Water, diffusion

Wriggers and Schulten, 1997b] Wriggers, W., and Schulten, K. Stability and dynamics of G-actin Back door water diffusion and behavior of a subdomain 3/4 loop. Biophys. J. 73 (1997b) 624-639... 

When the voltage force and source of water are removed, most of the injected water diffuses away and evaporates, and the tree disappears. This disappearance indicates that channels or paths close up, because if they did not, their appearance would be enhanced rather than diminished when the water is replaced by air which has a greater refractive index difference with respect to polyethylene. 

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. 

Monomer molecules, which have a low but finite solubility in water, diffuse through the water and drift into the soap micelles and swell them. The initiator decomposes into free radicals which also find their way into the micelles and activate polymerisation of a chain within the micelle. Chain growth proceeds until a second radical enters the micelle and starts the growth of a second chain. From kinetic considerations it can be shown that two growing radicals can survive in the same micelle for a few thousandths of a second only before mutual termination occurs. The micelles then remain inactive until a third radical enters the micelle, initiating growth of another chain which continues until a fourth radical comes into the micelle. It is thus seen that statistically the micelle is active for half the time, and as a corollary, at any one time half the micelles contain growing chains. 

Because of their greater thickness, CAA oxides serve to protect the metal surface from corrosion better than thinner oxides but the important factor for bond durability is the stability of the outer oxide structure when water diffuses to the oxide-polymer interphase. Accordingly, it would be expected that the performance of CAA treated adherends would be similar, although no better, than that of PAA, or BSAA. The wedge test data shown in Fig. 20 and other work [29,77,97,98] support this and demonstrate that when these processes are done correctly the wedge test crack will be forced to propagate entirely within the adhesive. Similar arguments are likely with BSAA adherends, also. 

Diffusion From Food Ingested Water Diffusion Metabolism... 

Perrin model and the Johansson and Elvingston model fall above the experimental data. Also shown in this figure is the prediction from the Stokes-Einstein-Smoluchowski expression, whereby the Stokes-Einstein expression is modified with the inclusion of the Ein-stein-Smoluchowski expression for the effect of solute on viscosity. Penke et al. [290] found that the Mackie-Meares equation fit the water diffusion data however, upon consideration of water interactions with the polymer gel, through measurements of longitudinal relaxation, adsorption interactions incorporated within the volume averaging theory also well described the experimental results. The volume averaging theory had the advantage that it could describe the effect of Bis on the relaxation within the same framework as the description of the diffusion coefficient. 

Another important factor in diffusion measurements that is often encountered in NMR experiments is the effect of time on diffusion coefficients. For example, Kinsey et al. [195] found water diffusion coefficients in muscles to be time dependent. The effects of diffusion time can be described by transient closure problems within the framework of the volume averaging method [195,285]. Other methods also account for time effects [204,247,341]. 

Phenomena Arising from the Heating of Glass A rapid evolution of adsorbed water first occurs on heating glass this is followed by a persistent evolution, due to gas (mostly water) diffusing from the interior. Above 300°C the two processes are fairly clearly separated. The adsorbed water is rapidly and completely removed, and the quantity of gas evolved by the persistent evolution... 

This problem illustrates the solution approach to a one-dimensional, nonsteady-state, diffusional problem, as demonstrated in the simulation examples, DRY and ENZDYN. The system is represented in Fig. 4.2. Water diffuses through a porous solid, to the surface, where it evaporates into the atmosphere. It is required to determine the water concentration profile in the solid, under drying conditions. The quantity of water is limited and, therefore, the solid will eventually dry out and the drying rate will reduce to zero. 

The rate of drug release (E) from the eroding matrix is controlled by (a) the chemical properties of the system - the hydrolytic and the neutralizing process at the boundary of the device, catalytic degradation of the polymer and the intrinsic backbone reactivity, and (b) several concomitant physical processes such as water diffusivity, water solubility, water partitioning, etc. 

No volume change of the system as water diffuses in. Poly(ortho ester)s are rather hydrophobic (5). 

The UWL permeability is nearly the same for drugs of comparable size, and is characterized by the water diffusivity (Daq) of the drug divided by twice the thickness of the layer (ftaq), Pu = Aiq / (2 h.Aq), in a symmetric permeation cell [40], The unstirred water layer permeability can be determined experimentally in a number of ways based on pH dependency of effective permeability [25,509,535-538], stirring rate dependence [511-514,552,578], and transport across lipid-free microfilters [25,546],... 

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. 

Stevenson, C. M., D. Wheeler, S. W. Novak, R. J. Speakman, and M. D. Glascock (2007), A new dating method for high-calcium archaeological glasses based upon surface-water diffusion Preliminary calibrations and procedures, Archaeometry 49, 153-177. 

All steps in fixation and tissue processing involve exchange of fluids in the three-dimensional space of the specimen. At the start of fixation, tissue fluid (mostly water) is inside the specimen, while fixative molecules are on the outside. Ignoring for the moment the actual structure of the tissue and the effect a fixative may have upon it, assume that the specimen is like a porous sponge filled with water. To enter this system, a fixative molecule must replace a molecule of water. Diffusion is the driving force when two different liquids meet, there is a gradual equalization in the distribution of their molecules. Ideally, at the end of the process, the concentration of one in the other will be the same both inside and outside the specimen. 

It is probable that capillary flow of water contributes to transport in the soil. For example, a rate of 7 cm/year would yield an equivalent water velocity of 8 x 10-6 m/h, which exceeds the water diffusion rate by a factor of four. For illustrative purposes we thus select a water transport velocity or coefficient U6 in the soil of 10 x 10 6 m/h, recognizing that this will vary with rainfall characteristics and soil type. These soil processes are in parallel with boundary layer diffusion in series, so the final equations are... 

Air-soil diffusion thus appears to be much slower than air-water diffusion because of the slow migration in the soil matrix. In practice, the result will be a nonuniform composition in the soil with the surface soil (which is much more accessible to the air than the deeper soil) being closer in fugacity to the atmosphere. 

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