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separate from the water and float on top of it. However, the sodium acetate portion loves water. It will dissociate into an acetate anion and a sodium cation, both of which will have strong favorable interactions with water molecules. Both portions of the surfactant molecule get what they want by forming micelles. Micelles are nanoscopic structures, that is, they are structures with a size on the order of nanometers. A common shape for a micelle is spherical, or near spherical, although a variety of shapes occur, depending on the surfactant and the concentration of surfactant in the water. Their sizes are typically about 10 nanometers, that is, 10 billionths of a meter in diameter. The size and structure of the surfactant molecules determine the size of the micelles.

Figure 15.8 shows a schematic illustration of a spherical micelle. The balls represent the acetate or other charged or hydrophilic portions of the surfactant molecules. The hydrophilic portion of a surfactant molecule is often referred to as the head group. The squiggly lines represent the hydrophobic hydrocarbon tails of the surfactants. The head groups are very soluble in water and form an outer shell. The hydrocarbon tails avoid contact with water by clustering together to form a nanodroplet of oil called the core of the micelle. The formation of micelles enables soap to readily dissolve in water.

FIGURE 15.8. A schematic of a spherical micelle. The balls represent the acetate anion head groups. Wiggly lines represent the hydrocarbon tails. The micelle is surrounded by water that hydrogen bonds to the head groups. The hydrocarbon chains clump together to form a nanodroplet of oil, which is protected from water by the head groups.

Soap Dissolves Grease

Now consider what happens when plates or hands, with grease or oil on the surface, are put into soapy water. In pure water, the hydrocarbons on a surface are repealed by the water. However, with soap micelles in the water, the situation is very different. The charged head groups of the micelles come in contact with the oily surface. The head groups want to avoid the oil, which causes the micelles to open up, exposing the surfactants’ hydrocarbon tails to the grease. The tails of surfactants are perfectly happy to be embedded in the oil and grease. The oily hydrocarbons become entangled with the surfactant tails. Helped by agitation, some of the oil hydrocarbons lift off from the rest of the oily surface. The surfactant head groups close up around the core, reforming a micelle. However, some of the oil and grease hydrocarbons have been incorporated into the micelle’s core. The containment of hydrocarbons in the interior of a micelle is shown schematically in Figure 15.9. The hydrocarbon tails of the surfactants are the double lines, while the oil hydrocarbons are single spotted lines. The oil and grease molecules remain in the micelle core as part of the oil nanodroplet. The additional hydrocarbons in the core make the micelles bigger. More surfactant molecules, which are in the water, can join a micelle to fully surround the enlarged oil nanodroplet. The charged head groups of one micelle repel those of other micelles, which prevents the grease from coagulating and forming grease globs that are not soluble in water.

Soap-like materials are reported to have been produced as early as 2800 BCE. True soaps, basically the same as those used today, were made by chemists in the Islamic world in the seventh century. Today, we hear a lot about the coming of nanotechnology, in which nanometer scale assemblies of molecules or atoms can perform very specialized functions. It is remarkable that soap in water is a nanoscopic material. Surfactants form nanometer-size micelles that can encapsulate grease and oil. The micelles with encapsulated hydrocarbons are soluble in water, which makes it possible for us to wash away otherwise water-insoluble molecules.

FIGURE 15.9. A schematic of hydrocarbons from oil or grease (single spotted lines) contained in the interior of a soap micelle.

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