There are however some surfactants used in sulfate free shampoos which are genuinely gentle (this is with the provision that the shampoo is placed in a pH range of 4-7 which reduces irritancy). This is all to do with the charge of the surfactants.
Good cleansers - negative charge - harsh to skin/hair
In the table, you may have noticed that the surfactants which generally produce some reaction are anionic (meaning they carry an overall negative charge). Surfactants like these are usually more efficient at cleaning hair because they are repelled from the surface of hair (which itself has slight negative charge). However they irritate skin and hair in part because they add to the negative charge but mostly because they are highly efficient when removing oil from the surface of hair and skin.
|Sulfate and Sulfate Free Shampoo Surfactants|
|Surfactant||Sulfate Free||Charge||%Skin Swelling (in 1hr)||Irritation Intensity|
|SLS||No||Anionic (-)||20-40%||Severe in 1 day|
|Olefin Sulfonates||Yes||Anionic |
|15-26%||Severe in 1 day|
|13-21%||Intense by day 4|
|11-17%||Moderate by day 5|
|Sorbitol esters (polysorbate/tween 20)||Yes||Non-ionic |
Why not use positively charged surfactants?
Generally surfactants with a positive charge will not tend to be used in shampoo at all. This is because they have a tendency to build up on the surface of hair instead of cleaning and removing oil. Many positively charged surfactants do also tend to be larger in size and more suited for a hair conditioner rather than a shampoo. So while positively charged surfactants would not irritate skin, they would also not tend to clean well enough
Weaker cleansers - no charge - gentle to skin/hair
The gentlest (in terms of irritation) sulfate free shampoos will have non ionic surfactants. This means the surfactant has no overall charge and therefore does not tend to cause the skin or hair to swell in general. However, some of these surfactants tend to not clean as well as the negatively charged surfactants, which often means that:
1.You will find many surfactants in the bottle as a combination is necessary to make the shampoo more effective.
2. A mix of anionic and non ionic surfactants is used to increase the cleaning power.
Something a little bit special - cocamidopropylbetaine
To help the issue of low cleaning with non ionic surfactants, many sulfate free shampoos contain one specific surfactant - cocamidopropylbetaine (cocobetaine /cocabetaine). This surfactant is special because it is zwitterionic (meaning it has both negative and positive charge). This means in essence it can clean hair a little bit better than non-ionic surfactants due to having a negative charge but at the same time be less irritating than a negatively charged cleanser as it also has a positive charge
Coming up next is a list of surfactants and whether they are anionic, non ionic or zwitterionic (so that you don't have to guess!). I will also have a table of where your shampoo fits in a gentleness index based on the surfactants in it (remember that your own test by using the shampoo is the best test, not just by looking at ingredients).