Sciencemadness Discussion Board

A beginner's trouble: Residual fat in soap, why?

Daddy - 21-11-2009 at 14:02

Some time ago I tried with my kids the classical experiment of making soap. First we tried with a small amount of homemade NaOH and vegetable oil. This worked considerably well. Then we decided to do it the way people made soap in the old times: with lye made from wood ash, and animal fat. The final concentration of the lye we used was about 4 M, according to titration with HCl. After heating it in a bowl inside boiling water, together with the fat, for about half an hour, we got a thick mass which smelled of soap. Also, when we added NaCl to a part of it, it solidified; so the substitution of K by Na seemed to work as well. (Lye from ash contains mostly K instead of Na.) But when we tested our "soap", we found that it was really greasy: instead of cleaning things, it left streaks of fat behind. Obviously the reaction was incomplete.

I could think only of two reasons why this might have happened:
- Had I made a mistake in calculating the concentration and/or amount of the lye?
- Or was the reaction temperature too low? (We live high up in the Peruvian highlands; water boils at 88ºC here, so this was the maximum temperature we had.)

I added some more lye and decided to sacrifice an enamel coated pan in order to boil the mixture directly, to achieve a higher temperature. (The pan survived.) But after another half an hour of boiling, there was still no change: we had still half soap, half fat - only that now it was also terribly alkaline. So what was the problem here? Is there some secret in ancient-style soap making we don't know? Are there certain kinds of fat which do not react? Or might our lye not have the right composition? (Somewhere I read it must be hardwood ash. Ours was mostly from eucalyptus; I suppose that is hardwood?) - There was obviously a certain amount of carbonate in our lye, since it produced small bubbles when titrating. Does this have a big influence on the reaction? But anyway, people made soap this way in ancient times, didn't they?

Advice is appreciated. Thank you very much in advance!

bbartlog - 21-11-2009 at 15:59

As far as possible mistakes in measuring or calculating the amounts, it would be hard to say without some more details about the amounts involved. However, the fact that you ended up with a very alkaline mixture suggests that a mere shortage of base was probably not the issue.
I'm not convinced the temperature is to blame, either; when I made soap with my kids, we never returned it to boiling after we mixed the NaOH solution and the oils (in our case lard and coconut oil). Just kept it fairly hot (maybe 60ºC though I didn't measure it).
Other things that seem more likely:
- you have a lot of K2CO3 in your lye (basic, but I'm not so sure it will make soap quickly, or at all...). I don't know enough about making lye from hardwood ash to say how to avoid this problem.
- you didn't adjust the weights of lye and fat to account for the greater molecular weight of animal fat, and your real problem is that you have a whole bunch of excess base, as well as the water that came with it, giving you a watery mix. This wouldn't really explain the unreacted fat, though...

Based on my limited experience, I would look more closely at your stirring methods than at trying to maintain a high temperature. An immersion blender will let you drive soap to 'trace' something like 50x faster than an old school wooden spoon.

sonogashira - 21-11-2009 at 16:02

I know very little of soap making... but...
Have you tried with this animal fat and sodium hydroxide to see if it the fat or base which is to blame?
Possibly you could add ethylene glycol (antifreeze) to increase the temperature of hydrolysis?

Or maybe the type of fat is to blame - though I don't know how much they differ - human fat produces the best soap (according to fight-club!!!) :D:D

Edit - and Mr bbartlog has a good point of the "wood ash" base being too weak due to carbonate instead of hydroxide? I don't know :)

[Edited on 22-11-2009 by sonogashira]

not_important - 22-11-2009 at 03:01

Strength of the alkali is important. If the concentration of NaOH is too high there will be a tendency to form a crust of sodium soap on the outside of oil droplets; too weak of lye will give a slow reaction. The high end strength is less critical with potassium as the soaps are more soluble.

When using the traditional wood ash source of alkali, you're dealing with a solution of mainly K2CO3 with some KOH, to a solution of mostly K2CO3 with a bit of KHCO3. The carbonate is in equilibrium with the glycerol esters, the saponification must be forced by removing the CO2. This was done by long boiling of the mix, generally 1/4 to 1/3 of a day, the boiling also providing some mixing. Note that a small amount of unsaponified fats as well as mono- and di- glyerides would be acceptable in colonial times, cleaning glass was not a common application while cleaning cloths and people was, both of which will tolerate a small amount of oil.

Using alkali carbonate does work, with those caveats, it's how soap was make until the LeBlanc process came into wide use in the last decade of the 18th century, for perhaps a third of a century longer in North America. It can be tricky, even with modern knowledge; from http://www.alcasoft.com/soapfact/history.html comes this
Quote:
From the diary of Elizabeth Ranch Norton, a niece of President John Adams, written in 1799, we learn how an exasperating job soap making could be. On one occasion Mrs. Norton had to make three batches of soap before she was able to make one barrel of soap fit for her family's needs.



S.C. Wack - 22-11-2009 at 06:09

The ash lye was never used as is, it was always causticized with lime. And so I'm in doubt that carbonate is useful at all. No doubt much information would be found in Muspratt and Thorpe, for those who wish to know. Probably entire books on soap at google books. Experimental making of soap is given in Gattermann and Cumming. And these JCE articles; 534 (1937), and 476 (1940).

http://ifile.it/16oiruj

not_important - 23-11-2009 at 01:58

Quote: Originally posted by S.C. Wack  
The ash lye was never used as is, it was always causticized with lime. ...


Thi is not true, there are plenty of 16th and 18th century writings detailing soap making by individuals in which the 'lye' is the concentrated extract of wood ashes, period.

I also refer you to http://sleekfreak.ath.cx:81/3wdev/VITAHTML/SUBLEV/EN1/SOAPMA... which is a modern document from CD3WD in which soap making using either commercial NaOH or K2CO3 from wood ash is used.

S.C. Wack - 23-11-2009 at 03:59

Not soap as we know it. Soap as the OP is talking about, maybe, but that's the problem.

EDIT: According to Roscoe, http://books.google.com/books?id=Jak_AAAAYAAJ&pg=PA688
the use of lime for causticization is known to go back to the second century...There are many books available on soap making, which may be better reading than websites.

http://books.google.com/books?id=Kh5_AAAAMAAJ
http://books.google.com/books?id=UQuAAAAAMAAJ
http://books.google.com/books?id=qcmEAAAAIAAJ
http://books.google.com/books?id=LRJKAAAAMAAJ
etc etc.

[Edited on 23-11-2009 by S.C. Wack]

not_important - 23-11-2009 at 05:39

True, not salted-out, milled, bar soap. But that is a 19th century product, and in North America not until several decades into the 19th century except for imported soaps.

Soap in Colonial times was not infrequently soft and kept in small barrels, pots, or similar containers; it was a potash soap with some unsaponified content. In towns and on wealthier estates the potash soap would be salted out, leading to a firmer product closer to current soaps and it was often made into blocks from which the amount needed by a customer would be cut or shaved off of. But in the towns this was soap made by commercial producers, done on a larger scale. Even then the cost of salt often precluded the salting out process, or some of the crude soap was salted out and then mixed with the soft potash soap.

Individual families just didn't have the time and resources to go about burning lime and causticising the potash, spending several days labour just to save a few hours of boiling. Even today in poorer regions, using woodash to produce a soft soap may be the most affordable way to obtain soap as it can be done using only local materials; thus documents such as that FAO one I gave the link to.