Interface water-hydrate shell

The anions and cations in solution are normally hydrated and although hydration is a dynamic process, it is usually possible to identify a small number of water molecules in the hydration shell immediately surrounding the ion whose exchange rate is slow in comparison with other processes that might take place as the ions approach and recede from the interface. The picture of Figure 1.6 is, in any case, intended to represent a dynamic... 

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 7. A schematic representation of the microscopic model for the metal/electrolyte solution interface. Shown from top to bottom are an ion that is contact adsorbed with partial loss of its hydration shell, an ion whose hydration shell partially consists of first layer of water molecules, and an ion that is not contact adsorbed.

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.)...

Consider a CO2 droplet of radius 3 mm injected at 600 m seawater depth with temperature of 5.2°C (Zhang, 2005b). Under these conditions, density and viscosity of seawater are 1026 kg/m and 0.00161 Pa s, and density of liquid CO2 is 916kg/m, or 20.82 mol/L. Because of the formation of hydrate shell, the solubility of CO2 in seawater should be that of CO2 hydrate, which is 1.00 mol/L (CO2 liquid solubility is significantly greater), or Wq = 0.0429. Because solubility of CO2 is small, density of the interface water is similar to the bulk seawater. Hence, the... 

For anionic monolayers, the reversal of the tt-A isotherms can be explained in terms of a competition between the anionic head groups and the alkali metal cations for molecules of water. If a modified Stern-type model of the plane interface is assumed, this interface will be composed of distinct adsorption sites, with counterions (cations) of finite size that can adsorb on these sites if the standard free energies of adsorption are favorable. If the anionic head group is more polarizable than water, as with carboxylic acids or phosphates, the hydration shell of the cation is incompletely filled, and the order of cation sizes near the interface is K+ > Na+ > Li+. When the polarizability of the anionic group is less than that of water, as with the sulfates, the lithium cation becomes the most hydrated one, and the order of cation sizes becomes Li+ > Na+ > K+. 

Figure 3.25 Cyclopentane hydrate formation from a water droplet (a) initial contact, (b) hydrate shell formation around the water droplet, (c) depressions formed on the hydrate shell, (d) conversion of interior water to hydrate, indicated by darkening, (e) almost completely converted hydrate. (From Taylor, C.J., Adhesion Force between Hydrate Particles andMacroscopic Investigation of Hydrate Film Growth at the Hydrocarbon/Water Interface, Master s Thesis, Colorado School of Mines, Golden, CO, (2006). With permission.)...

In summary, the microimaging technique provides a powerful tool to study directly the mechanism of converting water droplets to hydrate particles. The results reported indicate that provided the gas hydrate former can diffuse into the interior droplet, hydrate growth can proceed in the bulk interior droplet away from the hydrate shell-water interface, as well by growing out from the hydrate shell resulting in shell thickening. 

The second and third parts of this review develop, through correlation of the results described in the first part, a picture of the hydration process and the hydration shell (Section VI) and an assessment of how the hydration shell may modulate enzyme and other protein functions (Section VII). The literature on protein hydration is now rich enough to provide a comfortably detailed picture of the protein-water interface. The ways in which the interface enters into function are just beginning to emerge, and one purpose here is to point out directions in which one may move to understand better the relationship to function. Sections VI and VII are meant to stand alone as summary statements, and some overlap with the preceding sections describing results should be expected. 

Overtone infrared spectroscopy described by Luck [3] is an effective means for determining quantitatively the concentrations of water in nonbonded and hydrogen bonded OH groups. Interesting results have been obtained for a variety of situations, including salt solutions, water-organic solvent mixtures, interface effects, organic molecule hydration, and diffusion in polymeric substrates. From such studies. Luck classifies water structure as (a) first shell water hydrate, (b) second shell, disturbed liquid-like water, and (c) liquid-like water. For salt transport in membranes, for diffusion of dyes in fibers, and for life in plant and animal cells, water of types b and c are essential. 

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. 

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