Building Blocks for Surfactant Molecules

Synthetic surfactants and the natural fatty acid soaps are amphiphihc materials that tend to exhibit some solubility in water as well as some affinity for nonaqueous solvents. As a basis for understanding the relationship between surfactant structures and surface activity, it is useful to work through the simple example of how changes in the polarity (ie, the head group) for a specified hydrocarbon chain affects its solubility and surface activity. As an illustration, consider the simple, straight-chain hydrocarbon dodecane. 

If a terminal hydrogen in dodecane is exchanged for a hydroxyl group (—OH), the new material, n-dodecanol, 

If the original dodecanol is oxidized to dodecanoic acid (lauric acid) 

Similar solubility problems are encountered in acidic media where the soap will be neutralized to produce the free carboxylic acid, which will have little surface activity and probably precipitate from solution. In nonaqueous solvents, the polyvalent salts of carboxylate soaps show a significantly enhanced solubility and perform admirably in many surfactant functions. 

The inability of the carboxylate soaps to tolerate the presence of commonly encountered cations, as well as their sensitivity to pH changes, was one of the major driving forces for the development of synthetic surfactants (detergents) that are not so adversely affected by the common circumstances of hard water and cool washing temperatures, each detrimental to the effectiveness of the soaps. 

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