SOAPS & DETERGENTS

In olden days clothes were cleaned by beating them on rocks in the nearest stream. This practice is followed even today in many villages. Sometimes plants such as soap nuts are used as cleaning agents. Such plants contain saponins, chemical compounds that produce a soapy lather. These saponins were probably the first detergents used.

In modern commercial soap making, the fats and oils are often

hydrolysed with super heated steam. The fatty acids then are neutralized to make soap. The process takes place in large cylindrical vessel. The next step in the manufacture of soap is called graining or Saltingout. This involves the addition of common salt (NaCl). During this process the soap becomes insoluble in brine and separates from the solution. The soap may be washed several times with brine to rid it of free alkali.

The molten soap may be run into large frames from which bars may be cut, or it may be run over cold rollers, producing thin sheets which are scraped to form soap chips. The molten soap may also be squirted from a nozzle as a spray into hot air to form powdered soap.

Among the consumer products aggressively advertised,

bath soap’s are in the top of the list. Although, a both soaps basic function is to

clean the body, the advertisements confer exotic qualities to it. Soaps are being

advertised as

- Beauty soaps - Cinema stars are their brand ambassadors

- Health soaps - Sports person used to sponsor them

- Complexion soaps

- Deodorant soaps

- Freshness soaps- Baby soaps and

- Herbal or medicinal soaps

Cleansing Action of Soap

A soap molecule a tadpole shaped structure, whose ends have different polarities. At one end is the long hydrocarbon chain that is non-polar and hydrophobic, i.e., insoluble in water but oil soluble. At the other end is the short polar carboxylate ion which is hydrophilic i.e., water soluble but insoluble in oil and grease.

When soap is shaken with water it becomes a soap solution that is colloidal in nature. Agitating it tends to concentrate the solution on the surface and causes foaming. This helps the soap molecules make a unimolecular film on the surface of water and to penetrate the fabric. The long non-polar end of a soap molecule that are hydrophobic, gravitate towards and surround the dirt (fat or oil with dust absorbed in it). The short polar end containing the carboxylate ion, face the water away from the dirt. A number of soap molecules surround or encircle dirt and grease in a clustered structure called 'micelles', which encircles such particles and emulsify them.

The subsequent mechanical action of rubbing or tumbling dislodges the dirt and grease from the fabric. These get detached and are washed away with excess of water leaving the fabric clean.

Limitations of Soaps

Soaps do not wash well in hard water and does not form much lather or foam. The calcium, magnesium or iron ions of hard water form an insoluble sticky gray coloured precipitate called scum, which restricts the cleansing action of soap and makes washing more difficult. The scum formed also hardens and dis-colours the fabric. Thus, a large amount of soap is wasted and cleaning is not efficient.

• Ordinary soaps are not suited for fabrics such as silks, wool etc. The alkalis in them injure the fiber.
• If the water is slightly acidic in nature soaps cannot be used for cleaning purpose. The acid media change soaps into carboxylic acid and the action of soap becomes ineffective.

To overcome these drawbacks new types of chemical based cleansing agents were developed. These are called synthetic detergents or simply detergents.

Synthetic Detergents

A detergent is a non-soapy cleaning agent that uses a surface-active agent for cleaning a substance in solution. Synthetic detergents are described as soapless soaps. Unlike soaps they are effective even in hard or salt water, as they form no scum.

Modern synthetic detergents are alkyl or aryl sulphonates produced from petroleum (or coal) and sulphuric acid. They can be defined as 'the sodium or potassium salt of a long chain alkyl benzene sulphonic acid or the sodium or potassium salt of a long chain alkyl hydrogen sulphate that have cleansing properties in water'.

Cleansing Action of Detergents

Synthetic detergents have the same type of molecular structure as soaps i.e. a tadpole like molecule having two parts at each end i.e., one large non-polar hydrocarbon group that is water repelling (hydrophobic) and one short ionic group usually containing the 

     -   +

SO₃ Na  group that is water attracting (hydrophilic). Thus the cleansing action is exactly similar to that of soaps whereby the formation of micelles followed by emulsification occurs.

Advantages of Detergents 

Synthetic detergents clean effectively and lather well even in hard water and salt water (sea water). There is no scum formation.

• Since detergents are the salts of strong acids they do not decompose in acidic medium. Thus detergents can effectively clean fabric even if the water is acidic.
• Synthetic detergents are more soluble in water than soaps.
• They have a stronger cleansing action than soaps.
• As detergents are derived from petroleum they save on natural vegetable oils, which are important as essential cooking medium.

Disadvantages of Detergents

Detergents are surface-active agents and cause a variety of water pollution problems.

• Many detergents are resistant to the action of biological agents and thus are not biodegradable. Their elimination from municipal wastewaters by the usual treatments is a problem.
• They have a tendency to produce stable foams in rivers that extend over several hundred meters of the river water. This is due to the effects of surfactants used in their preparation. Thus they pose a danger to aquatic life.
• They tend to inhibit oxidation of organic substances present in wastewaters because they form a sort of envelope around them.