Carrier-assisted membrane transport

Table 9. Total Urea Fluxes Carrier-Assisted Urea Transport Through Supported Liquid Membranes, and (Adapted from ref. 69, 70) ...

Another possibility of constructing a chiral membrane system is to prepare a solution of the chiral selector which is retained between two porous membranes, acting as an enantioselective liquid carrier for the transport of one of the enantiomers from the feed solution of the racemate to the receiving side (Fig. 1-5). This system is often referred to as membrane-assisted separation. The selector should not be soluble in the solvent used for the elution of the enantiomers, whose transport is driven by a gradient in concentration or pH between the feed and receiving phases. As a drawback common to all these systems, it should be mentioned that the transport of one enantiomer usually decreases when the enantiomer ratio in the permeate diminishes. Nevertheless, this can be overcome by designing a system where two opposite selectors are used to transport the two enantiomers of a racemic solution simultaneously, as it was already applied in W-tube experiments [171]. 

For the separation of racemic mixtures, two basic types of membrane processes can be distinguished a direct separation using an enantioselective membrane, or separation in which a nonselective membrane assists an enantioselective process [5]. The most direct method is to apply enantioselective membranes, thus allowing selective transport of one of the enantiomers of a racemic mixture. These membranes can either be a dense polymer or a liquid. In the latter case, the membrane liquid can be chiral, or may contain a chiral additive (carrier). Nonselective membranes can also... 

Previously in our laboratory cation carrier assisted transport of lipophilic potassium salts has been studied [ref 34]. A preliminary investigation showed a significant transport of KH2PO4, a hydrophilic potassium salt, through a supported liquid membrane with receptor 26 [ref 33]. 

Theory and Mechanism. In Chapter 3, Reinhoudt and coworkers review recent mechanistic aspects of carrier-assisted transport through supported liquid membranes. Carriers for selective transport of neutral molecules, anions, cations, or zwitterionic species have been developed. Transport is described in terms of partitioning, complexation, and diffusion. Most of the mechanistic studies were focused on diffusion-limited transport, in which diffusion of the solute-carrier complex through the membrane phase is the rate-limiting step for transport. However, for some new carriers, the rate-limiting step was found to be decomplexation at the membrane phase-receiving phase interface. 

Transport Through a Bulk Liquid Membrane. All theoretical models concerning carrier-assisted transport through SLMs are based on the theoretical work for carrier-assisted transport through BLM systems reported by Reusch and Cussler (5). They described the transport of different alkali salt mediated by dibenzo-18-crown-6 through a BLM. 

Solid mixed ionic-electronic conductors (MIECs) exhibit both ionic and electronic (electron-hole) conductivity. Naturally, in any material there are in principle nonzero electronic and ionic conductivities (a i, a,). It is customary to limit the use of the term MIEC to those materials in which a, and 0, 1 do not differ by more than two orders of magnitude. It is also customary to use the term MIEC if a, and Ogi are not too low (o, a i 10 S/cm). Obviously, there are no strict rules. There are processes where the minority carriers play an important role despite the fact that 0,70 1 exceeds those limits and a, aj,i< 10 S/cm. In MIECs, ion transport normally occurs via interstitial sites or by hopping into a vacant site or a more complex combination based on interstitial and vacant sites, and electronic (electron/hole) conductivity occurs via delocalized states in the conduction/valence band or via localized states by a thermally assisted hopping mechanism. With respect to their properties, MIECs have found wide applications in solid oxide fuel cells, batteries, smart windows, selective membranes, sensors, catalysis, and so on. 

In the process of mediated transport, carrier proteins embedded within the plasma membrane assist in the transport of larger polar molecules into or out of the cell. When a given substance attaches to a specific binding site on the carrier protein, the protein undergoes a conformational change such that this site with the bound substance moves from one side of the plasma membrane to the other. The substance is then released. Mediated transport displays three important characteristics influencing its function ... 

An example of light-assisted transport of the first type involves (200) as the carrier in the liquid membrane. In this case, irradiation of the membrane alternatively with UV and visible light significantly increases the rate of K+ and Rb+ transport in the presence of picrate ion. This system also exhibits discrimination since the transport of K+ is favoured over Rb+ (Shinkai, Shigematsu, Sato Manabe, 1982). 

Most drugs that are fully charged or otherwise too polar for passive diffusion cross membranes with assistance from carrier or transport proteins. Carrier proteins span the membrane and can shuttle small molecules from one side to the other. These proteins are technically catalysts because they accelerate a process (membrane crossing) without being consumed. 

Type 2 facilitation is also known as carrier facilitated transport, since a carrier compound, that is, an extractant or complexing agent, solubilized in the organic phase is used to assist transfer across the membrane. In this simation, the solute of interest reacts with the carrier to form a complex that is only soluble in the membrane phase. The solute is de-complexed by a stripping solution contained in the internal phase. An example of such a process is the removal of a metal ion such as copper or zinc from wastewater by the extractant DEHPA (di-2-ethyUiexyl phosphoric acid, represented as HE) as shown in Figure 25.2. In this case, the carrier also enhances the selectivity as most extractants are specifically designed to extract particular metal ions... 

Facilitated diffusion is very similar to passive diffusion with the difference that transfer across membranes is assisted by the participation of carrier proteins embedded in the membrane bilayer. Again, the direction of passage will be from the side of the membrane with high concentration of a chemical to the side with low concentration this also occurs without energy expenditure by the cell. Such a process is somewhat specific in the sense that it applies to molecules that are able to bind to a carrier protein. Absorption of nutrients such as glucose and amino acids across the epithelial membrane of the gastrointestinal tract occurs by facilitated diffusion. Since a finite number of carriers are available for transport, the process is saturable at high concentrations of the transported molecules and competition for transport may occur between molecules of similar structure. 

Facilitated transport Transport of a molecular entity through a membrane that occurs via the assistance of a carrier or a channel. 

The use of an external cationic transporter to assist in the transport of anions by certain carriers has been explored by Gale and others recently. This was demonstrated by Gale in the case the triazole-strapped calix[4]pyrrole 36, which was combined with the potassium carrier valinomycin. The combination led to enhanced transport of chloride anions across POPC membranes (cf. Fig. 12.23) [71]. The overall transport process was consistent with a KVC1 symport mechanism. This dual host approach was employed by Sessler, Gale, Shin, and coworkers in the case of the pyridine-diamide strapped caUx[4]pyrrole Cl transporter 37 (cf. Fig. 12.23) [72]. It was found that the combination of 37 and... 

The control of surface functionality by proper selection of the composition of the LB films and/or the self-assembling (amphiphatic) molecular systems can mimic many functions of a biologically active membrane. An informative comparison is that between inverted erythrocyte ghosts (Dinno et al., 1991 Matthews et al., 1993) and their synthetic mimics when environmental stresses are imposed on both systems. These model systems can assist in mechanistic studies to understand the functional alterations that result from ultrasound, EM fields, and UV radiation. The behavior of carrier molecules and receptor site functionality must be mimicked properly along with simulating disturbances in the proton motive force (PMF) of viable cells. Use of ion/electron transport ionomers in membrane-catalyst preparations is beneficial for programs such as electro-enzymatic synthesis and metabolic pathway emulation (Fisher et al., 2000 Chen et al., 2004). Development of new membranes used in artificial organs and advances in micelle reaction systems have resulted from these efforts. 

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