What properties do soap molecules have?

1 Answer
Apr 24, 2017

Soaps and detergents are the SALTS of long chain fatty acids........


So what is a fatty acid? It is usually a long-chain carboxylic acid, of the form #RCO_2H#. The #R# group can be 10-20 carbons long. And to a first approx. we could represent a soap molecule as #RC(=O)O^-)""^(+)Na#. These beasts are ambiphilic. The carboxylate end can bind to ions and the water solvent, and the hydrocarbyl tail can bind to fats and greases, on the basis of polarity: like versus like etc.

And this is thus the origin of micelle formation:


And here the micelle binds the grease/dirt, and prevents it from redepositing back on your skin, or your clothes, or on the breakfast dishes.

Similar chemistry occurs with long chain sulfonic acids, #RSO_3^(-)""^(+)M#, and these are the basis of modern detergents and washing powders.