- Although water is a good solvent, it cannot dissolve oil and grease, and most of the dirt is cover by a layer of oil or grease.
- To eliminate the dirt from a surface, we need the help from soap.
- Soap is very effective in cleaning up some of the dirt that cannot be removed by water.
- The cleaning effect of soap is due to the special structure of its ion.
- Soap molecules dissolve in water to form sodium (or potassium) ions and soap ions.
- Let us examine the structure of the soap ion.
- The ion of soap can be divided into two parts, namely the long carbon chain, which is hydrophobic (water-hating or oil-loving) in nature, and the ionized part, which is hydrophilic (water-loving) in nature.
- The hydrophobic part is soluble in organic solvent like oil or grease but insoluble in water, while the hydrophilic part is soluble in water but insoluble in organic solvent like oil and grease.
- The cleaning of soaps results from their ability
- to lower the surface tension of water and (Wetting agent)
- to emulsify oil or grease. (Emulsifying agent)
Soap as Emulsifying Agent (Emulsifier)
- An emulsion is a dispersion of one liquid in a second, immiscible liquid. Examples of emulsion are milk and cream.
- Soap can help the formation of emulsion of water and oil.
- The hydrophilic heads like to be in the water, while the hydrophobic tails like to be in the oil. As a result, each of the droplets will be negatively charged.
- Since like charges repel, the soap ions helps to slow the rate of droplet coalescence.
Soap as Wetting Agent
Soaps increase the spreading and wetting ability of water by reducing its surface tension.
Cleaning Action of Soaps
- The hydrophobic part of the soap dissolves in grease.
- The hydrophilic part dissolves in water.
- The grease is loosen and been lifted off from the surface.
- The grease is disperse into smaller droplets.