Soaps

Soaps: Manufacture, Formulation Chemistry, and the Hard-Water Problem

Chemical identity. Soaps are the sodium or potassium salts of long-chain fatty acids, such as stearic, oleic, or palmitic acid.

Manufacture (saponification). Sodium soaps are manufactured by heating a fat - chemically, a glyceryl ester of a fatty acid - together with aqueous sodium hydroxide, in a reaction called saponification, which hydrolyses the ester bond. The soap that forms initially appears in colloidal form, and is then precipitated out of solution by adding sodium chloride; the remaining solution, which contains the glycerol released by the hydrolysis, can be separately recovered by fractional distillation. Only the sodium and potassium salts are water-soluble enough to be used for cleaning purposes, and of the two, potassium soaps are gentler on the skin than sodium soaps.

Formulation variants. Varying the raw fats/oils used and the details of the manufacturing process produces distinct commercial soap types:

• Toilet soaps use better-grade fats and oils, have excess alkali carefully removed, and are coloured and perfumed.
• Floating soaps have air bubbles beaten into them before the soap hardens.
• Transparent soaps are prepared by dissolving the soap in ethanol and then evaporating off the solvent.
• Medicated soaps have medicinal substances added to them.
• Deodorant soaps are formulated with agents specifically to control body odour.
• Shaving soaps contain glycerol, which slows down drying, together with rosin, which forms a lathering sodium rosinate to sustain the shaving foam.
• Laundry soaps contain fillers such as sodium rosinate, sodium silicate, borax and sodium carbonate.
• Soap chips, granules, powders and scouring soaps may contain abrasives such as powdered pumice or fine sand, along with builders such as sodium carbonate or trisodium phosphate that speed up the soap's cleansing action. …