Liquid membrane system description

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. 

Liquid membranes with an ionophore L function via complex formation between the ionophore and an ion in the aqueous phase. When this ion is a monovalent cation M" " the membrane phase contains the cation in the form ML" ", that is, completely complexed with the ionophore. The other components of the membrane are a hydrophobic solvent S which constitutes the majority of the liquid phase and a hydrophobic anion A. The concentration of free cation M within the membrane is very small but must be considered in assessing the Donnan equilibria on each side. The description of the membrane system is... 

The purpose of this chapter is to discuss the task of chemical model determination (CMD) not only with respect to solvent extraction (SX) and liquid membranes (LM) but also to give overview of the fundamentals and also to elucidate the problems of extraction equilibria computation more deeply. There is no intention of providing an exhaustive description of computation in extraction systems due to the limited space and because it is not necessary. Also no exhaustive review of the computer programs will be given here but will be limited to the most important questions. 

The theoretical description of the kinetics of transmembrane transport through a liquid membrane should be based on the principles of solvent extraction kinetics. It should be determined by the processes at both water/membrane interphases and should also involve the intermediate step of diffusion in the membrane. Thus the existence of all these three steps makes the membrane system and its description much more complicated than the relatively simple water/organic phase. However, even the kinetics mechanism in simpler extraction systems is often based on the models dealing only with some limiting situations. As it was pointed out in the beginning of this paper, the kinetics of transmembrane transport is a fimction both of the kinetics of various chemical reactions occurring in the system and of diffusion of various species that participate in the process. The problem is that the system is not homogeneous, and concentrations of the substances at any point of the system depend on the distance from the membrane surface and are determined by both diffusion and reactions. The solution of a system of differential equations in this case can be a serious problem. 

Water molecules are represented by three- or four-center models with fixed values of point charges. These models have been thoroughly tested in simulations of bulk water and aqueous solutions. So far, no attempts have been made to study water-membrane systems using polarizable models of water and lipid molecules. Since interfacial molecules experience an anisotropic environment very different from the bulk liquid, it may be expected that including polarization will yield an improved description of these systems. The extent to which this is the case remains to be explored. 

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. 

This type of approach was developed by Wei et al. [72], Ma et al. [73], Inoue et al. [74] for gaseous systems. Yoshida et al. [75,76] studied the use of combined liquid and surface flow for the description of dye permeation through a cellulose membrane. 

The following is restricted to membrane phases. However, the description is general enough that it stands for soft matter systems that possess internal disorder (liquid crystals, colloids, polymers, plastic crystals, etc.)... 

Mass spectrometry of liquid samples of the cathode outlet stream is another way of determining the methanol crossover flux. For mass spectrometric measurements of methanol crossover, a clear description of the respective system conld be achieved by measuring the background methanol signal of a cell filled with distilled water and equipped with the membrane sample, and subseqnently adding well-adjusted portions of aqueous or pure methanol to this liquid [25]. The slopes of mass signal vs. time curves are typical for diffusion-controlled processes and with the help of the calibration lines, the diffusion coefficient of methanol through the membrane can be calculated. Online analysis of the cathode exhaust gas with multipurpose electrochemical mass spectrometry can also be employed to determine methanol permeability. However, as mentioned, the assumptions that the entire permeated methanol is converted to CO and that there is no anodic CO contribution are contentious. 

The liquid crystalline phase is most relevant to membranes in living systems and represents a good description for the cell membrane, even though real membrane nanoscale structure may be highly complex and dynamic. In the following section, we explore some of the interesting membrane phase behavior that has been seen in mixed-lipid membranes and its relevance to real biological systems. 

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