The Softer Side of Science

When you stumble bleary eyed out of bed in the morning and into the shower, you’d be forgiven for not realising that the super-shine, ultra fruity hair cleansing product you reach for is in fact a form of soft matter. At 7 am such thoughts rarely go through anybody’s head.

The shampoo you liberally lather into your hair, like most detergent based products, is in fact made up of molecules called surface-active agents (or surfactants) which have a very distinctive composition. They tend to have a ‘head’ which is has a partial or full electronic charge and which is ‘hydrophilic’ or water-loving. This is attached to a ‘hydrophobic’ or water-hating ‘tail’ often made up of a long hydrocarbon chain. These surfactant molecules are a form of soft matter- the way they interact with each other and with solvents dictates the properties of the material.

Surfactants clean your hair and dishes by removal of ‘greasy’ dirt particles. When mixed with water and applied to a surface, the tails of the surfactant molecules head straight for the dirt. This is because both the tails and the dirt are ‘water hating’, so sticking together means that neither has to come into contact with water. The surfactant molecules thereby coat the surface of the material and, since the interactions between the surfactant and the dirt are more favourable than between the dirt and the water, the dirt is loosened from the surface. Once this occurs the surfactant can wrap right around the dirt and hold it in a suspension in the water with the tails interacting with the dirt and the water-loving heads happily surrounded by water so that when the water is thrown away the dirt goes with it. Hey Presto! Shiny hair and squeaky clean plates!

Surfactants also have uses in paints, dyes and printing inks due to their ability to reduce surface tension. Imagine a drop of water on a piece of plastic; it forms a dome shape rather than spreading out. This is because the water and the plastic do not interact favourably. Why? Well, the water is ‘polar’ i.e. the electrons in the molecule are not evenly spread out but gather more on the oxygen than the hydrogen. Plastic on the other hand, is composed of long hydrocarbon chains with fairly even electron distribution. As a result water molecules would rather interact with other water molecules as the slightly positive parts of one molecule are attracted to the slightly negative parts of another. This sort of electrostatic interaction cannot happen with the non-polar plastic and so interactions between water and plastic are not very favourable, forcing the water to try to reduce its contact with the plastic as much as possible.

If a surfactant is added the droplet spreads out and ‘wets’ the surface. How? Well, remember the Jekyll and Hyde personality of surfactants! The hydrophobic tails point down towards the plastic whilst the polar hydrophilic heads interact with the water by electrostatic interactions. This generates a sort of buffer layer between the water and the plastic, reducing the surface tension and allowing the water to spread out.

Somewhat surprisingly detergents aren’t the only form of soft matter you come into contact with on a regular basis; colloidal systems frequently encountered in daily life too. A colloid is a system made up of two different types of matter, or phases, where one is finely divided and dispersed in the continuous medium of the other. Sounds a bit foggy? Well, it should do! Fog is made up of water droplets that are dispersed in the continuous medium of air. In fact, this makes fog a liquid aerosol! What’s more, the white fluid you pour on your cereal is also a colloidal system. Yup, milk is soft matter as is blood believe it or not! Here the ‘continuous medium’ is a liquid called blood plasma which has microscopic solid particles, such as red blood cells and minerals, dispersed in it.

So next time you reach for your shampoo bottle, eat your breakfast and head out on a misty November morning, bear in mind that you have just had a close encounter with the softer side of science.