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Soaps
and detergents
Soaps and detergents are
used by all of us as cleaning agents in our everyday lives. These products
are sodium salts of organic acids. They are special organic acids that have
long chain hydrocarbons with hydroxyl groups. These types of acids are also
called fatty acids. We have seen earlier that the name fatty acid comes from
the fact that these acids, such as palmitic acid (C15H31COOH),
oleic acid (C17H33COOH) and stearic acid (C17H35COOH)
are obtained from hydrolysis of fats.
Soaps
Soapiness in a soap comes
from sodium salts of fatty acids like stearic acid, oleic acid and palmitic
acid. These chemicals are sodium stearate C17H35COO-
Na+, sodium oleate C17H33COO- Na+,
sodium palmitate C15H31COO- Na+.
The naturally occurring fatty acids such as palmitic acid (C15H31COOH),
oleic acid (C17H33COOH) or stearic acid (C17H35COOH)
is heated with sodium hydroxide (NaOH), soapy sodium salt along with an
alcohol is formed. This type of reaction we have seen while studying
hydrolysis of ester. The alcohol
called glycerol is formed when higher fatty acids are used in the reaction.
Preparation
of soap
The following raw materials
are required for the manufacturing of soaps : |
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Heating
fat along with NaOH makes soap. Sodium salt of a fatty acid and an alcohol
is formed. This process is called saponification. One such saponification
reaction is shown below.
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In
saponification, a water molecule is removed from a fatty acid or an ester.
When sodium stearate, sodium oleate or sodium palmitate is formed, it is
precipitated from the solution by addition of NaCl. The addition of sodium
chloride makes the sodium salt of fatty acid partially insoluble in water.
This product then separates out of the solution and starts floating on top.
The soap flakes are removed and pressed into moulds after addition of
perfumes and colours. The solution remaining has glycerol and sodium
chloride. Glycerol is recovered as it finds important use in many industries
like cosmetics, paints, etc. Soap can be made from many fatty ester or oils
such as vegetable oil, olive oil, coconut oil and other oils.
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Cleansing action of soap
We
all know soap is used to remove dirt and grime from substances. Generally
dirt and grime get stuck because they have an oily component which is
difficult to remove by plain brushing or washing by water. A soap molecule
has two parts: the long chain organic part and the functional group –COO-
Na+. A tail and a
head. It has to be remembered that this is not an ion, the atoms are all
covalently bonded, the electrical charges show how the charges get polarized
in the group. A soap molecule has a tadpole like structure shown below.
The
organic part is water insoluble but is soluble in organic solvents or in oil
or grease. The ionic part is soluble in water, as water is a polar solvent.
When soap is added to water in which dirty clothes are soaked, the
two parts of the soap molecule dissolves in two different mediums. The
organic tail dissolves in the dirt, grime or grease and the ionic head
dissolves in water. When the clothes are rinsed or agitated, the dirt gets
pulled out of the clothes in the water by the soap molecule. In this way the
soap does its cleaning work on dirty and grimy clothes or hands.
The
soap molecules actually form a closed structure because of mutual repulsion
of the positively charged heads. The structure is called a micelle. The
micelle pulls out the dirt and grime more efficiently.
Synthetic detergents
Soaps
have problems in use for hard water. Hard water has calcium and magnesium,
which does not create lather. Synthetic detergents overcome this problem.
Synthetic detergents do not have sodium salts of fatty acids. In
spite of this the detergents have all the properties of soap. Synthetic
detergents have long chain molecules such as sodium n-dodecyl benzene
sulphonate and sodium n- dodecyl sulphate. The chemical structure of these
molecules is shown below.
We
can see that the synthetic detergent molecule is similar to the soap
molecule. It has a long hydrocarbon chain (or tail) and a short ionic head.
These two parts are water repellent and water attracting respectively.
The cleansing function of a synthetic detergent molecule is similar
to that of the soap.
Synthetic
detergents are manufactured from long chain hydrocarbons obtained as a
byproduct of the petroleum industry. The hydrocarbons are treated with
concentrated sulphuric acid and sodium hydroxide. Neutral sodium salt is
obtained, which is the synthetic detergent.
Advantages and disadvantages of synthetic
detergents with respect to soaps
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1. Soap cannot be used in hard water.
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1.
Synthetic
detergents can be used in hard
water.
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2. Soap is made from vegetable/edible oils
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2.
Synthetic
detergents are made from byproducts of petroleum industry which
helps to conserve valuable edible
oils.
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3. Soaps cannot be used in acidic medium
as this would precipitate the
fatty acids.
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3.
Synthetic
detergents can be used in any
medium including acidic medium.
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4. Soaps have weak cleansing action.
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4.
Synthetic
detergents have strong cleansing
action.
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5. Soaps are not very soluble in
water.
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5.
Synthetic
detergents are highly soluble.
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6. Soaps are biodegradable and do not cause any pollution.
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6.
Some
synthetic detergents are not
biodegradable and cause water
pollution. This in some cases can
become
quite serious. |
Washing powders
Washing
powders are a combination of soaps, detergents and other chemicals.
Generally washing powders have about 15 to 30% of their weight in synthetic
detergents.
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chemicals included in washing powders and their functions are as follows : |
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Sodium
sulphate and sodium silicate are added to keep the powder dry.
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Sodium
triphosphate or sodium carbonate is added to make the solution alkaline.
This helps to remove dirt and also soften water.
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Carboxy
methyl cellulose is added so
that the dirt particles removed are kept suspended in the solution.
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For
obtaining sparkling white clothes, a bleaching agent is added. The
bleaching agent is in the form of sodium perborate.
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