So many people are using soap-related terminology incorrectly, or in specialized ways that turn words into jargon, that I feel compelled to straighten out the confusion and lay down the law.

If it were up to me, the term bubble bath would be dropped, because it has two meanings. The first is what can be called unambiguously foam bath, a bath with bubbles on top of the water, or a preparation for producing the same. The second is a bath with bubbles in the water, or a preparation or apparatus for producing such, such as effervescent bath salts (as in a bath "bomb") or an air pump. We might call the second generically a "fizz bath", but I don't know if that'll catch on as a catch-all for gas-generating bath fizzies and aerated spa baths. People have been disappointed when a foam-type bubble bath didn't fizz, and therefore didn't even foam because they tried using it by mixing it with still water.

CTFA is the Cosmetics, Toilet Goods, and Fragrances Association, which adopts names for the ingredients of such products. These are usually shorter than IUPAC names, and include substances other than pure chemicals. Unfortunately the CTFA has not helped matters with its nomenclature for ingredients. For example, why do they express alkanolamides redundantly? They shorten lauric diethanolamide (or lauryl diethanolamide) to "lauramide DEA", where the DEA must stand for either diethanolamine or diethanolamide. Why say it twice, guys? Why not just "lauric DEA" or "lauroyl DEA"? (But at least they come nowhere near pharmacologists when it comes to committee craziness in naming things.)

Curd: see soap curd.

Detergent as an adjective means "cleaning", and as a noun means "cleaning agent". (It comes from words meaning "wipe away".) If you use wet sand for cleaning, that's a detergent. Unfortunately many people are using the word improperly to mean things like "surfactant other than soap", even in cases where the material so named is not to be used for cleaning. If it's a cleaner, it's a detergent whether or not it's made of or with soap; if it's not a cleaner, it's not a detergent, regardless of any surfactants it contains.

Be aware that "EO" is commonly used as an abbreviation for "essential oil" by makers of small batch toiletries including soaps, and for "ethylene oxide" or ethoxyl residues of same by makers of other surfactants and polymers. It's usually clear from the context.

Glycerine, also spelled glycerin and known to academics as glycerol, IUPAC name 1,2,3-propanetriol, is a substance in its own right. Please don't use the word "glycerine" to mean glycerine soap (as in the melt-and-pour products), which is a mixture of glycerine and soap. (How much glycerine to how much soap qualifies as "glycerine soap"? No rule about that. And it doesn't have to be transparent, "glass" soap.)

Water "hardness" is its concentration of certain cations (positively charged ions) -- chiefly calcium, secondarily magnesium, in some cases others -- to the extent that they're available to react with anions (negatively charged ions) under conditions of interest. The word derives from the way water with a high concentration of calcium bicarbonate made beans soaked and cooked in it come out hard and crusty with calcium carbonate, and I tend to put it in quotation marks because taken literally "hard water" suggests ice. Another problem is that when one speaks of water's being "hard" or "soft", that can be in a relative sense (i.e. compared to other waters), or absolutely (i.e. whether it has any significant concentration of "hardness" minerals); and of course "water" here refers not to pure water, but water as supplied. A major effect of water "hardness" of interest here is its production of lime soap (q.v.)

IUPAC is the International Union for Pure and Applied Chemistry, who promulgate rules for naming any pure chemical. However, many substances have names other than those derived from the IUPAC rules. See CTFA.

Lime is oxidized calcium. Lime soap can refer to a soap scented with the fruit oil, but usually refers to calcium and magnesium soaps formed from soluble soaps and water "hardness". Lime soap, being insoluble in water, forms a curd (q.v.) which leaves bathtub ring at the water line with trapped water and dirt, and plates out as a scum or film on surfaces. Specific water-insoluble soaps in dry form have various uses in lubrication, water-proofing, and anti-caking applications.

Pulmonary surfactant is a surfactant (see below) produced in lungs to keep them from collapsing. It has very special properties regarding surface tension. Lack of it in premature newborns is a cause of respiratory distress syndrome, or hyaline membrane disease.

Chemically, a soap is a salt of a fatty acid. It need not have any cleaning properties: for example sodium acetate, or water-insoluble soaps (see lime soap). In commerce, "soap" also means detergent products consisting chiefly of soap, as in your ordinary cake of soap. Colloquially the word "soap" is often used imprecisely to refer to products having the characteristics we ordinarily associate with soap, even if they contain no soap. (It's hard to beat a 1- syllable 4-letter word like that when you don't need to be precise, so even I say it.) The U.S. Food & Drug Admin. has a restrictive regulatory definition of soap, but they have contradicted their own logic in formulating that definition by allowing products to be labeled as soap according to the colloquial consumer usage. Interested in soapmaking?

Soap curd is precipitated soap entrapping water and air, as when soap is salted out during manufacture, or lime soap (q.v.) forms when soap lather reacts with water "hardness".

"Soft water": see "water hardness".

Sometimes stearic acid refers to pure octadecanoic acid, but often to various commercial mixtures consisting chiefly of stearic and palmitic (octadecanoic and hexadecanoic) fatty acids. So if you're referring to the chemical species octadecanoic acid, best do it by that name; otherwise be aware of what grade of "stearic acid" is meant, if it might make a difference.

Use of the term stearin has led many astray. It should only be used to refer to the triglyceride (q.v.) fat, tristearin, but it seems as though roughly a century ago some started writing "stearin" or "stearine" to refer to the fatty acid stearic acid. Given that confusion, it'd be best if people used only the terms stearic acid and tristearin.

A surface-active agent (link to Surfactants Virtual Library) is one which when dissolved acts preferentially at surfaces of the solution. This characteristic may give such agents one or more of these properties in solution: detergency (cleaning ability), defoaming (ability to break bubbles), emulsifying (ability to suspend an insoluble material in a liquid), film forming (ability to form bubbles), reduction of surface tension, and wetting. A common example of a surface-active agent is the soap in your usual bars.

Surfactant (link to Surfactants Virtual Library) is an abbreviation for surface-active agent. However in so many medical contexts is "surfactant" presumed to mean pulmonary surfactant that the National Library of Medicine spells out "surface-active agent" for the more general meaning.

Syndet is a word that has slowly been going out of fashion as an abbreviation for "synthetic detergent". It refers to detergents other than soap, or not based on soap. Unfortunately soap is synthesized too, so the term is a misnomer. It would be far preferable for people to say simply "non-soap detergent". (Could've been worse: early on, the term "soapless soap" was put forward.)

Triglyceride is the common term for fatty acid tri-ester of glycerine (q.v.), although some snooty biochemists would rather we call triglyerides triacylglycerols.

to Robert Goodman's Painless Lather Bath, MarshWallow
to Robert Goodman's index page