Diffusion across polymeric membrane

SO Electrode. A gas-sensing SO2 electrode marketed by Ionics, Inc. was used to provide additional VLE data at 25°C as a function of composition. Aqueous SO2 equilibrates across a polymeric membrane with a filling solution containing about 0.1 M NaHSO-j. Ionic species do not diffuse across the membrane. A small combination glass electrode measures the pH of the filling solution. The SO2 activity (Pso ) is proportional to the activity of H+ (10"PH), because the bisulfite activity is constant ... 

Solution diffusion — gas dissolves in the membrane material and diffuses across it. The membranes used in most commercial appHcations are non-porous in structure where separation is based on the SD mechanism. This mechanism involves molecular-scale interactions of the permeating gas with the membrane polymer. The model assumes that each component is sorbed by the membrane at one interface, transported by diffusion across the membrane through the voids between the polymeric chains (the so-called free volume ) and desorbed at the other interface. According to the SD model, the flux of gas through a membrane is given by... 

In many processes, including chromatographic separations, the adsorption of drugs to surfaces, the equihbration of solutes between oh and water phases, and diffusion across polymeric or biological membranes, molecules start in one environment and end in another. Nucleation, aggregate or droplet formation, membrane and micelle formation, and protein folding have been modeled in terms of such processes. 

The transport properties across an MIP membrane are controlled by both a sieving effect due to the membrane pore structure and a selective absorption effect due to the imprinted cavities [199, 200]. Therefore, different selective transport mechanisms across MIP membranes could be distinguished according to the porous structure of the polymeric material. Meso- and microporous imprinted membranes facilitate template transport through the membrane, in that preferential absorption of the template promotes its diffusion, whereas macroporous membranes act rather as membrane absorbers, in which selective template binding causes a diffusion delay. As a consequence, the separation performance depends not only on the efficiency of molecular recognition but also on the membrane morphology, especially on the barrier pore size and the thickness of the membrane. 

Fig. 3 Release of drug from various shapes of pol5mer membrane permeation-controlled drug-delivery systems (A) sphere-type, (B) cylinder-type, and (C) sheet-type. In (D), the drug concentration gradients across the rate-controlling polymeric membrane and hydrodynamic diffusion layer exist in series. Both the polymer membrane, which is either porous or non-porous, and the diffusion layer have a controlled thickness and h, respectively).

Water vapor molecules selectively dissolve into the coating or membrane material, diffuse across it, and desorb into the environment, while organic vapor molecules are prevented from permeating through its polymeric material structure. 

Sousa et al [5.76, 5.77] modeled a CMR utilizing a dense catalytic polymeric membrane for an equilibrium limited elementary gas phase reaction of the type ttaA +abB acC +adD. The model considers well-stirred retentate and permeate sides, isothermal operation, Fickian transport across the membrane with constant diffusivities, and a linear sorption equilibrium between the bulk and membrane phases. The conversion enhancement over the thermodynamic equilibrium value corresponding to equimolar feed conditions is studied for three different cases An > 0, An = 0, and An < 0, where An = (ac + ad) -(aa + ab). Souza et al [5.76, 5.77] conclude that the conversion can be significantly enhanced, when the diffusion coefficients of the products are higher than those of the reactants and/or the sorption coefficients are lower, the degree of enhancement affected strongly by An and the Thiele modulus. They report that performance of a dense polymeric membrane CMR depends on both the sorption and diffusion coefficients but in a different way, so the study of such a reactor should not be based on overall component permeabilities. 

Because a vacuum is applied for the removal of the solutes on the membrane downstream face, this side of the membrane is ideally dry in comparison to the more swollen (if polymeric membranes are employed) and hence more flexible membrane upstream face resulting from the solute uptake. This anisotropy of the membrane in the direction of the diffusion of the solute always exists for polymeric membranes and results in a non-uniform diffusivity of solute within the membrane. In other words, the diffusion coefficient of solute i in the membrane, will be position-dependent and not constant across the membrane. 

The composition and the morphology of the membranes are key to effective use of membrane technology. The choice of membranes strongly depends on the types of applications (Koops and Smolders 1991). It is important to know which of the components should be separated from the mixture and whether the component is water or an organic liquid. In general, the component with the smallest weight fraction in the mixture should preferentially be transported across the membrane. The PV membrane can be considered as a dense homogeneous medium in which diffusion of species takes place in the free volume that is present between the macromolecular chains of the polymeric membrane material. 

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