Hydration primary

In general, anions are less strongly hydrated than cations, but recent neutron diffraction data have indicated that even around the halide ions there is a well defined primary hydration shell of water molecules, which, in... 

Figure A2.4.3. The localized structure of a hydrated metal cation in aqueous solution (the metal ion being assumed to have a primary hydration number of six). From [5].

The reaction corresponds to a proton transfer and not to a net formation of ions, and thus the AS is of minor importance in the whole series, especially for the two t-Bu derivatives. This last effect is believed to be due to a structure-promoting effect of the bulky alkyl groups in the disordered region outside the primary hydration sphere of the thiazolium ion (322). 

The coordination chemistry of the large, electropositive Ln ions is complicated, especially in solution, by ill-defined stereochemistries and uncertain coordination numbers. This is well illustrated by the aquo ions themselves.These are known for all the lanthanides, providing the solutions are moderately acidic to prevent hydrolysis, with hydration numbers probably about 8 or 9 but with reported values depending on the methods used to measure them. It is likely that the primary hydration number decreases as the cationic radius falls across the series. However, confusion arises because the polarization of the H2O molecules attached directly to the cation facilitates hydrogen bonding to other H2O molecules. As this tendency will be the greater, the smaller the cation, it is quite reasonable that the secondary hydration number increases across the series. 

Moreover there are nineteen water molecules forming the socalled primary hydration... 

Fig. 1.5 A scheme of hydration (1) cation, (2) primary hydration sheath (water molecules form a tetrahedron), (3) secondary hydration shell, (4) disorganized water, (5) normal water... 

Remarkable data on primary hydration shells are obtained in non-aqueous solvents containing a definite amount of water. Thus, nitrobenzene saturated with water contains about 0.2 m H20. Because of much higher dipole moment of water than of nitrobenzene, the ions will be preferentially solvated by water. Under these conditions the following values of hydration numbers were obtained Li+ 6.5, H+ 5.5, Ag+ 4.4, Na+ 3.9, K+ 1.5, Tl+ 1.0, Rb+ 0.8, Cs+0.5, tetraethylammonium ion 0.0, CIO4 0.4, NO3 1.4 and tetraphenylborate anion 0.0 (assumption). 

Fig. 1.7 Possible hydration modes of an ion pair (A) contact of primary hydration shells, (B) sharing of primary hydration shells, (C) direct contact of ions...

All the methods involve preparation of an initial, primary, hydrated, viscous solution referred to as base gel or linear gel. 

Alkali 10ns in aqueous solution are probably the most typical and most widely studied representatives of non-specific adsorption. The electrochemical term of non-specific adsorption is used to denote the survival of at least the primary hydration shell when an ion is interacting with a solid electrode. As pointed out previously, the generation of such hydrated ions at the gas-solid interface would be of great value because it would provide an opportunity to simulate the charging of the interfacial capacitor at the outer Helmholtz plane or perhaps even in the diffuse layer. 

Our model for the adsorption of water on silicates was developed for a system with few if any interlayer cations. However, it strongly resembles the model proposed by Mamy (12.) for smectites with monovalent interlayer cations. The presence of divalent interlayer cations, as shown by studies of smectites and vermiculites, should result in a strong structuring of their primary hydration sphere and probably the next nearest neighbor water molecules as well. If the concentration of the divalent cations is low, then the water in interlayer space between the divalent cations will correspond to the present model. On the other hand, if the concentration of divalent cations approaches the number of ditrigonal sites, this model will not be applicable. Such a situation would only be found in concentrated electrolyte solutions. 

Surface complexation models attempt to represent on a molecular level realistic surface complexes e.g., models attempt to distinguish between inner- or outer-sphere surface complexes, i.e., those that lose portions of or retain their primary hydration sheath, respectively, in forming surface complexes. The type of bonding is also used to characterize different types of surface complexes e.g., a distinction between coordinative (sharing of electrons) or ionic bonding is often made. While surface coordination complexes are always inner-sphere, ion-pair complexes can be either inner- or outer-sphere. Representing model analogues to surface complexes has two parts stoichiometry and closeness of approach of metal ion to... 

Alcohols are hydroxylated alkyl-compounds (R-OH) which are neutral in reaction due to their unionizable (OH) group (e.g., methanol, ethanol, isopropanol, and w-butanol). The hydroxyl of alcohols can displace water molecules in the primary hydration shell of cations adsorbed onto soil-solid and sediment-solid clay particles. The water molecule displacement depends mainly on the polarizing power of the cation. The other adsorption mechanisms of alcohol hydroxyl groups are through hydrogen bonding and cation-dipole interactions [19,65],... 

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... 

There is a conceptual model of hydrated ions that includes the primary hydration shell as discussed above, secondary hydration sphere consists of water molecules that are hydrogen bonded to those in the primary shell and experience some electrostatic attraction from the central ion. This secondary shell merges with the bulk liquid water. A diagram of the model is shown in Figure 2.3. X-ray diffraction measurements and NMR spectroscopy have revealed only two different environments for water molecules in solution of ions. These are associated with the primary hydration shell and water molecules in the bulk solution. Both methods are subject to deficiencies, because of the generally very rapid exchange of water molecules between various positions around ions and in the bulk liquid. Evidence from studies of the electrical conductivities of ions shows that when ions move under the influence of an electrical gradient they tow with them as many as 40 water molecules, in dilute solutions. 

The hydration of anions is regarded as being electrostatic with additional hydrogen bonding. The number of water molecules in the primary hydration sphere of an anion depends upon the size, charge and nature of the species. Monatomic anions such as the halide ions are expected to have primary hydration spheres similar to those of monatomic cations. Many aqueous anions consist of a central ion in a... 

The above ideas are not limited to species with central metal ions. They apply to the higher oxidation states of non-metallic elements. Many simple anions do exist with primary hydration spheres in which the positive ends of dipoles are attracted to the central negative charge. Table 3.8 gives examples of ions that may be thought about in terms of the hydrolysis of parent hypothetical hydrated ions. 

The more exact forms of the aqueous cations, with their primary hydration shells, are normally omitted from the diagrams. As is the case with Crvl, which in acid solution exists as the dichromate ion, Cr2072-, the forms of any oxo anions are indicated by their formulae in the diagrams. The diagram for chromium summarizes the following important properties ... 

In Cliapter 2 we learned how ions in solution are solvated. Some of the water molecules that form the solvation sheet are left behind when ions random walk and drift around, while others—the primary hydration molecules—show a stronger attrac -tion to the ion and follow it in its thermal, random movements. 

---