General Description of Liquid Membranes

Liquid membranes are made by forming an emulsion of two immiscible phases and then dispersing it in a third phase (the continuous or donor phase). Frequently, the encapsulated (or receiving) phase and the continuous phase are miscible. The membrane phsse must not be miscible with either if it is to remain 

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Another analogous relationship is that of mass transfer, represented by Pick s law of diffusion for mass flux, J, of a dilute component, I, into a second fluid, 2, which is proportional to the gradient of its mass concentration, mi. Thus we have, J = p Du Vmt, where the constant Z)/2 is the binary diffusion coefficient and p is density. By using similar solutions we can find generalized descriptions of diffusion of electrons, homogeneous illumination, laminar flow of a liquid along a spherical body (assuming a low-viscosity, non-compressible and turbulent-lree fluid) or even viscous flow applied to the surface tension of a plane membrane. 

An understanding of the properties of liquids and solutions at interfaces is very important for many practical reasons. Some reactions only take place at an interface, for example, at membranes, and at the electrodes of an electrochemical cell. The structural description of these systems at a molecular level can be used to control reactions at interfaces. This subject entails the important field of heterogeneous catalysis. In the discussion which follows in this chapter the terms surface and interface are used interchangeably. There is a tendency to use the term surface more often when one phase is in contact with a gas, for example, in the case of solid I gas and liquid gas systems. On the other hand, the term interface is used more often when condensed phases are involved, for example, for liquid liquid and solid liquid systems. The term interphase is used to describe the region near the interface where the structure and composition of the two phases can be different from that in the bulk. The thickness of the interphase is generally not known without microscopic information but it certainly extends distances corresponding to a few molecular diameters into each phase. 

The description given here can be applied in general. However, a distinction must be made for pressure-driven processes such as microfiltration, ultrafiliration and reverse osmosis. Here the feed consists of a solvent (usually water) and one or more solutes. In general, the concentration of the solute(s) is low and the separation characteristics of the membrane are always related to the solute(s). On the other hand, in liquid separation (pervaporation) and gas separation the terms solvent and solute are best avoided. 

In the following part of this section, we provide simple mathematical descriptions of a few common features of two-phase/two-region countercurrent devices, specifically some general considerations on equations of change, operating lines and multicomponent separation capability. Sections 8.1.2, 8.1.3, 8.1.4, 8.1.5 and 8.1.6 cover two-phase systems of gas-Uquid absorption, distillation, solvent extraction, melt crystallization and adsorption/SMB. Sections 8.1.7, 8.1.8 and 8.1.9 consider the countercurrent membrane processes of dialysis (and electrodialysis), liquid membrane separation and gas permeation. Tbe subsequent sections cover very briefly the processes in gas centrifuge and thermal diffusion. 

The water phase can also be recharged with fresh substrate in an emulsion reactor, where a hydrophilic membrane is used to cleave the emulsion (Fig. lOd) [49]. In this type of reactor as well as in the bimembrane reactor the enzyme is well separated from the organic phase. This is important as interphases, e.g., between two immiscible solvents or between a liquid and a gas differing widely in the dielectric constants, may lead to protein denaturation. A more detailed description of these reactors can be found in the references given or for membrane reactors in general [35,106-108]. 

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