There's nothing like a nice caustic soda facial scrub

A reader writes:

What's the chemical difference between soap and detergent? I know soap's made by reacting an oily substance and an alkali; is detergent made some other way?


Soap is, as you say, a class of substances with a clear chemical definition; a soap is a salt of a fatty acid. Soap molecules, in brief, are hydrophilic on one end and lipophilic on the other, and so allow oils to disperse in water.

Detergents are less clearly defined. One old definition of "detergent" says it's just any substance you use to clean something. By that definition, plain water is the most common detergent, and soap is detergent too. Sand counts as a detergent by this definition, because you can scour pots with it.

The standard home-and-garden definition of a detergent is, of course, much more limited; it covers dishwashing liquid, dishwasher powder, laundry liquid and powder, and so on. Any consumable cleaning substance that doesn't come in a solid bar, or in a pump-pack with "Soap" written on it somewhere.

These detergents contain one or more surfactants that do something similar to what soap does - often the ubiquitous sodium lauryl and/or laureth sulfate, which are not actually incredibly good at cleaning, but are very good at making the rich bubbly lather that consumers have been trained to equate with cleaning power. On top of that, there's "builders" that make the surfactant work better, and various other substances suited to the application. Oh, and in the case of liquid detergents, there's also quite a lot of water, which I've heard referred to by a formulator of shampoo and conditioner as "profitrol".

There are lots of substances that, like plain water, can fill the role of a detergent without containing surfactants. Sodium carbonate or bicarbonate, for instance, can be used as a fairly effective remover of oily substances all by themselves, essentially because they're alkaline and make a little of their own soap from whatever oils are on the thing to be cleaned. Sodium carbonate and bicarbonate are, however, not alkaline enough to be dangerous (unlike the standard soap-making alkali, sodium hydroxide), and also quite non-toxic, so you can use them straight as dishwasher powder with decent results if you're out of the detergent-y stuff.

(Note that some non-standard cleaning products actually have little or no effect at all. The classic example is the magical plastic laundry ball, which doesn't actually do anything, but may appear to work without any other detergent because plain detergent-less water will clean your clothes to some extent all by itself. See also, stone soup.)

Psycho Science is a regular feature here. Ask me your science questions, and I'll answer them. Probably.

And then commenters will, I hope, correct at least the most obvious flaws in my answer.

4 Responses to “There's nothing like a nice caustic soda facial scrub”

  1. Anon Says:

    I've found acetone to work really work for cleaning beakers and the like (or at least it tended to work where DHMO failed).

    Though detergent pretty much does mean surfactant (unless you want to call DHMO and acetone detergents which I don't think anyone does).

    According to my first year chemistry notes the difference between soaps and detergents is that detergents come from the petrochemical industry while soaps are natural but have less cleaning power (and form insoluble Ca and Mg salts whereas the Ca and Mg salts in detergents are soluble).

  2. NathanM Says:

    Acetone will work well when you're looking to remove organic substances. Instead of using water and a detergent to make the water better able to solubilise the organics, instead you're cutting the middle man and going straight to an organic solvent. Just make sure not to dump Acetone down the sink in any sizable quantity.

    I recall viewing an article with comparable efficacy of a range of detergents usable on human skin, and SLS rated amongst the best. SLES was less effective and so on. I'll have a look and see if I can find something to support this.

  3. Mohonri Says:

    This reminds me of a link Dan put in one of his articles a couple years back:
    Things I won't work with - written by a chemist who is just as entertaining in his description of lively chemicals as Dan is in his description of...well, whatever.

    BTW, Dan, have you found a suitable replacement for Tribes 2? Have you tried out the new Tribes? Does it allow for the same kinds of nefarious fun you had before?

  4. Anon Says:

    Yeah, I was using the acetone to deal with organics (and dumping the small amounts I used (didn't usually need much) down the sink of a chemistry lab, at least no one told me not to).

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