Science and Technology for UPSC Examinations [Chemistry] Part 21- Soaps and Detergents

Soaps are sodium or potassium salts of some long chain carboxylic acids. Sodium salts of fatty acids are known as hard soaps and potassium salts of fatty acids are known as soft soaps. Hard soaps are prepared from cheap oils, fats and sodium hydroxide. They contain free alkali and are used for washing purposes. Soft soaps are prepared from good oils and potassium hydroxide. They do not contain free alkali, produce more lather and are used as toilet soaps, shaving creams and shampoos.

Soap is prepared by heating vegetable oil or animal fat containing Glyceryl stearate with concentrated sodium hydroxide solution. Hydrolysis of fat takes place and a mixture of sodium salts of fatty acids and glycerol is formed. The salts of fatty acids thus formed are used as soap. The alkaline hydrolysis of oils and fats forming soaps is commonly known as saponification.

Hardness of water and soaps

The big disadvantage of soapy detergents is that their washing action is reduced by hardness in water. Water that contains calcium and magnesium compounds is said to be hard. These compounds destroy the soap by reacting with it and converting into insoluble compounds called scums that float to the surface. In hard water, a lot of soap is needed to get a good lather and a lot of scum is formed as well. This difficulty is overcome by detergents.

Detergents are sodium salts of benzene sulphonic acids. Thus instead of -COOH group in soaps, detergents contain -SO3H group. The detergents do not form precipitates with metal ions such as Ca2+ and Mg2+ present in hard water. Therefore, the cleansing action of detergents is better than soaps. Detergents are prepared by treating hydrocarbons obtained from petroleum with conc. sulphuric acid. The corresponding sulphonic acids are then converted into their sodium salts.

Washing powders available in the market contain about 15 to 30 percent of detergents by weight. Some other chemicals which are added to detergents for specific cause are as follows:

  • Sodium sulphate and sodium silicate added to keep the washing powder dry.
  • Sodium carbonate is added to maintain alkalinity which helps in removing dirt and also in softening water.
  • Carboxy-methyl cellulose (CMC) added to keep the dirt suspended in water.
  • A mild bleaching agent such as sodium perborate is added to produce whiteness in clothes.

Soaps Versus Detergents

Soaps are sodium salts of long chain fatty acidsDetergents are sodium salts of sulphonic acids.
The ionic part of a soap is -COO- Na+The ionic part in a detergent is -SO3 - Na+
They are biodegradableSome detergents are not biodegradable.
They are prepared from animal fats or vegetable oils.They are prepared from hydrocarbons obtained from coal and petroleum.
Soaps form insoluble salts called scums with calcium and magnesium ions which are present in hard water and hence cannot be used in hard water.Calcium and magnesium salts of detergents are soluble in water and therefore can be used even in hard water.

How soaps work?

When used for cleaning, soap allows otherwise insoluble particles to become soluble in water and then be rinsed away.

Firstly, a soap molecule contains two chemically distinct parts that interact differently with water. One part is a long hydrocarbon chain, which is non-polar and water hating (hydrophobic), while the other part is charged carboxylate group -COONa which is polar and water-loving (hydrophilic). The hydrophilic part makes the soap soluble in water. So, a soap molecule can be thought of as one having a long tail made of hydrocarbon and a short head made of carboxylate group. The long tail is dirt-loving and water-hating and the short head is water-loving. The hydrophobic part of the soap molecule traps the dirt and the hydrophilic part makes the entire molecule soluble in water. When a soap or detergent is dissolved in water, the molecules join together as clusters called micelles. Their long hydrocarbon chains attach themselves to the oil and dirt. The dirt is thus surrounded by the non-polar end of soap molecules. The charged carboxylate end of the soap molecules makes the micelles soluble in water. Thus, the dirt is washed away with soap.

Secondly, soap is an emulsification agent. Oil and grease are not soluble in water (they don’t dissolve in water), which is why, for instance, oil and water will separate when combined. This is because oils are non-polar molecules, but water is a polar molecule, and non-polar things do not mix well with polar things. In order to remove oil from clothes or from your dishes with water, the oils must be made soluble by the process of emulsification.

The non-polar end adsorbs the oil or other hydrophobic dirt. The ionic end is highly soluble in water. This allows for an emulsion to be formed. The alkali metal (sodium or potassium ion) does not play a role in the action of the soap.

The Total Fatty Matter in Soaps

TFM is one of the most important characteristics, which describes the quality of soap. It is defined as the total amount of fatty matter, mostly fatty acids, that can be separated from a sample after splitting with mineral acid, such as hydrochloric acid. The fatty acids most commonly present in soap are oleic, stearic and palmitic acids and pure, dry, sodium oleate has TFM 92.8%, rest may be moisture. Besides moisture, finished commercial soap also contains fillers used to lower its cost or confer special properties, plus emollients, preservatives, etc. Thus, in commercial soaps, the TFM can be as low as 50%. Fillers, which are usually dry powders, also make the soap harder, harsher on the skin and with greater tendency to become ‘mushy’ in water and so low TFM matter is usually associated with hardness and lower quality.

Detergents, like soaps, work because they are amphiphilic: partly hydrophilic (polar) and partly hydrophobic (non-polar). Their dual nature facilitates the mixture of hydrophobic compounds (like oil and grease) with water. Because air is not hydrophilic, detergents are also foaming agents to varying degrees.