Secrets of surface tension

Without most of us never paying attention to it, surface tension is acting all around us, everywhere and all the time, affecting our daily life in a number of ways.

In fact it is surface tension which keeps the billions of cells in our body functional, ensuring the proper organization of their biomolecules, proteins, lipids, and nucleic acids, into membranes and the various types of oreganelles.

Surface tension is a truly fundamental property of water, making it an ideal medium allowing for life as we know it to exist. On a more easily accessible scale, familiar to us all, it is surface tension what makes water drops spherical.

What causes surface tension?

In brief, surface tension arises from the strong interactions between water molecules, called hydrogen bonding. It is this strong interaction which also manifests in the other unusual property of water, its high boiling point.

In the bulk of a liquid, water molecule can make optimal number of H-bonds to other water molecules. On the surface, however, the interactions from the neighboring molecules are weaker, resulting in a higher energy and reduced intermolecular hydrogen bonding of the molecules. In nature, water has one of the highest surface tensions, 72.8mN/m (at 20 degrees Celsius), only exceeded by very few liquids, such as mercury, which has a surface tension of about 480mN/m.

Surface tension of water also manifests in the so-called hydrophobic effect, evident as the lack of mixing of oil and water.

The hydrophobic effect

The hydrophobic effect is the driving force for the formation of micelles by amphiphilic molecules and membranes by phospholipids. In these assemblies the polar parts of the amphiphiles seek interaction with water , while in contrast, their hydrophobic parts cluster to minimize contacts with water.

The hydrophobic effect can be easily studied through changes in the surface tension for an air/water interface. More specifically, measuring changes in the surface tension of the air-water interface, allows to monitor the efficiency of the accumulation of compounds such as drugs, surfactants and lipids into the surface. This gives the academic research and the pharmaceutical industry totally new possibilities and benefits to analyze drugs and conduct physicochemical profiling to support their QSAR and ADME/Tox research.

Measuring surface tension provides data reflecting thermodynamics of the compounds tested, and reveals fundamental physicochemical properties associated with processes such as adsorption, hydrogen bonding interactions, and self-assembly.

Why do we use soap?

Surface tension plays a big role also in our daily activities. Soaps and detergents include surfactants, that reduce the surface tension of the liquid. This allows the liquid to have a good contact with the material and to remove the dirt from it efficiently. Measuring changes in surface tension and determining the CMC (critical micelle concentration) values, the chemical industry can optimize the amount of surfactants in their products, bringing more environment friendly, yet efficient goods to the market.

Surfactants and oil reduce the surface tension of water. Accordingly, by measuring the surface tension of natural waters the presence of substances such as oils or detergents can easily be verified.