Surface active agents are molecules composed of an oil-soluble group and a water-soluble group. These chemicals lower the interfacial tension between the immiscible fluids. They also adsorb on rock surfaces and can alter the natural wettability of rock.
Surfactants are classified into four major groups depending on the nature of the water-soluble part of the molecule. These divisions are:
- Anionic (water-soluble end is anionic)
- Cationic (water-soluble end is cationic)
- Nonionic (do not ionize—one end of molecule is water-soluble, the other is oil-soluble)
- Amphoteric (water-soluble end may be anionic, cationic, or uncharged depending on the pH of the system)