Wednesday, July 3, 2013

How Does It Work? Soap

It would be possible to live in a world without soap. You could do it the modern way-- use the oil cleansing method, go "no poo", eat only on paper plates. You could do it the old-fashioned way and just kind of smell bad. However, since we've been using soap since at least ancient Babylon, you may find it helpful to know the chemistry behind this household staple.

Soaps are fatty acid salts. They are comprised of two distinct portions: a carboxylate head (those of you who remember your organic chemistry will recognize that as a CO2 group) and a long hydrocarbon tail (a tail that is simply carbons and hydrogens).
Sodium stearate is an example of a soap.
In chemistry, we often use the rule of thumb "like dissolves like" to talk about solubility. A nonpolar molecule, such as a hydrocarbon, will be uncharged, and will not dissolve in water, which is polar. A polar molecule, such as a carboxylate group, is very polar, and will dissolve in water.

Soap molecules have both of these traits put together. Near the carboxylate head, a soap molecule is polar. However, by the time you find yourself wandering down to the other end of the molecule, it's acting pretty hydrocarbon-y, meaning it will act non-polar. The head of a soap molecule is hydrophilic (water-loving). The tail is hydrophobic (water-fearing). As a result, soap can mix with both polar and non-polar solvents.

In a polar solvent, such as water, the soap molecules will form a sphere called a micelle, with the carboxylate groups surrounding the hydrocarbons on the inside. The outside, hydrophilic layer makes the whole shebang water soluble. (In a non-polar solvent, the reverse will occur.)
Micelle cross-section
When these micelles encounter a non-polar substance, they will begin to break up due to the "like dissolves like" solubility rule. This substance will be attracted to the hydrocarbon side of the soap molecules. However, the micelles will re-form-- with the grime in the center. Surrounded by the carboxylate groups, they have become water-soluble, and can be easily rinsed away.

This general mechanism also forms the basis of other specialized soaps, such as shampoo.


  1. So, does that mean that all those fancy, expensive micelle water facial cleansers they sell are basically just soap water?

    1. Not quite-- a lot of formulation goes into facial cleansers. For example, soap is very basic (pH=10-ish). You probably don't want to put anything on your face that isn't slightly acidic, just like your skin. However, the term "micelle"? Yeah, that's not fancy. Those are in every bar of soap you've ever used.

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