- Fatty acids are carboxylic acids that contain 4-28 carbon in the molecule.
- Soap is the sodium or potassium salt of fatty acid.
- The general formula of soap is RCOO–Na+ or RCOO–K+, where R is an alkyl group that has long carbon chain.
- Image below shows the general structure of a soup molecule.
- Image below shows the example of a soup molecule (Sodium Stearate)
Making of Soap – Saponificationts
- Alcohol react with carboxylic acid will form ester.
Ethanoic acid + ethanol → ethyl ethanoat (ester) + water
CH3COOH + C2H5OH → CH3COOC2H5 + H2O
This process is call esterification, and it is a condensation reaction (take out 1 water molecule and form a covalent bond).
- Inversely, the ester also can be decomposed to form alcohol and a salt by alkali such as sodium hydroxide.
ethyl ethanoat (ester) + sodium hydroxide → sodium ethanoat + ethanol
CH3COOC2H5 + NaOH → CH3COONa + C2H5OH
This process is a hydrolysis reaction (add 1 water molecule to the covalent bond and break the bond).
The Making of Soap
- As we learnt before, salt can be produced by the reaction between acid and alkali.
- Since soap is the sodium or potassium salt of fatty acid, we can produce soap by the reaction between fatty acid and alkali.
- Usually, the alkali that we use is sodium hydroxide (NaOH) or potassium hydroxide (KOH).
- Fat molecules are the molecules that form by a glycerol molecule and three fatty acid molecules.
- Since glycerol is an alcohol, while fatty acids are carboxylic acid, the process is call esterification, and it is a condensation reaction.
- Figure above shows that, we can separate the glycerol molecule from the fat molecule, and at the same time, produce three sodium salt of fatty acid.
- The hydrolysis of a fat or oil is given the particular name, saponification.
- The saponification reaction is the hydrolysis of fatty esters.
- The saponification reaction typically refers to the reaction that is carried out by a strong base.
- An example the saponification of tallow by sodium hydroxide. (tallow is the mixture of fats that are obtained by steam treating cow fat.)
- Summary of saponification reaction
- Normally some sodium chloride (NaCl) will be added into the solution to reduce the solubility of soap. As a result, we will obtain soap in solid form.
Cleaning Effect of Soaps
- 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.