Chemistry Question

[Nate5700]

Summon @DeeAnna.

So, I think I'm getting a better understanding of the chemistry involved in soap, but I got to thinking about something yesterday and it got me wondering. So here's how I got there (and please let me know if I'm misunderstanding any of this):

1. Someone said in a thread yesterday that the triglycerides that make up a fat molecule are composed of three fatty acids linked by glycerol. The fatty acids react with the Na+ from the lye and make a salt, which is soap. The (OH)- reacts with the glycerol to make glycerin.

2. Someone told me in a PM that when stearic acid is added directly and saponifies it does not create any glycerin. Presumably because it is composed of pure fatty acid without the glycerol linking the molecules in a triglyceride.

So, here's the question. When you make a soap using stearic acid, what happens to all the (OH)- that doesn't have any glycerol to react with? Do they just float around in the soap? Presumably that would raise the pH of the soap, but it seems counter intuitive to me that you would raise the pH of soap by adding acid.

This question is more out of curiosity than anything else, but I appreciate any help people can provide.

 

[steffamarie]

Your assumption about stearic acid not creating glycerin is correct. If I had to guess, I'd assume the hydroxyl groups combined with themselves to create H20 maybe? Although that would leave you with an excess of oxygen. If they do create H2o, the other product might be O2. Of course, I may be thinking through this all wrong!! Chemistry has been a long time ago lol

I'm sure someone (ideally DeeAnna) will turn up with a better answer

 

[DeeAnna]

@steffamarie -- Stearic acid does not produce glycerin when it saponifies. Only triglyceride fats contain glycerin, which is freed up when fats are saponified. Fatty acids do not contain glycerin. Water, not glycerin, is created when fatty acids are saponified.

@Nate5700 -- Stearic acid has an H+ where the glycerol used to be attached. When you add NaOH, the Na+ ion bumps the H+ off the acid molecule. The fatty acid plus the sodium ion combine to form the salt (aka soap). The OH- reacts with the H+ to make water.

If Ste-H is shorthand for stearic acid, then -

Ste-H + NaOH =>
Ste-H + Na+ + OH- =>
Ste-Na + H+ + OH- =>
Ste-Na + H2O where Ste-Na is the sodium salt of stearic acid, aka stearic soap

 

[Nate5700]

Stearic acid has an H+ where the glycerol used to be attached. When you add NaOH, the Na+ ion bumps the H+ off the acid molecule. The fatty acid plus the sodium ion combine to form the salt (aka soap). The OH- reacts with the H+ to make water.

Perfect answer as always. It makes sense that the acid would have the H+ there. Not sure why I didn't think of that initially.

Interesting to me that from what I understand stearic acid with NaOH is supposed to make a very hard soap, but the chemical reaction actually produces more water. Maybe it's the absence of the glycerin that makes the soap harder? That plus some of the extra water would evaporate as the soap cures. Just thinking out loud.

 

[DeeAnna]

Yes, less glycerin will tend to make the soap harder. Yes, some of the extra water will evaporate.

But you're totally ignoring the properties of the soap itself. Stearic and palmitic soap molecules create a stable, relatively insoluble, rigid crystal structure after cure compared with the crystal structure formed by other fatty acids typically found in soap.

 

[Nate5700]

Stearic and palmitic soap molecules create a stable, relatively insoluble, rigid crystal structure after cure compared with the crystal structure formed by other fatty acids typically found in soap.

Gotcha. So that would explain why people will use palm oil or lard to get more hardness in the soap, because they're bringing more palmitic acid to the party.

So while we're on the properties of different soaps, what differences could I expect between stearic soap and palmitic soap? I used stearic acid in my shave soap, but I know of another member who used a 50-50 blend of stearic and palmitic acids, and I wasn't sure what difference that would make.

 

[DeeAnna]

Commercial "stearic acid" is roughly 50-50 mix of palmitic and stearic acids.

As far as using 100% stearic, Lee Bussey (Silver Fox) has gone down that road, so ask him for his perspective. Or read the Songwind shave soap thread -- it's discussed there. https://www.soapmakingforum.com/threads/my-first-shaving-soap-is-a-success.34264/

 

[Susie]

Summon @DeeAnna.
1. Someone said in a thread yesterday that the triglycerides that make up a fat molecule are composed of three fatty acids linked by glycerol. The fatty acids react with the Na+ from the lye and make a salt, which is soap. The (OH)- reacts with the glycerol to make glycerin.

Life has been crazy lately (and not getting better until at least June), so I may have forgotten my soaping chemistry. If so, just tell me. But doesn't the fatty acids combine with the OH part to form the salt/soap?

 

[Nate5700]

Life has been crazy lately (and not getting better until at least June), so I may have forgotten my soaping chemistry. If so, just tell me. But doesn't the fatty acids combine with the OH part to form the salt/soap?

Not the way I understood it from DeeAnna, it's the Na+ ion that reacts with the fatty acid is what I got. Which makes sense when you think about her explanation of the reaction of pure stearic acid, she said that the fatty acid has a H+ ion where the glycerol normally would be attached. What that tells me is that when the fatty acid ionizes it has a negative charge, so it would repel the (OH)- ions and attract the Na+ ions. And vice versa, the glycerol molecule would have a positive charge after ionization and would attract (OH)- ions.

I mostly slept through college chemistry and somehow made a B in the class, but I'm still getting refreshed on a lot of this stuff. But once you break it down into ions it all comes back to electricity at that point, which is my thing.