Tuesday, July 20, 2010


Bar Soap

Soap is an item we use daily. It is made up of salt from strong base and fatty acids (carboxylic acids with long hydrocarbon chain). The structure of soap consists of a non-polar tail and a polar-ionic head. Sodium stearate is the most common component in bar soap.

C17H35COO-Na+ (Sodium Stearate)

There are three major types of soap, mainly: Lithium soap, which is hard with a high melting point and widely used in car lubricants; Sodium soap, more soluble in water and appearing as common bar soap; and Potassium soap, which has a low melting point and appears in liquid form.

How soap is made.

Soap can be prepared through the saponification process. During the process of soap manufacture, the ester group commonly found in lipids undergo hydrolysis with a strong base. In soap industrial manufacture, tallow (animal fat) or vegetable oil is commonly used to be heated with sodium hydroxide.

C17H35COOC2H5 + NaOH ---> C17H35COO-Na+ + C2H5OH

Once the saponification process is complete, sodium chloride is added into the soap to form a precipitate. Water layer is drawn from the top of the mixture and vacuum distillation method is used to recover the glycerol.
Crude soap that is produced from saponification contains sodium chloride, sodium hydroxide and glycerol. The purification process is repeated by boiling the crude soap curd in water and re-precipitating it with salt. Sand or pumice may be added to produce scouring soap.

The soap formed is made up of polar molecules with negative charges. The long non-polar hydrocarbon chain does not face water molecules but attach to each other via dispersion forces and formed micelles embedded with polar-ionic heads. Its polar-ionic head interacts with water molecules through ion-dipole interactions and H bonds and form negatively-charged spherical surface. Soap micelles repel each other in water due to this negatively-charged surface.

Polar-ionic Head & Non-polar Head Form Micelle.

How soap works.

Non-polar tails and polar-ionic heads of soap make it an excellent emulsifying agent and enable it to remove grease from our hand and clothes. Grease and oil which are non-polar and insoluble in water, can be removed by soap through agitation that breaks up the grease molecule into small aggregates of grease molecules, embedded with non-polar tail of soap. These aggregates can be easily flushed away by water due to polar-ionic heads that stick to water molecules.

Even though soap are excellent cleansers, they still have disadvantage like they canˇt work out efficiently in hard water (water contain of divalent cations, Eg. Ca2+ and Mg2+). Anions of fatty acids in soap will form insoluble salts with these divalent cations and stick on the clothes and washing machine.

2CH3(CH2)16CO2-Na+ + Mg2 ---->
[CH3(CH2)16CO2-]2Mg2+ + 2Na+

(Soap) + ( Divalent Cations)
(Insoluble salts)

No comments:

Post a Comment