Coordination number of water

Although, as will be seen in this section, the developments of neutron diffraction during the 1980s (Enderby and Neilson) have led to substantial advances in determining the coordination numbers of water molecules around ions, thae are stiU some hurdles to clear before one can use this powerful method ... 

The coordination of Cu + in zeolite rho with other adsorbates than water has also been studied by ESEM methods. Table I summarizes the adsorbate coordination number of water and other adsorbates for Cuz in rho zeolites with various cocations (6,7). 

The hydration shell sfructure and the local density around and OH show interesting features with temperature variation. Though the first-shell structures around the solute ions remain more or less intact over temperature changes, the second hydration shell and beyond become loose, and even disappear as temperature increases. It is found that the coordination number of water molecules around an OH is 4.5 at 300 K, which decreases slowly with increasing temperature. For example, it becomes 4 at 800 K. 

As temperature decreases ASW and WAW signal intensity gets somewhat smaller, which sng-gests that these types of water can be partially frozen. When ice formation process takes place, coordination number of water molecules inside of the crystallites is eqnal to four. Therefore for the nucleation process of ASW and WAW, the formation of 3D water clnsters is necessary. The formation of 3D crystallites in the narrow slits is hindered by steric factors. Before freezing ASW and WAW molecules possess certain mobility, which assists formation of ordered strnctnres of bound water from amorphous to crystalline as a result of rotational and translational diffusion of molecules. It can be suggested that ASW- and WAW-formed ice crystals have small size and slightly... 

As briefly described in O Sect. 18.2.3.2, soft-donor extractants have potential for selective separation of trivalent actinide ions. To understand the difference in extraction between Am " and trivalent lanthanide elements, Yaita et al. (2001) studied Am and Nd complexes with N-donor ligand benzimidazole by EXAFS. Although they found slight difference in the coordination number of water molecules and in the distance, Yaita et al. concluded that the coordination structure of Am - benzimidazole complexes is almost similar to that of Nd " complexes. 

The temperature dependence of the coordination number of water is also in sharp contrast to that of a simple fluid. According to computations made by Samoilov (1957), the coordination number of water increases slightly from 4.4 at 4°C to about 4.9 at 83°C. Whereas the coordination number of argon decreases substantially with rising temperature, Narten and Levy (1972), using more recent data on water, concluded that the coordination number of water is almost constant with temperature. [Note that differences in the definition of the concept of coordination number may exist. For a survey of the various definitions, the reader is referred to Pings (1968).]... 

Second, we pointed out the slight increase in the average coordination number of water as we increase the temperature (see Fig. 7.7), i.e.,... 

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