Comparative transport

The lengthy permeability chapter (Chapter 7) recounts the study of many different artificial membrane formulations, comparing transport results of each to human jejunal permeabilities. A very promising in vitro screening system was described the double-sink sum-Pe PAMPA GIT model. It is most applicable to molecules that are classified as soluble in the BCS scheme. 

In an attempt to justify the assumption of plasticization put forth in their interpretation of 3 in Eq (A-2), Raucher and Sefcik compare transport data and NMR data for the C02/pvC system This comparison has several questionable aspects To relate local molecular chain motions to the diffusion coefficient of a penetrant, one should use the so-called local effective coefficient, Deff O such as shown in Figure 5 rather than an average or "apparent" diffusion coefficient as was employed by these authors Deff(C) describes the effects of the local sorbed concentration on the ability of the average penetrant to respond to a concentration or chemical potential gradient in that region ... 

There is no bimolecular recombination. This assumption is in good agreement with experimental data for the solar cells. Combined with the assumption that both components (p- and n-t.ypc semiconductors) have comparable transport properties, hole and electron contributions to the overall current can be treated separately. 

The use of supports in asymmetric, supported membranes introduces a number of complications in the interpretation of permeation and separation data as well as in the optimalisation of membrane systems. If the flow resistance of the support is not negligible, there is a pressure drop across the support. This implies that the pressure and so the occupancy at the interface of separation layer and support is different from the (directly accessible) pressure at the support surface, usually the permeate side. Consequently, the driving force for permeation through the separation layer is different from the total driving force across the membrane system. In cases where one wants to calculate or compare transport properties of the separation layer material, it is necessary to correct for this effect (for illustration see below). 

System assumptions that should be valid for such applications include fluid flow in the porous media is isotropic and adsorption is fast, reversible, and linear (cf. Freeze and Cherry 1979). Given these constraints, the comparative transport of a conserved (nonadsorbed) tracer, such as Br , and an adsorbed or retarded species, such as Am, can be described as shown in Fig. 10.29. A comparison of migration distances of the two species after time t, is made at concentrations where C(measured)/Co(initial) = 0.5 for the conserved and adsorbed species. The migration distance X of the conserved species after time r is a measure of the average groundwater velocity (U), or X = vt. Similarly, the migration distance of the adsorbed species (X,) i related to its velocity of movement (v ) by Xf = vj. The retardation factor (/tj for the adsorbed species is then given by... 

Paulsen, I.T, Nguyen, L., Sliwinski, M.K., Rabus, R., and Saier, M.H., Jr. (2000) Microbial genome analyses comparative transport capabilities in eighteen prokaryotes. Journal of Molecular Biology, 301 (1), 75-100. 

Widimsky P, Groch L, Zelizko M, Aschermann M, Bednar F, Suryapra-nata H. Multicentre randomized trial comparing transport to primary angioplasty vs. immediate thrombolysis vs. combined strategy for patients with acute myocardial infarction presenting to a community... 

Fig. 6 shows the pore size distribution of transport-pore radii from LEPP for three porous samples. When comparing transport-pore radii from diffusion and pomeation measurements with these distributions the size and shape of confidence regions (Fig. 5) have to be taken into account as well as the above arguments concerning the validity of ()p. Therefore, the agreement of pore-size distribution of transport pores with ()d is excellent and ()p is a little worse. 

What transportation challenges does online grocer Peapod face Compare transportation costs at online grocers and supermarket chains. 

---