Cleaning with soap and soapless detergents.

Detergent comes from the Latin word detergere meaning to clean, it is defined as a cleansing agent. Therefore, water itself is a detergent. This essay looks at soap and soapless (or synthetic) detergents. Both substances we use everyday and have a big market commercially, they effect everyone. Soaps are made from natural products and soapless detergents are produced chemically, each having advantages and disadvantages.

Soap has a much longer history than it s relatively new synthetic version. There is evidence of soap made in Mediterranean countries around 2500 years ago. The basic process has not changed much although now the chemistry is understood. Soap is made from the process called saponification, the alkaline hydrolysis of fats and oils. It is essentially the reverse of esterification.

O O

R-C-O-R + NaOH ----> R-C-O-Na+ + R OH

Ester(fat) + base(caustic soda) ---- > salt of fatty acid(soap) + alcohol(glycerol).

Caustic potash (potassium hydroxide) can be used instead of caustic soda (sodium hydroxide)but is more expensive. The base used to come from wood ash containing potassium carbonate which formed potash as this was not plentiful it made soap a luxury. The cheapest source of the ester is animal and vegetable fats and oils.

H-H-H-H-H-H-H-H-H-H-H-H-H-H-H O

H-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C

H-H-H-H-H-H-H-H-H-H-H-H-H-H-H O-Na+

This is an example of a soap molecule. The hydrocarbon end is non polar and hydrophilic (water hating) and the carboxylate end is polar and hydrophilic (water loving). This the property which allows it to clean, it acts as an emulsifying agent. The soap disperses in water to form miscelles where a negatively charged surface is formed and hydrocarbon chains are in the centre. These miscelles surround droplets of dirt or grease suspending them in the water so they can be washed away.

In soft neutral water soap works very well. However in hard water those containing ions (calcium and magnesium ions) the soap reacts with the ions forming insoluble salts, scum which settles on fabrics and around the bath. A scum is also the result when soap is used in acidic water. Soap is also affected by the nature of the dirt, for example perspiration breaks down the soap reducing the washing power. There are other disadvantages of soap, it deteriorates on storage lacks cleaning power and doesn t rinse out completely.

The production of synthetic detergents are an example of a standard chemical approach. If a useful substance has some undesirable properties an attempt is made to make a near copy synthetically which will perform better. Detergents were first developed in the 1940 s. The criteria was a molecule with a long non polar hydrocarbon chain and a polar group on one end. This was achieved and the new substance is closely related to ordinary soap. An important base in the manufacture of detergents is sodium alkyl benzene sulphonate.

O O

R-C becomes R-S

O-Na+ O O-Na+

The detergents produced this way work equally well in hard, soft, acid, hot or cold water. They don t leave scum and are very powerful cleaning agents. However, the new product is so stable that it persistent in water waste and does not break down. There were problems with excessive foaming in natural and sewage waterways. The solution to this was to replace non biodegradable branched chain with a linear hydrocarbon chain. The second problem, is the phosphate additives, these are fertilisers and when released into the waterways feed plants including algae which take oxygen out of the water, killing fish and producing a bad hydrogen sulfide smell. There are now regulations on phosphate levels.

The cleaning action of soap and synthetic detergents are very similar, they are both surfactants. They act as emulsifying agents and reduce the surface tension of water allowing it to wet better. Each have their own disadvantages and advantages. I think the production of synthetic detergents is a good thing especially since all the time improvements are being made to make them environmentally friendly.