Learning Centre

Soap vs Detergent - What's the deal?

Posted Tuesday, September 05, 2017, Author - Nathan Smith

Often when we read descriptions of pet shampoos in catalogues we see the assertion “soap-free”. Sometimes we see, “no detergents.” What do these mean?

Before we get to the answer, let’s take a moment to look at these statements as marketing tools or techniques. A marketing technique is a device of words or images intended to produce a favorable impression and a desire to buy. The statement “soap-free” is intended to convey that soap is a bad ingredient and this product has none.


What’s so bad about soap? Soap is the simplest surfactant; it is formed by the action of caustic ash or lye on fats. The first soap was probably accidentally formed when a cave person tossed some animal fat into the fire and when things cooled down they found...soap. Today’s soaps are created by the action of a strong alkali, usually sodium hydroxide (lye) on fats or oils.

—This basic chemical reaction is called saponification.

Oils + Lye = Salt (soap) + Glycerin

You will see soaps listing their ingredients as “coconut oil”, “palm oil” or “olive oil”, etc. Technically, this is not accurate. Even the description “saponified oils of coconut, palm and olive oils” is not the INCI identification(International Nomenclature of Cosmetic Ingredients). The accurate names would be Sodium Cocoate, Sodium Palmate, and Sodium Olivate for soaps from these oils. Sodium Tallowate, or soap from beef tallow is still a common ingredient in commercial soaps.

Soap has served humans for hundreds, even thousands, of years. They are good cleaners and degreasers. Soaps are cheap to make and are manufactured from renewable resources. They are biodegradable and don’t pollute rivers.

Sounds good so far, right? But here’s the rub (soap joke): soaps have a fatal flaw. Soap has a chemical affinity to the minerals in water and soil, and will combine to form a hard and sticky precipitate known as soap “curd” or “soap scum”. This is especially troublesome for hair, as it does not rinse off and will build up over time. It leaves a troublesome dull film on the hair. Soap scum also clogs plumbing.

There’s more: Soap has a highly alkaline pH of 9.5-10. This is in the same range as chemical straighteners and permanent wave solutions. This makes soap very harsh on hair, raising the cuticle and leaving it open and susceptible to moisture loss. This can be very problematic for fine or thin hair, as well as porous hair. Porosity can cause cuticle damage and moisture loss from the inner cortex. To close the hair cuticle after shampooing with soap, using an acidifying rinse is recommended. This can be as simple as vinegar, ¼ cup in a pitcher of water, or and acidifying rinse, such as ShowSeason Results Rinse, that will also improve combability and decrease drying time.

Hair shaft with open, porous cuticle

Figure 1 Hair shaft with open, porous cuticle

Pictured above is a hair shaft with an open, rough cuticle, such as might be expected by the use of soap on hair without a pH correction.


OK, so much for soap. What is a detergent? Technically, a detergent is any surface acting agent (surfactant) that cleans. That means that in scientific terms, soap IS a detergent. But for purposes of clarification, most chemists distinguish between

soaps and detergents, in that if it cleans and it is not soap.... ta da.... it is a detergent.

DETERGENTS can be derived from some of the same fatty acids as soaps, such as coconut or palm oil. They undergo a more complex synthesizing process, called sulfation and sulfonation. Chemically, detergents are often sulfates or sulfonate salts. Sodium Lauryl Sulfate is a prime example, found in hundreds of cleansing and personal care products. Fear mongers who make claims about it causing scary things from birth defects to cancer have crucified Sodium Lauryl Sulfate (SLS) on the Internet. These claims have no scientific base.

The truth is SLS is quite harsh and aggressive; it is an excellent cleanser degreaser, but it is also responsible for cases of irritation and even eczema, and it can damage eye tissue. It is our association of detergents as harsh chemical ingredients that leads the marketing people to utilize the concept of “no detergents.” There are, however, many less harsh and very mild detergents available to shampoo formulators. Moreover, the formulation of pet shampoos has evolved so that products that utilize SLS, or Ammonium Lauryl Sulfate or TEA Lauryl Sulfate combine these more aggressive surfactants with co-surfactants such as Lauramide MEA, Cocomide DEA, and Cocomidopropyl Betaine that lessen the irritation potential of the strong cleanser without sacrificing cleaning ability.

Ethoxylated Detergents

Some surfactants undergo an additional process, called “ethoxylation” where they are treated with ethylene oxide to form larger molecules. This makes the detergent less harsh without interfering with the cleaning ability. Ethoxylated surfactants can be recognized by their names. They contain the suffix “…eth”, as in Sodium Laureth Sulfate, or “PEG” as in PEG-80 Sorbitan Laurate. Some manufacturers use these ethoxylated detergents as less harsh alternatives to the lauryl sulfates, such as Sodium Lauryl Sulfate or Ammonium Lauryl Sulfate. The ethoxylates are less likely to cause shampoo irritation.

The cosmetic fear mongers have a heyday with ethoxylated surfactants. During the process of ethoxylation, a potent carcinogen, 1,4 dioxane is released. It is a very undesirable by-product of a beneficial process. Early in the history of ethoxylated ingredients, it was discovered that many final products were contaminated with 1.4 dioxane at significant levels. YIKES, shampoos that can cause cancer!!! This prompted a fear campaign that has not let up, even though most chemical suppliers have instituted rigorous vacuum stripping techniques to remove the 1.4-dioxane before the ingredient is distributed to shampoo manufacturers.

Another distinction amongst detergents is between “synthetic” and “naturally derived” detergent surfactants. Surfactants can be made from petrochemicals or oleo chemicals (oils, fats). For marketing purposes, those derived from oleo chemicals have been labeled “natural” and those from petroleum as “synthetic”. The truth is that both are treated chemically to synthesize into surfactant molecules and the difference in the resulting carbon chains derived from petro or oleo chemicals is very slight. They are all synthetic.

A product that is labeled “no detergent” might be a soap-based shampoo, in which case it would leave a residue, or more likely, the company is redefining “detergent” to suit their own marketing. Most often they mean “no ethoxylated detergent”. Sometimes they mean “no petroleum derived detergent”. Remember, detergency is a property. It is the ability of a surfactant to clean. If a product has no detergency, it cannot clean. It does no good to argue with a sales rep about the definition of a detergent (been there/done that). The way to get to the bottom of the conversation is to ask, politely, “What is your primary surfactant?” You can expect it to be a chemical compound.

Milder Detergents

There are hundreds of available detergent surfactants for use in personal care and cleaning products. Some formulating chemists will go to great lengths to find the most obscure and least used, in order to allow the product in which it is used to be marketed as containing “no this”, “no that”, “nothing you’ve seen before.” Of course with pet products, it hardly matters, because they don’t have to show us what’s in there anyway. A current trend is away from sulfate surfactants and substitution with detergents that previously were mostly used in ultra mild, baby shampoos, such as Sodium Cocoamphoacetate and Decyl Glucoside. The downside of these very mild detergents is that they do not clean as well, and can’t be diluted nearly as well as the sulfates. They are often formulated with two or three together, sometimes with a little Sodium Laureth Sulfate slipped in to add cleaning power. Remember, less detergency means more mildness but less cleaning ability.

Of the cleaning surfactants available, shampoos must contain either soaps or detergent surfactants.