Diffusional permeability

Effects of annealing are also observed on the water transport properties. Both the diffusional permeability and the permeability under osmotic pressure decrease in comparison with the sample before annealing (Table IV). Moreover, the g ratio also decreases, which, in terms of the equivalent pore radius, means that the membrane becomes tighter upon annealing. 

Passive diffusional permeability was the same along the entire gastrointestinal tract, as the experiments were performed at the same luminal pH value. 

Figure 3. Flow and diffusional permeabilities of He and N, in sample. Key O, observed points A, diffusion experiment and-----------------, predicted.

In the case of the movement of labelled water (THO), the diffusional permeability coefficient, P., in cm/s can be calculated as follows ... 

Probably the most systematic and complete study on the influence of temperature on water transfer has been performed on mammalian red cells [10,20,28]. The dependence on temperature of both the tracer diffusional permeability coefficient (cotho) 3 nd the hydraulic conductivity (Lp) of water in human and dog red-cell membranes have been studied. The apparent activation energies calculated from these results for both processes are given in Table 2. The values for the apparent activation energies for water self-diffusion and for water transport in a lipid bilayer are also included in the table. For dog red cells, the value of 4.9 kcal/mol is not significantly different from that of 4.6-4.8 kcal/mol for the apparent activation energy of the water diffusion coefficient ( >,) in free solution. Furthermore, it can be shown that the product L — THOV )rt, where is the partial molar volume of water and the viscosity of water remains virtually independent of temperature for dog, hut not for the human red-cell membrane [20]. The similarity of the transmembrane diffusion with bulk water diffusion and the invariance of the... 

Here the problem is to decide whether these solutes permeate by dissolution in the membrane fabric or by crossing the membrane through polar pathways aqueous pores . It is worth pointing out from the start that there is no conclusive evidence which enables one to decide unequivocally between these two possibilities. As will become clearer by the end of this discussion, the data at hand in certain cases (such as mammalian red cells) are merely more consistent with the postulate that these molecules permeate partly through polar pathways and not entirely by dissolution in the membrane fabric. Probably the strongest evidence for this hypothesis is summarized in Table 4. The values of the diffusional permeability coefficient for egg lecithin spherical bilayers are included in the table for comparison. Based on their lipid solubility these molecules should be quite impermeable. In fact, in certain... 

Pelaez, L., Vdzquez, M.I. and Benavente, J. 2010. Interfacial and fouling effects on diffusional permeability across a composite ceramic membrane. Ceram. Int. 36 727 801. 

Here (v) is the observable transmembrane solvent velocity, and P is the membrane solute diffusional permeability. The permeability in turn is defined as the ratio of the molar flux of solute transport, moles/area-time, to the solute concentration difference causing this transport The most familiar examples of low-Pe devices are blood oxygenators and hemodialyzers. High-Pe systems include micro-, ultra-, and nano-filtration and reverse osmosis. The design and operation of membrane separators is discussed in some detail in standard references [Ho and Sirkar, 1992 Noble and Stern, 1995], and a summary of useful predictions is provided in Section 5.4. 

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). 

The permeability of the cell monolayer consists of parallel transcellular and paracellular pathways. In passive diffusional transport, it is generally taken that uncharged molecules are capable of partitioning into the cell membrane and... 

The permeability coefficient of the diffusional-bioconversion pathway can be delineated with the aid of Eq. (Ill) once the permeability coefficients of the ABL, filter support, and paracellular routes are known (Table 18). It is seen that 98% of the diester molecules passing through the cell monolayer take the intracellular route. 

Figure 31 Scheme for the protein-binding, diffusional, and partitioning processes and barriers that are encountered by a highly lipophilic and membrane-interactive drug (D) as it permeates through a cell within a continuous monolayer, h and h, thicknesses of the aqueous boundary layers. kd and ka, dissociation and association binding constants, respectively. P, protein molecule. Permeability coefficients Effective, Pe aqueous boundary layer, PABL and PW apical membrane, Pap basolateral membrane, Pbl. 

The effectiveness factor rj, defined in equation 8.5-5, is a measure of the effectiveness of the interior surface of the particle, since it compares the observed rate through the particle as a whole with the intrinsic rate at the exterior surface conditions the latter would occur if there were no diffusional resistance, so that all parts of the interior surface were equally effective (at cA = cAs). To obtain T], since all A entering the particle reacts (irreversible reaction), the observed rate is given by the rate of diffusion across the permeable face at z = 0 ... 

The inhibition of amino-acid transport has been regarded as the main toxic effect of mercury compounds [82], The biochemical mechanism underlying the inhibition is unclear. In unfertilized sea-urchin eggs an interaction with the amino-acid carrier was found, whereas in fertilized eggs inhibition of amino-acid transport was indirect and might result from an elevation of the Na + content of the egg caused by the inhibition of the Na+ pump [83]. The action on the diffusional process could be mediated by an effect on membrane phospholipids or on membrane proteins, or by interaction with Ca2+ which stabilizes membrane structure. Mercuric chloride in skate liver cells inhibited amino acid transport, decreased Na + /K + -ATPase (adenosinetriphosphatase) activity, impaired volume regulatory mechanisms and increased the permeability of the plasma membrane to potassium [84]. It has been suggested that... 

Several modifications of Poiseuille s equation have been attempted by various authors to describe permeability in the transitional region between viscous and diffusional flow. The assumptions underlying these modifications are often questionable and the results obtained offer little or no theoretical or experimental advantage over the BET theory for surface area measurements. Allen" " discusses these modifications as well as diffusional flow at low pressures. 

Water Vapor transmission. Table 3 lists water vapor transmission (WVTR) values for selected polymers. Comparison of Tables 1 and 3 shows that often there is a reversal of roles. Those polymers that are good oxygen barriers are often poor waler-vapor barriers and vice versa. This can be rationalized as follows. Barrier polymers often rely on dipole-dipole interactions to reduce chain mobility and, licncc, diffusional movement of pcrmcants. These dipoles can be good sites for hydrogen bonding. Water molecules are attracted to these sites. Polymer molecules without dipole—dipole interactions, such as polyolefins, dissolve very little water and have low. WVTR and permeability values. The low values of S more than compensate for the naturally higher values of D. 

Boundary layers also contribute to the effect of intestinal fluid hydrodynamics on drug absorption by both diffusional- and carrier-mediated processes. In a well-defined isolated in situ model such as perfused intestine of the rat, a good estimate of the gut wall permeability, which is the vector of convective diffusive mass transfer, passive diffusion and carrier-mediated transport, can be accomplished [99,100]. 

When the barrier has two or more independent diffusional pathways present in parallel, the total permeability coefficient Ptotal is the sum of P for each individual pathway as... 

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