Halides, hydration numbers

A fair number of mixed solvates, compounds containing molecules of crystallization of two different solvents, are also known. Generally, these are obtained either by recrystallizing halide hydrates from a nonaqueous solvent, or by crystallizing a halide from an appropriate solvent mixture, such as an alcohol intentionally or unintentionally containing significant amounts of water. Examples include... 

Anions have a much weaker tendency to coordinate water molecules than cations and precise values for hydration numbers are difficult to obtain from X-ray data. Although a hydration sphere around an anion is often included in the least-squares analysis of an intensity curve for a metal salt solution, meaningful values are usually obtained only for the bond lengths. Coordination numbers and rms variations are often kept constant at assumed values. For the halide ions the bond lengths found show no significant deviations from values in crystal structures. 

Hydration Numbers for Some Alkali Metal and Halide Ions Obtained from MD Calculations and X-Ray and Neutron-Diffraction Experiments ... 

Summary of Primary Hydration Numbers for Aikaii Metai and Halide Ions... 

The trivalent transplutonium halides have been extensively studied. Several reviews deal specifically with actinide halides. " In aqueous solution the mono- and bis-complexes have been characterized, with the formation of the latter decreasing down the halide series. For example, AmF and AmF2" have been studied, but only a very weak monochloride complex AmCP has been reported. These species are reported to have coordination numbers as high as 11, although recent EXAFS studies show that the hydration number decreases with increasing halide concentration (and ionic strength) at concentrations below which the halo complexes form. These data suggest the coordination numbers of the mixed aquo halo complexes are probably seven to ten. 

Hydration numbers (cont.) for alkaline metals, 144 and coordination numbers, 140 determined by various methods, 143 function of, 296 for halides, 144 from IR spectroscopy, 76 — iM3- easMemeHts-rf-tUelechic constants. 

Fig. 27. Top Average hydration number of alkali and halide ions as a function of their negative distance from the mercury surface (at z = 0). The Cl" hydration number is offset by -4 to improve readability. Data are from four series of simulations of one ion dissolved in 259 water molecules. Bottom The normalized oxygen density profile for the same system.

Spohr [190] studied the adsorption of on the Pt(lOO) surface. The free energy barrier towards iodide adsorption that is produced by the layers of adsorbed water is associated with a significant intermediate increase in coordination number, before the hydration number decreases at short ion-metal distances for geometrical reasons. Philpott and Glosli [109] observed in a series of MD studies of ion adsorption on charged electrodes that the Li+ hydration shell structure in the vicinity of a model metal surface does not depend on the halide counterion (F, Cl , Br , I ). In this study, no specific interactions between the metal surface and water molecules or ions were employed. 

Contrary to the larger halide ions the coordination number of F remains constant in the whole region of thermodynamic stability (see Fig. 22). At distances smaller than about 4 A from the surface, excluded volume effects lead to a similar reduction of the hydration number as for the other anions. 

In the present work, Eq. (17) was used to calculate the ion-dipole interaction energies for the alkali halide water dipole system. Here, water was treated as a simple spherical molecule of radius 0.14nm with a point dipole of moment 1.85D. Also, based on steric and hydration effects, three cases—A, B, and C—were used to calculate the ion-dipole interaction energies. (See Table 6.) Case A considers the alkali halides KF, RbF, CsF, CsCl, CsBr, and Csl, which have an alkali ion radius close to that of the water molecule and a ratio of hydration numbers of alkali ion to halide ion of less than 2. Case B considers the alkali halides LiF, NaF, LiCl, LiBr, NaBr, Lil, Nal, and Kl, which have an alkali ion radius less than that of the water molecule and a ratio of hydration numbers of alkali ion to halide ion of more than 2. Finally, case C considers the alkali halides KCl, RbCl, KBr, RbBr, and Rbl, which have an alkali ion radius close to that of the water molecule and a ratio of... 

FIG. 4 Case A, suggested for KF, RbF, CsF, CsCl, CsBr, and Csl, alkali halides having an alkali ion radius close to that of the water molecule and a ratio of hydration numbers of alkali ion to halide ion of less than 2. (From Ref. 9.)... 

It has been pointed out (67) that the suggestion of hydration numbers near zero for the halide ions appears contrary to experimental evidence. Although the physical concept of a primary hydration number is reasonably clear, the precise nature of the hydration index is not nearly as well defined. Furthermore, the separation of ionic activity coefficients embodied in Equations 15 and 16 is rather insensitive to the choice of hydration indexes. For example, Bagg and Rechnitz s studies of cells with liquid junction (68) lead to a value of h=0.9 for chloride ion, instead of h=0. This difference produces a change of only about 0.015 in pM, the negative logarithm of the cation activity, for Na" " in 2m NaCl and less than 0.001 for Ca2+ in 2m CaCl2 (W. 

It is worth noting, in passing, that Mootz s work is predated by that of G. A. Jeffrey and coworkers who, in a series of papers [669, 670], have described an extensive series of quaternary ammonium halide hydrates with low-melting points. A number of these have been structurally characterised, but as they melt to give liquids that are not conprised solely of ions (indeed, they are aqueous solutions), they are only briefly mentioned in the following text. 

Max, J.-J., and Chapados, C. 2001. IR spectroscopy of aqueous alkali halide solutions Pure salt-solvated water spectra and hydration numbers. J. Chem. Phys. 115 2664. 

Halides. AH of the anhydrous and hydrated binary haUdes of iron(Il) and iron(Ill) are known with the exception of the hydrated iodide of iron(Ill). A large number of complex iron haUdes have been prepared and characterized (6). 

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