Carrier facilitated transport, salts

Behr and Lehn (14) first demonstrated carrier-facilitated transport of amino acids through "liquid membranes" composed of a toluene layer floating on top of two isolated aqueous solutions. The carriers used were the quaternary ammonium salt Aliquat 336 (trioctylmethylammonium chloride) and the alkylated arylsulfonic acid dinonylnapthalenesulfonic acid. Amino acids were transported in the form of anions or cations, respectively, with the above carriers. The process is an ion-exchange process, as the carrier must exchange an ionized atom or molecule each time it forms a new ion pair (Figure 2). The net result of this type of carrier-facilitated transport process is that a solute ion is transported into the LM while an equal number of counterions are transported out of the LM. 

Figure 2. Carrier-facilitated transport of anions and cations in LMs (a) Anion transport by e.g. a quaternary ammonium salt (b) Cation transport by e.g. a phosphate diester.

Lamb, J. D. Christensen, J. J. Izatt, S. R. Bedke, K. Astin, M. S. Izatt, R. M. "Effects of Salt Concentration and Anion on the Rate of Carrier-Facilitated Transport of Metal Cations through Bulk Liquid Membranes Containing Crown Ethers" J. Am. Chem. Soc., 1980 102 (10), p. 3399. 

J.D.Lamb, J.J.Christensen, S.R.Izatt, K.Bedke, M.S.Astin, R.M.Izatt, Effects of Salt Concentration and Anion on the Rate of Carrier Facilitated Transport of Metal Cations Through Bulk Liquid Membrane Containing Crown Ethers, J.Am.Chem.Soc., 102, 3399 (1980) H.Tsukube, Effects of Cation on Transport Efficiency and Selectivity of Amino Acid Derivative Anions, Bull.Chem.Soc.Jpn.,... 

Chrisstoffels, L. A. J. de Jong, F. Reinhoudt, D. N. Sivelli, S. Gazzola, L. Casnati, A. Ungaro, R. Facilitated transport of hydrophilic salts by mixtures of anion and cation carriers and by ditopic carriers. J. Am. Chem. Soc. 

The typical concentration profile of solute in an SLM system with quaternary ammonium salt as carrier is schematically shown in Fig. 6. To model the facilitated transport within a supported liquid membrane [58,59], the following assumptions are usually made ... 

It is clear that numerous facilitated transport processes may still be set up, especially for anions, salts or neutral molecules, and that the active research in receptor chemistry will make available a variety of novel carrier molecules. Of special interest are those transport effectors, derived from coreceptors, that allow coupled transport (cotransport) to be performed. 

Concurrently with the work on carbon dioxide and hydrogen sulfide at General Electric, Steigelmann and Hughes [27] and others at Standard Oil were developing facilitated transport membranes for olefin separations. The principal target was the separation of ethylene/ethane and propylene/propane mixtures. Both separations are performed on a massive scale by distillation, but the relative volatilities of the olefins and paraffins are so small that large columns with up to 200 trays are required. In the facilitated transport process, concentrated aqueous silver salt solutions, held in microporous cellulose acetate flat sheets or hollow fibers, were used as the carrier. 

Hepatocytes make up 60-70% of the total number of liver cells. They have a well-organized intracellular structure with huge numbers of cell organelles to maintain the high metabolic profile. At the apical side or canalicular membrane the cell is specialized for the secretion of bile components. There are several ATP-dependent transport carriers located on this side of the membrane, which transport bile salts, lipids and xenobiotics into the canaliculus. On the sinusoidal side, the cells specialize in uptake and secretion of a wide variety of components. To increase the surface of the membrane for this exchange with the bloodstream, the sinusoidal domain of the membrane is equipped with irregular microvilli. The microvilli are embedded into the fluid and matrix components of the space of Disse and are in close contact with the sinusoidal blood because of the discontinuous and fenestrated SECs. To facilitate its metabolic functions numerous membrane transport mechanisms and receptors are situated in the membrane. 

Cation-anion cotransport was effected by an optically active macrotricyclic cryp-tand that carried simultaneously an alkali cation and a mandelate anion and displayed weak chiroselectivity [4.23a], as did also the transport of mandelate by an optically active acyclic ammonium cation [6.39]. Employing together a cation and an anion carrier should give rise to synergetic transport with double selection by facilitating the flow of both components of a salt (see the electron-cation symport below). Selective transport of amino acids is effected by a convergent dicarboxylic acid receptor [4.24b]. 

The phenomena accompanying interphase transfer of chemical materials are universally observed in biological systems. A number of carriers and various transportation methods are used and the selective transportation of materials contributes to controlling the biochemical reactions in vivo. In synthetic chemistry, however, the carriers as well as the methods are very limited. Phase-transfer catalysts (PTCs cf. Section 4.6.1) such as crown ethers or onium salts are limited to the transportation of anions from an aqueous or solid phase into an organic phase nevertheless, the PTCs contributed to the development of synthetic chemistry. The most important point is that these catalysts have enabled the biphasic reactions of lipophilic molecules with inexpensive inorganic salts and at the same time facilitated the separation of products. 

Other ann is filled with water. Because of the concentration difference, the salt will diffu.se from thecpncentrated. oIution tothe pure water,phase. Howevenfin the absence the transport of salt us extremely low because its solubility in fKe organic phase (e.g.-chloroform) is very low. Adding a carrier to. the organic phase that is capable to form a reversible complex with the salt (e.g. diphenyl-18-crown-6) causes transport of potassium from one side of the U-tube to the other. After a finite time the pure water phase will now contain a certain amount of KCl (note that to maintain electroneutiality the anion chloride has to diff e along with the carrier complex). This U-tube experiment is very suitable to demonstrate the existence of facilitated or canier-mediated traiisptHt... 

Calixarenes may be used as carrier molecules for facilitated ionic transport. The equilibrium constant for KC104-complexation is 9.2 10 1/mol and for NaCI04-complexaiion is 270 1/mol. The diffusion coefficients of the carrier-salt complex is 2 lO cm2/s for both salts. The carrier concentration is lO- M. 

Optimization of the flux is investigated through the variation of external parameters which affect transport, such as the membrane polarity, viscosity, anion used and operating temperature (see the section on Optimization of the Flux External Influences on Transport). The influence on the transport kinetics has been investigated as well. Until recently, the transport of salts has only been facilitated through the use of a cation carrier. The cation is complexed by the carrier while the anion accompanies the complex as a free ion, hence a lipophilic anion is used to accompany the complex. 

---