As explained above, bulk liquid water consists of a seething mass of various-sized chain-like groups and that flicker in and out of existence on a time scale of picoseconds. But in the vicinity of a solid surface or of another molecule or ion that possesses an unbalanced electric charge, water molecules can become oriented and sometimes even bound into relatively stable structures.

Water in ionic hydration shells

Water molecules interact strongly with ions, which are electrically charged atoms or molecules. Dissolution of ordinary salt (NaCl) in water yields a solution containing the ions Na+ and Cl –. Owing to its high polarity, the H2O molecules closest to the dissolved ion are strongly attached to it, forming what is known as the inner or primary hydration shell. Positively charged ions such as Na+ attract the negative (oxygen) ends of the H2O molecules, as shown in the diagram below. The ordered structure within the primary shell creates, through hydrogen bonding, a region in which the surrounding waters are also somewhat ordered; this is the outer hydration shell, or cybotactic region.

Some recent experiments have revealed a degree of covalent bonding between the d-orbitals of transition metal ions and the oxygen atoms of water molecules in the inner hydration shell.

Bound water in biological systems

It has long been known that the intracellular water very close to any membrane or organelle (sometimes called vicinal water) is organized very differently from bulk water, and that this structured water plays a significant role in governing the shape (and thus biological activity) of large folded biopolymers. It is important to bear in mind, however, that the structure of the water in these regions is imposed solely by the geometry of the surrounding hydrogen bonding sites.

Water can hydrogen bond not only to itself, but also to any other molecules that have -OH or -NH2 units hanging off of them. This includes simple molecules such as alcohols, surfaces such as glass, and macromolecules such as proteins. The biological activity of proteins (of which enzymes are an important subset) is critically dependent not only on their composition but also on the way these huge molecules are folded; this folding involves hydrogen-bonded interactions with water, and also between different parts of the molecule itself. Anything that disrupts these intramolecular hydrogen bonds will denature the protein and destroy its biological activity. This is essentially what happens when you boil an egg; the bonds that hold the egg white protein in its compact folded arrangement break apart so that the molecules unfold into a tangled, insoluble mass that, like Humpty Dumpty, cannot be restored to their original forms. Note that hydrogen-bonding need not always involve water; thus the two parts of the DNA double helix are held together by H—N—H hydrogen bonds.

 
The picture above, taken from the work of William Royer Jr. of the U. Mass. Medical School, shows the water structure (small green circles) that exists in the space between the two halves of a kind of dimeric hemoglobin. The thin dotted lines represent hydrogen bonds. Owing to the geometry of the hydrogen-bonding sites on the heme protein backbones, the H2O molecules within this region are highly ordered; the local water structure is stabilized by these hydrogen bonds, and the resulting water cluster in turn stabilizes this particular geometric form of the hemoglobin dimer. More diagrams, with commentary, can be found on Prof. Royer's Web site.

In 2003, some chemists in India found that a suitable molecular backbone (above) may even cause water molecules to form a "thread" that can snake its way though the more open space of the larger molecules. What all of these examples show is that water can have highly organized local structures when it interacts with molecules capable of imposing these structures on the water.

"Clustered", "Unclustered" and other structure-altered waters

The "alternative" health market is full of goofy products which purport to alter the structure of water by stabilizing groups of H2O molecules into permanent clusters of 4-8 molecules, or alternatively, to break up what they claim are the larger clusters (usually 10-15 molecules) that they say normally exist in water. The object in either case is to promote the flow of water into the body's cells ("cellular hydration"). This is of course utter nonsense; there is no credible scientific evidence for any of these claims, many of which verge on the bizarre. There are even some scientifically absurd U.S. Patents for the manufacture of so-called "Clustered Water™". At least 20 products that we are aware of, of this kind, are offered to the scientifically naive public through hundreds of Web sites and late-night radio "infomercials". None of these claims is supported by credible evidence.

Does water have "memory"?

According to modern-day proponents of homeopathy, it must. Diluting solutions of various substances, so greatly that not even a single molecule of the active substance can be expected to be present in the final medication, makes homeopathic remedies. Now that even the homeopaths have come to accept this fact, they explain that the water somehow retains the "imprint" or "memory" of the original solute.

Homeopathy

In 1985, the late Jacques Benveniste, a French biologist, conducted experiments that purported to show that a certain type of cellular immune response could be brought about by an anti-immunoglobulin agent that had been diluted to such an extent that it is highly unlikely that even one molecule of this agent remained in the aqueous solution. He interpreted this to indicate that water could somehow retain an impression, or "memory", of a solute that had been diluted out of existence. Proponents of homeopathy immediately embraced these experiments as the justification for the dogma that similarly diluted remedies could be effective as alternative medical agents.

The consensus today among most chemists is that any temporary disruption of the water structure by a dissolved agent would disappear within a fraction of a nanosecond after its removal by dilution, owing to the vigorous thermal motions of the water molecules. Unfortunately, other scientists have never convincingly replicated Benveniste’s results.

References:

Water Clusters. K. Liu, J.D. Cruzan, and R.J. Saykally. Science 1996 929-993 - A summary of experimental data on the structures, energetics and dynamics of small clusters, and comparisons with theoretical predictions.

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