Surface active agents or surfactants have a distinct molecular structure that gives rise to their molecular properties. [extra background]
Soaps such as sodium oleate are examples of a class of molecules called surfactants. The name "surfactant" comes from somebody who we don't think would spell very well - it actually stands for SURFace ACTive AgeNT. Both detergents and soaps are classed as surfactants.
Surfactants are characterised by the essential features that we saw in sodium oleate. A surfactant molecule has a hydrophilic (water-loving) head and a long hydrophobic (water-hating or oil-loving) tail. For this reason, we often describe surfactants is being amphiphilic molecules - they love everything.
Surfactants are said to have a "head" and a "tail". The head is hydrophilic which means that it is water loving, and it is generally depicted as a circle. The tail is generally a long hydrocarbon chain and is hydrophobic, which means water-hating (therefore oil-loving). The tail may be depicted either as a straight line or a wavy tail.
Properties of Surfactants
The molecular structure of surfactants means that they have unusual properties, leading to both widespread and highly specialised applications. The properties of surfactants fall into two broad categories: adsorption and self-assembly.
Adsorption is the tendency for a surfactant molecule to collect as an interface. Adsorption is very different to absorption (don't get them confused!):
the taking up of a gas or liquid at the surface of another substance, usually a solid (for example, activated charcoal adsorbs gases). It involves molecular attraction at the surface.
the taking up of one substance by another, such as a liquid by a solid (ink by blotting paper) or a gas by liquid (oxygen dissolving in water).
The adsorption properties of surfactants mean that surfactant molecules are usually found at the interface between an oil phase and a water phase or a water phase and an air phase. This molecular property leads to the macroscopic properties of wetting, foaming, detergency and emulsion formation.
Surfactant molecules tend to adsorb to the surface of oil droplets. The hydrophilic heads stick out into the water phase, while the hydrophobic tails happily stick into the oil phase.
Self-assembly is the tendency for surfactant molecules to organise themselves into extended structures in water. This includes the formation of micelles, bilayers and liquid crystals. These structures are formed by when the hydrophobic tails of the surfactants cluster together, forming small aggregates such as micelles, or large layer structures (bilayers) which are similar to a cell wall.
These properties make surfactants very interesting study, and are areas of current research.
Surfactants can aggregate to form micelles. Forming a micelle allows the hydrophobic tails to get out of the water but still allows the hydrophilic heads to stay in the water. There is usually between a few dozen to a couple of hundred surfactant molecules in a micelle.
Surfactants also aggregate to form extended structures in water, such as the surfactant bilayer depicted here.