Inner hydration shell

Tetrahedral (Mootz and Staben, 1992) and octahedral (Rustad etal., 2003) coordination environments are known for HO in crystalline hydrates. Evidently the numbers and arrangements of water molecules coordinating an HO ion are flexible enough to be decided by a crystal environment. Therefore development of the self-consistent molecular field models suggested by Section 7.8 would be valuable. Proximity of a specific cation is an issue, in general, for crystals. But it is interesting that, in crystalline NaOH hydrates beyond the monohydrate, the counter-ion is excluded from the inner hydration shell of both Na and HO (Rustad et al., 2003). The latter work used the PBE electron density functional, and found overall excellent results for crystalline NaOH hydrates. So that electron density functional model is able to properly characterize higher coordination structmes where they are known to exist. 

When a metal salt dissolves in water, the cation and anion are hydrated we discuss the energetics of this process in Section 6.9, but for now, we consider the interactions between the individual ions (freed from their ionic lattice on dissolution) and the solvent molecules. Consider the dissolution of NaCl. Figure 6.5a shows a schematic representation of the formation of the inner hydration shell around Na. The O Na interaction can be described in terms of an ion-dipole interaction, while the solvation of the anion can be described in terms of the formation of hydrogen bonds between Cl and H atoms of surrounding H2O molecules. 

After A.M. BlohkI969). Structure around ion A - inner hydration shell. B - outer hydration shell, C - unbroken water structure. State of solution a- diluted b - critical c - quasi-orderly. 

In the binary water-PC system the values for tightly bound water and the inner hydration shells show a much larger spreading (relative to those for... 

The rate-determining step in the formation of both complexes again appears to be water loss from the inner hydration shell of the metal, and the adenine and cytosine nucleotides of a given charge type exhibit the same kinetic behaviour (Table 6). 

Some recent papers permit an exciting outlook on the degree of sophistication of experimental techniques and on the kind of data which may be available soon. In the field of NMR spectroscopy, a publication by Hertz and Raedle 172> deals with the hydration shell of the fluoride ion. From nuclear magnetic relaxation rates of 19F in 1M aqueous solutions of KF at room temperature, the authors were able to show that the orientation of the water molecules in the vicinity of fluoride ions is such that the two protons are non-equivalent. A geometry is proposed for the water coordination in the inner solvent shell of F corresponding to an almost linear H-bond and to an OF distance of approximately 2.76 A, at least under the conditions chosen. 

The values of JSn generally decrease as the metal ion becomes larger and its effective charge lower, i. e. as the electric field around the metal ion becomes weaker. This is certainly to be expected as a weaker field implies less well-ordered inner an outer hydration shells and consequently less increase of entropy when the water molecules are liberated. 

Although OH reacts at near-diffusion-controlled rates with inorganic anions [59], there seems to bean upper limit of ca. 3 x 10 dm mol sec in the case of simple hydrated metal ions, irrespective of the reduction potential of M"". Also, there is no correlation between the measured values of 43 and the rates of exchange of water molecules in the first hydration shell of, which rules out direct substitution of OH for H2O as a general mechanism. Other mechanisms that have been proposed are (i) abstraction of H from a coordinated H2O [75,76], and (ii) OH entering the first hydration shell to increase the coordination number by one, followed by inner-sphere electron transfer [77,78]. Data reported [78] for M" = Cr, for which the half-life for water exchange is of the order of days, are consistent with mechanism (ii) ... 

The rough water-hydrocarbon surface of the core introduced in Figure 8.3c suggests that the core of the micelle should really be considered as two distinct regions an inner core that is essentially water-free and a hydrated shell between the inner core and the polar heads. This partly aqueous shell is sometimes called the palisade layer. The extent to which the hydrocarbon chains protrude into the water is problematic, but we can get an idea of the volume of the palisade layer as follows. 

Figure 2.13 illustrates what is currently a widely accepted model of the electrode-solution interphase. This model has evolved from simpler models, which first considered the interphase as a simple capacitor (Helmholtz), then as a Boltzmann distribution of ions (Gouy-Chapman). The electrode is covered by a sheath of oriented solvent molecules (water molecules are illustrated). Adsorbed anions or molecules, A, contact the electrode directly and are not fully solvated. The plane that passes through the center of these molecules is called the inner Helmholtz plane (IHP). Such molecules or ions are said to be specifically adsorbed or contact adsorbed. The molecules in the next layer carry their primary (hydration) shell and are separated from the electrode by the monolayer of oriented solvent (water) molecules adsorbed on the electrode. The plane passing through the center of these solvated molecules or ions is referred to as the outer Helmholtz plane (OHP). Beyond the compact layer defined by the OHP is a Boltzmann distribution of ions determined by electrostatic interaction between the ions and the potential at the OHP and the random jostling of ions and... 

In coordination chemistry two types of complex can occur between metals and complexant ligands. Outer-sphere complexes are relatively weak electrostatic associations between a hydrated metal ion and a complexant ligand, and in which both of the charged species retain a hydration shell. In contrast, inner-sphere complexes are stronger interactions in which a covalent bond is formed between a metal ion and a ligand. 

The site bound ions accounts for its hydration state and are grouped either as outer-sphere or as inner-sphere [9, 11, 14]. In the later case, it is assumed that the water molecules in the hydration shell do not participate. The ions directly interact with the phosphate charges and anionic ligands [9, 11, 14], Since both outer- and inner-sphere interactions lead to formation of ion pairs, site bound ions are describable in terms of an association constant satisfying the law of mass action [9, 11, 14—16]. 

Examination of the structures of Ln(III) hydrates in crystals and our knowledge of Ln(III) complexes in solution now throws up a problem which the above equations do not readily meet. There is no certain distinction between inner and outer sphere for ions such as Ln(III). Firstly the inner sphere is constantly switching between 8- and 9-coordination but 9-coordination is not far from 6-innermost water molecules which can distort to an octahedron and 3-outermost water molecules. The steps of kinetics can involve multiple re-arrangements of the cation hydration shell which is itself variable in the series of Ln(III). The model equations above are only guides to thinking. 

In some cases, involving inert complexes of chromium(III) and cobalt(III), it has been possible to distinguish unambigously between inner and outer sphere complexes (33,34). In the latter type of complexes, no water of hydration is displaced by the ligand on complex formation. No bonds directly involving the metal are thus broken or formed, nor are any large number of water molecules set free from the hydration shells. If those interpretations are true which have been given above for the... 

The edge charges can also bond ions with opposite charges. This process, however, is not directed clearly by electrostatic forces chemical properties play an important role. The ions are sorbed with no hydrate shell, that is, inner-sphere complexation occurs. These reactions and the surface complexation models for their quantitative treatment are shown in general in Chapter 1, Table 1.7. 

Even in the case of strong interactions between solvent and solute, the life time of each solvate is brief since there is continuous rotation or exchange of the solvent shell molecules. The time required for reorientation of hydrates in water is of the order 10 ... 10 " s at 25 °C [91]. If the exchange between bulk solvent molecules and those in the inner solvation shell of an ion is slower than the NMR time scale, then it is possible to observe two different resonance signals for the free and bound solvent. In this... 

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