Tonya Mckay of NaturallyCurly writes;

Many people with naturally curly hair are practitioners of shampoo free hair care routines. This is an abbreviated term for a regimen that eliminates or reduces the use of traditional shampoos for hair cleansing. While many curlies have known for years that shampooing too often can be detrimental to our fragile hair, the idea of drastically reducing the frequency of shampooing or eliminating it altogether became popularized with the publication of Lorraine Massey’s book, “Curly Girl”.

The reason Massey advises skipping the shampoo is that curly hair is already fairly moisture-deprived due to its unique shape and structure. If hair is not especially oily (which we know our curly hair usually is not), traditional shampoo can strip needed oil and moisture away from the hair and raise the cuticle of the hair making the surface very rough, which leads to tangling and breakage. The primary ingredients responsible for the removal of oils from the hair are known as surfactants.

Surfactants possess the trait of having one distinct portion of the molecule that is polar and hydrophilic (water-loving) and one portion that is non-polar and hydrophobic (water-fearing). This dual nature is the basis for detergency—the removal of oil from a surface. At sufficiently high concentrations in water, surfactant molecules group together to form three-dimensional structures known as “micelles”. These structures are clusters of molecules with an oily center made up of the non-polar tail, surrounded by a shell formed by the polar portion of the molecule. These micelles absorb oils from your skin, hair or clothes, and trap them inside until they are removed from the surface by the rinsing phase of the process. Another very important property of surfactants is their ability to produce significant foaming effects, an attribute considered to be desirable by many product developers.

The most commonly used materials for this purpose are called “anionic surfactants”, which have a negatively-charged head group (sulfate, sulfonate, isethionate), with a positively-charged counterion (typically sodium or ammonium). By learning the conventions for naming these surfactants, one can learn to recognize what they are. The accepted cosmetic nomenclature system (INCI) adheres to the following format for naming anionic surfactants: positive counterion name, followed by a term that denotes the structure of the non-polar tail portion, ended by the name of the anionic head group (example: ammonium lauryl sulfate).

Perhaps the harshest anionic surfactant, and also the one most commonly used in shampoos until recently, is sodium lauryl sulfate (SLS). “Lauryl” means 12 carbons in the nonpolar portion of the molecule, and is the shortest length of chain used in most surfactants. This surfactant is extremely efficient at removing oils from the hair, and can lead to dry, brittle hair. Sodium laureth sulfate (SLES) is a modified version of SLS, containing 2-3 ether units in the molecule. This modification reduces the efficiency of the detergency action, decreasing its drying tendencies compared to SLS. Ammonium lauryl sulfate and ammonium laureth sulfate are the same surfactant molecules as SLS and SLES, respectively, simply with a different positively-charged counterion (ammonium vs. sodium). Thus, ALS and ALES can be expected to give very similar results to SLS and SLES in terms of removing oils from the hair.

Some anionic surfactants can provide comparatively gentle cleansing to the hair because they do not remove as many oils and fats. Anything with a carbon count above 12 (in even increments) is considered to be less harsh. Some examples of this are sodium myreth sulfate and sodium C14-16 olefin sulfonate. Also, sodium coco sulfate, derived from coconut oil, contains a mixture of chains containing anywhere from 8-18 carbons. This makes it gentler than SLS. There are also numerous nonionic surfactants, such as sorbitol, decyl glucoside, laureth 4-20, and decyl polyglucose, which contain no positively or negatively-charged groups. These surfactants are considered to be much less drying to the hair.