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Not big on chemistry so this question might be a bit out there. It is my understanding that a soap molecule is made up of a free fatty acid and lets say a sodium ion. Correct? Would it be possible to modify the free fatty acid (with a solid or liquid substance) in such a way that when it comes in contact with the sodium ion it wouldn't be able to form a soap molecule?

Thanks

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  • $\begingroup$ What really matters is the counterion that the sodium is associated with. If the counterion is something (like hydroxide OH-) that will remove the acid proton from the free fatty acid then you get the sodium salt of the fatty acid formed - this is the soap. If the counterion is something that does not remove the acid proton (chloride for instance) then you don't get the soap. $\endgroup$
    – Waylander
    Commented Jan 17, 2019 at 21:45
  • $\begingroup$ That's not what is called saponification. $\endgroup$
    – Ivan Neretin
    Commented Jan 17, 2019 at 22:28
  • $\begingroup$ Waylander are you saying if chloride is added to a free fatty acid it won't form a soap molecule even if later on both come in contact with sodium (Na+)? $\endgroup$
    – okonolapota
    Commented Jan 17, 2019 at 23:00
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    $\begingroup$ @okonolapota you have to have a base (an anion that grabs protons) to get the salt, if there is no base present then all you have is a mixture of free acid plus salt $\endgroup$
    – Waylander
    Commented Jan 18, 2019 at 8:50
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    $\begingroup$ You can do a lot of chemistry with the fatty acid. But to what purpose? You only say what you don't want. $\endgroup$
    – Karl
    Commented Jan 18, 2019 at 10:56

2 Answers 2

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In principle You can replace the Sodium ion with any other cation. But all You get is another kind of soap: Potassium gives a smoother substance, Calcium/magnesium gives the soap-stone you see in washing machines, etc.

Another possibility is to esterify the fatty acid with an alcohol. This is widely used in the industry to produce non-ionic surfactants, and these do not react with cations. But off course, then it really isn’t a fatty acid anymore.

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  • $\begingroup$ That's what I'm looking for, something that wouldn't be a fatty acid anymore and thus wouldn't be able to become a soap molecule. Is there anything else besides alcohol? What about precipitating the fatty acid? I read that free fatty acids can be precipitated as alkali soaps. What does this mean? $\endgroup$
    – okonolapota
    Commented Jan 18, 2019 at 18:28
  • $\begingroup$ What about xylitol? $\endgroup$
    – okonolapota
    Commented Jan 19, 2019 at 0:07
  • $\begingroup$ In chemistry an alcohol is basicly any organic molecule that contains an -OH group: ethanol, isopropanol etc, but also xylitol, glucose and glycerol are alcohols. There is a group of detergents called Alkyl plyglycosides (APGs) that are esters of fatty acids and (poly) glycose-units. $\endgroup$
    – FrankS
    Commented Jan 20, 2019 at 9:04
  • $\begingroup$ So basically by adding an alcohol to a fatty acid you get a surface active ingredient? $\endgroup$
    – okonolapota
    Commented Jan 20, 2019 at 10:09
  • $\begingroup$ Very basically: Yes. But in reality it is more complicated. With simple alcohols just mixing it could yield a significant amount of ester (Fisher reaction), A catalyst, such as a strong (sulfuric)acid, is however needed as well as elevated temperature. If water is present (and water is formed during the reaction) it will hydrolyze the ester and thus reducing the yield. $\endgroup$
    – FrankS
    Commented Jan 21, 2019 at 12:49
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Saponification is the process of creating the free fatty acids from the triglycerides that carry fatty acids around in the body. Triglycerides are glycerol esters of fatty acids and are the major constituent of bodily fat in animals.

Soap is created by breaking down the triglyceride esters releasing the free fatty acids and is usually accomplished by reacting the fat with sodium or potassium hydroxide. The resulting compounds–the sodium or potassium salts of the free fatty acids–are what we call soap.

Soaps act as surfactants which can solubilise dirt and fat in water. Their primary action as surfactants results from the long chain fatty acids having a polar end that likes water and a non-polar end that likes the fatty and water-insoluble elements in the dirt. It is the fatty acid not the counter-ion that dominates this effect. Swapping the counter-ion (sodium or potassium, etc.) for something else make a small difference to the character of the soap but not a large difference to the surfactant action as that is dominated by the free fatty acid.

So swapping the counterion isn't the point of saponification: the point is to get the salt of the fatty acid free from the triglyceride. And the resulting salts are ionic compounds where the counter ion of the acid salt is not hugely important to the function as a soap.

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  • $\begingroup$ Thanks for explaining what is saponification but it did not help with my question. $\endgroup$
    – okonolapota
    Commented Jan 18, 2019 at 18:34
  • $\begingroup$ @okonolapota Given what saponification actually is your question "Is it possible to prevent saponification by modifying free fatty acids?" doesn't really make any sense. Free fatty acid salts are soaps. Esters of free fatty acids are fats not soaps. Changing the metal counter ions only makes a marginal difference to their soapiness. Different fatty acids make different soaps. Some non fatty acid compounds make good surfactants. The fats that fatty acids are derived from are not soaps. What is the question? $\endgroup$
    – matt_black
    Commented Jan 18, 2019 at 18:42
  • $\begingroup$ I said that the question "may be a little out there" implying that what I asked wasn't even possible. Still got a few interesting answers. If you have a FFA without the sodium/potassium ion it isn't a soap it is just a FFA right? Let's say you have a situation where a free fatty acid which will eventually bind with a sodium ion - this is something you cannot control, it just will happen. My question was if it were possible to modify the FFA with a substance (one answer was alcohol) so that when it eventually does comes into contact with the sodium ion it won't be able to form soap. $\endgroup$
    – okonolapota
    Commented Jan 18, 2019 at 20:52

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