Diffusion Reference

Volume diffusion refers to the transport of atoms through the body of a solid. It is also called lattice or bulk diffusion. In amorphous or glassy solids and in cubic crystals, the speed of diffusion in all directions is the same and is said to be isotropic. In all other crystals, the rate of volume diffusion depends upon the direction taken and is anisotropic. Volume diffusion is usually much slower than short-circuit diffusion, which refers to diffusion along two- and three-dimensional imperfections in the material. 

Short-circuit diffusion refers to diffusion along ... 

An adsorption unit is to be designed to dry air using silica gel. A moving-bed design is considered in which silica gel moves down a cylindrical column in plug flow while air flows up the column. Air enters the unit at the rate of 0.129 kg of dry air/m2s and with a humidity of 0.00267 kg water/kg dry air. It leaves essentially bone dry. There is equilibrium between air and gel at the entrance to and the exit from the adsorption zone. Experiments were carried out to find the relative resistances of the external gas film and pellet diffusion. Referred to a driving force expressed as mass ratios then ... 

Wheeler s treatment of the intraparticle diffusion problem invokes reaction in single pores and may be applied to relatively simple porous structures (such as a straight non-intersecting cylindrical pore model) with moderate success. An alternative approach is to assume that the porous structure is characterised by means of the effective diffusivity. (referred to in Sect. 2.1) which can be measured for a given gaseous component. In order to develop the principles relating to the effects of diffusion on reaction selectivity, selectivity in isothermal catalyst pellets will be discussed. 

In their study of the NMR T2 and T2 of crosslinked cis-polyisoprene sheets under extension, von Meerwall and Ferguson 65) found that T2 of the rubber had much smaller anisotropy ( magic angle effect) than that of trace penetrants at the same extension ratio X < 3. However, the penetrant diffusion (referred to the strained dimensions) was within experimental error isotropic these findings are equally valid for C6F6 and n-hexadecane as penetrant. The authors concluded that segment orien-... 

The major source of band-broadening in CZE is longitudinal diffusion. Longitudinal diffusion refers to the axial diffusive spreading of the solute from the solute zone into the bulk solution as it travels down the capillary. The variance in peak width contributed by longitudinal diffusion is given by... 

In this book we are concerned only with mass transport, or diffusion, in solids. Self-diffusion refers to atoms diffusing among others of the same type (e.g., in pure metals). Interdiffusion is the diffusion of two dissimilar substances (a diffusion couple) into one another. Impurity diffusion refers to the transport of dilute solute atoms in a host solvent. In solids, diffusion is several orders of magnitude slower than in liquids or gases. Nonetheless, diffusional processes are important to study because they are basic to our understanding of how solid-liquid, solid-vapor, and solid-solid reactions proceed, as well as [solid-solid] phase transformations in single-phase materials. 

The data shown here for diffusion refer to the system polyethyl acrylate-benzene, and have been determined by Kishimoto and Enda (unpublished) by using the sorption technique. Values of D calculated from both absorption and desorption measurements agreed with each other over the range of benzene concentration studied at each temperature of the experiment, and the D0 values were determined by appropriate graphical extrapolation of those D values. In Fig. 15 are shown... 

Longitudinal diffusion refers to the natural spreading of a solute band from regions of high concentration to those of lower concentration as it passes through a chromatographic system [1], It is a simple process which is dependent on the time that the solute spends on the chromatographic system, which, in turn, is related to the flow rate of the mobile phase. 

In addition to the filtration and secretion processes, the concentration of a diuretic in the luminal fluid of the more distal segments of the tubule is determined by the agent s lipid/water partition coefficient and pK, as well as the pH of the distal luminal fluid. These factors modulate the concentration of diuretic at sites 3 and 4. Weakly acidic diuretics. whose undis.sociated forms possess a favorable balance of lipid and water solubility, may undergo pH-dependent diffusion (referred to as nonionic back diffitsion) from the distal tubular luminal fluid back into the bloodstream. This frequently decreases the luminal fluid concentration and the renal excretion rate of the diuretic but prolongs its plasma half-life. Diuretics that are weak bases... 

Let us now mention two examples in which the above conditions are violated and anomalous diffusion takes place. It is worth remarking that here the term anomalous diffusion refers to asymptotic nonstandard diffusion. Sometimes in the literature the term is used also for long (but nonasymptotic) transient behaviors. 

For a more detailed descriptions of Koc and diffusivity refer to TPHCWG (1997b). 

D - stands for diffuse refers to the diffuse bands D1 and D2 on the SDS-PAGE gel Dl- the 32-kDa polypeptide subunit of Photosystem II encoded by the chloroplastpsbA gene D2- the 34-kDa polypeptide subunit of Photosystem II encoded by the chloroplastp feD gene Da - daltons... 

The basic concept of diffusion refers to the net transport of material within a single phase in the absence of mixing (by mechanical means or by convection). Both experiment and theory have shown that diffusion can result from pressure gradients (pressure diffusion), temperature gradients (thermal diffusion), external force fields (forced diffusion), and concentration gradients. Only the last type is considered in this book that is, the discussion is limited to diffusion caused by the concentration difference between two points in a stagnant solution. This process, called molecular diffusion, is described by Pick s laws. His first law relates the flux of a chemical to the concentration gradient ... 

Diffusion refers to random Brownian motion of solutes from a region with a higher concentration to one with a lower concentration. 

Longitudinal diffusion refers to diffusion of solute molecules in the direction of flow and, since solute diffusivity is low in Uquids (providing that the analysis time is not excessively long), it is a neghgible problem. 

The mechanism of vacancy diffusion refers to atoms moving into ... 

Figure 9.29. The plot of hydrodynamic volumes determined from diffusion NMR measurements and van der Waals volumes computed from 1 1 ion pairs in the coiresponding crystal structures for a series of metallocenes. The point labelled ref. is for the internal diffusion reference Si( -tolyl)4 (adapted with permission from [45], Copyright 2003, American Chemical Society).

Diffusion refers to the randomizing effect of molecular motions. It is the only mechanism enabling contact between individual molecules and is therefore the precursor to a chemical reaction. Thus whereas diffusive mixing is of less importance than convective mixing in some duties (e.g. blending of lubricating oils to achieve a required viscosity), it is always important in chemical reactions. 

There are mainly two types of Slurry seepage paths named cylindrical and surface-shaped, during the slurry diffusion process, cylindrical diffusion refers to slurry along the approximate space of the cylinder movement. The planar diffusion model of the slurry, is refers to the slurry along the approximate plane of the spatial movement... 

Diffusion refers to the transport of a species due to a gradient in chemical potential (concentration). 

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