Soap
One of
the organic chemical reactions known to ancient man was the preparation of
soaps through a reaction calledsaponification. Natural soaps are sodium or potassium salts of fatty
acids, originally made by boiling lard or other animal fat together with lye or
potash (potassium hydroxide). Hydrolysis of the fats and oils occurs, yielding
glycerol and crude soap.
In the
industrial manufacture of soap, tallow (fat from animals such as cattle and
sheep) or vegetable fat is heated with sodium hydroxide. Once the
saponification reaction is complete, sodium chloride is added to precipitate
the soap. The water layer is drawn off the top of the mixture and the glycerol
is recovered using vacuum distillation.
The
basic structure of all soaps is essentially the same, consisting of a long
hydrophobic (water-fearing) hydrocarbon "tail" and
a hydrophilic (waterloving) anionic "head":
CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 COO − or
CH 3 (CH 2 ) n COO −
The
length of the hydrocarbon chain ("n") varies with the type of fat or
oil but is usually quite long. The anionic charge on the carboxylate head
is usually balanced by either a positively charged potassium (K + )
or sodium (Na + ) cation. In making soap, triglycerides in
fat or oils are heated in the presence of a strong alkali base such as sodium
hydroxide, producing three molecules of soap for every molecule of glycerol.
This process is called saponification and is illustrated in Figure 1.
Like
synthetic detergents, soaps are "surface active" substances ( surfactants )
and as such make water better at cleaning surfaces. Water, although a good
general solvent, is unfortunately also a substance with a very high surface
tension. Because of this, water molecules generally prefer to stay together
rather than to wet other surfaces. Surfactants work by reducing the surface
tension of water, allowing the water molecules to better wet the surface and
thus increase water's ability to dissolve dirty, oily stains.
In studying how soap works, it is useful to
consider a general rule of nature: "like dissolves like." The nonpolar hydrophobic
tails of soap are lipophilic ("oil-loving") and so will
embed into the grease and oils that help dirt and stains adhere to surfaces.
The hydrophilic heads, however, remain surrounded by the water molecules to
which they are attracted. As more and more soap molecules embed into a greasy
stain, they eventually surround and isolate little particles of the grease and
form structures called micelles that are lifted into solution. In a micelle,
the tails of the soap molecules are oriented toward and into the grease, while
the heads face outward into the water, resulting in an emulsion of
soapy grease particles suspended in the water.
With
agitation, the micelles are dispersed into the water and removed from the
previously dirty surface. In essence, soap molecules partially dissolve the
greasy stain to form the emulsion that is kept suspended in water until it can
be rinsed away .
As
good as soaps are, they are not perfect. For example, they do not work well in
hard water containing calcium and magnesium ions, because the
calcium and magnesium salts of soap are insoluble; they tend to bind to the
calcium and magnesium ions, eventually precipitating and falling out of
solution. In doing so, soaps actually dirty the surfaces they were designed to
clean. Thus soaps have been largely replaced in modern cleaning solutions by
synthetic detergents that have a sulfonate (R-SO 3 − )
group instead of the carboxylate head (R-COO − ).
Sulfonate detergents tend not to precipitate with calcium or magnesium ions and
are generally more soluble in water.
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