Hydration secondary

The secondary hydration sheath has also been studied using vibrational spectroscopy. In the presence of... 

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. 

It has been suggested by Ikegami (1968) that the carboxylate groups of a polyacrylate chain are each surrounded by a primary local sphere of oriented water molecules, and that the polyacrylate chain itself is surrounded by a secondary sheath of water molecules. This secondary sheath is maintained as a result of the cooperative action of the charged functional groups on the backbone of the molecule. The monovalent ions Li", Na and are able to penetrate only this secondary hydration sheath, and thereby form a solvent-separated ion-pair, rather than a contact ion-pair. Divalent ions, such as Mg " or Ba +, cause a much greater disruption to the secondary hydration sheath. 

At the molecular level, a number of features are associated with the phenomenon of gelation or precipitation. In particular the disruption of the secondary hydration sheaths around the polyacrylate chains appears... 

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... 

B. SECONDARY HYDRATION, REARRANGEMENT AND OXIDATION TO PARABANIC ACID... 

Thus, one may expect two changes in the dielectric constant of water due to the presence of ions (1) lower dielectric constant in the primary and secondary hydration... 

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. 

Figure 2.3 A diagram showing lhe primary and secondary hydration spheres around an ion...

In aqueous solutions, in which the most probable ligand is the water molecule, most of the lower oxid ation states (i.e. + 2, + 3 and some of the + 4 states) of transition metal ions are best regarded as hexaaqua complex ions, e.g. [Feu(H20)6]2 +. In these ions the six coordinated water molecules are those that constitute the first hydration sphere, and it is normally accepted that such ions would have a secondary hydration sphere of water molecules that would be electrostatically attracted to the positive central ion. The following discussion includes only the aqua cations that do not, at pH = 0, undergo hydrolysis. For example, the iron(III) ion is considered quite correctly as [Fe(H20)6]3 +, but at pH values higher than 1.8 the ion participates in several hydrolysis reactions, which lead to the formation of polymers and the eventual precipitation of the iron(III) as an insoluble compound as the pH value increases, e.g. ... 

Ions in aqueous solution have primary and secondary hydration shells the former can generally be related to the coordination shell about the metal ion in hydrates containing aquo cations (cf. previous section). NMR studies of aqueous solutions containing slow-exchange cations, at low... 

In recent years, X-ray diffraction studies of aqueous solutions have established primary hydration numbers for several fast-exchange cations 45,187-190 the timescale of X-ray diffraction is very much shorter than that of NMR spectroscopy. Octahedral hydration shells have been indicated for Tl3+,191 Cd2+, Ca2+, Na and K+, for example. For the lanthanides, [Ln(OH2)9]3+ is indicated for La, Pr and Nd, but [Ln(OH2)8]3 for the smaller Tb to Lu.192,193 Sometimes there are difficulties and uncertainties in extracting primary hydration numbers from X-ray data. Thus hydration numbers of eight and of six have been suggested for Na+ and for K+,194 and for Ca2+,195 and 8 and 9 for La3+, 196 In some cases rates of water exchange between primary and secondary hydration shells are so fast as to raise philosophical questions in relation to specific definitions of hydration numbers.197... 

If we calculated with the idealized co-operative model by the content of spectroscopic determined Op values the number Nei of H-bonded water molecules we would get — with different 1 molar salt solutions — the result of Fig. 11. The values Nei with salt additions depend strongly on the salt concentrations because of the disturbance of the big H-bonded system3At small concentrations the Nel-N0 numbers (7V0 association number in pure water) of structure-makers are in size of the order of Debye-Sack s or Azzam s calculations. They are of the same size of order as the secondary hydration numbers calculated by solubility measurements of organic substances in water (Chapter b) or as the hydration numbers of hydrophilic organic molecules (Chapter lld-e) or biopolymers (Chapter III). 

The ion content of the organic phase of ethylenoxid-products indicate that under saturation conditions there are some water molecules whose properties are not too different from normal water. Polypropyleneoxide products which contain much less water, release under similar conditions water with a reduced ion content147. From this experience one gets a working hypothesis for the mechanism of semipermeable membranes. The membranes should have some secondary hydrate shell with movable water but by reason of solubility or steric effects, not too much secondary hydrate water to avoid normal water with common solubility properties. 

The bound water of the primary hydrate 1 -6 water shows with deuteron magnetic resonance a rotational correlation time of rc = 10 7 sec, the secondary hydration shell to 11—12 water/mole lipid has rc 8 10-10 sec and the trapped water to 11—13 water/mole lipid Tc < 3 10 10sec162. ... 

He estimated a thickness of about 25 A of the bond water layer. This value is similar to the extension of the H-bonds in bulk water and would mean in a simple model that the biopolymer surface prevents the flickering of the defects in water. Garlid has found too that the bound water on mitochondria is a solvent of H-bonding organic molecules275. If his interpretation is correct, solutes in the secondary hydrate sphere would get more immobile. The time for transfer in cells is increased. —... 

Summarizing Chapter 3 we can conclude there are many indications that biopolymers may have different hydrations 1. Primary hydrate 1 -2 H20 per unit 2. bonded water 10-20 H20 3. secondary hydrate shells up to 50 H20 and 4. bulk water. The secondary hydrates may solve ions. 

Primary and secondary hydration shells around DNA. The hydration of DNA is not homogeneous. It can be described in terms of two shells, as suggested by sedimentation equilibrium studies [857-859, 861], isopiestic measurements [860], gravimetric and infrared spectroscopic investigations [853-855, 862]. 

James and Armitage have analyzed the far IR spectra of some ionic solutions and attempted to distinguish waters in the primary hydration shell (those waters that stay with the ion as it moves) from waters ( secondary hydration ) which, although affected by the ion, are not attracted by it enough to move with it. 

Thus, a major misunderstanding is committed by those who confuse solvation numbers with the number of solvent molecules in contact with an ion, the coordination number. It has already been implied and indeed spelled out that the term solvation number implies a dynamic concept. Solvation numbers reflect the dynamic situation of the ion as it moves around in the solution. Thus, two hydration numbers may be described, but only one of these is open to numerical determination. This is the so-called primary hydration number, that is, the number of water molecules that have lost their own freedom of translational motion and move along with the ion in its random movements in the solution. A secondary hydration number rtitts to the water molecules in the area around the ion that are affected by the ion s presence. Clearly, this second quantity depends entirely on the degree of the effect on the solvent molecules outside the first and second layers and hence on the sensitivity of the method being used. It includes waters in the structurally broken-down region out from the first layer of waters attached to the anion. 

The nature of ions in aqueous solution has been studied using a wide variety of techniques, including X ray and neutron diffraction, and quasielastic neutron scattering, NMR, IR, and UV spectroscopies. The ions are generally considered to have primary and secondary hydration spheres, although there is relatively little quantitative information available concerning the second sphere in solntion. The rate of exchange of the... 

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