History
Soap has been manufactured for thousands of years and was most probably discovered by accident, when it was noticed that animal fats dripping onto hot ashes made a waxy substance useful for cleaning. By the third century AD, Greek and Roman writers had included recipes for soap in their works. Early settlers to Australia made soap in their own homes, although it was too harsh and smelly to use on skin.
Historically, soap was manufactured from animal fats such as beef tallow, lard or whale oil. Increased crop production meant that vegetable oils such as palm, coconut or olive could be used. Since the 1950s, detergents have been manufactured from petroleum byproducts. Detergents have an advantage over soaps as they do not require animal products and because of their different chemistry, they are less prone to making scum when used in hard water.
How do detergents work?
Soaps and detergents are known
as surfactants and work in a couple of different ways to help clean different
materials. The main characteristic that all share is that the molecules are
long chains, with a charge at one end. This makes the charged "head"
attracted to the water, as water molecules have a charge dipole. This
is said to be hydrophilic (water loving). The tail of the molecule has
no charge and is thus hydrophobic (water fearing). An example of this
is sodium stearate, which has a hydrocarbon “tail”
and a negatively charged “head”.
Detergents reduce the surface tension of water. In high concentrations, micelles form from the repulsion of the hydrophobic end of the molecules by water. Adding a detergent to water means that it is more likely for water to be adsorbed into the fibres on clothes, rather than forming droplets on the surface. Detergents also help in the removal of grease and oils from materials, as the hydrophobic end of the molecule can help to break up and attract droplets of oil. With agitation, large droplets can be broken up into samplers ones. The droplets of oil, surrounded by the detergent molecules form an emulsion in the water and are able to be washed away.
Making
soap
Fats and oils contain triglycerides. Triglycerides (such as those from beef fat, palm oil and olive oil) react with sodium hydroxide to form glycerol and soap. Soap is a mixture of various carboxylic acid salts, each of which can be represented by the formula RCOO-Na+, where R represents a hydrocarbon group. Initially, much of the soap that is made in this experiment is in solution. By adding salt, the concentration of Na+ ions is increased, causing a net forward reaction. Much of the soap is then precipitated.
The most common carboxylic acid salt in soap formed from beef fat is sodium stearate. These molecules are typically chains of around 18 carbon atoms. Too long chains cause the soap to be insoluble, too short chains can cause irritation to the skin.
Other additives
Often there are a number of additives added to detergents, depending on their use. Washing powders may have fragrances, additives to soften water and blue colours to give a psychological perception of cleanliness. Salts are used to bulk up the volume of powders, as the actual amount of detergent required is quite small. Car washing detergents may contain waxes that bond to the surface of the car. Shampoos may contain a number of vitamins and proteins, that perform a questionable duty given that hair is made up of dead cells.