AN INTRIGUING LIQUID: WATER

Everybody knows water is H2O, but there is a little more to it: There is a good reason why an oxygen atom and two hydrogen atoms join in a family of three. The hydrogen atoms have what the oxygen atom always wanted an electron to fill its outer electron shell; and the hydrogen atoms are only too glad to stick their single and exposed electrons into the offered shelter, for they, too, are highly erudite in physical chemistry.

The hydrogen electrons are now shared, that is, they orbit around both the hydrogen and oxygen nuclei. But they spend more time going round the bigger oxygen atom, which has the effect of making the oxygen end of the molecule negative, and the end with the two hydrogen atoms positive. The resulting water molecule looks something like the figure below, with the big ball representing the oxygen atom.

If now two water molecules meet, they will attract each other: The negative oxygen end of one will attract one of the positive hydrogen atoms of the other. This bond (called a "hydrogen bond" and shown by the dashed line above) between two different molecules is not as strong as the chemical bond that holds a single molecule together; the H is held strongly to the O in its own home, but is only friends with the O in the neighboring molecule. When the water is warm enough to be liquid, its molecules are very agitated by the random heat motion; the homes will hold together, but the friendships between them will mostly be broken, and the hydrogen bonds will not hold, or hold only occasionally.

In ice, on the other hand, the hydrogen bonds are stronger than the thermal motion breaking them, which results in a rigid structure of an oxygen atom always being tied to one hydrogen atom of a neighboring molecule by a hydrogen bond. To figure out what this structure must look like in three dimensions is somewhat nightmarish, but it has all been worked out by the good people who write chemistry books, and the figure above right shows what it looks like. The important point is that this structure can be joined by other ice molecules, because they have the right form and polarity to attach themselves to this jigsaw puzzle, but not by alien molecules, such as salt, which is electrically neutral and incapable of hydrogen bonding.

When water freezes, all of its molecules arrange themselves into this hydrogen bonded structure. But even in liquid water there are such icy structures in places and at times. As was discovered in 1933, about half of all water molecules at any moment are in these icelike clusters, though they form and disintegrate quickly. Thus, half of what looks like water is, in a sense, ice. In real ice, of course these structures are extensive and permanent.

Now we can return to reverse osmosis membranes. They do not have pores from one side straight through to the other, but tiny spaces between their polymer fibres where small volumes of water are taken up. There it assumes the hydrogen-bonded structure of ice and will allow nothing but more ice to join it; salt molecules will not fit. When pressure is now applied to one side of the membrane, the ice will melt away into pure water on the other side and make way for new icy members to join this strictly segregated club on the entrance side. Thus, impurities and salt cannot even get into the door, let alone through it.

The only molecules other than water that can penetrate the membrane are those capable of hydrogen bonding with water. One of these is methanol, which, significantly, will also penetrate ordinary ice, and for the same reason. Fortunately, the trouble with sea water is not methanol, but salt.

Although desalination by reverse osmosis is still in its infancy, its small energy investment may well turn out to make it one of those "little' inventions that made a lot of difference like the clock escapement, which paved the way to precise time measurement, navigation, and the great discovery voyages of the Renaissance.

[More: "Synthetic Membrane Technology" by H.P. and C.D.Gregor, Scientifc American, July 1978; P.Eblin, Elements of Chemistry, Harcourt, Brace & World, 1965 (from which the picture of the ice molecule cluster is taken).]