There are various chemicals used as cleansers. There are various ways for how those cleansers are used. Often times people get confused with all the terminology.
Surfactants and detergents
A surfactant is the short-hand name for any chemical that functions as a "surface active agent." A surfactant allows chemicals to mix together that would not otherwise be able to mix.
Surfactants do many jobs. Some surfactants called wetting agents help water easily combine with finely powdered chemicals, such as garden pesticides. Other surfactants are good at creating lots of foam; still others are good at preventing foam. Some types of surfactants are used as emulsifiers to make products such as hand lotion, water-based paints and adhesives, and thick, creamy foods such as ice cream and mayonnaise.
The surfactants we soap makers are most familiar with are surfactants used for for bathing, washing dishes, washing clothes, and general cleaning. Surfactants that are good cleansers are efficient at helping water to mix with and remove grease, oil, and dirt particles.
A detergent is a surfactant that is used as a cleanser. All detergents are surfactants, but not all surfactants are detergents. The lye-based soap we make is a detergent -- in other words, a surfactant that cleans.
Detergents are divided into several classes, based on the electrical charge the surfactant has that helps it bond with water.
Antionic (an-eye-ON-ick) detergents have a negative charge. This class of detergents includes true soap, sodium lauryl sulfate (SLS), sodium laureth sulfate (SLeS), sodium lauryl sulfoacetate (SLS), sodium cocoyl isethionate (SCI), and sodium C14-C16 Olefin sulfonate (Bioterge).
Cationic (cat-eye-ON-ick) detergents have a positive charge. Examples include
Amphoteric (am-fo-TEH-rick) detergents have a positive or negative charge depending on the pH. A familiar amphoteric surfactant is cocamidopropyl betaine (CAPB) which acts as a cationic detergent in an alkaline mixture, such as soap. These are also called zwitteronic detergents.
Nonionic (non-eye-ON-ick) detergents have no electrical charge. The polysorbates (polysorbate 20, polysorbate 80, etc.) are nonionic detergents.
Soap and synthetic detergents
Soap is made by reacting fats or fatty acids with an alkali (sodium hydroxide for example). The soap we make is a widely used cleanser, but there are other types of soap that cannot be used as cleansers. These insoluble soaps are used as lubricants, thickeners in paint, and other industrial uses and are not much on our normal soapy radar.
The chemical reaction that creates "true soap" is called saponification. Saponification is a process that sometimes occurs in nature, and, in a sense, soap is the only "natural" detergent.
The word "soap" is loosely used by many people to mean any cleanser, whether it is a true soap or a non-soap detergent. But true soap and other non-soap cleansers are not the same, so it is confusing when having a technical discussion to use the word "soap" to mean any type of cleanser.
Non-soap cleansers are cleansers other than true soap. Non-soap cleansers, just like true soap, are surfactants (surface active agents) and more specifically they are detergents (surfactants that clean).
Non-soap cleansers are made using chemical reactions such as esterification and ethoxylation rather than saponification. These chemical reactions are not found in nature, so non-soap cleansers are often called synthetic detergents or "syndets" to distinguish them from true soap.
Shampoo, body wash, cleansing bars, and beauty bars
Shampoo is a cleanser intended for cleaning the hair and scalp. Shampoo can be 100% true soap or 100% syndets or a blend of the two. The diff"shampoo" rather than a cleanser for general bathing can be mainly what it's called on the label, not necessarily what's inside the bottle.
A shampoo is best made with syndets, because the high pH of soap is damaging to hair. It lifts the cuticle (scales on the hair strand) making the strand weaker and more prone to breakage. Since hair can't repair itself like living skin can, this damage is cumulative.
It is true that soap was used to clean hair before syndets were available; the first syndet shampoo was introduced in 1930. Before that time, hair was washed a lot less frequently, so the cumulative damage from using soap was minimized. Even so, an acidic rinse was often recommended to help repair the damage after washing the hair with soap. We wash our hair a lot more frequently nowadays. An acidic rinse only reduces the damage; it doesn't eliminate the damage.
This is the reason why all commercial shampoos are formulated with syndets, not soap. And this is why many small scale makers also make syndet-based shampoos, not soap labeled for washing the hair. Again, as others are pointing out, there are other small scale makers who make soap and label it for cleansing the hair, but there is a real risk that this type of product will damage hair, especially with long-term use. That's why most of us syndet shampoo makers avoid saying "shampoo" when talking about soap used to wash the hair.
Body wash is any liquid cleanser used for cleaning the body. It too can be based on soap or synthetic detergents or perhaps a mix of the two. A body wash could have exactly the same formulation as a shampoo; it's just labeled differently.
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