Soap & Detergent: What is the difference?

Last weekend, I found myself trying to explain EXACTLY what the difference between soap and detergent was. Wow! That was a lot harder to answer than I thought it was. My original thought was that while a soap and detergent may have a bar or liquid appearance, they are different due to the ingredients.

I went looking for answers and I began to feel as though I had just opened a can of worms that would have kept many fishermen happy!

To start at the beginning, what is the legal definition of soap? According to the FDA, soap is a product in which most of the nonvolatile matter consists of an alkali salt of fatty acids and whose detergent properties are due to these alkali-fatty acid compounds. This definition was written for the purposes of excluding soap from being regulated as a cosmetic.

To understand this definition a little more, what is an alkali salt of fatty acids? A good example of an alkali would be Potassium or Sodium Hydroxides. These are considered alkaline or basic. To be classified as such, these items must have a pH that is greater than 7. We need these alkalis to convert our oils to soap. In order to create a reaction between the two, the oils we use should be on the other side of the pH scale. This means these are called acidic ingredients. Now I hope the FDA definition of soap makes more sense.

So, what is a detergent? I couldn’t phrase it better than Harold Hopkins in an article published in February 1979 titled as “ALL THAT LATHERS IS NOT SOAP.”

If the bar you use for bathing does not claim to be a soap, it’s probably a synthetic detergent product. The FDA defines a cosmetic as an article intended to be used on the body for cleansing, beautifying, promoting attractiveness, or altering the appearance; thus, a nonsoap product intended for any of these purposes is automatically classified as a cosmetic.

Soaps and synthetic detergent cleansing agents function in water in somewhat the same way; that is, they break down the resistance barrier between the water and the dirt, grime, oil, or other material, allowing it to be wetted and washed away. Soap works well in soft water, but in hard water, which contains a relatively high amount of calcium in solution, the calcium and soap react to form a gummy material called soap scum, which includes dirt and other matter. This gummy stuff is what forms the familiar ring in the bathtub.

The increasing number of synthetic detergent bars on the market is due largely to their more efficient functioning in water, regardless of hardness, and because they don’t form gummy deposits as does soap. There are many types of synthetic detergents, ranging from strong to mild; usually the milder types are used for personal cleansing. Some of the harsher detergents are capable of causing eye irritation or injury and manufacturers normally avoid using these in personal bathing bars. There are consumers who may experience irritation or allergic skin reactions from some synthetic detergents. Some consumers also may be allergic to fragrances, colors, or other substances added to either soaps or synthetic detergent bars.

Thank you Mr. Hopkins. You saved me from trying to rack my brains for a way to re-write this.

I also found another article published by the FDA in July of 2002 titled Is It a Cosmetic, a Drug, or Both? (Or Is It Soap?). This article is another great resource for information about soap.

Soap is a category that needs special explanation. That’s because the regulatory definition of “soap” is different from the way in which people commonly use the word. Products that meet the definition of “soap” are exempt from the provisions of the FD&C Act because — even though Section 201(i)(1) of the act includes “articles…for cleansing” in the definition of a cosmetic — Section 201(i)(2) excludes soap from the definition of a cosmetic.
How FDA defines “soap”

Not every product marketed as soap meets FDA’s definition of the term. FDA interprets the term “soap” to apply only when –

  • The bulk of the nonvolatile matter in the product consists of an alkali salt of fatty acids and the product’s detergent properties are due to the alkali-fatty acid compounds, and
  • The product is labeled, sold, and represented solely as soap [21 CFR 701.20].

If a cleanser does not meet all of these criteria…

If a product intended to cleanse the human body does not meet all the criteria for soap, as listed above, it is either a cosmetic or a drug. For example:

If a product –

  • consists of detergents or
  • primarily of alkali salts of fatty acids and
  • is intended not only for cleansing but also for other cosmetic uses, such as beautifying or moisturizing,

it is regulated as a cosmetic.

If a product –

  • consists of detergents or
  • primarily of alkali salts of fatty acids and
  • is intended not only for cleansing but also to cure, treat, or prevent disease or to affect the structure or any function of the human body,

it is regulated as a drug.

If a product –

  • is intended solely for cleansing the human body and
  • has the characteristics consumers generally associate with soap,
  • does not consist primarily of alkali salts of fatty acids,

it may be identified in labeling as soap, but it is regulated as a cosmetic.

Now before you tell me that I “broke your brain”, let’s look at a product that can be found in the grocery store and is a combination of soap and detergent. Let’s take a look at the ingredients listing for a Dove White Moisturizing Beauty Bar.



Sodium Cocoyl Isethionate, Stearic Acid, Sodium Tallowate, Water, Sodium Isethionate, Coconut Acid, Sodium Stearate, Cocamidopropyl Betaine, Sodium Cocoate or Sodium Palm Kernelate, Fragrance, Sodium Chloride, Titanium Dioxide, Tetrasodium EDTA, Trisodium Etidronate.

Ingredient with INCI Name: About the Ingredient: Purpose of Ingredient:
Sodium Cocoyl Isethionate Sodium Cocoyl Isethionate is an ingredient derived from coconut oil. It is usually found in the following forms; noodles, flakes, prills or even powder. This product is used as a surfactant*.
Stearic Acid Stearic Acid is an ingredient that is derived from either animal tallow or various vegetable fats This product is used as a secondary emulsifier, emollient and lubricant.
Sodium Tallowate Sodium Tallowate is the salts of the fatty acids of Tallow. It is a soap and is used in the formulation of bath soaps and detergents. This product is used as a surfactant*.
Water Water is primarily used as a solvent in cosmetics and personal care products in which it dissolves many of the ingredients that impart skin benefits, such as conditioning agents and cleansing agents. Water is a solvent that is used to combine all the ingredients together.
Sodium Isethionate Sodium Isethionate is a synthetic detergent. Technically it contains the following: Ethanesulfonic acid, 2-Hydroxy-, Monosodium Salt. This product is used as a surfactant*.
Coconut Acid This is simply a derivative of Coconut Oil. This product is used as a surfactant*.
Sodium Stearate Sodium Stearate is the salts of the fatty acids of Stearic Acid. It is a soap and is used in the formulation of bath soaps and detergents. This product is used as a emulsion stabilizer and opacifying agent.
Cocamidopropyl Betaine Cocamidopropyl Betaine is derived from coconut oil and dimethylaminopropylamine. It has some antiseptic properties which makes it popular for use in toiletries. This product is used as a surfactant*.
Sodium Cocoate or Sodium Palm Kernelate Sodium Cocoate is the salts of the fatty acids of Coconut Oil. Sodium Palm Kernelate is the salts of the fatty acids of Palm Kernel Oil. Both are soaps and are used in the formulation of bath soaps and detergents. This product is used as a surfactant*.
Fragrance Fragrances are used in a wide variety of products to impart odor or to mask the odor of another ingredient used in the formulation of a product. This product is used as an additive.
Sodium Chloride This is regular table salt. This product is used to precipitate the soap.
Titanium Dioxide Titanium Dioxide is the oxide of titanium This product is used as an opacifying agent and a colorant.
Tetrasodium EDTA Tetrasodium Ethylenediamine Tetraacetic Acid is a salt of ethylenediamine tetraacetic acid (Edetic Acid). Tetrasodium EDTA has the ability to bind with and inactivate metallic ions to prevent their adverse effects on the stability or appearance of cosmetic products. It was originally developed to counteract the effects of hard water and heavy metal ions in the manufacture of textiles. This product is used as a sequestering agent, has the ability to bind with and inactivate metallic ions to prevent their adverse effects on the stability or appearance of cosmetic products.
Trisodium Etidronate Trisodium Etidronate is a double duty ingredient, it works as a water softener as well as a preservative This product is used as a preservative and water softener.

*What is a surfactant? A surfactant is a cleansing agent that are used for either cleaning the skin and/or hair as well as emulsifiers in products. So to put it basically, surfactants make water wetter, emulsify or solubulize body oils and suspend dirt and grime. Most surfactants also add foaming and lathering characteristics to products containing them.

Wow! I don’t know about you, but my brain is starting to hurt a little. ;) I do understand more about the difference between soaps and detergents. Do you?

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5 Comments

  • CanfieldFive says:

    Thank you so much for posting this! I understood the basic differences between soap and detergent, but now I feel much more confident that I could explain it to someone else.

    I enjoyed the breakdown of the ingredients of the Dove bar- I had been planning to do research on just that (And with the ingredients of a dove bar, too! *lol*) but now I don’t have to! :-)

    I did have one question- which ingredients make the “1/4 moisturizing cream” in a Dove bar? I would love to be able to duplicate the creaminess that Dove bars are known for, but in a bar of real soap. :-)

    Thanks again!

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  • Jackson Maru says:

    Great article. You did a good job explaining the difference between the two.

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  • Keith Nealy says:

    So I take it that the Dove “soap” is a combination and is regulated as a cosmetic?

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  • Keith Nealy says:

    BTW, the interesting thing about soap is that the molecule is hydrophilic at one end and hydrophobic at the other. One end binds to grease while the other to water, thus lifting off the oils and washing away in the rinse. A remarkable molecule.

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