Hydration shell of ion

A primary hydration number of 6 for Fe + in aqueous (or D2O) solution has been indicated by neutron diffraction with isotopic substitution (NDIS), XRD, 16,1017 EXAFS, and for Fe " " by NDIS and EXAFS. Fe—O bond distances in aqueous solution have been determined, since 1984, for Fe(H20)/+ by EXAFS and neutron diffraction, for ternary Fe " "-aqua-anion species by XRD (in sulfate and in chloride media, and in bromide media ), for Fe(H20)g by neutron diffraction, and for ternary Fe -aqua-anion species. The NDIS studies hint at the second solvation shell in D2O solution high energy-resolution incoherent quasi-elastic neutron scattering (IQENS) can give some idea of the half-lives of water-protons in the secondary hydration shell of ions such as Fe aq. This is believed to be less than 5 X I0 s, whereas t>5x10 s for the binding time of protons in the primary hydration shell. X-Ray absorption spectroscopy (XAS—EXAFS and XANES) has been used... 

Del Terzo et al. [64] compared the iontophoretic and passive diffusion profiles of n-alkanols and reported an enhanced iontophoretic transport of the small-MW solutes (methanol, ethanol, and butanol) over passive diffusion, equivalent transport of hexanol, and a decreased iontophoretic transport of the longer-chained solutes (octanol and decanol). In terms of electroosmosis, the hydrophobic molecules have greater difficulty in associating with water molecules, which form the hydration shells of ions in solution. Therefore, the iontophoretic transport of alkanols with increasing chain lengths should decrease [64]. 

In addition to the concentration of the electrolyte solution, another barely examined factor influencing the structure and size of the hydration shells of ions is temperature. Invest tions of spin-lattice relaxation rates of and Li nuclei in aqueous LiCl solution as a function of concentration and temperature indicate that heating may destroy the tetrahedral water structure with inserted lithium cations that exists at temperatures below 30°C and enable the ions to construct a surrounding octahedral with the coordination number of 6 at temperatures above 40°C [165]. However,... 

The rate of exchange of water molecules between the hydration shells of ions and bulk water was considered by Samoilov (1957) to indicate the strength of the hydration and indirectly the effects of the ions on the water structure. The ratio of the average residence time of a water molecule near another one in the hydration shell of the ion, Ti, to that in the bulk (tw = 17 ps) was obtained from the activation Gibbs energy of the exchange, This, in turn, was obtained from the temperature... 

The same water model, SPC/E at 25 °C was used by other authors too. Chowd-huri and Chandra (2001) employed 256 water molecules per ion, as well as lower ratios at increasing concentrations, and reported the average residence times of water molecules near ions in ps Na+ 18.5, K+ 7.9, and Cl 10.0. Guardia et al. (2006) also reported residence times in ps of the water molecules in the first hydration shells of ions Li+ 101, Na+ 25.0, K+ 8.2, Cs+ 6.9, F 35.5, Cl 14.0, and I 8.5, compared with 10 1 for water molecules in the bulk. These values, resulting from detailed considerations of the hydrogen bond dynamics in water and near the ions, can be compared with experimental values derived from NMR. According to Bakker (2008) these are Li+ 39, Na+ 27, K+ 15, Cl 15 (by definition the same as for K+), Br 10, and 5 ps, and for water molecules in the bulk 17 ps, calculated from the self diffusion coefficient. 

DEVELOPMENT OF SOLUTION CHEMISTRY UNTIL 1957 IONIC HYDRATION STRUCTURE OF HYDRATED IONS SECOND HYDRATION SHELL OF IONS... 

DYNAMIC PROPERTIES OF WATER MOLECULES IN HYDRATION SHELLS OF IONS IONIC HYDRATION UNDER EXTREME CONDITIONS IONIC SOLVATION IN NONAQUEOUS SOLVENTS PREFERENTIAL SOLVATION OF IONS IN MIXED SOLVENTS THEORETICAL CONSIDERATION OF IONIC SOLVATION CONCLUSION REFERENCES... 

Information on the second hydration shell of ions is much poorer than that of the first hydration shell (see Table 3). The quantitative analysis of radical distribution curves in the long r range is difficult and much less reliable. Reliable information in the structure of the second shell may be obtained by the isomorphous substitution method in XRD measurements and the isotopic substitution method in ND measurements. Of course, such methods cannot be applied to every electrolyte solution. In most works in which some attempts have been made to determine the structure of the second hydration shell of ions, suitable structural models have been introduced. In some other cases fitting procedures between experimental and theoretical G r) functions have been continued until a more or less satisfactory agreement is obtained between them. Details about the distance between a central ion and water molecules in the second hydration shell, mo(2), the rmsd, /mo(2), the number of water molecules in the second hydration shell of the ion, mo(2), and /o(i) o(2) and /o(i) o(2) values between water molecules in the first and second hydration shells are given in Ohtaki and Radnai (1993). ... 

DYNAMIC PROPERTIES OF WATER MOLECULES IN HYDRATION SHELLS OF IONS... 

Table 4 Residential time of water molecule in the bulk and of oxygen and hydrogen atoms of hydrated water molecules in the first hydration shell of ions (th o, tq, and th) (for detaUs see Ohtaki and Radnai,...

Repulsion of water molecules from the electric double layer or loss of water molecules from the hydration shell of ions as a result of electrostatic adsorption [61,94,99]. 

Hydration numbers, hi, are the time-average numbers of water molecules residing in the first (and second, if formed) hydration shell of ions. If directional coordinate bonds are formed with the water molecules in the first hydration shell, hi equals the coordination number. 

Neutron-scattering experiments demonstrated that the electrostriction effects are small in the secondary hydration shell of ions and beyond [275], This conclusion probably holds for the hydration layer on the hydrophilic surfaces of surfactant microstructures. 

The residence time of water in the primary hydration shells of ions, x is defined by... 

In concluding this section, we stress that the local structure of solvent near its interface and the structure of the hydration shells of ions introduce significant specificities in ion adsorption at liquid interfaces, especially in H-bonded solvents. Thus, both the structure of the primary hydration shell of ions and the "structure-broken region (Frank and Evans, 1945) just outside the primary shell will also be important in ion adsorption since the properties of water in this region are known to be ion-specific, but these factors will be significant only near the interface. 

Model of the electrochemical double layer with potential drop within the diffuse double layer and A<(>h within the Helmholtz layer, (a) The centers of the closest hydrated ions define the OHP. (b) The centers of the contact adsorbed ions define the IHP. The hydration layer of the metal surface and the hydration shells of ions... 

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