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Soaps

Soaps are the sodium and potassium salts of long chain carboxylic acids (fatty acid).

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 and 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 has a large non-ionic hydrocarbon group and an ionic COO-Na⁺ group. So for simplicity the structure of soap can be represented as

Where () represents the hydrocarbon group and ⊝ represents negatively charged carboxyl group. Some of the examples of soaps are: sodium stearate, C₁₇H₃₅COO-Na⁺, sodium palmitate, C₁₅H₃₁COO-Na⁺ and sodium oleate, C₁₇H₃₃COO-Na⁺.

Hard water is not suitable for washing with soap

Soaps of metals other than sodium and potassium are usually water insoluble and do not find application as a cleansing agent. Therefore hard water, which contains salts of magnesium and calcium, reacts with soap to form magnesium salt of fatty acid and calcium salt of fatty acid.

These calcium and magnesium salts of fatty acids are insoluble in water and separate as curdy white precipitate and are useless as cleansing agent. These precipitates stick onto the fibre of cloth as gummy mass and thus are a hinderance to good washing. Dye does not absorb evenly on cloth washed with soap using hard water because of these precipitates. Moreover, a lot of soap is wasted if water is hard.

Preparation of soap

Soap is prepared by heating oil or fat of vegetable or animal origin with concentrated sodium hydroxide solution (caustic soda solution). Hydrolysis of fat takes place and a mixture of sodium salts of fatty acids and glycerol is formed. Since the salts of fatty acids thus formed are used as soap, therefore, alkaline hydrolysis of oils and fats is commonly known as saponification.

Soap which is formed as a result of alkaline hydrolysis of oil or fat is separated from the solution by the addition of sodium chloride (common salt). Salt decreases the solubility of soap which is therefore, released from the solution. Now, soap being lighter than water floats on its surface from where it is removed.

Soap is then mixed with desired colours, perfumes and sometimes with chemicals of medicinal importance, for example as in carbolic soap, neem soap, etc. Then the soap is moulded into soap cakes.

Soaps that float in water are made by beating tiny air bubbles into the product before it hardens.

Transparent soaps are made by dissolving the soap in ethanol and then evaporating the excess of solvent.

Toilet soaps are made by using better grades of oils and fats and care is taken to remove excess alkali.

After the removal of soap the solution which is left behind contains glycerol and sodium chloride. Glycerol is then recovered from this mixture as it is an important chemical which finds use in drugs, paints cosmetics and explosives.