The effectiveness of soaps as cleaning agents lies in their dual
hydrophobic-polar structure. In the bulk of the liquid, soap molecules
remove their hydrocarbon tails form the water by forming spherical
droplets or micelles, with hydrocarbon chains pointing to the interior
and negatively charged heads on the surface (see right). Particles
of grease, oil, and other hydrocarbons can be picked up and incorporated
into the interior of the soap micelles, where the particles are
isolated from the water environment. The grease-laden soap micelles
then can be flushed away with water, with their negative surface
charges helping to keep them apart.
Natural soaps are sodium or potassium salts of fatty acids, which
are combinations of hydrocarbon and carboxyl groups. Other molecules
can be manufactured that are combinations of hydrocarbon and sulfate
or some other negatively charged group. These artificial detergents
have many of the same properties as soaps, and we will discuss them
in the next section.