Surfactants

When chemists hear the word “surfactant,” they first think of soaps or detergents. Although soaps and detergents are the most familiar of the surfactants, they are not the only ones. There are different kinds of surfactants—many of which have commercial uses. Even your body produces surfactants.

A surfactant is one of a class of chemical structures with a “dual personality” in respect to solubility. Surfactants are usually large molecules. One end is soluble in water; the other end is soluble in the typical organic solvents. This dual solubility is due to a highly polar segment at one end of the molecule and a nonpolar segment at the other end.
Now look at soaps and detergents. A typical soap is the sodium or potassium salt of a long-chain carboxylic acid. Potassium stearate, CH3(CH2)16COOK, is a soap molecule. The “head” of the molecule, the ionic portion, is water-soluble; the “tail” of the molecule, the hydrocarbon chain, is water-insoluble. Chemists call the head the hydrophilic portion of the molecule and the tail the hydrophobic portion.

Adding a soap molecule to water results in the formation of small agglomerations of molecules with long hydrophobic tails. These tails are dissolved in one another and the polar portions pointing outward into the water. These agglomerations are called micelles. Figure 4.8 illustrates the structure of a micelle.

When you wash your skin or clothing, you want to remove two basic types of materials: dry particles and grease. Usually, you can readily rinse dry particles off. Washing grease off is much harder because it is insoluble in water. Here the surfactants in soap help. The tails of the soap molecules are a solvent for the grease. So they dissolve the grease. The heads of the soap molecules project out of the grease particle. Because the heads are soluble in water, they dissolve in the water, and the water carries the soap and the grease away. The interaction of the ionic ends of the surfactant molecules with the surrounding water holds the micelles suspended in the water (see Figure 4.8). The suspended micelles form an emulsion of the grease-soap solution and water.

A difficulty with soaps is that they don't work well in hard water. The most common cations in hard water are calcium, magnesium, and iron ions. When you put soap into hard water, a precipitate of calcium, magnesium, or iron salts forms from an interaction of the cations with the carboxylate ions in the soap. This precipitate is the “bathtub ring” that you must scrub from your tub after you bathe using soap in hard water.

To solve this problem, chemists have developed a variety of surfactants with more soluble calcium or magnesium salts. One is sodium dodecanyl sulfate (sodium lauryl sulfate), CH3(CH2)11OSO3- Na⊕. The textile industry uses sodium lauryl sulfate as a detergent. It is also used as a wetting agent in photography and toothpaste. In an aqueous solution, a wetting agent works by lowering the surface tension of the solution below that of pure water.

A very important surfactant that lowers the surface tension of a solution is found in the transfer of oxygen to the bloodstream. This surfactant is necessary for life itself. A complex mixture of lipids and water coats the interior of the lungs. Simply described, this mixture consists of an aqueous solution of diapalmitoylphosphatidylcholine (DPPC), whose molecules have a hydrophilic head and a hydrophobic tail.

DPPC lowers the surface tension of the interior of the lungs, thereby increasing the rate of oxygen absorption. Polluted air and, particularly, cigarette smoke reduce the ability of the lungs to produce DPPC and thus inhibit the transfer of oxygen into the body.
DPPC also keeps your lungs inflated. DPPC coats the inside of the alveoli in your lungs and this coating lowers the surface tension on the inside of the alveoli. Outside the alveoli is the blood, which has a much higher surface tension. This difference in surface tension pulls the alveoli into a spherical shape.
Before birth, the fetus makes some respiratory-type movements, but its lungs remain collapsed. Immediately after birth the infant makes several strong inspiratory movements, and the lungs expand. The surfactant in the lining of the lungs keeps them inflated. However, when an infant is born before the surfactant system is functional, their lungs will not properly inflate or remain inflated. This problem, called hyaline membrane disease, is especially serious for premature infants who don't have a functional surfactant system. To help fight this condition, premature infants are given surfactants until their own surfactant system begins to function properly.