What happens when you mix alum with soap?

Question

I was handling alum one day and accidentally I touched toilet soap after that. They reacted with each other and formed a rubberlike sticky substance (like dried glue on palm). I am curious what was the chemical reaction and the substance which was formed. I re-tried the same thing with detergent and got similar results.

Answer 1

I think you’ve made a so called “aluminium soap”. It is a compound of the form $\ce{(RCOO)3Al}$ where $\ce{RCOO-}$ is a fatty acid chain. The $\ce{Al^3+}$ ions from the alum you handled would have displaced the $\ce{Na+}$ ions in soap to form the thick gel like aluminium version of soap.

This is similar to how $\ce{Mg^2+}$ or $\ce{Ca^2+}$ ions form an insoluble scum (which consists of $\ce{(RCOO)2Mg}$ or $\ce{(RCOO)2Ca}$) when soap is used in hard water.

I assume here that you’re talking about potash alum $\ce{KAl(SO4)2.12H2O}$ as @BenNorris has mentioned in his comment above.

Any other aluminium containing alum will also have the same effect. But chrome alums (containing chromium instead of aluminium) might have a different effect on soap.

These “aluminium soaps” are used as a thickening agent in oils and greases and also in napalm (as Georg has mentioned in his comment above).

Answer 2

This reminds me of two things in my professional life, I have been involved both in solvent extraction chemistry and coordination polymers.

The carboxylic acid salts in a typical bar of soap would be able to react with the aluminium ions to form the aluminium carboxylate (aluminium soap). In solvent extraction what you can attempt to do is to have a mixture of the extractant (such as DEHPA or a carboxylic acid) in an inert organic solvent (diluent) such as aliphatic kerosene.

Normally aluminium forms a series of water soluble complexes or hydroxides, these are normally very insoluble in non polar solvents. However the complexes of aluminium with liophilic anions such as deprotonated DEHPA or long chain carboxylates are normally soluble in non polar diluents.

If you look at figure 9 in this cogent chemistry paper you will see how the distribution ratio of aluminium (and the early lanthanides) follows the same shaped line when plotted against the sulfuric acid content of the lower phase. We can write the following equation for the distribution ratio of aluminium (first approximation).

D = [DEHPA]^3[H+]^-3

Now if the solvent extraction experiment is conducted without adding sufficent organic solvent (diluent) to dissolve the metal complex, and where the extractant is added either as a pure liquid or as an aqueous solution then it is possible for the metal organic ligand complex to separate from the water as either a greasy (lipophilic) liquid or as a crystaline solid. Conisder for a moment what happens if you add acetylacetone (H-ACAC) to a solution of copper sulfate in aqueous ammonia, What will happen is that the complex [Cu(ACAC)2] will form a insoluble solid.

If a dilute solution of ACAC is used then it is possible to use it to extract metals from aqueous solutions with modest pH values into organic solvents. For example one of my coworkers did part of their PhD by doing an experiment on the extraction of thorium with ACAC or a similar ligand.

I also see that what can happen is that the metal and the carboxylate can form a polymeric or polynuclear coordination compound. These can be very low in solubility and can separate out from the water.