How to Spot a Psychopath

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?

Jun

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.