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Soap is made by a saponification reaction, where a fat reacts with hydroxide ions to form a surfactant and glycerol.

To make a solid soap $\ce{NaOH}$ is used, while $\ce{KOH}$ is used for liquid soaps.

I don't understand why the alkali metal has such a great impact on the state of matter. Usually the argumentation is based on intramolecular interactions, such as Van-der-Waals forces or hydrogen bonds, but if the same fat is used once with $\ce{NaOH}$ and once with $\ce{KOH}$ the resulting surfactants are basically the same, so the interactions shouldn't differ too much.

The only reason I could think of is the size of the alkali metal. Potassium has an atomic radius of 231 pm which is quite a bit more than the radius of sodium, being 186 pm. But why the atomic radius should have an impact on the state of matter of the soap is still unclear to me. Maybe I'm also completely wrong with this assumption.

A while ago, this question was already asked here on this forum, but I wonder if the given explanation is the only reason for the different state of matter. It's absolutely true, that the reactivity of the alkali metals increases from top to bottom, but can this solely explain the phenomenon? @rch provides the solubility of $\ce{NaOH}$ and $\ce{KOH}$ to back up his answer, but I don't think that this is sufficient. You cannot simply change the hydroxide ion by a fatty acid and assume, that there are no substantial amendments in the reaction behavior, can you?

Does anyone can explain this in more detail?

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    $\begingroup$ related chemistry.stackexchange.com/questions/19663/soap-versus-shampoo $\endgroup$ – Mithoron Aug 31 '17 at 20:47
  • $\begingroup$ The answer to that older question is total nonsense. Reactivity of the metal has nothing to do with the physical properties of the finished product. Afaik the reason is mostly that potassium soaps are rather crude mixtures, and further the slightly larger ion seems to not fit well into any viable crystal structure of the carboxylate salt. $\endgroup$ – Karl Aug 31 '17 at 21:41
  • $\begingroup$ @Karl I thought so myself, that's why I reposted the question and am also asking for a better and more detailed answer. Could you explain, what you mean be crude mixture? $\endgroup$ – Sam Aug 31 '17 at 22:07
  • $\begingroup$ Well, most fats are wild mixtures of glycol triesters, so after saponification you have a dozen or so different carboxylate salts in your pot. $\endgroup$ – Karl Aug 31 '17 at 23:01
  • $\begingroup$ @Mithoron Although this is a duplicate of the given question, it would be very useful not to close this question. The thread 'Hard' soap vs 'Soft' soap: Why do they work this way? was already answered and has been marked as solved, even though the given answer is not meaningful. With my question I want to go into the phenomenon more closely. By your mark, other chemists can no longer answer and so the question will not be solved again. $\endgroup$ – Sam Sep 1 '17 at 13:15
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I don't know if anyone is still looking for the answer but here I go anyway because I spent 40 minutes researching this for an assignment

TL;DR - The better solubility of potassium salts is the key factor, but not in the way one would initially suspect. Industrial processes and the efficiency of large-scale soap making explains the choice.

The reason why potassium-based soaps are 'soft' or liquid is to do with the manufacturing process and the difficulty in separating them (R-COO-K) from the glycerin/glycerol after reaction with the soap lye (The alkaline salt solution used to make the soap e.g. KOH). The better solubility of the potassium salts contributes to this significantly.

Glycerin/glycerol has a lot of good properties for making a liquid soap too. It's colourless, viscous, non-toxic to humans and even has some antiviral and antimicrobial properties. It makes sense to utilise these and make the soap liquid instead of trying to get it out. Also it is often cost ineffective to dissolve out the glycerin with large amounts of expensive KCl or other K salt. Comparing this to sodium-based soaps, it takes smaller amounts of cheaper Na salts to separate the by-products.

It all comes down to money I guess.

Here are my sources, read them if you want to know more:

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I can't answer your question. I thought I could, but once I looked into it, I couldn't find the data to support it. That said, consider Na & K stearates (C18). BOTH have mp's well above 200°C. That is, neither of them are liquids. I believe these are reasonable models for fatty acid salts relevant to your question. Once you accept (tentatively) the idea that one isn't a liquid and the other a solid at room temperature, you're left trying to understand why the saponification reactions (apparently) result in products with different physical states (I'd have to see a bit more rigorous a study than anecdotal testimony, no offense.) I am unable to do more than speculate on this question. The reason which most suggests itself to me is that the degree of reaction (completion) is less with KOH than with NaOH. That is, the (anhydrous) products are less pure and so have less tendency to crystallize. (I suspect side-reactions aren't important, but I could be wrong.) The other "reasons" I can imagine (larger propensity of the K salt to hold onto water, less propensity of the K salt to continue to hydrolyze due to the lower level of KOH in the product (leading to a soap which is more stable (ages better)), etc.) all seem a bit far out. Finally, it is possible that the difference in size of the two ions results in the product with K+ to be substantially less ordered (more disordered) than with Na+. Believe it or not, it is unusual to have C12 -C18 fatty acids which are pure, the far most common situation is that there are distributions of both C number as well as saturation and branching. What this means as far as your question goes is that the data for the pure compounds is quite difficult to locate (if it has even been measured). (and so, without the "clean" data, everyone can have their pet hypothesis.)

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  • $\begingroup$ Thanks for your answer. The statement is based not only on anecdotal testimony but on experimental findings. I've tried the saponification with coconut fat using $\ce{KOH}$ and $\ce{NaOH}$ and the products were liquid respectively solid. Your objection with the sodium or potassium stearate is interesting, but according to PubChem potassium stearate is liquid. $\endgroup$ – Sam Sep 1 '17 at 13:39
  • $\begingroup$ The type of alkali metal used determines the kind of soap product. Sodium soaps, prepared from sodium hydroxide, are firm, whereas potassium soaps, derived from potassium hydroxide, are softer or often liquid. That's what Wikipedia says, but unfortunately there is not given any source to back up the statement. I can remember having read this in a textbook, but I don't know in which anymore. $\endgroup$ – Sam Sep 1 '17 at 13:40
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I think we are thinking too chemically, i.e., too basically. From my (limited) experience, soft soaps contain more water. The correspondence of viscosity or some other measure of strength or hardness with water content is not exactly identical for the sodium and potassium soaps, but it is fairly close. If you make a potassium soap with low water content, it will be "hard"; if you make a sodium soap with more water it will be soft. All of the variables mentioned in previous answers are important, but there is (at least) one more...

The stability of the soaps is a different matter: a sodium or potassium soap with low water can be manufactured to be hard, but the potassium soap will be more expensive, and apparently potassium confers no great advantage. Hard soaps are stable.

However, a soft soap made can be made over a wide range of compositions and viscosities reproducibly with potassium, and will be stable and will not separate on storage. A soft soap made from sodium has much less stability, or a smaller stability range, and thus is more limited in what can be manufactured easily. Considering that soft soaps tend to be higher cost (cosmetic) soaps with higher cost (fragrances, colors, special feel, etc.), the extra potassium expense is a minimal burden.

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