Soap is a mixture of sodium salts of various naturally occurring
fatty acids. Air bubbles added to a molten soap will decrease
the density of the soap and thus it will float on water. If the
fatty acid salt has potassium rather than sodium, a softer lather
is the result.
Soap is produced by a saponification or basic hydrolysis reaction
of a fat or oil. Currently, sodium carbonate or sodium hydroxide
is used to neutralize the fatty acid and convert it to the salt.
General overall hydrolysis reaction:
fat + NaOH ---> glycerol + sodium salt of fatty acid
Although the reaction is shown as a one step reaction, it
is in fact two steps. The net effect as that the ester bonds
are broken. The glycerol turns back into an alcohol (addition
of the green H's). The fatty acid portion is turned into a salt
because of the presence of a basic solution of the NaOH. In the
carboxyl group, one oxygen (red) now has a negative charge that
attracts the positive sodium ion.
Types of Soap: The type of fatty acid and length of
the carbon chain determines the unique properties of various
soaps. Tallow or animal fats give primarily sodium stearate (18
carbons) a very hard, insoluble soap. Fatty acids with longer
chains are even more insoluble. As a matter of fact, zinc stearate
is used in talcum powders because it is water repellent.
Coconut oil is a source of lauric acid (12 carbons) which
can be made into sodium laurate. This soap is very soluble and
will lather easily even in sea water.
Fatty acids with only 10 or fewer carbons are not used in
soaps because they irritate the skin and have objectionable odors.