First hydration shell

As is suggested frequently , this term might well result from the restriction of the hydrogen bonding possibilities experienced by the water molecules in the first hydration shell. For each individual water molecule this is probably a relatively small effect, but due to the small size of the water molecules, a large number of them are entangled in the first hydration shell, so that the overall effect is appreciable. This theory is in perfect agreement with the observation that the entropy of hydration of a nonpolar molecule depends linearly on the number of water molecules in the first hydration shell ". ... 

Fig. 20. Schematic representation of the unrolled major groove of the MPD 7 helix showing the first hydration shell, consisting of all solvent molecules that are directly associated with base edge N and O atoms. Base atoms are labeled N4,04, N6,06 and N7 solvent peaks are numbered. Interatomic distances are given in Aup to 3,5 A represented by unbroken lines, between 3,5-4,1 A by dotted lines. The eight circles connected by double-lines represent the image of a spermine molecule bound to phosphate groups P2 and P22. There are 20 solvent molecules in a first hydration layer associated with N- and O-atoms l58)...

E. Hydrated divalent cation approaching a channel with a slightly larger diameter than in D, but the energy of interaction with the divalent cation is sufficient to deform the channel drawing the walls in to make lateral coordination with the divalent cation. Since the channel is too small for a monovalent cation to pass through with its first hydration shell and since the monovalent cation channel interaction is insufficient to make the channel small enough for lateral coordination of the monovalent cation, the channel is selective for divalent cations. (Part E reproduced with permission from Ref. 68 )... 

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... 

Molecular dynamics calculations have been performed (35-38). One ab initio calculation (39) is particularly interesting because it avoids the use of pairwise potential energy functions and effectively includes many-body interactions. It was concluded that the structure of the first hydration shell is nearly tetrahedral but is very much influenced by its own solvation. 

Another, related effect leading to non-bulk response in the first hydration shell is electrostriction[131], which is the change in solvent density due to the high electric fields in the first solvation shell of an ion. 

A fourth solvent structural effect refers to the average properties of solvent molecules near the solute. These solvent molecules may have different bond lengths, bond angles, dipole moments, and polarizabilities than do bulk solvent molecules. For example, Wahlqvist [132] found a decrease in the magnitude of the dipole moment of water molecules near a hydrophobic wall from 2.8 D (in their model) to 2.55 D, and van Belle et al. [29] found a drop from 2.8 D to 2.6 D for first-hydration-shell water molecules around a methane molecule. 

We summarize this section by emphasizing that we have identified a host of effects, and we have seen that they are mainly short-range effects that are primarily associated with the first solvation shell. A reasonable way to model these effects quantitatively is to assume they are proportional to the number of solvent molecules in the first hydration shell with environment-dependent proportionality constants. 

After this computer experiment, a great number of papers followed. Some of them attempted to simulate with the ab-initio data the properties of the ion in solution at room temperature [76,77], others [78] attempted to determine, via Monte Carlo simulations, the free energy, enthalpy and entropy for the reaction (24). The discrepancy between experimental and simulated data was rationalized in terms of the inadequacy of a two-body potential to represent correctly the n-body system. In addition, the radial distribution function for the Li+(H20)6 cluster showed [78] only one maximum, pointing out that the six water molecules are in the first hydration shell of the ion. The Monte Carlo simulation [77] for the system Li+(H20)2oo predicted five water molecules in the first hydration shell. A subsequent MD simulation [79] of a system composed of one Li+ ion and 343 water molecules at T=298 K, with periodic boundary conditions, yielded... 

Integration of the Li+-0 pair correlation function (see Fig. 4, curve labeled 2-body) shows that in the first hydration shell the Li+ is surrounded by 6 water molecules. 

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... 

An interesting combined use of discrete molecular and continuum techniques was demonstrated by Floris et al.181,182 They used the PCM to develop effective pair potentials and then applied these to molecular dynamics simulations of metal ion hydration. Another approach to such systems is to do an ab initio cluster calculation for the first hydration shell, which would typically involve four to eight water molecules, and then to depict the remainder of the solvent as a continuum. This was done by Sanchez Marcos et al. for a group of five cations 183 the continuum model was that developed by Rivail, Rinaldi et al.14,108-112 (Section III.2.ii). Their results are compared in Table 14 with those of Floris et al.,139 who used a similar procedure but PCM-based. In... 

Water molecules are absent from the hydrophobic interior, but both the choline and the phosphate headgroups are fully solvated [41]. Similarly, the first hydration shell of the sulfate headgroup of SDS is formed rather by water molecules than by sodium ions. Because of hydration the charge density due to the lipid headgroups is overcompensated by the water dipoles, thereby reducing the transmembrane potential by 50-100 mV across the lipid water interface and resulting in a negative potential at the aqueous side [42]. 

Figure 8. The structure of hydrated Na and CP ions at the water/Pt(IOO) interface (dotted lines) compared with the structure in bulk water (solid lines). In the two top panels are the oxygen ion radial distribution functions, and in the two bottom panels are the probability distribution functions for the angle between the water dipole and the oxygen-ion vector for water molecules in the first hydration shell. (Data adapted from Ref. 100.)...

X 10 cm /s at room temperature) and that the diffusion of protonated water molecules makes some contribution to the total proton conductivity (vehicle mechanism " ). This is --"22% when assuming that the diffusion coefficients of H2O and H3O+ (or H502 ) are identical. However, as suggested by Agmon, " the diffusion of H3O+ may be retarded, because of the strong hydrogen bonding in the first hydration shell. 

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) ... 

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