What Is Surface Tension?

Hydrogen Bonds, Organized Water and Maximized Entropy

Jul 21, 2009 Art Ayers

Water at the surface is frozen in a stretched sheet of optimally bonded molecules, as molecules shake into the minimal energy configuration.

Each water molecule, consisting of one oxygen and two hydrogens (H20), can form hydrogen bonds with four other water molecules. Hydrogen bonds are very rigidly oriented, so that the hydrogen nucleus is directly in line with the oxygens on either side. This rigid orientation is temporary in liquid water, as hydrogen bonds between rapidly moving water molecules are made and broken thousands of times a second. But the action of the water molecules at the surface of the liquid, facing air, is very different.

Surface Tension Forms In the Following Steps

1. The number of hydrogen bonds formed by rigidly orienting water molecules at the surface is maximized.
2. Entropy is maximized by minimizing the number of water molecules organized in the surface film.
3. Tension of the surface film of organized water molecules is increased as high energy molecules of the film move to the low energy of the liquid.

Air is Hydrophobic, It Can’t Hydrogen Bond

Water molecules on the surface cannot form hydrogen bonds with air, because air is just widely spaced gas molecules. The surface water only occasionally comes in contact with molecules of nitrogen and oxygen as they collide. No bonds are formed and the gas molecules return to the air.

Surface Water Molecules Become Organized Into a Film

Randomly oriented water molecules exposed to air will have either an electron pair of the oxygen, or a hydrogen pointing toward the air and unable to make hydrogen bonds. Continued motion will eventually orient the molecules so that hydrogen bonds form. More motion will only lead to one of the hydrogen bonds breaking and a high energy state, so the water molecules become trapped in an organized surface film of structured water with the maximum number of hydrogen bonds. The energy released in hydrogen bond formation is transferred to the energy of motion, kinetic energy, of the liquid water. A molecular film of frozen water has formed on the surface.

Randomly Oriented Water is High in Entropy and Low in Energy

The randomness of the water, its entropy, was sacrificed as hydrogen bonds were maximized. This is a trade-off. Bond formation releases energy and organization of the film takes up energy in the form of lower entropy. The lowest energy state is reached when the water molecules shake into the maximum number of bonds in the organized surface film.

Minimizing Organized Molecules Maximizes Entropy

The surface film flattens as organization of the surface layer continues and fewer molecules of low entropy and high energy are included in the surface. Every time the surface molecules shake into a structure with fewer molecules, the energy in the surface lowers. Conversely, water molecules must have unusually high kinetic energies to blast their way into the organized, high energy surface layer. As a consequence the surface area decreases.

A Tighter Surface Film is Lower in Energy

Fewer molecules are needed if a slight stretch is applied to the hydrogen bonds of the surface film, so each random motion that applies tension to the layer, results in more molecules recruited into the lower energy of the liquid. Thus, the pounding of molecules of the liquid below tightens the film and shakes more molecules out of the surface. It is this tightness of the surface film of organized water molecules that is observed as surface tension.

The copyright of the article What Is Surface Tension? in Chemistry is owned by Art Ayers. Permission to republish What Is Surface Tension? in print or online must be granted by the author in writing.

Surface Tension, Hydrogen Bonds, Entropy

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