Hydration shell penetration

Figure 6. Two possible modes of hydration shell penetration in the conceptual outer sphere complex formation (24).

At this time diffraction data for ion-ion distributions in aqueous solutions of moderate concentration are beginning to become available. In aqueous NiCl2 solutions very refined neutron diffraction studies indicate that the Ni2+-Cl pair correlation function has a peak near 3.l8 under conditions in which the Cl does not penetrate the Ni(H20)g2+ unit. (J+2 ) It is reported that EXAFS studies give the same result. (1 3) While the information is most welcome it is puzzling because a geometrical calculation indicates that the closest center to center distance for the Ni2+ and a Cl that does not penetrate the hydration shell is closer to 3.98. (7)... 

Mechanism 4 also was bom in this laboratory (1-6,24). A rise in AV could be determined by the penetration and orientation of the ions of electrolyte, H20, and their products of interaction. Such ions could be part of either the hydration shell of the lipid s molecular organization or the thick interfacial viscosity layers that are located above and/or below the mathematical line (25) of surface tension. The measurements of IR absorption and surface radioactivity (see Figures 6,7,8... 

The second equilibrium involves hydrated 1 ions in equilibrium with E" complexes. In the forward step, an iodide ion donates an electron to the working electrode and is hence oxidized to a I atom. We may speculate that the 1 ion remains outside the double layer and that an electron tunnels through the double layer. However, from many experimental results and molecular dynamic simulations (see Section 4.7.2), it became clear that this is not the case. Instead, a solvated ion penetrates the double layer and becomes chemisorbed as a 1" ion (<5 < 1) on the metal surface, losing about half of its hydration shell [18, 19]. Moreover, there is a local restructuring of the double layer. Here also, the electrochemical reaction does not involve tunneling of an electron through the double layer. 

Conversely, the dissolved inorganic electrolytes cause only a minor increase in the surface tension of water (Fig. II-3, curve 2), which according to the Gibbs equation corresponds to negative adsorption, i.e., a deficiency of solute within the surface layer as compared to the bulk (c(s) < c). Such a deficiency can be easily explained it is unfavorable for the hydrated ions to reach the surface closer than the distance equal to their hydration shell radius (the penetration of ions into the surface layer is thermodynamically unfavorable since additional energy is required for dehydration). 

The results of these calculations imply that none of the ions would be contact adsorbed when no specific interactions between ions and metal are taken into account in the model. The Li+ ion, believed to be nonspecifically adsorbing, would be able to approach the surface more closely than the anions, mostly because of its small size, which allows it to penetrate the surface layers without displacing water molecules. The simulation results thus indirectly demonstrate the importance of specific chemical interactions for realistic models of the electric double layer. Apparently, also some specific features of the hydration shell structure of the ions must be taken into consideration in order to fully understand the adsorption of ions. 

It is relevant to this discussion that membrane equilibrium measurements have shown that under certain conditions the lithium ion is bound more strongly to long-chain phosphates than the sodium ion (46), as this reversal of the normal binding order implies penetration of the phosphate groups through the hydration shell of the alkali metal ions, and thus site binding. 

The model system has also showed much lesser hydration of phosphate groups, a feature probably contributing to easier penetration by water, since the water molecules do not have to remain bound within the hydration shells. The atom charge distribution across the membrane showed that phospholipid head-groups provide an electrostatic environment conductive to penetration of the headgroup zone by water molecules and sodium ions. This conclusion was also borne out by the fact that water diffused faster in the interface region of the DTPS membrane than in the DPPC membrane. 

Calcification involves the seeding of the calcium apatite crystal and the growth of the crystals. It would be more convenient to review the second of these mechanisms first. This can best be done by assuming for the moment that the bone matrix, connective or cartilaginous, is bathed in a medium supersaturated in calcium phosphate. Intrinsic to the matrix, there is a mechanism that precipitates calcium phosphate in the form of tiny apatite crystals. If there is no interference with the process of calcification, either by deficient absorption of calcium (vitamin D deficiency) or by active dissolution of the calcium crystals (parathormone), most of the process of mineralization can probably be explained by the physicochemical properties of the apatite crystals. Thus, the divalent cations and anions, PO4 and calcium, penetrate the hydration shell reaching the surface of the crystals where they are crystallized, thereby increasing the size of the crystals. As mineralization proceeds, the amount of bound... 

Some data are shown in Table 6.1. Of course the larger the metal ion, the larger the radius of the hydration shell. An important corollary is that the attraction of a water layer around K+ (as measured by the free energy of hydration AG ) is considerably smaller than a similar layer around a smaller alkali ion. Thus, potassium ions throw off their accompanying water molecules much more easily than sodium ions, if this is necessary to enter a channel of a cell membrane in living matter. Potassium penetrates the cell wall more easily in an ionic channel, since it may pass the channel without a load of water molecules around it. 

The ability of different salts to promote binding to HlC media is ranked according to the Hoffmeister series (Table 3), and this can be understood in terms of their relative abilities to penetrate the hydration shell. Salts high in the lyotropic series (NH4, K, Na, POg , S04 ) are strongly excluded from protein and media surfaces and therefore promote binding, whereas chaotropic salts (Ba, Ca, Mg, SCN ) tend to penetrate the hydration shell, reduce local hydration, and diminish binding. The most commonly used salt is ammonium sulfete, but it requires care to control the pH and alternatives such as potassium... 

Micelles of ionic surfactants are aggregates composed of a compressive core surrounded by a less compressive surface structure/ and with a rather fluid environment (of viscosity 8-17 cP for solubilized nitrobenzene in SDS and cetyltrimethylammonium bromide micelles). Copper ions attached to micelles have essentially the same hydration shell near the micellar surface as in the bulk phase, and do not penetrate into the nonpolar part of the micelle. In addition, it is known that the volume change caused by binding of divalent metal ions to micelles is very small. The rate of rotation of the hydrated Na ion at the micellar surface is unlikely to change by more than 35% upon adsorption from the bulk to the Stem layer of SDS micelles. ... 

Experiments on other electrolyte solutions have so far revealed that the Ca and Na ions are surrounded by hydration shells closely resembling that of Ni, the main difference being that the peaks are not as sharp. A surprising result, and one that conflicts with other data, is that the g nature of the cation Ca about cation-cation distributions, but it is claimed that the experimental results at reasonable concentrations clearly show that the Cl does not penetrate the cation primary hydration shell. 

For many pairs of ions in aqueous solution the reaction distance a has been found to be about 7.5 A. This distance typically includes the hydration shells of the reacting partners. In the process of reacting with one another, the two charged partners must shed or penetrate their solvent shells in a multistep process. In the initial stages of this process the reaction partners have an infinite number of possible reaction distances and orientations with respect to one another. It is only in the late stages of the recombination process when the collision complex has been formed that the distance o- has a significance and the splitting out of the last few water molecules becomes resolvable with respect to time as discrete steps. 

Fe(II) penetrates inside the spherical shell by the hydrophilic channels. After an oxidation on ferroxidase sites, located on H subunits, Fe(III) iron ions migrate to a nucleation site, situated on L subunits, where a crystal of hydrated iron oxide grows. Up to 4500 Fe(III) can be stored inside this mineral phase (31). The number of iron atoms contained in the ferritin molecule is called the loading factor (LF). 

For the Tar—Tar kissing loops, the P—B calculations are unable to discern their propensity to accumulate counterions accumulation at the loop—loop interface (data not shown). This is because the fully hydrated ions as defined by the Stem layer cannot penetrate into the central cation binding pocket (data not shown). Similarly, the axial spine of counterion density observed in the A-RNA helix (Fig. 20.5) is not captured by the P—B calculation (Fig. 20.7). No noticeable sequence specificity is observed in the counterion accumulation patterns in the P—B calculations, even though the sequence effects are explicitly represented in the P—B calculation through the appropriate geometry and assignment of point-charges. This is because the sequence specificity observed in the molecular dynamics simulations usually involves first shell interactions of base moieties with partially dehydrated ions, which cannot be accurately represented in the P—B framework. 

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