Interphase transport membranes

There are a variety of membrane structures available. Correspondingly, the variety of transport expressions or transport models is considerable. The nature and magnitude of the driving forces and the frictional resistances can vary, leading to enormous variations in individual species fluxes. It is not possible to cover the whole spectrum of such behavior here. Rather we focus on a few cases of considerable use in practical separation techniques/processes to illustrate integrated flux expressions for membranes. 

An additional category studied is a liquid-porous charged membrane in an electrical field. 

We study first liquid separation through practically nonporous membranes and then we move on to porous membranes. Of the known techniques using nonporous membranes, (reverse osmosis, dialysis, liquid membrane permeation and pervaporation), we select the most common, reverse osmosis, to begin our study of integrated flux expression development. Pervaporation is considered next There is one feature which is, however, common to almost all nonporous membrane processes i.e. the additional phase, the membrane phase, is stationary in general (except in cases of rapid transient membrane swelling or emulsion liquid membranes). This is in contrast to molecular diffusion processes in a gas or liquid where all species can diffuse (they may or may not). 

1 Liquid permeation through nonporous membranes reverse osmosis (RO) and pervaporation 

Liquid permeation through practically nonporous membranes can take place under a variety of conditions. These conditions are defined by the nature of the phase on the downstream side of the membrane (liquid or vapor/gas), the nature of the two components in the feed mixture (volatile or nonvolatile) and the level of pressure on the feed side and that on the permeate side (especially including a vacuum). Of the number of combinations possible from this set of variables, the following combinations are of principal interest to us here. 

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A few distinguishing features of transport of any species through a membrane with respect to interphase transport in two-phase systems are as follows. 

Interphase transport In two-phase systems with phase barrier membranes... 

To determine the rate of interphase transport of any species being absorbed from the gas into the liquid (or desorbed from the liquid into the gas) through such a gas-liquid interface in a microporous/porous hydrophobic membrane, consider the concentration profile of species A shown in Figure 3.4.10. The flux of species A being absorbed at steady state may be written down for the three regions (gas film, membrane pore and liquid film) as follows ... 

Quite generally, the interphase between an organism and its environment encompasses the elements outlined in Figure 1 of Chapter 1. The scheme shows that the cell membrane, with its hydrophobic lipid bilayer core, has the most prominent function in separating the external aqueous medium from the interior of the cell. The limited and selective permeabilities of the cell membrane towards components of the medium - nutrients as well as toxic species - play a governing role in the transport of material from the medium towards the surface of the organism. 

Separations in membrane processes are the result of differences in the transport rates of chemical species through the membrane interphase. The transport rate is determined by the driving force or forces acting on the individual components and their mobility and concentration within the interphase. The mobility is primarily determined by the solute s molecular size and the physical structure of the interphase material, while the concentration of the solute in the interphase is determined by chemical compatibility of the solute and the interphase material, the solute s size, and the membrane structure. The mobility and concentration of the solute within the interphase determine how large a flux is produced by a given driving force. 

If an electrical potential difference is established between the electrodes all charged components will be removed from an aqueous interphase between the two ion-exchange layers. If only water is left in the solution between the membranes further transport of electrical charges can only be accomplished by protons and hydroxyl ions which are available in very low concentrations in completely de-ionized water. Protons and hydroxyl ions removed from the interphase arc replenished because of the water dissociation equilibrium. A bipolar membrane thus consists of a cation- and anion-exchange layer laminated together. 

According to Wikipedia [1], a membrane is a thin, typically planar structure or material that separates two enviromnents or phases and has a finite volume. It can be referred to as an interphase rather than an interface. Membranes selectively control mass transport between phases or environments. Again, according to Wikipedia, membranes can be divided into three groups (1) biological membranes, (2) artificial membranes, and (3) theoretical membranes. 

Although a nuclear membrane undoubtedly exists during interphase, the relative permeability of that membrane is not known. Is the nuclear membrane a perforated envelope that prevents the extrusion of chromosomes from the nuclei into the cytoplasm, but otherwise allows at least some cytoplasmic components to flow freely from cytoplasm to nucleus and vice versa The porous structure of the membrane seems to favor such a concept however, the pores are not necessarily true openings and, therefore, the problem of relative permeability remains crucial. Permeability has been investigated in various ways. Small molecules or ions pass freely from cytoplasm to nucleus. In fact, the exchange of sodium is so rapid that up to the present time it has been impossible to follow its transfer. Little is known of the transfer of other molecules such as glucose, amino acid, etc. Whether they are transferred by passive diffusion or by active transport is not known. Deoxyribonucleosides have been reported to accumulate within the nucleus in the course of liver regeneration. 

In using such equations, the transport process is considered as being macroscopic and the membrane as a black box. The factor membrane structure can be considered as an interphase in which a permeating molecule or particle experience a friction or resistance. 

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