Soaps are sodium or potassium salts of higher fatty acids such as palmitic, stearic, oleic, etc. Sodium salts are generally known as hard soaps, while potassium salts are named as soft soaps. Both of these types are soluble in water, while soaps of other metals (e.g., ca, Mg, Al, Cr, etc.) are usually water insoluble2 and hence are not used for cleansing purposes. Hard soaps contain a higher proportion of salts of saturated acids (palmitic, stearic), while soft soaps contain a higher proportion of salts of unsaturated acids (oleic acid). Hard soaps are sodium salts and contain free alkali and are used as laundry soaps; while soft soaps are potassium salts and do not contain free alkali and are used as toilet soaps.
Soap is manufactured by treating oil with a 10% solution of caustic soda (lye) in an iron pan at room temperature (cold process) or in a steel tank known as kettle at high temperature (hot process). salts of higher fatty acids separate out due to common ion effect and float on the surface (salting out When saponification is complete, some common salt (brine, NaCl) is added. Soaps which are sodium of soap). The filtrate containing glycerol, NaCl, unused alkali and some soluble soap is known as spent bye and used for the recovery of glycerol.
In modern processes (Ittner and Twitchel processes), oils are first hydrolysed by steam in presence of catalyst and free acids are then neutralised separately with NaOH to form soaps.
Structure : A soap molecule has two ends, viz. a polar end comprising the salt end, -COO- Na+, and a non-polar end comprising the long hydrocarbon chain. The polar end is water-soluble (hydrophilic) while the non-polar end is oil soluble (lyophilic or lipophilic).
