MadSci Network: Chemistry

Re: How to make shampoing and liquid soap

Date: Wed Sep 9 08:31:16 1998
Posted By: David Barker, Grad student, Synthetic Organic Chemistry, University of Sydney
Area of science: Chemistry
ID: 902480797.Ch

Before we look at how shampoos and liquid soaps are made lets just look at 
the difference between them, as they contain totally different chemicals 
and are made by different methods.

Liquid Soap is just a thick water solution of normal soap, just like you 
find in any bar of soap. The liquid soap may or may not contain other 
ingredients such as :preservatives, colours and fragrances.

So how is liquid soap, or any soap for that matter made.
All soap comes from the reaction of fat and sodium hydroxide. Just about 
any type of natural (animal or vegetable) fat or oil can be used and as the 
picture below shows the reaction of 3 molecules of NaOH with one fat 
molecule generates 3 molecules of soap and 1 molecule of glycerol.

Soaps have been in use since 600BC but their main disadvantage are that 
they are quite harsh, they are often very basic and react easily with metal 
ions found in most water to form insoluble scums.

In recent years chemists designed detergents that act in exactly the same 
way as soap does but are much less harsh and do not form the insoluble 
salts that soaps do.

Detergents are not made from fats such as animal fat like soaps are but are 
made from the petroleum industry. The reactions are quite complicated but 
involve the reaction of molecules of sulfer trioxide or sulfuric acid on 
petrolium to form detergents like the one show below.

Most cleaning products these days use detergents instead of soap as their 
cleaning agent due to its more desirable properties. Infact many products 
that call themselves as soaps, especially for cleaning skin and faces 
contain detergents as well as soaps.

Most shampoos are a mixture of many detergents as well as colours and 
fragrances. Detergents are used in shampoos as soaps are considered too 
harsh for hair.

There are many sites on internet dealing with the hobby of making soaps you 
can find a good selection of sites on soap making and detergents in 
general at:
The Soap and Detergent Association

David Barker

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