Surfactant is a shortened version of the term surface active agent. A surfactant is a molecule that is preferentially adsorbed at water/oil interfaces. Based on different chemical structures, the surfactants can be classified as ionic and nonionic surfactants. Both kinds of surfactants contain hydrophilic (water loving) heads and lipophilic (oil loving) tails (also referred to as hydrophobic end), which allow them to perform three general functions, i.e. surface-wetting, detergency, and emulsification. For example, sodium dodecyl sulphate (CH3(CH2)10OSO3-Na+), a typical ionic surfactant, contains a ROSO3- group. The SO3- is the hydrophilic head of the group and the R (CH3(CH2)10) is the hydrocarbon hydrophobic tail or chain. Dodecylhexaoxyethylene glycol ether (C12H25(OCH2CH2)6OH) is a nonionic surfactant, which contains an ethylene oxide-alcohol (OCH2CH2)6OH) hydrophilic head and a hydrocarbon hydrophobic tail or chain. Depending on the structure of the surfactant, some are better at surface-wetting and are referred to as wetting agents, though they also have detergency and the ability to emulsify. Similarly, some may exhibit better detergency and be called a detergent and some may react with air rapidly to form foam and be called a foaming agent. Typically, a surfactant has multiple functionality and is classified by which function it performs best.
Figure 1 illustrates the two ends of a typical surface active molecule commonly known as the head group and the tail group in surfactant chemistry. The head group is the polar or ionic portion, which strongly interacts with water via dipole-dipole or ion-dipole interactions. Consequently, the head group is said to be hydrophilic. The tail group, however, is formed by the hydrocarbon chain portion, which interacts only very weakly with water molecules. This hydrocarbon tail is usually called hydrophobic. When the surfactant with hydrophilic heads and hydrophobic tails is added to water containing a small amount of oil, the hydrophilic heads will attach to the water molecules under agitation conditions, forming oil emulsion in the water, as shown in Figure 2.
In this figure, the oil emulsion consists of many very fine oil droplets (down to 0.5 - 10 microns), surrounded by the hydrophilic heads and hydrophobic tails of the surfactants. The hydrophobic tails favor to orient themselves to the interfacial surfaces of the water and the oil droplets, while the hydrophilic heads orient outwards from the interfacial surfaces. As a result, this interfacial reaction between polar and unpolar molecules forms a very stable oil emulsion in the water (each oil droplet is coated with a layer of the hydrophilic heads of the surfactant to prevent the droplets from conglomerating). Conventional coalescers are made with hydrophobic materials, such as polypropylene plates to attract oil droplets (hydrophobic). When the oil droplets are coated with a hydrophilic layer, the coalescing function is totally ineffective (an analogy is water droplets beading on wax paper).