The surfactant molecules reversibly assemble into polymolecular aggregates called micelles. However, when the concentration is increased an interesting change occurs. Very small amounts of these surfactants dissolve in water to give a random dispersion of solute molecules. The use of such compounds as cleaning agents is facilitated by their surfactant character, which lowers the surface tension of water, allowing it to penetrate and wet a variety of materials. Note that each of these molecules has a nonpolar hydrocarbon chain, the "tail", and a polar (often ionic) "head group". The most common examples of such compounds are soaps and detergents, four of which are shown below. Substances that accumulate at water surfaces and change the surface properties are called surfactants.Īlkali metal salts of fatty acids are more soluble in water than the acids themselves, and the amphiphilic character of these substances also make them strong surfactants. This behavior is illustrated in the diagram on the right. Unlike paraffin or other alkanes, which tend to puddle on the waters surface, these fatty acids spread evenly over an extended water surface, eventually forming a monomolecular layer in which the polar carboxyl groups are hydrogen bonded at the water interface, and the hydrocarbon chains are aligned together away from the water. Fatty acids made up of ten or more carbon atoms are nearly insoluble in water, and because of their lower density, float on the surface when mixed with water. Such molecules are termed amphiphilic (Gk. \)Ĭarboxylic acids and salts having alkyl chains longer than eight carbons exhibit unusual behavior in water due to the presence of both hydrophilic (CO 2) and hydrophobic (alkyl) regions in the same molecule.
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