Hydration second shell

Fig. 21. Schematic representation of the second hydration shell in the major groove of MPD7 including all the water molecules interacting with those of the first shell (broken circles). Interacting phosphate groups are marked by numbered dots1581...

Figure 21 shows the arrangement of the water molecules in the second hydration shell in the major groove of MPD 7. It contains all the water molecules associated with those of the primary layer indicated by broken circles, and those interacting with one another in a network. 

For Ca and Ba, whose n values are larger than 10, however, it is thought that some hydrated water molecules not only in the first hydration shell but also in the second hydration shell are cotransferred into NB. Accordingly, it can be supposed that some water molecules in the first hydration shell (i.e., in the vicinity of the ion) are covered with the second hydration shell, so that they cannot be associated with outer solvent... 

EXAFS has been used to determine the second hydration shell of zinc in aqueous solution. Aqueous solutions of zinc nitrate over a range of concentrations were examined and a Zn—O distance of 2.05 A for the first shell of the six-coordinate zinc center found, which is unaffected by concentration. The second hydration shell shows a Zn—O distance which has no systematic trend but an average distance of 4.1 A. The coordination number for the second shell is 11.6 1.6 with unusual behavior for the most concentrated 2.7 M solution, which has a decrease in coordination number to 6.8 1.5 340... 

The DFT results of Table II (which include the zero point energy correction) have been computed by considering the lowest values of the two sets of Table I. The results are clearly good for n=l and n=2, but wrong for higher n a clear indication that the minima we have reached are far from being close to the absolute ones. Therefore, the question remains whether for n=5, one water molecule is in a second hydration shell. 

Inset a) refers to the starting configuration, t=0 fs, with the 5 water molecules in the first hydration shell. Inset b) refers to t=70 fs some rearrangement starts to occur, especially for the left most water molecule. At t=l 10 fs (inset c)) one ion-water distance is above the threshold value, the water starts to leave the first hydration shell. Finally, at t=210 fs, one water molecule is in the second hydration shell and the remaining four... 

Structure diffusion (i.e., the Grotthuss mechanism) of protons in bulk water requires formation and cleavage of hydrogen bonds of water molecules in the second hydration shell of the hydrated proton (see Section 3.1) therefore, any constraint to the dynamics of the water molecules will decrease the mobility of the protons. Thus, knowledge of the state or nature of the water in the membrane is critical to understanding the mechanisms of proton transfer and transport in PEMs. 

In some cases the pH dependence of the relaxivity is associated with changes in the structure of the second hydration shell. Two such systems have been reported by Sherry s group. The first case deals with a macrocyclic tetraamide derivative of DOTA (DOTA-4AmP, Chart 12) that possesses an unusual ri vs. pH dependence (131). In fact, the relaxivity of this complex increases from pH 4 to pH 6, decreases up to pH 8.5, remains constant up to pH 10.5 and then increases again. The authors suggested that this behavior is related to the formation/disruption of the hydrogen bond network between... 

Starting with [Cr(OH2)6]3+, the nature of the second hydration shell has been probed with a variety of techniques including IR, XRD, EXAFS, and neutron diffraction (4). Surprisingly consistent results have been obtained, with n = 13 1 in [Cr(OH2)6]3+ (H20) and a Cr— distance of 4.02 A for the water molecules in the second hydration shell. 

Water tends to hydrate many solutes. The hydration number is the number of water molecules bound to solute sufficiently strongly so as to become part of it. The hydration number varies with different solute molecules Na+, 3.9 0.5 Ca2+, 12 + 2 Al3+, 22 2 glycerol, 2 0.5 sucrose 5 0.5 Fe3+ 18 2. Water molecules form a hydration shell around the solute molecules. In case of molecules such as Al3+, the large number of water molecules is unlikely to be accommodated in the first hydration shell. X-ray diffraction has shown a highly ordered second hydration shell around Al3+ (Zavitsas, 2001). 

Conventional QM/MM MD simulations at MP/2 level [7,48,66], restricting the QM region to a one-shell treatment due to the substantial increase of the computational effort, have indicated that an extended quantum mechanical zone (i.e. first plus second hydration shells of an ion) at Hartree-Fock level is more important for the quality of results than the treatment of a smaller system (i.e. an ion with its first hydration shell only) at correlated level - the inclusion of many-body and polarisation effects extending beyond the first shell is more crucial than the partial correction of electron correlation. 

Figure 10-5a and b depicts the ion-oxygen radial distribution functions (RDFs) of Al(III) and Zn(II) in aqueous solution obtained from a conventional QM/MM [50,51] and a QMCF MD simulation. In both cases the first and second hydration shells have been included in the QM region, in the QMCF studies the ion and its first hydration shell formed the core region . In general all important structural features like first and second shell distances and coordination numbers are very similar. 

Figure 10-7b displays the Hg(I)-oxygen and Hg(I)-hydrogen radial distribution functions. Two well-defined peaks representing the first and second hydration shells are centred at 2.4 and 4.7 A in the Hg(I)-0 RDF and at 3.0 and 5.35 A in the Hg(I)-H RDF, respectively. The mean Flg(I)-Hg(I) bond length was found to be 2.63 A. This structural description is in good agreement with data obtained from diffraction experiments [73],... 

The second hydration shell is permeable to cations. At subzero temperatures, it crystallizes in the form of ice I and therefore resembles bulk water [856]. Because Donnan-type equilibria could have an influence on the structure of this water layer around the DNA polyelectrolyte, it is believed that this second hydration layer is different from an outer layer of bulk water further away from the DNA. 

The violet hexaaquo ion [Cr(OH2)6] is a regular octahedron with Cr-0 distances in the range 191.5-199.1 pm, depending on the particular compound used for the structure determination. EXAFS measurements on dilute solutions have provided information about the second hydration shell it appears to contain an average of 13.5 water molecules at a Cr-0 distance of 402(2) pm. ... 

The interatomic distances and the hydration number of Li" " were determined by a least-squares fitting procedure applied to the r -weighed GLi(r). Characteristic Li-0 and Li-D distances are 2.02 0.05 A and 2.61 0.05 A respectively, independent of temperature. The number of water molecules coordinated to Li is 4 1 at the temperatures down to 213 K, but decreases to 3 1 at 173 K when the second hydration shell is established. 

This approach has been proven able to describe successfully the structural features of the hydration complex of several cations [129-132,215,216]. Table 5 collects some results relevant to the first and second hydration shell, for those metal ions where the latter can be clearly identified. It is also possible to note in Table 5 the effect of including many-body terms in the average way allowed by PCM-based potentials. 

Figure 4-12. The water network in a single tube (top and side views). The top view (left) shows 8 bridging water molecules in red, 8 first-hydration shell water molecules in blue, 12 second-hydration shell water molecules in yellow, and 4 third-hydration shell water molecules in gray, while the side view shows twice those in the top view. Reproduced by permission of American Chemical Society Ref. [54].

Fig. 1 Schematic of a hydrated ion-eluater. The three regions indicated are (i) the first hydration shell, (ii) the second hydration shell and (iii) the cluster interior, respectively. Reproduced with permission from Ref. 10, Fig. 1. Copyright 1982, American Chemical Society.

In concentrated salt solutions, the vapor pressure is lower than that of pure water, and hence it exhibits reduced water activity. This phenomenon is explained by the fact that a considerable fraction of the water molecules are associated with the hydration of the salt ions. The binding energy of these water molecules (which forms the first and the second hydration shells) to the center ion is larger than 10 kcal/mol therefore, they are less likely to participate in the hydration of the newly formed proton. To observe successful proton dissociation, the thermodynamic stable complex must be formed within the ion-pair lifetime. The depletion of the solution from water molecules available for this reaction will lower the probability of the successful dissociation. As demonstrated in Figure 9, this function decreases with the activity of the water in the solution. 

---