Self-assembly of surfactants: An overview on general aspects of amphiphiles

Surfactants are molecules (ionic or nonionic) that upon adsorption at the air-water interface reduce surface tension of water. Therefore, surfactants are surface-active agents. Surfactants are normally amphiphilic molecules with a long hydrocarbon tail and a polar head group. The head group may be anionic, cationic or nonionic and accordingly the surfactants are classified as anionic, cationic or nonionic. There are some surface-active amphiphilic molecules that contain both anionic and cationic centers at the head group. These are called zwitterionic surfactants. Surfactants can also have two hydrocarbon chains attached to a polar head and are called double chain surfactants. Also, surfactants containing two hydrophobic and two hydrophilic groups, called "gemini" surfactants. The gemini surfactants can be thought of "twin" surfactants as being made up of two typical surfactant molecules chemically linked at or near the head group. Amphiphilic molecules can also have two head groups (both anionic, both cationic or one anionic and the other cationic) joined by hydrophobic spacer. These types of molecules are termed "bola-amphiphiles" commonly known as "bolaforms". Surface activity of these molecules depends on both the hydrocarbon chain length and the nature of head group(s). Amphiphiles with longer hydrocarbon chains are found to be more surface-active compared to those having shorter hydrocarbon tail. It is observed that amphiphiles with fluorocarbon chain are more surface-active than those with hydrocarbon chain. This is because the fluorocarbon chain is more hydrophobic than hydrocarbon chain.