WATER, H2O

as regarded by a biophysicist

Part I.

General properties

We refer to “water” as the condensed phase of the molecule H2O at approximately 1 bar

pressure.

Why waste valuable time on water?

Most biophysics texts either

- ignore the water solvent or

- mutter darkly that water is very complex.

In fact, we all have been taught that the condensed phase of water in its liquid form is a very

complex liquid. Is it really complicated in comparison to say, liquid Helium 3?

The answer is a sound “Yes”, but in a different sense than the mathematical and quantum

mechanical complexities of Helium. Water is a liquid with very strong many-body effects,

but it is hot compared to Helium 3 and so many of the problems are almost classical in nature.

We must understand to some degree water, since it is the primary solvent for biomolecules. This

isn’t too surprising, since water is the only naturally occurring inorganic liquid on the surface of

the earth so life on earth has to play with the hand it was dealt. The water’s unique properties

seem quite important in terms of allowing life at all to exist. It is altogether wonderful that the

only liquid present in abundance is also one of the most complex and useful in its properties.

People have speculated in the past that on other planets silicon may substitute for carbon

chemistry, but they also should consider well the primary role that water plays in biology. It may

well be the truly crucial ingredient that cannot be replaced. We seek simplicity, yet we find that

even the solvent for biomolecules is very complex.

Bizarre chapter in the history of science: the polywater craze.

We will see that water is a highly correlated liquid, held together by a network of

hydrogen bonds. It was proposed that under certain conditions water could

“polymerize” into a highly viscous form called “Polywater”. Obligingly, an obscure

Russian chemist found that by exhaustive distillation of distilled water a highly viscous

“form” of water could be isolated. Of course, ultimately the whole bubble burst as it was

revealed that the isolators of polywater were purifying either

-highly concentrated salt solutions or

- silica gels

derived from the powerful solvent capability of water.

Water IS so unusual that it isn’t too hard to ascribe truly fantastic properties to it.

General characteristics of water

that do not concern the molecular properties directly, but are very important for biology:

1) It is the only inorganic substance and chemical compound that is found in all three phases –

gas, liquid, and solid – in nature.

2) 93% of blood plasma, 80% of muscle, and 60% of red cell mass is water.

3) 80% of fresh water is in the frozen state (e.g. glacial ice).

4) < 3% of all water on the surface of the earth is fresh.

5) 95% of the fresh water that is in the liquid state, is in ground deposits.

6) 2/3 of the planet is covered by water.

7) 105 km3 H2O passes through the photosynthetic pathway.

H2O is the most reduced form of O, and

CO2 is the most oxidized form of carbon.

These two molecules are the end products of the metabolism in the mitochondria

C6H12O6 + 3O2 + 38ADP + 38Pi → 6CO2 + 6 H2O + 38ATP

An average human produces 300 gm of H2O per day, and used 7600 kJ/day to produce ATP.

8) Some interesting physical characteristics of H2O: • High dielectric constant = 78.5 D (Debye); MeOH has 32.6 and H2S has 9.3. So, it is a very polar solvent. Electrically charged molecules are easily dissociated in H2O. The dielectric constant of ice is greater for ice than it is for liquid water. This is very unusual.

• Water has a very high heat capacity – it can act as a thermal buffer. This is required to keep the temperature in biological cells constant.

The relatively large heat capacity of water “disappears” with larger concentration of ions; this points to the reduction of entropy when the ions are added to the solution. This is usually attributed to the structuring effect of water around the ions. The normal H-bonds of water do not (supposedly) exist around the ions. In water there are a large number of microstates (high entropy) available for the water molecules in bulk water. Remember the enthalpy of hydration of these ions is highly negative, because of the number of primary and secondary hydration sheaths around the ions, as well as the H-bond energy that is lost when the bulk water structure is perturbed.

Also, the existence of an ionic atmosphere where the H-bonds do not exist not only accounts for the fact that the heat capacity decreases as the ion concentrations increase, but also accounts for the smaller volume of the water surrounding the ions (this is usually called electrostriction). The hexagonal “structure” of the water is perturbed.

• Very high heat of vaporization (40 kJ/mol). This is due to the large extent of H-bonding in bulk water (all solvents that have such extensive H-bonding, have relatively high heats of vaporization). This is of course important for cooling by perspiration.

It is interesting to compare the heat of vaporization of water to other compounds that have the same number of electrons and those where the hydride structures are the same, and the atoms of the hydrides are from the same group as oxygen. This emphasizes the unusual properties of water.

• Liquid water has a density larger than ice. This has important consequences for fish and aquatic

life, and probably for evolution. Also, the action and movement of glaciers depends on this. In

addition, this is very important for the ability of water to slowly destroy rock structures, which is

a major process forming the structure of the surface of the earth.

• Water has a very high surface tension, and this has an important effect of minimizing the

surface areas of water. This is also important for capillary action. In lungs, surfactants must be

present to open alveolar spaces for respiration. These are biological molecules that are made by

the organisms. If these are not present, or not enough of them, this leads to respiratory diseases,

and even to death.

• Water has a larger conductivity (proton conductivity) than would be expected from its

ionization constant (which is very low). The conductivity of ice is just as high. This is probably

due to a brigade mechanism for transport through large conjugates of H-bonds.

Is water really unusual?

The first clue to the unusual nature of water can be found in the melting and boiling points for water. The melting and boiling points of water are significantly higher than would be expected for a liquid made from a molecule as light as H2O. Clearly, there are strong associative forcesbetween water molecules. However, many molecules have strong associative forces.

There is another aspect to water that is quite unusual: the latent heats of fusion and evaporation and the specific heat. The latent heat of evaporation for liquid water, 40.5 kJ/mol, is indeed large (as we would expect), but it is not amazingly large: for example, liquid sodium at 107 kJ/mol is considerably larger. What is strange is the amazingly small latent heat of fusion of water which is often misrepresented as being very large! Note that water’s latent heat of fusion, 5.98 kJ/mol, is very small, in fact smaller than the latent heat of fusion of liquid argon, an inert element! One can easily conclude that either liquid water is very highly structured, rather like ice, or that ice is rather highly disordered, like water. In reality, both viewpoints have elements of truth to them. This is clear from the unusually large specific heat of liquid water, 75.2 J/mol-K, which is far larger than similar materials, and twice the heat capacity of ice. Appearently there is much structure in “liquid” water, which is being broken down with increasing temperature.

Other physical parameters of water are not so amazing.

For example,

the viscosity of water, while high, is not outrageous, nor are

the self diffusion constants of water highly unusual.

However, as we probe water in other sections on a microscopic scale we will find that other

unusual properties of water will manifest themselves on a molecular level. The parameter which

probably best characterizes the interaction of water molecules with each other is

the latent heat of evaporation. To cast that into more usable units, it is 40.5 kJ/mol or about 0.5

eV/molecule.