Water structure in presence of solutes

Water tends to hydrate many solutes. The hydration number is the number of water molecules bound to solute sufficiently strongly so as to become part of it. The hydration number varies with different solute molecules Na+, 3.9 0.5 Ca2+, 12 + 2 Al3+, 22 2 glycerol, 2 0.5 sucrose 5 0.5 Fe3+ 18 2. Water molecules form a hydration shell around the solute molecules. In case of molecules such as Al3+, the large number of water molecules is unlikely to be accommodated in the first hydration shell. X-ray diffraction has shown a highly ordered second hydration shell around Al3+ (Zavitsas, 2001). 

In case of non-ionic surfactants in water, the behaviour of the water structure outlines three main concentration regions, which closely coincide with the three phases intersected by the experimental isotherms. In the micellar solution phase, no significant changes in the water structure are indicated, while, in the lamellar phase, rapid destruction of the tetrahedral hydrogen bond network occurs due to the confinement of the water between the hydrophilic surfaces of the lamellae. The dehydration of the surfactant head groups was found to start near the border between the lamellar and the reverse micellar solution phases. At higher concentrations, water demonstrates its trend to form clusters of tetrahedrally bonded molecules even at the very low content in the system. The results with surfactant solutions have been obtained by Raman spectroscopy (Marinov el al., 2001). 

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