How Does It Work? Soap
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).
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.)
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.