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Surface tension is the energy it takes to increase the surface area of a liquid per unit area. Because these forces vary depending on the nature of the liquid (for example, water versus gasoline) or the solutes it contains (for example, surfactants such as detergent), each solution has different surface tension values.
Let’s look at an example: every time a glass of water is filled almost to the brim, it can be observed that the level of the water in the glass is actually higher than the height of the glass. You can also see that the water that has spilled has formed puddles that rise above the surface. The two phenomena described are due to surface tension.
More intuitively, surface tension is the tendency of a liquid to occupy as little surface area as possible. This tendency is the key factor in capillary action or capillary movement . Capillary action is a consequence of the cohesive forces between molecules, that is, the tendency of molecules to stay together and adhere to each other.
Cohesion forces and adhesion forces
Cohesion forces and adhesion forces are highly related to surface tension. These forces appear when substances have mass, that is, they are macroscopic properties, so they do not come into play when taking individual atoms or molecules into account.
- Cohesion forces . They are the forces that hold molecules together. If the cohesive forces are strong, a liquid will have a tendency to form droplets on a surface.
- Adhesion forces . They are the forces that are exerted between the molecules of the liquid and a surface. If the adhesion forces are strong, a liquid will have a tendency to spread across a surface.
Thus, if the cohesion forces are stronger than the adhesion forces, the liquid will maintain its shape, but if the opposite occurs, the liquid will spread, thus increasing its surface area. Any substance added to a liquid that increases its surface area is called a wetting agent.
Wetting agents are substances that reduce the surface tension of a liquid and cause it to spread in the form of drops on a surface, increasing the dispersion capacity of said liquid.
molecular perspective
In a water sample there are two types of molecules: those on the outside of the sample (outer molecules) and those on the inside (inner molecules). The inner molecules are attracted to all the molecules around them, while the outer molecules are only attracted to other molecules on the surface and those below them. This makes the energy state of the interior molecules less intense than that of the exterior molecules. Thus, the molecules maintain a minimal surface area, which allows more molecules to have a lower energy state. This phenomenon is a consequence of surface tension, and one of the best ways to verify its existence.
Water molecules are attracted to each other due to the polar property of water. The hydrogen ends are positive, while the oxygen ends are negative, and they bond together, negative oxygens with positive hydrogens. To break these intermolecular bonds a certain amount of energy is needed, which is precisely the surface tension. The same goes for other liquids, even ones that are hydrophobic , such as oil. There are other forces that act in the liquid such as Van der Waals forces, which are exerted between the molecules of the liquid.
Continuing with the example of water, its surface tension is very high. in fact, the surface tension of water can cause materials even denser than water itself to float on it. Thanks to their surface tension, certain organisms can literally walk on top of water. An example is the water mosquito or shoemaker, which can run across its surface due to the intermolecular forces of the water molecules and because the weight of the mosquito is distributed between its legs. Surface tension also allows for the formation of droplets that we constantly see in nature.
Other Examples of Surface Tension
An alcoholic beverage forms small grooves in the glass due to the interaction between the different surface tension values of ethanol and water, and the faster evaporation of alcohol compared to water.
Oil and water separate because the surface tension of these liquids is different. In this case, the term is “interface tension,” but it is simply a type of surface tension between two liquids.
