# Reasons for solid or liquid soap

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?

• – Mithoron Aug 31 '17 at 20:47
• 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. – Karl Aug 31 '17 at 21:41
• @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? – Sam Aug 31 '17 at 22:07
• Well, most fats are wild mixtures of glycol triesters, so after saponification you have a dozen or so different carboxylate salts in your pot. – Karl Aug 31 '17 at 23:01
• @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. – Sam Sep 1 '17 at 13:15

• 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. – Sam Sep 1 '17 at 13:39