第11章溶液とコロイド

Pioneers made soap by boiling fats with a strongly basic solution made by leaching potassium carbonate, K₂CO₃, from wood ashes with hot water. Animal fats contain polyesters of fatty acids (long-chain carboxylic acids). When animal fats are treated with a base like potassium carbonate or sodium hydroxide, glycerol and salts of fatty acids such as palmitic, oleic, and stearic acid are formed. The salts of fatty acids are called soaps. The sodium salt of stearic acid, sodium stearate, has the formula C₁₇H₃₅CO₂Na and contains an uncharged nonpolar hydrocarbon chain, the C₁₇H₃₅— unit, and an ionic carboxylate group, the —${\text{CO}}_2{}^{\text{−}}$ unit (Figure 11.31).

Detergents (soap substitutes) also contain nonpolar hydrocarbon chains, such as C₁₂H₂₅—, and an ionic group, such as a sulfate—${\text{OSO}}_3{}^{\text{−}},$ or a sulfonate—${\text{SO}}_3{}^{\text{−}}$ (Figure 11.32). Soaps form insoluble calcium and magnesium compounds in hard water; detergents form water-soluble products—a definite advantage for detergents.

The cleaning action of soaps and detergents can be explained in terms of the structures of the molecules involved. The hydrocarbon (nonpolar) end of a soap or detergent molecule dissolves in, or is attracted to, nonpolar substances such as oil, grease, or dirt particles. The ionic end is attracted by water (polar), illustrated in Figure 11.33. As a result, the soap or detergent molecules become oriented at the interface between the dirt particles and the water so they act as a kind of bridge between two different kinds of matter, nonpolar and polar. Molecules such as this are termed amphiphilic since they have both a hydrophobic (“water-fearing”) part and a hydrophilic (“water-loving”) part. As a consequence, dirt particles become suspended as colloidal particles and are readily washed away.