Secondary hydration shell

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... 

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... 

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... 

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. ... 

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]. 

The practical independence of Li-0 and Li-D distances with temperature shows the strong orientational correlation of water molecules around Li it is most likely that an average orientation of the coordinated water molecules is such that the four atoms in a Li -D20 unit is pyramidal. The strong orientational correlation of the bound water molecules rationalizes the anisotropic reorientational motion of the water molecules in the hydration shell found by nuclear magnetic relaxation data of supercooled LiCl solutions. The evolution of the secondary hydration shell of Li may be a hint of nucleation of ice, glass transition, and partial recovery of hydrogen bonds in the supercooled state. 

Therefore and as previously developed by Sung et al. (47) and reported by other authors, a three-step imbibation process can be postulated. The hydrophilic sites within the PDMAEMA-/-PCL conetwoiks first attract and bind an amount of water, followed by the formation of a secondary hydration shell of water that is preferentially oriented around the bound water and the polymer network structure. Further absorbed water exists as free or bulk-like water. 

According to their amphoteric nature, aluminum hydroxides have their minimum solubdity in water at a close-to-neutral pH value, that is, in the pH range of 6—7. At a pH larger than 8.5, the solubihty increases, and the [A1(0H)4] anion is formed. At a pH smaller than 4, the aquo cation [A1(0H2)6] is reported to be predominant, whereas at a pH between 4 and 6, its dissociated form [Al(OH2)5(OH)] prevails (36,37), likely both with less tightly bound water molecules in the secondary hydration shell. However, the speciation of aluminum in aqueous environments is very complicated. Depending on the conditions, such as pH, concentration (or hydrolysis ratio), and anions present, a variety of polynuclear species can be found in solution. In pure water, polynuclear species include dimers... 

Differential scanning calorimetric studies have shown that Nafion contains at least three types of water—one tightly bound to the ions, one weakly bound in the secondary hydration shell to the ion or another component of the fluorocarbon polymer (e.g., to the ether oxygens), and one free from strong interactions with ions or polymer chains. Thus, the K" " form of Nafion contains 3.8 water molecules per fixed ionic group in the primary hydration shell, 0.15 molecules as loosely bound water, and 3.8 water molecules as free water per each ionic group [49]. 

Hydration studies for higher oxidation state actinides are reported for some AnOj salts (UO2 or NpO ) but not for any AnOj systems. The Raman spectra of aqueous uranyl solutions were interpreted to show the presence of six water molecules coordinated in the primary plane perpendicular to the O-U-O axis (Sutton 1952). However, similar hydration numbers have been obtained by methods in which the secondary hydration shell may also contribute to the value. For example, the activity coefficient measurements suggest a hydration number of 7.4 [relative to an assumed hydration number of zero for Cs(I) (Hinton and Amis 1971)]. Similarly, a hydration number of seven has been derived from Gusev s conductivity method (Gusev 1971, 1972,1973). 

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. 

Ever since Franklin and Gosling [1] examined the first fibers of DNA it has been known that DNA occurs in vivo in hydrated form. Experiments involving sedimentation equilibrium studies [2-4], isopiestic measurements [5], gravimetric [6], X-ray fiber diffraction, infrared [7-9] and NMR spectroscopic investigations [10-12] lead to the conclusion that DNA is heavily hydrated. The hydration is not homogeneous around the DNA and can be described in the terms of two discrete lays representing primary and secondary hydration shells. 

Primary hydration shell Secondary hydration shell... 

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