Water diffusion constant

Osmotic Control. Several oral osmotic systems (OROS) have been developed by the Alza Corporation to allow controUed deHvery of highly water-soluble dmgs. The elementary osmotic pump (94) consists of an osmotic core containing dmg surrounded by a semi-permeable membrane having a laser-drilled deHvery orifice. The system looks like a conventional tablet, yet the outer layer allows only the diffusion of water into the core of the unit. The rate of water diffusion into the system is controUed by the membrane s permeabUity to water and by the osmotic activity of the core. Because the membrane does not expand as water is absorbed, the dmg solution must leave the interior of the tablet through the smaU orifice at the same rate that water enters by osmosis. The osmotic driving force is constant until aU of the dmg is dissolved thus, the osmotic system maintains a constant deHvery rate of dmg until the time of complete dissolution of the dmg. 

We finish this section by comparing our results with NMR and incoherent neutron scattering experiments on water dynamics. Self-diffusion constants on the millisecond time scale have been measured by NMR with the pulsed field gradient spin echo (PFGSE) method. Applying this technique to oriented egg phosphatidylcholine bilayers, Wassail [68] demonstrated that the water motion was highly anisotropic, with diffusion in the plane of the bilayers hundreds of times greater than out of the plane. The anisotropy of... 

Table 3 Diffusion Constants and Rotational CoiTelation Tunes of Water Molecules from an MD Simulation of a Fully Flydrated Fluid Phase DPPC Bilayer ...

The diffusion constant of water is consistent with the bulk water value for milk, however, it is slightly reduced in the cream sample and significantly reduced in the... 

Figure 7-3. Active site properties of CAII from SCC-DFTB/MM-GSBP simulations [91]. (a) The root mean square differences between the RMSFs calculated from GSBP simulations (WT-20 and WT-25 have an inner radius of 20 and 25 A respectively) and those from Ewald simulation, for atoms within a certain distance from the zinc, plotted as functions of distance from the zinc ion that die center of die sphere in GSBP simulations is the position of the zinc ion in the starting (crystal) structure, (b) The diffusion constant for TIP3P water molecules as a function of the distance from the zinc ion in different simulations...

Diffusion constants are enhanced with the approximate inclusion of quantum effects. Changes in the ratio of diffusion constants for water and D2O with decreasing temperature are accurately reproduced with the QFF1 model. This ratio computed with the QFF1 model agrees well with the centroid molecular dynamics result at room temperature. Fully quantum path integral dynamical simulations of diffusion in liquid water are not presently possible. 

More recent quantum-based MD simulations were performed at temperatures up to 2000 K and pressures up to 30 GPa.73,74 Under these conditions, it was found that the molecular ions H30+ and OH are the major charge carriers in a fluid phase, in contrast to the bcc crystal predicted for the superionic phase. The fluid high-pressure phase has been confirmed by X-ray diffraction results of water melting at ca. 1000 K and up to 40 GPa of pressure.66,75,76 In addition, extrapolations of the proton diffusion constant of ice into the superionic region were found to be far lower than a commonly used criterion for superionic phases of 10 4cm2/s.77 A great need exists for additional work to resolve the apparently conflicting data. 

Much less attention has been paid to the dynamic properties of water at the solution/metal interface (or other interfaces). Typical dynamic properties that are of interest include the diffusion constant of water molecules and several types of time correlation functions. In general, the time correlation function for a dynamic variable of interest A(t) is defined as... 

The diffusion constant of water molecules at the water/Pt(100) and water/Pt(l 11) interface at different locations is given in Table 2, which is taken from the article by Raghavan et al. In this table, the diffusion constants in the direction perpendicular and parallel to the Pt surface are... 

Table 1.2 Diffusion constants of CO, O, and in air and water and their solubility in water at 20 °C. Reprinted from Paul FA, Clark FE (1989) Soil microbiology and biochemistry. Academic Press, New York 234 pp. Copyright 1989 with permission of Elsevier...

This difference is largely accounted for by the difference in thickness of the homogeneous membranes involved, such that the product of water permeation constant and membrane thickness is about the same for both membranes. The constant arises from the diffusion model of permeation in which ... 

Rotational diffusion constant Density extrapolated to 20°C, water Density... 

I wish to ask about the concept of a unit of length in a biological cell. My concern is with the numerical value to be used for the diffusion coefficient. Most of biological space is heavily organized even in a single cell, and therefore a diffusion constant is not a simple property. To put my question simply, it is easiest, however, to consider enzyme sites that are otherwise identical in two situations (1) with water between them, (2) with a biological membrane between them. Is it not the case that the unit of length is quite different, for the diffusion in (1) is virtually free diffusion whereas in (2) the diffusion is constrained most probably as a series of activated hops ... 

Dr. Thomas drew our attention to the fact that, although biological membranes are thin in comparison to his preparations, their diffusion constants may be long. I have measured permeability coefficients (transmembrane "diffusion ) for a number of solutes for artificial phospholipid bilayers (liposomes). The values follow and are to be compared for calculated permeabilities for a solute diffusion across a comparable thickness of water. 

In practice, the diffusion constant is modified to reflect the complex nature of the biofilm. In the development of synthetic hydrogels, the hydrophilicity of the polymer in part defines the concept of equilibrium moisture. We discussed this concept earlier when we described determination of equilibrium moisture. The first practical application is the diffusion constant. As the equilibrium moisture approaches 100%, the diffusion constant approaches that of water. 

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