Clean feels good. It smells good. Clean means fewer microbes are
around to hurt us. Clean clothes feel good. Clean dishes make
food safer and more attractive.
For thousands of years, soap was the last word in clean.
The first soaps were probably the saps of certain plants, such as
the Soap Plant (Chlorogalum pomeridianum), whose roots can
be crushed in water to form a lather, and used as a shampoo.
Other plants, such as Soapbark (Quillaja saponaria),
Soapberry (Sapindus mukorossi), and Soapwort
(Saponaria officinalis) also contain the same main
ingredient, a compound called saponin, which forms the
foamy lather, and is also a toxin used to stupefy fish in
streams to make them easy to catch.
Later, people learned that fats would react with alkalies in
the ashes left over from a fire to produce saponified compounds
such as sodium stearate
and the related potassium stearate.
Today, soaps are made from fats and oils that react with
lye (sodium hydroxide). Solid fats like coconut oil,
palm oil, tallow (rendered beef fat), or lard (rendered pork fat),
are used to form bars of soap that stay hard and resist dissolving
in the water left in the soap dish.
Oils such as olive oil, soybean oil, or canola oil make softer soaps.
Castile soap is any soap that is made primarily of olive oil,
and is known for being mild and soft.
As warm liquid fats react with lye and begin to saponify, they
start to thicken like pudding. At this point dyes and perfumes
are often added. The hardening liquid is then poured into molds,
where it continues to react, generating heat. After a day, the bars
can be cut and wrapped, but the saponification process continues
for a few weeks, until all of the lye has reacted with the oils.
Soaps are often superfatted, so after all of the lye has
reacted with the fats, there are still fats left over. This is
important for two reasons. First, the resulting soap is easier
to cut, and feels smoother on the skin. Second, the extra fats
make sure that all of the lye reacts, so no lye is left to
irritate the skin, and the resulting soap is not too alkaline.
The saponification process results in about 75% soap, and 25%
glycerine. In homemade soaps, the glycerine is left in, as it
acts as an emollient (skin softener) and adds a nice feel to the
soap. In commercial soaps, the glycerine is often removed and
sold separately, sometimes showing up in skin moisturizers that
remedy the damage done by drying soaps.
Commercial bar soaps contain sodium tallowate,
sodium cocoate, sodium palmate and similar
ingredients, all of which are the results of reacting solid
fats (tallow, coconut oil, and palm kernel oil respectively)
To these ingredients, they add fatty acids such as coconut acid
and palm acid (the fats in coconut oil and palm kernel oil) as
the extra fats needed to ensure the lye is completely reacted,
and the soap has a good feel.
Polyethylene glycols such as PEG-6 methyl ether may be added as either surfactants,
detergents, emulsifiers (to make the dyes and perfumes blend
evenly), or as thickeners.
Glycerine is added as an
emollient and texture enhancer.
Sorbitol is another emollient
used along with glycerine. It is often added to help make
glycerine soaps more transparent.
Titanium dioxide is added
to make the soap opaque.
tetrasodium etidronate and
tetrasodium EDTA are added as water
and to protect the dyes and perfumes from the effects of metal
ions in the mixtures. These compounds lock up calcium and magnesium
in the water, preventing them from reacting with the soap to form
insoluble soap scum.
Not all bars that lather contain just soap. Many contain the same
detergents that you find in
shampoo, along with soap.