Is Water Special?

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28 Jun 2004 13:33:37 -0000

Is Water Special?

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ISIS Press Release 28/06/04

New age of water

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Water has come of age. It is cool on everyone's lips.

Decades of research on water is giving us remarkable

insights into its dynamic collective structure, and changing

our big picture of life and living process.

 

Organisms are seventy to eighty percent water. Is this water

necessary to life? What vital functions does it serve?

Entire biochemistry and cell biology textbooks are still

being written without ever mentioning the role of water. It

is simply treated as the inert medium in which all the

specific biochemical reactions are being played out.

 

Instead, recent findings are raising the possibility that it

is water that's stage-managing the biochemical drama of

life. Water is life, it is the key to every living activity.

Some people will even say it is the seat of consciousness.

 

ISIS brings you the latest revelations on water in this

extended series that starts from the basics. The articles

will not be circulated consecutively, so do watch out for

them.

 

Is Water Special?

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Water has a collective structure that's extremely flexible

and dynamic, which may explain some of its 'anomalies'. Dr.

Mae-Wan Ho explains Sources for this report are available in

the ISIS members site

http://www.i-sis.org.uk/full/IsWaterSpecialFull.php.

Full details here

http://www.i-sis.org.uk/membership.php

 

Water is simple, isn't it?

 

There is nothing simpler than water as a molecule. Its

chemical formula, H2O, is almost the first thing in

chemistry that one learns in school. However, its structure

in the bulk is multifarious and changeable. There are 13

known crystalline structures of ice that appear under

different temperatures and pressures. As a liquid, water

forms dynamic 'flickering clusters' or networks of joined up

molecules, with intermolecular bonds that flicker on and off

at random. The basis for all this complexity lies in the

ability of a water molecule to join up with its neighbours

through a special kind of chemical bond, the hydrogen bond.

 

The hydrogen-bond

 

To understand how the hydrogen bond comes about, picture the

water molecule consisting of an oxygen atom bonded to two

hydrogen atoms. The water molecule has a shape approximating

a tetrahedron, a three-dimensional triangle with four

corners. The oxygen atom sits in the heart of the

tetrahedron, the hydrogen atoms point towards two of the

four corners and two 'electron clouds' belonging to the

oxygen molecule point towards the remaining corners of the

tetrahedron. The 'electron clouds' are negatively charged,

and result from the atomic structures of oxygen and hydrogen

and how they combine in the water molecule.

 

Oxygen has eight (negatively charged) electrons disposed

around its positively charged nucleus, rather like the layer

of the onion, two in an inner shell and six in the outer

shell. The inner shell can only accommodate two electrons,

so its capacity is filled. The outer shell, however, can

hold as many as eight electrons. The hydrogen atom happens

to have only one electron, so oxygen, by combining with two

hydrogen atoms, completes its outer shell, while the

hydrogen atoms each completes its first electron shell with

two electrons, which it shares with the oxygen atom. That is

how the usual 'covalent bond' of chemistry arises.

 

The oxygen nucleus has more positive charges than the

hydrogen, so the shared electrons are slightly more

attracted to the oxygen nucleus than to the hydrogen

nucleus, which makes the water molecule polar, with two

'electron clouds' of negative charge at the opposite poles

to the two hydrogen atoms, which are each left with a slight

positive charge. (Though quantum mechanical calculations

have shown that the two electron clouds are not really

separate from each other.)

 

The positively charged hydrogen of one water molecule can

thus attract the negatively charged oxygen of a neighbouring

water molecule to form a hydrogen-bond (H-bond) between

them. Each molecule of water can potentially form four H-

bonds. Two in which it 'donates' its hydrogen atoms to the

oxygen atoms of two other water molecules, and two in which

its oxygen atom 'accepts' one hydrogen atom from each of two

other water molecules. In other words, each molecule is

capable of acting as hydrogen 'donors' and 'acceptors' for

two other water molecules, so it has four bonded neighbours,

or a '4-coordination'.

 

Ice structures

 

Water molecules in ordinary hexagonal ice crystals are close

to the ideal tetrahedral structure described above. The

hydrogen-bonded O-O distances are almost identical, varying

between 2.759 Å and 2.761 Å (an angstrom is 10-10m), while

the O-O-O angles also vary only slightly between 109.36o and

109.58o, which is close to the H-O-H angle of 104.52o of the

individual water molecule.

 

However, there are many more forms of ice crystals (at least

12 others known) under different temperatures and pressures,

where the bond lengths and angles vary much more widely. For

ice II, which forms under moderate pressure of about 5 kbar

(1kbar is equivalent to a pressure of ~ 1 000 atmospheres),

the basic four-coordinated motif is maintained. But the bond

length varies between 2.74 Å and 2.83 Å, while the bond

angle varies between 80 o and 129 o.

 

In liquid water, there is much less constraint compared to a

solid crystal lattice, and so the variations in bond length

and bond angles take on a much wider continuous range.

Instead of the regular hexagonal (6-member) ring structure

of ordinary ice, a snapshot of the hydrogen-bonded network

shows five, six and seven-member rings, and even smaller or

larger rings. Instead of the 4-coordination motif, 2-, 3-

and even 5-coordinations are possible, with the H of some

water molecules in a 'bifurcated' schizophrenic state,

seemingly bonded to two different neighbours.

 

Why is water special?

 

Why is water so special that life cannot exist without it?

According to John L Finney of University College, London,

the basic tetrahedral structure of the water molecule is

central to the structural versatility of water in the

condensed state (solid and liquid). It enables water to form

extended, flexible networks of H-bonded molecules in liquid,

allowing rapid coordinated molecular motions to take place.

This same extended network also supports proton conduction,

a flow of positive electricity that occurs much faster than

the diffusion of ions. Other substances might have some of

those special characteristics, says Finney, but only water

has them all, and that might be enough to make water

especially 'fit' for life.

 

New insights into water structure

 

The picture of the structure of water just described has

been obtained with powerful measurements techniques such as

x-ray and neutron diffraction, which involve firing x-rays

or neutron beams at water, and looking at the way the beams

are deflected or scattered to make a diffraction pattern,

which gives information about the structure of the atoms.

These experimental techniques are combined with computer

simulations (molecular dynamics) to give a consistent

picture, which is supposed to form a firm molecular basis

for all other investigations. But in April 2004, an

international team of scientists from universities and

research institutes in the United States, The Netherlands,

Sweden and Germany, have challenged this picture with the

next generation of an even more powerful measurement

technique.

 

They reported the behaviour of liquid water on a timescale

of less than one femtosecond (one femtosecond is 10-15s)

using a new x-ray absorption spectroscopy technique. This

involves firing x-rays of different frequencies at water,

and from the spectrum of frequencies absorbed - which is

characteristic of each atom - making inferences concerning

the structure of the water molecules.

 

They found that most molecules in bulk liquid water at room

temperature are like those at the ice surface, with only two

strong hydrogen bonds. The proportion of molecules with 4-

coordination similar to bulk ice is very small. The

contributions of the two different species - molecules with

two H-bonds and those with 4 H-bonds - are 80% and 20% at

room temperature, and increases to 85% and 15% at 90C with

uncertainties of +15% and +20% in both cases.

 

As consistent with earlier results, the bond lengths and

bond angles are found to vary widely from those in

tetrahedral ice, attesting to the flexibility of the water

structure in liquid.

 

They concluded: " Water is a dynamic liquid where H-bonds are

continuously broken and reformed. The present result that

water, probed subfemtosecond time scale, consists mainly of

structure with two strong H-bonds, one donating and one

accepting, nonetheless implies that most molecules are

arranged in strongly H-bonded chains or rings embedded in a

disordered cluster network connected mainly by weak H-

bonds. "

 

So, in a sense, it doesn't really alter the picture too

much. But are these methods focussing too much on the

individual molecules to reveal anything interesting? A

growing number of water scientists are beginning to think

so, and for good reasons.

 

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