Water and its properties

WATER AND ITS PROPERTIES.

Structural and dynamic features of liquid water

Physico-chemical characteristics of water.
The main chemical property in the constitution of organism is water. Not only active cells consist by 60-65 % of water, but also resting cells and tissues, for instance spores and seeds, include not less than 10-20 % of water [Levi A., Sikevitz F., 1971]. Modern biochemistry and bio-organic chemistry has enriched our minds with the knowledge of superfine details of structure of large albumen and nuclein acid molecules, and, at the same time, the notions of characteristics of water, a medium within which all the bio-chemical processes take place, of its structural features are quite limited. Water specialists acknowledge the following: "We do not deal with one, two or several, but we deal with many theories of water structure, which continue to emerge extremely fast. The way out should be found, and not only because these theories are imperfect, but also on account of their being successful" [Horn P, 1972]. Co-existence of many water structure models is likely to indicate the real multiformity of fundamental living substance, and it may be assumed that such a multiformity is necessary to realise the vital activity processes normally.

Judging nearly by all its physico-chemical properties water is a unique substance. If density of all the liquids grows with the temperature fall, water density reaches its maximum at 4oC and then decreases. Hence, ice remains on the water surface, preventing from absolute frost penetration into the water bodies. Water possesses an extremely high heat capacity: it heats up slowly, gets cold slowly and keeps much more heat energy than the other liquids under thermal change. These characteristics of water provide for maintenance of average temperatures comfortable for life on the Earth and for protection of living organisms from abrupt temperature drops. It is not quite well known that thermal dependency of water heat capacity, as well as of density, is non-monotone, either. At temperature of about 37oC heat capacity of water is minimal. This lucky "fortuity" enables us to liberate surplas heat quicker under body temperature rise higher than normal (36-37 oC).

Water has very high permittivity, which fact gives an opportunity to dissolve a large number of compounds in it. The importance of water surface tension is rarely high. This property, on the one hand, provides for the accumulation of immense energy on the surface of "water-air" separation, and, on the other hand, inhibits gas exchange between water and gas medium.
All the hypotheses, put forward to explicate nontrivial water characteristics, give consideration to the fact that apart from covalent bonds between H and O atoms inside the molecule itself there are also intermolecular hydrogen bonds (H-bonds). Hydrogen atom connected with a strongly electronegative element (nitrogen, oxygen, fluorine, etc.) has deficiency in electrons and thus is able to interact with an unshared electron pair of other electronegative atom of this or another molecule. Consequently, a hydrogen bond appears, graphically represented by the three points:

Bond energy: 25 kJ/mol 10 kJ/mol

This bond is much weaker than all the other chemical bonds (the energy of its formation is 10-40 kJoule/mole, whereas the energy of covalent bonds O-H exceeds 280 kJoule/mole) and, as until recently considered, is defined by electrostatic or donor-acceptor interactions.

The atoms of other elements do not form bonds like hydrogenous, because the electrostatic attraction forces of opposite ends of dipoles of polar bonds (O-H, N-H, etc.) are quite weak and only work on small distances. Having the smallest atom radius, hydrogen enables such fields to come close to each other so that attractive forces become apparent.

One of the most "customary" water models, given nearly in all physical chemistry manuals, is the model of Frank and Wen [Frank H.S., Wen W.Y., 1957]. According to it, hydrogen bonds in liquid water are being continuously formed and broken off. Moreover, these processes pass cooperatively within the limits of short-lived groups of water molecules, called "flickering clusters". Time of their life is determined over the range from 10-10 to 10-11 sec. Such an idea plausibly illustrates a high degree of liquid water mobility and its low viscosity. It is considered that owing to such characteristics water serves as one of the universal dissolvents.

Water Memory »

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