Soap, detergents or shampoo are a part of everybody's life. What would we do without them for cleaning our house, washing our clothes or taking care of our appearance ?

All these products have something in common: they are surfactants. One of their best-known properties is their ability to make oil and water mix to form an emulsion. They are also wetting agents: they make it easier for a solution to spread across a solid. All these properties are due to their amphiphilic character.

A molecule is called amphiphilic when it has both a hydrophilic head (attracted to water) and a hydrophobic tail (repulsed by water).

In a solution (aqueous medium) amphiphilic molecules such as surfactants behave in a particular way. Above a given concentration, they form small aggregates called micelles. In the spherical micelles, the hydrophobic tails pack inside the core of the structure to escape water, so that only the hydrophilic heads are in contact with the solution.

Laundry detergents clean clothes by capturing hydrophobic dirt inside micelles. Dirt is thus solubilised and then removed from the clothes by water.

In chemistry, many scientists are interested in surfactant properties. They try to better understand and optimise their characteristics for the needs of industry: for example, they try to improve the development of foam.

The amphiphilics encourage the production of foam and foaming agents, emulsifiers and dispersants. They can be used in many other fields of application than just cleaning: foam is used to facilitate industrial drying because it contains a very small percentage of water, shaving products need foam, fire firefighters use foam to prevent fire from spreading, etc.

Surfactants are also investigated in the cosmetic, medical, environmental, petroleum and mineral industries.

They can be used to produce cosmetic creams, sunscreens and medical products. They are especially interesting to help premature babies who have immature lungs.

They are also promising agents for environmental "remediation". They can play an active role in the containment and the clean-up contaminated groundwater. Some surfactants substantially increase the soil's sorption capacity for contaminants and reduce their mobility.

They are already taking a role in the mineral flotation process required for the cleaning of fine coal, for example.

Surfactants are therefore present everywhere and it is a great challenge for scientists to better understand their structure. Industry is expecting a lot from these products and their outstanding properties.

by Mathilde Volpert, ESRF

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