To know what is required to achieve
effective cleaning, it is necessary to have a basic understanding of the
chemistry of soaps and detergents.
Water, the liquid, which is primarily used for cleaning, has a
characteristic known as surface tension. Under the surface of water, each
water molecule is surrounded and attracted by other molecules. At the
surface, however, these molecules are surrounded by only those water
molecules that are on the water side. As the water molecules at the surface
are pulled inside the body of water, a tension is created. This tension
makes the water to bead up on surfaces (fabric, glass), as a result of which
the wetting of surface and hence the cleaning process slows down.
For effective cleaning, the surface tension should be reduced in order to
make water spread and wet surfaces. Chemicals that can do this effectively
are known as surfactants or surface-active agents.
Surfactants also execute some other important cleaning functions, such as
loosening, emulsifying (dispersing in water) and holding the soil in
suspension till the time it can be rinsed away. These chemicals also provide
alkalinity that has found to be effective in removing acidic soils.
Also known as surface-active agents, surfactants can also be classified on
the basis of their ionic (electrical charge), anionic (negative charge),
nonionic (no charge), cationic (positive charge) and amphoteric (either
positive or negative charge) properties in water.
Soap is an anionic surfactant while main ingredients in todays' detergents
are other anionic as well as nonionic surfactants. Now lets have a look at
the chemistry of surfactants -
Soaps
Soaps are water-soluble potassium or sodium salts of fatty acids, which are
made by the chemical treatment of fats (or oils), or their fatty acids with
a strong alkali. Now let us first examine the composition of fats, oils and
alkalis.
Fats and Oils
Fats and oils that are used in the manufacturing of soaps come from plant
or animal sources. Each fat or oil molecule is made up of a typical mixture
of different triglycerides.
In a triglyceride molecule, one glycerin molecule is attached to three
molecules of a fatty acid. There are several types of fatty acids, with each
having a particular combination of fatty acids. Components of fats and oils,
which are used in soap making, fatty acids are weak acids that are composed
of two parts.
A carboxylic acid group consists of two oxygen (O) atoms, one carbon (C)
and one hydrogen (H) atom with a hydrocarbon chain attached to the
carboxylic acid group. Typically, a fatty acid is made up of a long straight
chain of carbon (C) atoms in which each carbon atom carries two hydrogen
atoms.
Alkali
An alkali is a soluble salt of an alkali metal, such as potassium or
sodium. Earlier, the alkalis that were used in soap manufacturing were
obtained from the ashes of plants but now they are commercially
manufactured. Now days, the term alkali described a substance, which is a
base (chemically) and that reacts with and neutralizes an acid.
Usually the alkalis that are used in soapmaking are potassium hydroxide
(KOH), also known as potash (KOH) and sodium hydroxide (NaOH), also known as
caustic soda.
Surfactants in Detergents
A detergent is a better cleaning product as it contains one or more
surfactants. Due to their chemical makeup, the surfactants that are used in
detergents can be engineered to function well under different conditions.
Such types of surfactants are less sensitive to hardness minerals in water
than soap and most of them do not form a film.
Detergent surfactants were developed during World War I in response to a
shortage of animal and vegetable fats and oils. The discovery of surfactant
was also driven by the need of a cleaning product resistant to hard water to
make the cleaning more effective. At that time, petroleum was found to be a
readily available source for the manufacture of detergent surfactants.
Today, detergent surfactants can be produced with a variety oleochemicals
(derived from fats and oils) and/or petrochemicals (derived from petroleum).
Now let us first examine the composition of petrochemicals, oleochemicals
and alkalis prior to the review of how surfactant detergents work.
Petrochemicals and Oleochemicals
Like the fatty acids that are used in soap manufacturing, both the
petroleum and fats and oils contain hydrocarbon chains, which are repelled
by water but attracted to grease and oils in soils. The hydrocarbon chain
sources are used to form the water-repelling end of the surfactant molecule.
Other Chemicals
Other chemicals include chemicals, like sulfur trioxide, sulfuric acid and
ethylene oxide, that can be used to produce the water-attracting end of the
surfactant molecule.
Alkalis
As like soapmaking, an alkali is used to form detergent surfactants.
Potassium and sodium hydroxide are the most commonly used alkalis.
