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The Chemical Structure of Soap
It is difficult to wash an oil spot out of clothing with plain water, because oil is a hydrocarbon that does not
dissolve in water. Oil and water actually repel one another, so that oil adheres even more strongly to clothing
in the presence of water. The addition of soap or detergent to water changes the situation; soapy water can
dissolve oil from clothing and rinse it away. What is special about the structure of soaps that makes them
effective cleaning agents for oils and greases?
Most soaps are soluble sodium or potassium salts of carboxylic acids. The most common commercial soap is
sodium stearate, Na[C17H35CO2]. It dissolves in water, forming the sodium and stearate ions. Even though
most of the stearate ion is a hydrocarbon chain, it dissolves in water because of the carboxylate group. The
carboxylate end is called hydrophilic (water-loving), and the hydrocarbon tail is called hydrophobic (water-
fearing).
It is the long hydrocarbon chains of the stearate anions that dissolve the oils and greases. If water containing
dissolved soap is mixed with oil, the hydrocarbon chains strongly attract the oil, while the ionic ends keep the
soap dissolved into water. The oil spot is broken up into small droplets and dispersed into the water. The
"tails" of many soap anions are needed to remove each oil droplet.
While the sodium salt of stearate ions and the anions of other soaps are soluble in water, the calcium and
magnesium salts are not. Hard water contains these metal cations, so the metal salts precipitate, reducing the
oil-dissolving efficiency of the soap. "Bathtub ring" originates from the precipitation of soap by hard water.
Thus, soaps do not clean well in hard water until most of the metal cations have been precipitated by reacting
with the soap. In recent years, this problem has been solved by replacing soaps with detergents, which are
generally compounds with long hydrophobic tails and the charged sulfate group such as sodium dodecyl
sulfate, Na[CH3(CH2)11OSO3]. The calcium and magnesium salts of detergents generally remain soluble in
water.
Reger/Goode/Mercer: Chemistry Principles and Practice, 2/e, p. 995
STRUCTURE OF
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DETERGENTS
A detergent consists of two parts:
Hydrophilic part (water soluble)
Hydrophobic part (oil soluble)
Hydrophilic part
Hydrophilic part is sodium salt which is readily soluble in water. e.g. –SO3-, –OSO3-,
OH- or NR4.
This part of a detergent is ionic and is attracted by polar water molecules.
Hydrophobic part
hydrocarbon part of detergent is called hydrophobic part. It is non-polar .
Hydrophobic part is insoluble in water but it is soluble in oil. This part consists of a
hydrocarbon segment and can dissolve oil or grease.
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CLEANING ACTION
When a greasy cloth is put into aqueous solution of a detergent, The hydrophilic
part of detergent is dissolved in water while hydrophobic part dissolves grease or oil
like substances on the cloth. On slight agitation grease is readily removed from the
cloth.
ADVANTAGE OF
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DETERGENT
See difference between soap and detergent.
DISADVANTAGE OF
DETERGENT
Hydrocarbon chain of detergent does not broken by bacteria and bacteria remain in
the solution.
DIFFERENCE BETWEEN SOAP AND DETERGENT
SOAP DETERGENT
Detergents are sodium salts of long
Soaps are sodium salts of long
chain alkyl benzene sulphonic acids or
chain carboxylic acids.
alkyl sulphate.
It is obtained by natural resources
Detergents are synthetic materials.
i.e. fats and oils.
Calcium and magnesium salts of Calcium and magnesium salts of
soaps are insoluble in water. detergents are soluble in water.
In hard water it produces scum Hard water does not affect its cleaning
which affect its cleaning action. action
Lipid MiniTopics
LIPIDS I Lipids II Elmhurst College
Review
Fatty Acids Triglycerides Bilayer Membrane Chemistry Department
Wax Phosphoglycerides Soap Virtual ChemBook
Soap
Introduction:
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
Click for larger image 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.
Cleansing Action of Soap:
The cleansing action of soap is determined
by its polar and non-polar structures in
conjunction with an application of solubility
principles. The long hydrocarbon chain is of
course non-polar and hydrophobic (repelled
by water). The "salt" end of the soap
molecule is ionic and hydrophilic (water
soluble).
Monolayer: When soap is added to water,
the ionic-salt end of the molecule is
attracted to water and dissolved in it. The
non-polar hydrocarbon end of the soap
molecule is repelled by water. A drop or two
of soap in water forms a monolayer on the
water surface as shown in the graphics on
the left. The soap molecules "stand up" on
the surface as the polar carboxyl salt end is
attracted to the polar water. The non-polar
hydrocarbon tails are repelled by the water,
which makes them appear to stand up.
Soap vs. oil vs. water:
Water alone is not able to penetrate
grease or oil because they are of opposite
polarity.
When grease or oil (non-polar
hydrocarbons) are mixed with a soap-
water solution, the soap molecules work as
a "bridge" between polar water molecules
and non-polar oil molecules. Soap
molecules have both properties of non-
polar and polar at opposite ends of the
molecule.
The oil is a pure hydrocarbon so it is non-
polar. The non-polar hydrocarbon tail of
the soap dissolves into the oil. That leaves
the polar carboxylate ion of the soap
molecules are sticking out of the oil
droplets, the surface of each oil droplet is
negatively charged. As a result, the oil
droplets repel each other and remain
suspended in solution (this is called an
emulsion) to be washed away by a stream
of water. The outside of the droplet is also
coated with a layer of water molecules.
The graphic on the left although not strictly
a representation of the above description is
a micelle that works in much the same
fashion. The oil would be a the center of
the micelle. Click for more information on a
micelle.
Micelle - Chime in new window
QUES. Use the solubility principles to
complete a diagram showing many soap
molecules as "bridges" between water and
oil. Label and explain the diagram further.
Effect of Hard Water:
If soap is used in "hard" water, the soap
will be precipitated as "bath-tub ring" by
calcium or magnesium ions present in
"hard" water.
The effects of "hard" water calcium or
magnesium ions are minimized by the
addition of "builders". The most common
"builder" used to be sodium
trimetaphosphate. The phosphates react
with the calcium or magnesium ions and
keeps them in solution but away from the
soap molecule. The soap molecule can
then do its job without interference from
calcium or magnesium ions. Other
"builders" include sodium carbonate,
borax, and sodium silicate are currently in
detergents.
