Membrane-bathing solution interface

It is the hyperbolic relations (Ag )(Br ) = K and (M )(Y ) = (M+)(Y)/KZ that provides t e basic analogy be Seen the two kinds of systems. In the latter, K is the ionic salt partition coefficient relating membrane and bathing solution activities at an equilibrium interface. The latter form can also be derived for insoluble salt membranes. However the salt activities (super bar quantities) are constant and so are hidden in the value of the solubility product... 

The observed photoelectric effect must therefore result from an imbalance of ionic charge carriers across the membrane. On one side of the bathing solution, the formation of an electrical double layer (after reduction of Fe ) at the solution-membrane interface occurs, comprising a layer of anions in the solution and a layer of Ch (ion-radicals) in the membrane. The most likely positive ion carriers on the other side of the bathing solution are assumed to be The observed photoelectric voltage E p would be given by... 

The gradual increase in the formamlde fraction to solution increased the elapsed time before schlieren patterns appeared. For the solution composition formamlde/acetone (40/60), slow convection flow appeared suddenly after 650 seconds, but the formation of the pellicle at the nascent membrane Interface could be clearly seen 15 seconds after submersion in the water bath. Such pellicles could not be discerned for solutions cast from THF or pure acetone. 

When the glass membrane is exposed to water, a hydrated layer, approximately 50-100 nm thick, is formed at its interface. In addition to water, the chemical composition of the glass in this layer is the same as that in dry bulk. The concentration of the anionic binding sites is estimated between 3 and 10 M. The membrane is usually blown into a bulb of a typical thickness of the wall 50-200 jitm. The optimum thickness of the wall is a compromise between mechanical stability and the electrical resistance. The latter is typically on the order of 10MQ. The interior of this bulb is sealed and contains the internal reference electrode. Thus, the glass membrane is bathed on both sides by solution and a similar hydrated layer develops on the inside of the glass bulb as well (Fig. 6.14). 

Equation 7.15 is a statement of the equilibrium partitioning at the reservoir-pore liquid interface with m.12 the partition coefficient. Equation 7.16 indicates that the solute concentration is expected to be finite at the bottom of the reservoir. Equation 7.17 displays the continuity of the agent flux across the reservoir-pore interface, whereas Equation 7.18 accounts for the material leaving the membrane and entering the surrounding water bath. The quantity 2 is the membrane area at the outer wall (cm ). 

Loeb and Sourirajan invented the first integrally-skinned membrane in 1960 for desalination by phase inversion of cellulose acetate sols (1). In the interrally-skinned membrane, the skin and substructure are composed of the same material. The skin layer determines both the permeability and selectivity of the bilayer, whereas the porous substructure functions primarily as a physical support for the skin. Differences in density between the two layers are the result of interfacial forces and the fact that solvent loss occurs more rapidly from the air-solution and solution-nonsolvent bath interfaces than from the solution interior (2). 

In a wet or dry-wet process of phase inversion, the thermodynamic properties of the polymer solution and gelation medium give us some information on the overall porosity of a final membrane but not on the pore size and its distribution. The pore size and its distribution are mainly controlled by kinetic effects. This means that upon the immersion of polymer solution into a coagulation bath, mass transfer mainly determines the asymmetric structure of the membrane. The mass transfer is normally expressed by the exchange rate of solvent/nonsolvent at the interface between the polymer solution and the gelation medium. This exchange rate depends upon the nonsolvent tolerance of the polymer solution, the solvent viscosity and so on [14]. 

If the solutions (a reference solution of fixed Na " concentration and a test solution of variable Na" " concentration) bathing each side of the membrane contain equal concentrations of sodium cations then the magnitude of the charge separation at each interface will be equal. However if the concentrations are unequal this will not be the case and a potential will be developed across the membrane. At equilibrium the electrochemical... 

The entire phase inversion process of a polymeric solution is represented by the path from A to D. The original polymeric solution is at point A, where no precipitation agent (nonsolvent) is present in the solution. After the immersion of the polymeric solution into a nonsolvent coagulation bath, the solvent diffuses out of the polymer solution, whereas the nonsolvent diffuses into the solution. In the case when the solvent flux is higher than the nonsolvent flux, the polymer concentration at the interface would increase, and at some point, the polymer starts to precipitate (as represented by point B). The continuous replacement of the solvent by the nonsolvent would result in the solidification of the polymer-rich phase (point C). Further solvent/nonsolvent exchange would cause shrinkage of the polymer-rich phase and finally reach point D, where the two phases (solid and liquid) are in equilibrium. A solid (polymer-rich) phase that forms the membrane structure is represented by point S and a liquid (polymer-poor) phase that constitutes the membrane pores filled with nonsolvent is represented by point L. 

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