hair strands to neatly lay on top of one another, therefore, this charge must be neutralised and this normally happens with a cationic (positively charged) surfactant. The positive end of the molecule sticks to the negatively charged sites on the individual hair strands and thus the static charge is neutralised. The organic part of these molecules are normally long chain fatty acids that then lie along the length of theindividual strandsd,giving themt a smooth feel.
The reason that the shampoo and conditioner must be applied separately, with these formulations, is that because if they were in the same formula the negatively charged (anionic) surfactant would bind with the positively charged (cationic) surfactant and neither would be available to clean your hair. It’s exactly the same argument as to why fabric softeners are added separately to laundry detergents when washing clothes (section 5.2).