Soaps and Detergents

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Soaps and Detergents (OpenStax Chemistry 2e)

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 —CO2−CO2− unit (Figure 2).

This figure shows a structural formula for soap known as sodium stearate. A hydrocarbon chain composed of 18 carbon atoms and 35 hydrogen atoms is shown with an ionic end with 2 oxygen atoms and a negative charge. A positively charged N a superscript plus is also shown at the ionic end.
Figure 2. Soaps contain a nonpolar hydrocarbon end (blue) and an ionic end (red). The ionic end is a carboxylate group. The length of the hydrocarbon end can vary from soap to soap. Source: OpenStax Chemistry 2e

Detergents (soap substitutes) also contain nonpolar hydrocarbon chains, such as C12H25—, and an ionic group, such as a sulfate —OSO3, or a sulfonate —SO3 (Figure 2). Soaps form insoluble calcium and magnesium compounds in hard water; detergents form water-soluble products—a definite advantage for detergents.

This figure shows a structural formula for a detergent known as sodium lauryl sulfate. A hydrocarbon chain composed of 12 carbon atoms and 25 hydrogen atoms is shown with an ionic end involving a negatively charged sulfur and four oxygen atoms at the ionic end of the chain. A positively charged N a superscript plus is also shown at the ionic end.
Figure 3. Detergents contain a nonpolar hydrocarbon end (blue) and an ionic end (red). The ionic end can be either a sulfate or a sulfonate. The length of the hydrocarbon end can vary from detergent to detergent. Source: OpenStax Chemistry 2e

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 4. 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.

This figure shows a drop of oil in which approximately thirty hydrocarbon tails are oriented toward the center of the drop with ionic ends indicated as tiny red spheres on the surface of the oil drop. Solvated cations are indicated as purple spheres surrounded by clusters of H subscript 2 subscript O molecules shown as tiny clusters of red central oxygen spheres with two white hydrogen spheres attached.
Figure 4. This diagrammatic cross section of an emulsified drop of oil in water shows how soap or detergent acts as an emulsifier. Source: OpenStax Chemistry 2e

Source:

Flowers, P., Theopold, K., Langley, R., & Robinson, W. R. (2019, February 14). Chemistry 2e. Houston, Texas: OpenStax. Access for free at: https://openstax.org/books/chemistry-2e

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