What's the difference?
What's the difference between a surfactant and soap? In general terms, the difference can be likened to the difference between cotton and nylon. On the one hand, soap and cotton are produced from natural products by a relatively small modification. On the other hand, synthetic surfactants and nylon are produced entirely in a chemical factory. Synthetic surfactants are not very new, either. Back in 1834 the first forerunner of today's synthetic surfactants was produced in the form of sulphated castor oil, which was used in the textile industry.
The development of the first detergents in an effort to overcome the reaction of soaps with hard water provides a good illustration of one of the standard chemical approaches. If a useful substance has some undesirable property, an attempt is made to prepare an analogue, a near chemical relation, which will prove more satisfactory.
The petroleum industry had, as a waste product, the compound propylene, CH3-CH=CH2, which used to be burnt off. By joining four of these propylene molecules together and if benzene is attached at the double bond, the resulting compound reacts with sulphuric acid. Then sodium hydroxide is added to neutralise the sulfonic acid and a sodium salt is obtained. The new substance is closely related to an ordinary soap, and is an excellent detergent.
1.5 Soaps and detergents
Soaps and detergents are used by all of us as cleaning agents in our everyday life. These products are sodium salts of organic acids. They are special organic acids which 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.
Fig.1.2. Structure of soap
CH3(CH2)16COONa (sodium stearate)
Fig. 1.3. Structure of soapless detergent CH3(CH2)11(C6H4)SO3Na
There are two types of detergents: 1) soap detergents (usually just called “soap”) which are made from animal fats or vegetable oils; 2) soapless detergents (usually just called “detergents”) which are made using chemicals from oils.
1.6 Soaps Soapiness in 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 soap manufacture: |
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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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At 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. Addition of sodium chloride makes sodium salt of fatty acid partially insoluble in water. This product then separates out of the solution and starts floating on top. Soap flakes are removed and pressed into moulds after addition of perfumes and colours. The solution remained contains glycerol and sodium chloride. Glycerol is recovered as it has wide 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. |
