New state of water molecule discovered

Picture this: There’s a molecule in state A. And let’s say, said molecule in state A wants to be in state B. But passing from state A to state B would cost the molecule some energy. According to classical mechanics, the molecule should pass over a hypothetical energy barrier, if it is to reach the state B. What’s worth noting here is that this crossover from state A to state B can occur only if the molecule possesses energy sufficient to overcome the aforesaid energy barrier. Quantum mechanics, on the other hand, presents an altogether new picture. It posits that the molecule in state A can attain the state B even if it does not possess the required energy for crossover to take place; it reaches the new state just by tunneling through the hypothetical energy barrier.

A new state of water scientists at Oak Ridge National Laboratory in the United States, in collaboration with a couple of other research groups, have reported on the discovery of a new state of water in an article recently published in Physical Review Letters. The researchers studied water confined in the channels of the mineral beryl. These channels — hexagonal in shape — are about 0.5 nm in diameter (one nm is equal to one billionth of a metre). Using First Principles calculations, they found that the water molecules occupy six symmetrical orientations in the channels of beryl.

While these orientations were found to be in agreement with the crystal structure of water, what was interesting was that these six orientations were separated from each other by about 50 meV. Furthermore, despite the energy difference between the different orientations, it was found that the hydrogen atoms in the water molecules could freely tunnel between them. This unhindered tunneling of the protons between the six orientations reduces their ground state energy into multiple levels. Neutron scattering studies carried out by the team complemented the ab initio results: the multiple peaks obtained in the scattering experiments were consistent with the energy differences among the levels.

Another key finding: kinetic energy calculations of water protons in the beryl nanochannels showed that it was 30 per cent lower than in bulk water. This was attributed to a strong delocalisation of the protons over the six symmetrically equivalent positions in the channel’s hexagonal faces. Further credence to this rationale was lent by the elongation of the covalent bond in the beryl-confined water molecule, and the disappearance of its dipole moment.

What is this thing called confined water, anyway Water in a river, a glass, or even a drop of water is bulk water. This is water we can see. As opposed to this, there is water encapsulated in the interstices of minerals, rocks, biological cells etc. that has markedly different properties from bulk water. This water is held in nano-sized cavities, and is called confined water. It therefore makes sense to say that if one is interested in studying the structure and dynamics of water, say, in cells in the human body, then one would need a suitable nanocontainer for the water that would mimic its natural environment. To meet this objective, a wide array of confining media like reverse micelles, microemulsions, zeolite cages, clays, fullerenes, carbon nanotubes etc. are used. In all these cases, water is sequestered in the nano cavities of these media, which has drastic effects on its properties. To wit: confined water shows elevation of boiling point, depression of freezing point, a distortion in its hydrogen bond network and so on.

What’s the take home And why is this important One: Water protons in the channels of the mineral beryl tunnel between six symmetrically equivalent positions. Two: Protons in this case have an anomolously low kinetic energy due to spatial delocalisation. Because of this tunneling behaviour depicted by water, the team call it a “new state of the water molecule”.

Water is easily the most researched molecule not just on earth, it has relevance ranging from geochemistry to astrobiology. And given that water in nature occurs as confined water, it only becomes imperative that we investigate it in a bid to obtain its comprehensive picture. This study is a big step further in that quest in its finding of water that tunnels in confinement.

(The author teaches Chemistry at Women’s Christian College, Chennai. She is a scientist by training, and a logophile by temperament.)