What is the precipitate that results from blending Dr.Bronner Lavender Castile Soap and Magnesium Sulfate?

Question

When I mix these two in solution a precipitate forms at the bottom. It is a waxy substance with a seemingly higher concentration of lavender oil than the castile soap normally has. My hypothesis is that the epsom salt is dissolving the water soluble parts of the soap into the solution and leaving behind the lipophilic and phenolic compounds in the soap.

Answer 1

This is a really nice question.

The main constituent of soap is sodium stearate, possibly mixed with the sodium salts of other fatty acids. In fact, the word "soap" in chemistry means any dispersible salt of a long chain fatty acid. Notice that I said "dispersible", not "soluble". This is because soaps in water form colloidal aggregates called micelles, with the negative charge outwards and the hydrocarbon chains pointing inwards. This is connected with their surfactant and grease-removing action. Perhaps you know all this already.

Magnesium stearate does not disperse, but forms a soft waxy precipitate. (So does calcium stearate, which gives you the scum on bathwater in hard water areas.)

I think it is brilliant of you to notice that the lavender oil tends to attach itself to the magnesium stearate. This is a result of the hydrophobic attraction between the stearate hydrocarbon chain and the organic molecule.

There is actually quite a lot of research on the use of systems related to this one in the removal of unwanted organics from water.