Surfactants in Emulsions


Surfactants, being surface-active agents, adsorb at the oil/water interfaces. The patterns in which they adsorb are well defined due to the properties of each end of the molecule. The hydrophilic heads like to be in the water, while the hydrophobic tails like to be in the oil. It is said that surfactants form oriented, stabilising films.

Since the heads and tails of the surfactants line up on the oil/water interface, the film is highly oriented.

In the case of an anionic surfactant, each of the droplets will be negatively charged; a cationic surfactant would give positively charged droplets. Since like charges repel, the surfactant helps to slow the rate of droplet coalescence. This is known as electrostatic stabilisation.

An anionic surfactant gives the emulsion droplets a negative charge.

Nonionic surfactants are a little bit more complicated. Rather than forming a charged film, nonionic surfactant heads are polymeric chains dissolved in water. These polymeric chains can be thought of as little springs that push off other droplets, stopping them from coalescing. This is known as steric stabilisation.

As you can see, the surfactant is crucial to creating and maintaining the emulsion. For this reason surfactants are often called emulsifying agents or emulsifiers.

The surfactant film can stabilise an emulsion for hours (e.g. salad dressing), months, or years (e.g. medicinal cream). However, an emulsion is only ever kinetically stable. This means that over time the droplets in an emulsion will coalesce and separate out into two layers.

The kinetic (rather than thermodynamic) stability of emulsion means that emulsion systems are not in equilibrium. Once it has been formed, there is not a dynamic situation in which droplets are being continually formed and coalescing. This is why you have put energy into a system to make it in emulsion (e.g. by shaking). Equilibrium concepts cannot be applied to emulsions - equilibria rely on thermodynamics whereas emulsification is a kinetic effect. (This is why your dirty dishes don't spontaneously clean themselves in cold, soapy water - more's the pity!)