Hydration water diffusion

Accordingly, for each mole of chalcanthite (molecular weight 249.71) that dissolves, 1 mol of CuS04 (molecular weight 159.63) and 5 mol of hydration water diffuse through the liquid film from the surface of the crystal to the bulk of the liquid phase. This is shown schematically in Figure 1.10 (substance A is CuS04 and substance B is... 

In addition, protein motion reduces the retardation of the water dynamics, because the dimension of the water translational space is increased and at the same time the decay of the orientational correlation is accelerated. In spite of this accelerated dynamics, hydration water diffusion remains anomalous for a thermalized protein. 

FIGURE 23 Hydration layer in obsidian. When obsidian is broken into two or more pieces, new surfaces are created. As a new surface is exposed to the environment, water (from atmospheric humidity, rain, or the ground) penetrates the surface gradually, the water diffuses into the bulk and forms hydrated obsidian, that is, obsidian containing water. With time, the thickness of the hydration layer, as such a layer is known, gradually increases the rate of increase is affected by such factors as the vapor pressure of the water in the atmosphere, the environmental temperature, and the composition of the surrounding environment as well as of the obsidian. If the hydration layer reaches a thickness of 0.5 microns or more, it becomes discernible under a microscope, the thickness can be measured, and the age of the surface calculated. The microphotograph shows an hydration layer on obsidian. 

Of potentially greater significance is surface hydration which occurs concurrently with alkali diffusion at relatively low temperature. The average activation energy of water diffusion in obsidian can be estimated at 75kJ between 95° and 245°C (25). A nuclear resonance hydration profile of obsidian at 25°C has yielded a diffusion coefficient of 5xlO-20 cm2-s 1... 

For comparison purposes, the proton mobility. Do (for Nafion solvated with water), which is closely related to the self-diffusion coefficient of water, is also plotted. At low degrees of hydration, where only hydrated protons (e.g., H3O+) are mobile, it has a tendency to fall below the water diffusion coefficient (this effect is even more pronounced in other polymers), which may be due to the stiffening of the water structure within the regions that contain excess protons, as discussed in Section 3.1.1. . Interestingly, the proton mobility in Nafion solvated with methanol (Da(MeOH) in Figure 14a) is even lower than the methanol self-diffusion (Z ieon). This may... 

When it comes to the equilibration of water concentration gradients, the relevant transport coefficient is the chemical diffusion coefficient, Dwp. This parameter is related to the self-diffusion coefficient by the thermodynamic factor (see above) if the elementary transport mechanism is assumed to be the same. The hydration isotherm (see Figure 8) directly provides the driving force for chemical water diffusion. Under fuel-cell conditions, i.e., high degrees of hydration, the concentration of water in the membrane may change with only a small variation of the chemical potential of water. In the two-phase region (i.e., water contents of >14 water molecules... 

Figure 16. Self-diffusion coefficient of oxide ion vacancies in different perovskite-type oxides,which equals the chemical water diffusion coefficient in the fully hydrated state (see text). (Figure reproduced with the kind permission from Elsevier.)...

Fig. 3. Schematic representation of the topological space of hydration water in silica fine-particle cluster (45). The processes responsible for the water spin-lattice relaxation behavior are restricted rotational diffusion about an axis normal to the local surface (y process), reorientations mediated by translational displacements on the length scale of a monomer (P process), reorientations mediated by translational displacements in the length scale of the clusters (a process), and exchange with free water as a cutoff limit.

This bimodal dynamics of hydration water is intriguing. A model based on dynamic equilibrium between quasi-bound and free water molecules on the surface of biomolecules (or self-assembly) predicts that the orientational relaxation at a macromolecular surface should indeed be biexponential, with a fast time component (few ps) nearly equal to that of the free water while the long time component is equal to the inverse of the rate of bound to free transition [4], In order to gain an in depth understanding of hydration dynamics, we have carried out detailed atomistic molecular dynamics (MD) simulation studies of water dynamics at the surface of an anionic micelle of cesium perfluorooctanoate (CsPFO), a cationic micelle of cetyl trimethy-lainmonium bromide (CTAB), and also at the surface of a small protein (enterotoxin) using classical, non-polarizable force fields. In particular we have studied the hydrogen bond lifetime dynamics, rotational and dielectric relaxation, translational diffusion and vibrational dynamics of the surface water molecules. In this article we discuss the water dynamics at the surface of CsPFO and of enterotoxin. 

In aqueous food materials Tj and T2 relaxation behavior of water are related to different aspects of the interaction and motion of the water molecules. The relationship is not so simple, especially in heterogeneous food materials [63-65]. There are at least four types of protons to be considered, namely free (or bulk) water, bound (or hydrated) water, exchangeable macro-moleculc protons such as those found in hydroxyl and amino groups, and unexchangeable macromolecule protons. Under such circumstances measurement of Ti is more reliable than T2 measurement, but can be complicated by the spin diffusion, while T2 relaxation can be complicated by slow translational diffusion and proton exchanges. 

Davidson and Ripmeester (1984) discuss the mobility of water molecules in the host lattices, on the basis of NMR and dielectric experiments. Water mobility comes from molecular reorientation and diffusion, with the former being substantially faster than the water mobility in ice. Dielectric relaxation data suggest that Bjerrum defects in the hydrate lattice, caused by guest dipoles, may enhance water diffusion rates. 

This is one distinguishing feature between hydrates and ice water molecules diffuse two orders of magnitude slower in hydrates than in ice. As shown in Table 2.8, ice water molecules diffuse almost an order of magnitude faster than they reorient about a fixed position in the crystal structure. In direct contrast, hydrate water molecules reorient 20 times faster than they diffuse. As for all... 

The self-diffusion of the individual components is strongly affected by the formation of micelles in the solution. This applies to the surfactant, the counterion, the water, and to solubilized molecules. As illustrated in Fig. 2.11 for sodium dodecyl sulfate, surfactant and counterion diffusion are very weakly dependent on concentration below the CMC while a marked decrease in the micellar region is observed for the surfactant and a less marked one for the counterion37. Water diffusion shows a stronger concentration dependence below the CMC than above it. Self-diffusion studies using radioactive tracers have been performed to obtain information on CMC, on counterion binding, on hydration and on intermicellar interactions and shape changes. 

Kausik et al.,67 Pavlova et al.56 and Ortony et al.84 that the translational diffusion of hydration water on a given protein, macromolecule or lipid vesicle surface is two- to four-fold slower compared to bulk water is still valid. 

Starch changes during cooking of pasta are reported to vary from a hydration-driven gelatinization process in the outer layer to a heat-induced crystallite melting in the center.525 It is speculated that both the state of the starch and the surface structure contribute to the development of the elastic texture and stickiness of pasta. Interactions between starch and the surrounding protein matrix are evident in the outer and intermediate layer. In the center of cooked pasta, wheat starch granules retain their shape due to limited water diffusion, and the protein network remains dense. 

In the presence of water, the Li oxide is readily hydrated, so water diffuses through the Li oxide and then is reduced by the active metal at sites close to the Li-film interface. This process is continuous and leads to thickening of the Li20 films as H20 is reduced to Li20 and H2 as the final and most stable products. 

Below we will discuss each of these formulation principles in terms of basic release mechanisms and the advantages or drawbacks associated with the different formulations and manufacturing processes. However, drug release from all kinds of ER formulations starts with hydration of the formulation and water diffusion into the system. The presence of water in the formulation facilitates the start of the dissolution process of the drug, whereby the dissolved drug can be released from the formulations. 

There is some evidence that the degree of hydration of alkali hydroxide ions affects their ability to enter and swell cellulose fibers [310]. At low concentrations of sodium hydroxide, the diameters of the hydrated ions are too large for easy penetration into the fibers. As the concentration increases, the number of water molecules available for the formation of hydrates decreases and therefore their size decreases. Small hydrates can diffuse into the high order, or crystalline regions, as well as into the pores and low-order regions. The hydrates can form hydrogen bonds with the cellulose molecules. 

Mossbauer spectroscopic measurements suggest that the hydration water of myoglobin and the internal motions of the protein are coupled. [ Fe]Ferricyanide diffused into the solvent of myoglobin crystals exhibits (x ) values equal to those for the heme iron for temperatures below 250 K, and greater than those for the heme iron at higher temperatures (50% greater at 300 K) (Parak, 1986). The [ Fe]ferricyanide in the crystal monitors motions of the hydration water [ Fe]ferricyanide in bulk water shows no Mossbauer spectrum. 

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