Diffusional kinetics

In Section III, emphasis was placed on flux kinetics across the cultured monolayer-filter support system where the passage of hydrophilic molecular species differing in molecular size and charge by the paracellular route was transmonolayer-controlled. In this situation, the mass transport barriers of the ABLs on the donor and receiver sides of the Transwell inserts were inconsequential, as evidenced by the lack of stirring effects on the flux kinetics. In this present section, the objective is to give quantitative insights into the permeability of the ABL as a function of hydrodynamic conditions imposed by stirring. The objective is accomplished with selected corticosteroid permeants which have been useful in rat intestinal absorption studies to demonstrate the interplay of membrane and ABL diffusional kinetics (Ho et al., 1977 Komiya et al., 1980). 

Let us systematically delineate the transport pathways of the nondissociated and protonated species of the P-blockers by applying Eq. (82). The insignificance of the mass transfer resistance of the ABL on the overall transport process, as evidenced by the lack of influence of stirring on Pe, indicates that the passive diffusional kinetics are essentially controlled by the cell monolayer and filter. Therefore, Eq. (82) simplifies to... 

The unambiguous identification of the extraction rate regime (diffusional, kinetic, or mixed) is difficult from both the experimental and theoretical viewpoints [12,13]. Experimental difficulties exist because a large set of different experimental information, obtained in self-consistent conditions and over a very broad range of several chemical and physical variables, is needed. Unless simplifying assumptions can be used, frequently the differential equations have no analytical solutions, and boundary conditions have to be detemtined by specific experiments. 

Finally, it has to be emphasized that both the hydrodynamic parameters and the concentrations of the species involved in the extraction reaction simultaneously determine whether the extraction regime is of kinetic, diffusional, or mixed diffusional-kinetic type. It, therefore, is not surprising that different investigators, who studied the same chemical solvent extraction system in different hydrodynamic and concentration conditions, may have interpreted their results in terms of completely different extraction regimes. 

In view of the highly unusual nature of these results and the lack of a routine method for transport measurements unambiguously establishing that rapid transport was indeed a real manifestation of the system, our studies on rapid polymer transport remained unreported in detail. However, in a recent article 46> we have demonstrated that rapid polymer transport actually occurs in these systems due to the formation of ordered macroscopic structures which move rapidly. This rapid transport has been shown to be not the result of bulk convection since normal diffusional kinetics was observed for solvent markers such as [l4C]sorbitol. The striking feature of this new type of transport process is that it is accompanied by ordered structured flows in the... 

A detailed kinetic analysis of the rapid transport of PVP 360 in the solution of dextran has been made with the aid of a newly developed diffusion cell48) in which horizontal boundaries are formed by a shearing mechanism in a cylinder 5 mm in diameter and 10 mm high (Fig. 4). The transport of radiolabelled [3H]PVP 360 over the boundary does not follow normal diffusional kinetics, i.e. it is not linear with the square root of time (Fig. 5). Instead, the transport appears to be linear with time up to about 3 h, then, it levels off. Within the first 3 h, about 70-80% of the PVP... 

E. B. Krissinel and N. V. Shokhirev, Differential approximation of spin-controlled and anisotropic diffusional kinetics (Russian), in Siberian Academy Mathematical Methods in Chemistry, Preprint 30 (1989) Diffusion-controlled reactions 22, in DCR User s Manual 11-20-1990. 

The mass transfer at the liquid-liquid interface is composed of diffusional, kinetic and convective elements. Most of these applications are concerned with the extraction... 

An asset of lamellar clays as supports for oxidants is their effective surface dimensionality. This leads to fast diffusional kinetics on the clay surfaces, which translate through the Smoluchowsky-Debye equation into high collision rates and, in turn, through the preexponential term, into high kinetic rates. - ... 

S-,Di/k is a dimensionless number that arises in diffusional kinetics problems. In reality, the best form for k is Eq. (85), since Eq. (74) may now be written as... 

Ganguly J., Tirone M., and Hervig R. L. (1998) Diffusional Kinetics of Samarium and neodymium in garnet, and a method for determining cooling rates of rocks. Science 281, (5378), 805-807. 

J.M. Berty, S. Lee, K. Sivegnanam, and F, Szeifert, Diffusional kinetics of catalytic vapor-phase reversible reactions with decreasing total number of moles, Inst. Chem. Eng., Sympos. Ser. 87 455 (1984). 

On a more active and homogeneous surface, the authors of a number of papers observed only purely diffusional kinetics (Table 7). 

Th parameter p compares the diffusional flux of mediator and the diffusional flux of substrate the parameter y compares the electrode kinetics of mediator and substrate at the catalytic microparticle with the corresponding spherical diffusional kinetics. Concentration profiles for mediator A and substrate S are shown in each case. 

---