Soaps are sodium or potassium fatty acids salts, prepared from the hydrolysis of fats is called saponification. Each soap molecule has a long hydrocarbon chain, it’s called as ' tail', with a carboxylate' head'. In water, the sodium or potassium ions float with a negatively- charged head. Soap is an excellent cleaner because of its capability to act as an emulsifying agent.
Story of soap:
The oldest recorded evidence of the production of soap materials dates around 2800 BC in ancient Babylon. A formula for making soap was written on a Sumerian clay tablet around 2500 BC; the soap was produced by heating a mixture of oil and wood ash (alkali), This is the oldest recorded chemical reaction, and also used for washing woolen clothing.
We can find the first soap or at least the first use of soap in the Roman legends. According to the Roman legends, soap was named after Mount Sapo, an ancient site of animal sacrifices. Every time after an animal sacrifice, rain would wash animals' fat and burned ash that collected under the ceremonial altars down to the banks of the Tiber River in Italy.
Women washing clothes in the river noticed that if they washed their clothes in certain parts of the river Tiber after a rainfall their clothes were much cleaner than ever.
Chemical reaction behind the cleaning process:
The hydrocarbon chains are attracted to each other by dispersion forces and bind together, forming structures called micelles. In these micelles, the carboxylate groups form a negatively- charged spherical surface, with the hydrocarbon chains inside the sphere. Because they're negatively charged, soap micelles repel each other and remain dispersed in water.
Grease and oil are nonpolar and insoluble in water. When soap and oils are mixed, the nonpolar hydrocarbon portion of the micelles break up the nonpolar oil molecules.
A different type of micelle also forms, with nonpolar
soiling molecules in the center. Therefore, grease and oil and the' dirt'
attached to them are caught inside the micelle and can be rinsed away.
Preparation of soap:
Reaction between glycerol & fatty acids to form triglyceride
Reaction between triglycerides and NaOH
Reaction between coconut oil & sodium hydroxide to form soap.
Raw Materials used for The Soap Preparation:
a) Oils & fats
b) Caustic Soda / Caustic potash
c) Brine solution (for glycerin recovery)
d) Additives like as secondary products.
A) Oils & Fats:
The natural fats like triglycerol used for soap making.
Determines the suitability of triglycerol for soap product in saponification
process using two main chemical parameters. They are explained below.
i) Iodine Number:
The Iodine number of a fat indicates the weight of Iodine in
grams which can be fixed from 100g of fats. It shows the presence of double
links, thus the degree of unsaturation of the carbonic chain. Its value varies
between 10 to 200. Soaps made from high Iodine Number fat has the tendency to
be soft.
ii) Saponification Value:
The Saponification Value of a fat is based on the quantity
of caustic potassium (KOH) in mg needed to transform 1g of KOH in soap. An
easier conversion of the fat into soap indicated by a high Saponification
value.
The INS factor (Iodine Number-saponification Value), which
represents the difference between these indicates, is an essential parameter
and it is characterized as a soap derived from fat. When the value of this
factor increases, the following properties can be observed.
* The fat tends to become solid.
* The soap tends sharply to be rancid.
* The solubility of soap may decrease.
* The soap made from this will be rough.
* The bleaching and forming properties will decrease.
B) Soda or Potash:
We need to put strong alkali solution in the water for
direct saponification of neutral fats. Caustic soda (NaOH) and caustic potash
(KOH) are most used for this goal.
Caustic soda (NaOH)
Sodium soaps are harder and less soluble than potassium
soaps, and these are the most commonly used alkali for neutralizing fat due to
its ability to return air humidity. It should be handled with caution because the
concentrated caustic soda solutions are harmful to the skin and can cause
serious burns.
Caustic potash (KOH)
It’s also a strong base, which like caustic soda, allows
direct saponification of neutral fats. KOH is a type of lye specifically used
for making liquid soap, and also, Potassium hydroxide is used in baby soaps
because it is more environment friendly, water-soluble, and gentle for babies.
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C) Brine:
A concentrated NaCl solution is added to separate the
glycerin from the soap solution. Soaps which made from fats such as copra,
palm, or castor oil contain high levels of salt. In this case, salt can be used
as a filler material for soap. Also, the addition of salt to soda soaps can
lead to very strong soaps. This method is based on the fact that saltwater is
not soapy.
D) Additives in soap:
These are added to soap in order to either increase the
quantity or to give it a hard consistency. For that purpose, usually kaolin or
clay or starch or silicates of soda or potassium are added. To produce transparent soaps, additives like
alcohol, sugar, and glycerin are also added.
i) Adding colors to soap
Various chemical compounds are used to produce color in
soaps. Natural pigments also be used. Among them the extracts from niebe leaves
for the green color and extracts from red sorgo for colors going from red-brown
to orange also be used.
ii) Scents
For The fragrance purpose various chemical compounds are
used and the extract from plants such as citrus and limes used as fragrances.
iii) Skin protecting agents
Soaps made from certain fats are harmful to the human skin.
For that reason, it is necessary to add skin protection agents. For this
purpose, natural wax (about 3-5% of beeswax) are added.
TFM:
The most important factor in soap quality is its TFM (total fatty matter).
Higher the TFM quantity in the soap, its quality also good.
As
per BIS (Bureau of Indian standards),
* Grade 1 soaps should have minimum 76% TFM
* Grade 2 soaps should have minimum 70% TFM
* Grade 3 soaps should have minimum 60% TFM
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Reference:
1) https://en.wikipedia.org/wiki/Soap
2) https://www.britannica.com/science/soap
3) https://www.thoughtco.com/how-dos-soap-clean-606146
4) https://www.reagent.co.uk/blog/chemistry-of-soap-making/
5) https://www.wilsonchemical.com/5-properties-of-a-high-quality-soap/
