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Wikipedia's Soap gives sodium stearate as an example of soap, and apparently I've been eating it:

Sodium stearate is the sodium salt of stearic acid. This white solid is the most common soap. It is found in many types of solid deodorants, rubbers, latex paints, and inks. It is also a component of some food additives and food flavorings.

What would be the smallest or simplest molecule that we could reasonably call a soap? Perhaps a functional definition would be that it could perform some of the functions of soap in the same way that soap does.

In Why does bleach feel slippery? and its follow-up Is it known for sure that bases feel slippery because of the production of soap/surfactant? the saponification of other existing molecules is discussed, and I'm not looking for that here.

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It boils down to the definition of soap. Wikipedia defines a soap as the salt of a fatty acid. IUPAC claims the smallest fatty acid can be considered to have 4 carbons. Therefore the simplest soap molecule would be a (generally sodium) salt of butyric (butanoic) acid, i.e. sodium butyrate.

Now apart from the chemical definition, a soap must adhere to its function in order to be defined as such. Therefore, before calling something a soap, we would have to know if the substance does in fact act as a surfactant in a oil-water surface (reducing the interfacial tension) - but that will depend on the nature of the oil in question and the purity of the water. Only for a well defined system we can then conclude that such molecular salt is acting as a soap in it.

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    $\begingroup$ There is another dimension to Vinicius' answer. I doubt there is a real yes or no answer to if a compound is a surfactant. It is more a matter of how good of a surfactant a particular compound is. Since vinegar is often used as a cleaning solution, I'd guess that sodium butyrate would remove a finger print off glass very well. $\endgroup$
    – MaxW
    Commented Nov 14, 2018 at 8:47
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    $\begingroup$ @uhoh: For practical purposes, I'd say the biggest issue with using sodium butyrate as a cleaning agent would be its rather nasty rancid smell. Basically, imagine something like the concentrated essence of stinky cheese, gym socks and vomit. $\endgroup$
    – Ilmari Karonen
    Commented Nov 14, 2018 at 13:45
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    $\begingroup$ @MaxW But is it the surfactant qualities of vinegar which are being used in cleaning, or is it some other property? (e.g. the acidity) -- I wonder if sodium acetate would work as well. $\endgroup$
    – R.M.
    Commented Nov 14, 2018 at 16:02
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    $\begingroup$ @uhoh When I started to write the new question, of what is the shortest fatty-acid molecule, I found that this question had been asked as part of another question on this site. Unfortunately the only answer doesn't actually go further than my comment above. Perhaps the answer is that there is no actual bound other than the obvious 1-carbon limit, but that it is rather arbitrary what counts as a 'proper' fatty acid. I suggest answering or commenting on that question if anyone has any views. $\endgroup$
    – David Robinson
    Commented Nov 15, 2018 at 0:09
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    $\begingroup$ @A.K. - Understand but a "large" change in interface surface would indicate a surfactant, but a "small" change wouldn't. The problem is that the difference between "large" and "small" is fuzzy. $\endgroup$
    – MaxW
    Commented Nov 15, 2018 at 4:24
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While I agree that the definition matters I disagree with the definition of soap as "as the salt of a fatty acid". For one SDBS, AOT, SDS, Cocamide DEA (not even a salt) and CTAB are all popular soaps that are not fatty acid salts. I also do not think that soap and surfactant are interchangeable words. Surfactants lower surface tension, soaps create micelles either by them selves or with oils. Essentially all soaps are surfactants but not all surfacants are soaps. For example, ethanol may be considered a surfactant as it does reduce the surface tension of water, but it does not help form micelles in water appreciably and thus should not be considered a soap.

What would be the smallest or simplest molecule that we could reasonably call a soap?

This is somewhat a matter of estimate, but given that propionic acid is not micible with water and acetic acid is, I would assert that sodium (or other alkali metal) propionate is the simplest soap since it has an aliphatic group to interact with an oil phase, yet has carboxylate salt "head" to retain interaction with the water phase. It is possible sodium acetate may do so as well but empirical data is needed. I suppose technically lithium salts would be the smallest since the lithium ion is smaller than the sodium ion, but that is perhaps too technical and may not interact with water as strongly as sodium salts.

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  • $\begingroup$ Ýou can distinguish between detergents and soaps, the latter being of natural origin, although many if not most people will use the words as synonyms. $\endgroup$
    – Buck Thorn ♦
    Commented Mar 22, 2019 at 9:49
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As always, there is no sharp border and no minimal molecule. You can make the head piece more charged, which means the molecule becomes more hydrophillic and water soluble and you can make the tail piece shorter, so it will not bind fats very well anymore and fails to form micelles at some point.

Consequently, the question makes no sense, because the issue is a task-specific trade-of and the term soap is too broad. It is not even clear, whether you want a detergent for water or a e.g. fluorinated oil.

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  • $\begingroup$ "the question makes no sense" Apparently it makes some sense to several people. $\endgroup$
    – uhoh
    Commented Nov 15, 2018 at 10:03
  • $\begingroup$ No it does not make any sense, because of a too drastic oversimplification. The correct question would be, "what influences the property of a "soap" in a certain solvent". $\endgroup$
    – dgrat
    Commented Nov 15, 2018 at 10:07

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