Sticking Together

Water: Surface Tension, Hydrogen Bonds and Entropy

The interactions of water molecules are complicated, because water is a dipole with areas that are positively or negatively charged and when the regions of opposite charge on two molecules are close enough to attract and in the correct orientation, a hydrogen bond can form. Like shock-corded tent poles (with elastic cord threaded through hollow poles) that can be shaken until they assemble into an interlocked pole, water molecules are held together, elastically, until they orient and form hydrogen bonds (with release of energy) or a knocked apart by other bombarding molecules.
Water in liquid in jossled around continuously by collisions. Forming hydrogen bonds tends to keep two molecules together a little longer than just the attraction of their two weak opposite charges, but the continuous bombardment and shaking due to thermal kinetic energy causes the molecules to change bonding partners thousands of times per second. Sometimes large aggregates of dozens of

water molecules form into little icebergs, but these are quickly broken up in seconds.

At the surface, water molecules cannot hydrogen bond with air, because the air molecules (oxygen and nitrogen) are symmetrical molecules that are not charged and they are very few and far between. Thus, the water molecules on the surface cannot hydrogen bond with water molecule on both sides as they can if they are randomly oriented. If all of the water molecules on the surface get organized just right, then a maximum number of hydrogen bonds will form and the surface is at its lowest energy. Thus exposing water to a hydrophobic, non-hydrogen bonding surface, results in a layer of organized water that is much more stable than random water. This organization has an energy cost in the form of lower entropy, i.e. less random, and therefore from an entropy persective the surface layer is at a higher energy than random water. Thus, the surface is a trade off between having the maximum number of hydrogen bonds and the minimum amount of organized water. The layer is further optimized by stretching the bonds to the maximum to have the fewest molecules involved. Thus surface layers are under tension, i.e. surface tension. The tension slowly builds up after the surface is exposed and can be released by adding molecules, such as soap, that have functional groups that can face the air, but don’t form strong lateral bonds.

The little soap-powered boat works by the continuous release of soap molecules at the rear so that the pull of the organized, stretched water at the front pulls the boat forward. The tension is replenished as water molecules in the slackened region ahead are lost and the lower energy state of stretched water is regained. Forward motion stops when the soap has made a single layer of soap molecules across the surface behind the boat. The total volume of the soap slick surface area times the thickness of one molecule is the same as the volume of soap applied. The soap layer maintains the tension across it, because separation of the soap molecules requires the same force as the water, because it exposes water molecules beneath the soap layer that will not be able to hydrogen bond. Thus, the water below the soap will also be organized and tense, with the hydrophilic head of the soap molecules bonding in the layer of organized water molecules..

A soap bubble is just like two layers of surface water, back-to-back with partially randomized water in between. The hydrophobic air is on the inside and outside of the bubble. Random water can drain down by gravity and drip out the bottom. When the water layer is depleted at the top of the bubble, the top bubble membrane thins, fewer water molecules are available to form lateral bonds and the bubble ruptures from the top.