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Surfactants are compounds that lower the surface tension of a liquid, allowing easier
spreading, and lowering of the interfacial tension between two liquids, or between a
liquid and a solid. Surfactants may act as: detergents, wetting agents, emulsifiers,
foaming agents, and dispersants.
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Surfactant (Surface Active Agent)
Surfactants are usually organic compounds that are amphiphilic, meaning they contain both
hydrophobic groups (their tails) and hydrophilic groups (their heads). Therefore, a surfactant
molecule contains both a water insoluble (or oil soluble component) and a water soluble
component. Surfactant molecules will migrate to the water surface, where the insoluble
hydrophobic group may extend out of the bulk water phase, either into the air or, if water
is mixed with an oil, into the oil phase, while the water soluble head group remains in the
water phase. This alignment and aggregation of surfactant molecules at the surface, acts
to alter the surface properties of water at the water/air or water/oil interface.
A micelle—the lipophilic tails of the surfactant molecules remain on the inside of the
micelle due to unfavourable interactions. The polar "heads" of the micelle, due to favourable
interactions with water, form a hydrophilic outer layer that in effect protects the hydrophobic
core of the micelle. The compounds that make up a micelle are typically amphiphilic in nature,
meaning that not only are micelles soluble in protic solvents such as water but also in aprotic
solvents as a reverse micelle.
Surfactants reduce the surface tension of water by adsorbing at the liquid-gas interface.
They also reduce the interfacial tension between oil and water by adsorbing at the
liquid-liquid interface. Many surfactants can also assemble in the bulk solution into
aggregates. Examples of such aggregates are vesicles and micelles. The concentration at
which surfactants begin to form micelles is known as the critical micelle concentration
(CMC). When micelles form in water, their tails form a core that can encapsulate an oil
droplet, and their (ionic/polar) heads form an outer shell that maintains favorable
contact with water. When surfactants assemble in oil, the aggregate is referred to as
a reverse micelle. In a reverse micelle, the heads are in the core and the tails maintain
favorable contact with oil. Surfactants are also often classified into four primary groups;
anionic, cationic, non-ionic, and zwitterionic (dual charge).
Thermodynamics of the surfactant systems are of great importance, theoretically and
practically. This is because surfactant systems represent systems between ordered and
disordered states of matter. Surfactant solutions may contain an ordered phase (micelles)
and a disordered phase (free surfactant molecules and/or ions in the solution).
Ordinary washing up (dishwashing) detergent, for example, will promote water penetration
in soil, but the effect would only last a few days (many standard laundry detergent powders
contain levels of chemicals such as alkali and chelating agents, which can be damaging to
plants and should not be applied to soils). Commercial soil wetting agents will continue
to work for a considerable period, but they will eventually be degraded by soil micro-organisms.
Some can, however, interfere with the life-cycles of some aquatic organisms, so care should be
taken to prevent run-off of these products into streams, and excess product should not be washed down.