How Soap Works

[Guest guest]

Hey Everyone,

I thought this was a really interesting article to share with the group.

I've always wondered the science of how it was that Soap could get oil and

dirt off of you in the shower!

Erin Guajardo

Link to this article: http://www.bbc.co.uk/dna/h2g2/alabaster/A283259

*How Soap Works*

 

Soap is a curious substance, designed to solve an intriguing problem. Most

dirt that will not simply wipe off or be shaken out is in fact some form of

fat or grease. In most households the most common cleaning agent is tap

water. The problem is that grease and water fall into two different and

largely incompatible chemical groups. Drop oil into water, and it will tend

to float or form discrete droplets. Pour water into oil and you will see the

same effect. Additionally, substances such as salt and sugar that dissolve

in water will not dissolve in oil, whereas something like petrol will only

float on water but is quite capable of dissolving oil.

 

*The Chemistry of Oils*

 

This difference in behaviour is due to the nature of the molecules involved.

Water is largely polar, that is, water molecules tend to separate into

fragments with opposite electrical charges, one positive and one negative.

Chemicals such as table salt that happen to be made up of collections of

charged fragments, or ions, find it easy to dissolve in water because the

positive ions in the salt are attracted to the negative ions in the water,

and vice versa. Similarly, the charged nature of water means that water is a

good conductor of electricity.

 

Fats and oils, on the other hand, tend not to be polar. Their molecules have

no particular electrical charge, and so are not attracted to polar

substances such as salt. Instead, they prefer to bond with other non-polar

substances. Fats and oils tend to be electrical insulators.

 

*Washing Up*

 

This, then, returns us to the washing-up. You have a greasy dish in a bowl

of water, but the grease is showing no inclination to dissolve in the water

because the water is polar and the grease is not. Attack the grease with a

cloth and most of what you achieve is to move it around on the plate,

because it is trying to flatten itself against the surface of the plate in a

effort to get away from the water molecules.

 

The soap molecule is a halfway house. It consists of a long strand with an

ionic water-loving, grease-repelling group on one end, and a non-polar

grease-loving, water-repelling group on the other. If you drop soap into

clean water, all the molecules gather on the surface with their water-loving

(hydrophilic) ionic ends stuck in the water and their fat-loving

(lipophilic) ends waving in the air. Slide a dirty dish in, however, and the

lipophilic end of each molecule sticks to the grease as it slips past. As

the dish sinks, it takes the soap molecules with it, attached by their heads

to the grease but still waving their hydrophilic tails in the water like

microscopic tadpoles.

 

All you have to do now is bash at the dirt with a sponge or cloth, and it

can be persuaded to leave the plate, for as it lifts off the surface it

becomes insulated from the water as new soap molecules rush in and try to

bury their heads in it. The end result is a small blob of grease completely

surrounded by a layer of soap molecules, all with their lipophilic heads

pointing inwards and their hydrophilic tails pointing outwards. As far as

the grease is concerned, all it can see are lipophilic molecules, and as far

as the water is concerned, all it can see is a rather large hydrophilic

lump.

 

Eventually, of course, all the soap molecules are used up, and you have to

tip out the washing-up water and start again. Pass the tea-towel.