As is expected the density of light water does not show any appreciable change due to the very small quantity of deuteriated water at the 1 - 150 ppm level. Surface tension (ST) on the other hand is the result of weak inter-molecular interactions in liquid water. The decrease of ST with increasing temperature is usually associated with a decrease in density due to the increase of intermolecular distances. However, it should be noted, all experimental measurements indicate that the changes of ST far exceed the corresponding change in density. For example, in the temperature range from 0 to 100°C, the volume is changed by 4% while ST is reduced by 22%. This significant change of ST can not be explained only by the change in the density or the interaction of individual molecules, but can be tackled on the basis of the cluster model. With increasing temperature, the average cluster size decreases. Also, the increase in temperature enlarges the intercluster distances, which entails a reduction of unbound O-H groups in the surface layer, that causes a decrease of ST.
At a constant isotopic content the size of the clusters depends on the temperature. The gradual increase of the temperature also leads to an increase of intercluster distances and the breaking of large clusters into the smaller ones which reflects an increase of free hydrogen bond O-H groups. However at selected constant temperatures, the fraction of NHB H atoms varies as a function of the concentration of deuterium, which is manifested in a change in the characteristics of the viscosity and the surface tension. This change in the physicochemical properties of water means that the deuterium introduced into the system affect the distribution of the bound and unbound OH groups. The question arises is how? From the data conducted for experiments with a light water one sees that the surface tension has a maximum value for a content of deuterium ≤ 96.5 ppm. The atoms of deuterium having mass twice of the protium make more stronger O-D bonds than O-H bonds. Our theoretical calculations show stable spherical structures with encapsulated guest molecules [23]. We anticipate that the deuterium atoms form the core of the cluster, around which the light molecules are arranged symmetrically in a strictly defined mosaic order. Upon reduced concentration of deuterium (≤155 ppm) the clusters can be expected of a larger size than in extreme cases of pure light and heavy water, which means fewer NHB H atoms and hence a greater surface tension. Because the larger clusters have a lower “mobility” in comparison with small water clusters, then the dynamic viscosity must also to increase with increasing cluster size and the registered deviations of kinematic viscosity values must be in consistency with the corresponding deviations of dynamic viscosity at the condition of constant density of water.
Dilution of the light water with the heavy water content should lead to the increase of the cluster size. This trend reaches its maximum at D/H ∼150 ppm inherent to the normal water. Then, however, the further deuterium atoms compete with each other in the process of cluster formation, which leads to a decrease of the clusters size under the excess of the deuterium isotope. Let’s try to answer the question what is really behind this mechanism? In our earlier work [23] we investigated the homologous series of icosahedral water clusters with various inclusions (CO2, CH4, (H2O)
n
) (see Figure 1). Among these series the smallest spherical cluster, which can accommodate, for instance a deuterated hydronium cation, is dodekahedra (b) with 20 water molecules composed from 12 pentamers (a). Next to it is a cluster (c) consisting of 100 water molecules on the surface of which there are 30 unbound hydrogen atoms. If 4 additional deuterated active centers appear then the cluster can break down into five basic dodecahedra, which together will produce 50 unbound hydrogen atoms (NHB H atoms). The next largest cluster (d) contains 280 water molecules and only 60 NHB H atoms. The collapse of such a cluster into 14 elementary subclusters will make possible the formation of 140 NHB H atoms. Thus an increase in deuterium content leads to a decrease in the size of the clusters, which in its turn leads to an increase of NHBs, and the last in our opinion, causes a decrease of surface tension and viscosity under conditions of constant temperature.
When the ice melts at 0°C into the liquid water it absorbs an energy of 80cal/g [24]. This energy 80cal/gm does not lead to an increase in temperature. We may speculate that the major part of this energy is spent for cleavage of 13% of the hydrogen bonds in the structure of ice, while the 87% of the bonds may remain intact. Therefore, the cluster sizes must be huge in order to accommodate 87% hydrogen bonds “inherited” from the structure of ice. Small clusters mean a large number of broken hydrogen bonds. The experimental data display the decrease in the size of the clusters with the increase in temperature [25]. Also theoretical calculations [26, 27] show the broadening of oxygen-oxygen radial distribution function upon temperature increase. Apparently the elongation of inter-cluster distance contributes to the water density decrease. The increase in inter-cluster distances with increasing temperature also leads to a decrease in the number of NHB H atoms per unit area and, consequently, ST reduction.
Finally, considering the role of isotope effect on physical properties of water one can not neglect the O18/O16 ratio. Based on results obtained from our investigation we may conclude that the concentration of the heavy isotope of oxygen is closely related with the concentration of deuterium. The increase of deuterium connected with the increase of 18O/16O ratio: at 4 ppm of D/H the O18/O16 is 910 ppm,; at 52,53% D/H the 18O/16O concentrations are 1479,1552 ppm correspondingly. This indirectly signifies the bound character of these two heavy isotopes. The simultaneous increase or decrease of 18O and 2H quantities become possible during formation of covalent bonds between them which give appearance to the D218O, HD18O molecules. It should be noted that these molecules make hydrogen bonding stronger than the 1H216O or HD16O species due to the lower energy of zero point vibrational level. In our opinion the heavy isotopes of oxygen when they participate in cluster formation play a static role and make an additional contribution to the stability of the cluster while the deuterium atoms govern the whole process of water structuring and hence causes the qualitative changes of physicochemical properties of water.
For the first time the experimental studies have been conducted on deuterium depleted water. The anomalous properties of water at 4°C are found to be due to the heavy isotope of protium, which is responsible for the water cluster formation. The developed conception of determining factor of deuterium is in a good agreement with the experimental data obtained for the water samples with various deuterium content at the same conditions. The represented model provides a comprehensive assessment of the “isotope composition - water structure” relations.