Pro's Corner

Surface Tension

The properties of water are amazing. A simple mixture of hydrogen and oxygen, yet without some of the unusual properties of water, life on earth could not exist. For example, most substances become denser and heavier as become colder. For water, that is true up to a point, but as the water gets close to freezing, the structure changes. It gets lighter and floats. If ice sank, then lakes, streams, and other bodies of water would freeze from the bottom up. Eventually, all water would be frozen and we’d no longer see liquid water.

The same molecular structure of water also is responsible for surface tension, and the bane of most commercial carpet cleaners, wicking or capillary action. Understanding surface tension can help us understand how detergents, surfactants and fabric protectors work. 

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We know water is composed of one oxygen and two hydrogen atoms. The arrangement is not balanced. The two hydrogen atoms and their associated electrons are closer to one end of the water molecule. Thus, one end has a greater negative charge and the other end a more positive charge. This is called a dipole or polar effect.

This polarity, having positive and negative poles, causes water and other polar molecules to attract each other and repel non-polar molecules. Within liquid water, the forces of attraction on each molecule come from all sides and balance out. However, on the surface or at other places where liquid water meets air or some other material, the attractive forces are only downward or inward. This creates surface tension.

This attraction pulling water molecules toward each other creates round drops. Add the effect of gravity pulling down and the water takes on the classic raindrop shape. 

There is a similar effect with the molecules of many solids. With a solid, the term is surface energy. We can also use “surface activity” which includes both surface tension and surface energy. When the surface tension of a liquid is much higher than the energy of the surface it is on, the water tends to bead up. If the surface tension of the water is lower than the material it is on, the water tends to spread out across the surface. This is called “wetting” the surface.

One method to measure the difference in surface energy is to measure the angle between a drop of liquid and the surface it contacts. A bead of liquid will have a high angle, but if the liquid wets the surface, the angle will be very flat. 

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Many properties of liquids are affected by temperature. This includes surface tension. The standard for measuring surface tension is 20OC or 68OF. As the water gets colder, surface tension increases. Conversely, when water is hotter, surface tension decreases. Reduced surface tension is one of the reasons hot water cleans better.

Let’s compare the surface tension of some liquids. If you have seen mercury, you recall how it forms into small balls or beads. It also climbs the inside of the capillary tube in thermometers. The surface energy is very high 487 dynes / cm. Water at 68OF is 72.8. Glycerine is a little lower at 63. Glycol Ether, a surfactant, comes in at 26. Ethyl alcohol is 22. Good surfactants have a surface tension of between 25 and 30 dynes / cm. Water near freezing is around 75 and drops to 59 near boiling.

Now let’s look at the surface energies of some of the surfaces we clean. Olefin comes in at 31. Polyester is 43 and nylon 46. They all resist being wet with just water. Cotton is just over 72. Water can wet out cotton. The outer surface of wool resists wetting, but once that layer is penetrated, water wets the surface and is absorbed by wool more readily.

The lower the surface tension of a liquid, the better it can spread across a material or even penetrate into that material. When the surface energy of the material is lower it keeps the liquid out. Like golf, the lower number wins. In the accompanying photos, polished granite has a lower surface tension than honed travertine. You can clearly see that the granite does a better job of keeping liquids out. If a sealer was added, surface energy would be even lower.  

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There are three common avenues for a cleaning product to remove soil from whatever is being cleaned. These include alkalinity, solvents, and surfactants. The product may use one or more of these. Surfactants will be the most expensive ingredients in any formulation. Fortunately, even a small amount of surfactant can have a significant influence on the cleaning.

Surfactants – a mash-up of “surface-active agents” – change the surface tension of water. The use of surfactants is key to separating soil from the substrate or surface the soil is attached to.

Surfactants consist of a water-loving (hydrophilic) head and a water-hating (hydrophobic) or oil-loving (oleophilic) tail.  The water-hating tail is attracted to anything that is not water. Therefore, they tend to accumulate at any interface where water meets air or any other substance such as soil.  In reaching for the air or soil, the hydrophobic tails get between water molecules and break the attraction they have for each other. 

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By interfering with the polar forces that attract one water molecule to the next, surfactants allow a cleaning solution to penetrate into porous surfaces and get between a surface and the attached soil.  Thus, the soil is separated from the surface. It is held suspended in the liquid by surfactant molecules with their water-hating ends attached to the soil and the other end in the water.

Surfactants aid cleaning by lowering the surface tension of the cleaning solution. This allows the solution to spread across the surface being cleaned and penetrate between the soil and the surface it is attached to.

Protectors for carpets, upholstery, grout, or stonework work, in part, by lowering the surface energy of the surface to which they are applied. In the case of very porous materials, including grout and stone, the sealer also works by filling in pores and capillaries. The lower surface energy of the protected surface makes it difficult for staining material to penetrate.

Special Thanks to Everard Paynter of Actichem / Applied Products of Australia for support in producing this article on Surface Tension.

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