What is Soap?
Definition: Soap is a cleansing agent made from the interaction
of fats and oils with alkali.
History
Records mentioning the use of soapy materials date from ancient
times. Soap making was common in Italy and Spain during the 8th
century. By the 13th century, the soap industry had traveled into
France. Most soap was produced by using the tallow of goats with
beech ash (furnishing the alkali). The French devised a method of
making soap from olive oil. In 1783, a Swedish chemist accidentally
simulated the reaction that occurs in the present-day boiling
process of soap making. He produced a sweet-tasting substance that
is now known as glycerin. In 1823, a French chemist discovered the
chemical nature of the ingredients used in soap.
Ingredients
Oils and fats for soap are compounds of glycerin and a fatty
acid. When oils are mixed with an alkali, they form glycerin and the
sodium salt of the fatty acid. The fatty acids required for soap
making are supplied by tallow, grease, fish oils, and vegetable
oils. The hardness, lathering qualities, and transparency of soap
vary according to the combinations of fats and alkalis used as
ingredients. An experienced soap crafter uses many combinations of
oils.
How does it work?
Most soaps remove grease and dirt because some of their
components are surfactants (surface-active agents). Surfactants have
a molecular structure that acts as a link between water and the dirt
particles. This loosens the particles from the underlying fibers or
surfaces to be cleaned. One end of the molecule is hydrophilic
(attracted to water), and the other is hydrophobic (attracted to
substances that are not water soluble). This peculiar structure
allows soap to adhere to substances that are otherwise insoluble in
water. The dirt is then washed away with the soap.
Chemistry 101 - The Scientific
Explanation
We have talked a bit about water solubility but have not really
discussed why some things are soluble in water while others are not.
We can in general divide compounds into ionic compounds (like salt,
potash, and lime), polar compounds (like water and alcohol), and
non-polar compounds (like fats, oils, and gasoline).
Let us begin by talking about the structure of water. Water
molecules consist of 2 hydrogen atoms and an oxygen atom with the
oxygen in between the two hydrogens and a bond angle of about 104
degrees. Oxygen is far more electronegative than hydrogen and so it
tends to hog more of the electrons. Consequently the water molecule
is polar, with a positive charge at one end of the molecule
and a negative charge at the other. In the first figure, the
molecule on the left shows two hydrogen atoms and an oxygen atom
bound together into a water molecule. The molecule on the right
shows the distribution of charges on the water molecule. A red color
denotes a negative charge, while a blue color denotes a positive
charge. The positive end of one water molecule will be strongly
attracted to the negative end of another water molecule. When an
ionic compound, like sodium chloride, dissolves in water, oxygen
(negative) end is attracted to the cations (positive ions) while the
hydrogen (positive) end of the molecule is attracted to the anions
(negative ions). The solubility of a substance in water is largely
determined by the relative strength of the attraction of water to
the substance compared to the strength of the attraction between
water molecules.
In contrast to oxygen, carbon has almost the same
electronegativity as hydrogen and the carbon-hydrogen bond is
non-polar. For example, the octane molecule (a component of
gasoline) consists of 8 carbon atoms in a chain, with 2 hydrogens
attached to the interior carbons and 3 hydrogens on the end carbons.
Since the electrons are not hogged by any of the atoms, the molecule
is electrically neutral along its entire length. In the second
figure, the molecule on the left shows eight carbon atoms and
eighteen hydrogen atoms bound together into an octane molecule. The
molecule on the right shows the distribution of charges on the
octane molecule. No regions of red and blue show up because there
are no strongly negative or strongly positive regions in the
molecule. Instead, the molecule is green, which denotes neutrality
in this figure.
The simplest way to understand solubility is to remember the rule
"like dissolves like," that is polar and ionic substances are
soluble in polar and ionic substances while non-polar substances are
soluble in non-polar substances. Thus salt dissolves in water but
not in gasoline. Oil dissolves in gasoline but not water.
Now, living cells need both polar and non-polar substances. The
cell uses non-polar substances, fats and oils, to make up the cell
membrane which separates the interior of the cell from the exterior.
If the cell membrane were soluble in water, it would dissolve away
and soon there would be nothing to divide the cell from the
non-cell. But in order to get to the cell in the first place, all
the parts of the cell must be water soluble because that's how
materials get transported from place to place. What nature needs is
a non-polar material that can be dissolved, moved around, and then
made non-polar again. This material is known as a lipid, or
triglyceride.
A lipid consists of two parts, a fatty acid, and a type of
alcohol called glycerol, or glycerine. The fatty acid by itself and
the glycerol by itself are both water soluble because of the polar
oxygen atoms on the ends of these molecules. In a lipid, three fatty
acids are bonded to the three oxygens on the glycerol. Although the
oxygens are still there, they are now buried way down inside the
molecule and the lipid is essentially non-polar and therefore
insoluble in water.
Now fatty acids and glycerol may seem pretty exotic, but they are
variations on molecules with which we are already familiar. Glycerol
(aka glycerine) is simply a tri-alcohol, i.e. an alcohol with three
OH groups. It has chemistry similar to that of ethanol. Whereas
ethanol is C2H5OH, glycerol is
C3H5(OH)3. The chemistry is
dominated by the properties of the OH group. Because the OH group is
polar, alcohols tend to be soluble in water.
We are also familiar with an organic acid, acetic acid, the
acidic component of vinegar. Were as acetic acid is
CH3COOH, a fatty acid has formula
CnH2n+1COOH. The chemistry is dominated by the
properties of the COOH group. Because this group is polar, fatty
acids tend to be soluble in water. Octanoic acid,
C8H17COOH, is just one of a very large number
of fatty acids. In fact, most fatty acids are longer than octanoic
acid. Two very common components of lipids are palmitic acid
(C15H31COOH) and stearic acid
(C17H35COOH). Solid lipids are generally
called fats.
All the fatty acids we have discussed so far are
saturated, i.e. they have 2n+1 hydrogens for every n
carbons. Another class of fatty acids are the unsaturated
fatty acids, with less than 2n+1 hydrogens for every n carbons.
Oleic acid, for example, has formula
C17H33COOH and linoleic acid has formula
C17H31COOH.
Saturated fats contain saturated fatty acids and are solids at
room temperature. Lard, and butter are examples of saturated fats.
Soap made from these fats tends also to be solid at room
temperature. Unsaturated fats contain unsaturated fatty acids and
are liquids at room temperature. Generally called oils,
examples include corn oil and safflower oil. These oils produce
liquid soap. While unsaturated fats are generally more healthy than
saturated fats, many times a liquid fat is not convenient. For
example, margarine is made from unsaturated plant oils (e.g. corn
oil) which has been hydrogenated to produce a saturated (solid)
fat.
To make soap, we must break the fat into its fatty acid and
glycerol constituents. The fatty acid has a long hydrocarbon tail
which is soluble in fats, and a polar oxygen end which is soluble in
water. Thus a fatty acid in solution acts as a soap by dissolving
fats in one end of the molecule and water in the other. When we use
a strong base, such as lye to break apart or hydrolyse the
fat, the fatty acid is present as a large cation which is polar at
one end and non-polar at the other. Just as we can have sodium
chloride and sodium carbonate which are soluble in water, we can
have sodium octanoate, the sodium salt of octanoic acid, which is
also soluble in water.
Let's take a fat derived from palm oil (containing palmitic acid)
and hydrolyse it using sodium hydroxide. Saponification is
the term applied to the hydrolysis of fats using a strong alkali
like lye. The reaction is
[C15H31CO]3[C3H5O3](s)
+ 3 NaOH(aq) -----> 3 C15H31COONa(aq) +
C3H5(OH)3(aq)
fat(s) + 3 lye(aq)
-----> 3 sodium palmitate(aq) + glycerol(aq)
While this
reaction may appear intimidating because of the long formulas, it
is, in fact, quite simple. It could be written generally
as
[RCO]3[C3H5O3](s)
+ 3 NaOH(aq) -----> 3 RCOONa(aq) +
C3H5(OH)3(aq)
Where "R" is some
long carbon hydrogen chain. If you look on a list of ingredients on
a soap, you will find things like "sodium stearate," "sodium
palmitate," or, generally, "sodium somebiglongnameate." This is
simply specifying the particular fatty acids present in the soap.
When fat is introduced to a soap solution, the non-polar tail of
the fatty acids dissolves in the non-polar fat, leaving the
water-soluble oxygen end at the surface of the fat globule. With
enough soap, these fat globules become covered with a water-soluble
coating and disperse throughout the solution, as in the last figure.
They are not truly dissolved since individual fat molecules are not
dispersed in the solution. Rather, we say the fat is
emulsified. Notice the glycerol molecule in the upper right
hand corner of the figure.
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