What is the Difference between Soap and Detergent?

Castellet Soap – Image: Stock Photo Exchange

A difference between soap and detergent? Though both clean what they are designed to clean, they possess a different chemistry and functionality.

Soap is defined as a metallic salt of a fatty acid—the metal often being an alkali metal such sodium or potassium, or an alkaline earth metal, such as calcium or magnesium. A fatty acid is often of plant or animal origin, and contains an aliphatic or straight chain carbon skeleton, with a carboxylic acid group at its end.

Detergents, on the other hand, are generally of synthetic origin. They’re generally not made insoluble by mineralized—so-called hard—water. Instead of a carboxylic acid group, the detergent may contain a sulfate or sulfonate group. In addition, detergents can contain an aromatic ring or rings. Detergents possess additional properties—and may serve as surfactants and foaming agents.

An example of a soap is sodium palmitate: CH₃(CH₂)₁₄-COO⁻ Na⁺

An example of a detergent is sodium lauryl sulfate: CH₃(CH₂)₁₂-OS(O)₂-O⁻ Na⁺

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4 Comments

  • Barbara Reply

    I did not know these facts. Quite fascinating.

  • MegL Reply

    In the UK, most shampoos and shower gels are detergents, as is what we term “washing up liquid”, which I think Americans call “dish soap”. Mt granddaughters love bubble bath (detergent) but it is so hard to get the bubbles to drain after the bath but I have found that washing your hands with a piece of soap in the detergent water helps get rid of the bubbles. They seem to work against each other!

  • Styra Avins Reply

    I wish there were more information here. In my experience, detergent is much better at dissolving fats, without leaving any residue. If you wash your hair with soap, you will find it very difficult to rinse thoroughly. If you have been in poison ivy and need to wash the irritant oil off your skin, detergent is far more effective than even soap made with lye. (East coast of the USA, I have years of experience on this point!). On the other hand, soap is much nicer! I use brown soap for wiping my kitchen sink and surfaces. But my question is still, in what way does detergent work differently from soap? What is the mechanism of the soap molecule vs. the detergent molecule?
    Thank you.

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