Proton-coupled transport, crown

Bartsch, R.A. Charewicz, W.A. Kang, S.I. Walkowiak, W. Proton-coupled transport of alkali metal cations across liquid membranes by ionizable crown ethers. In Liquid Membranes Theory and Applications Noble, R.D., Way, J.D., Eds. ACS Symp. Ser. No. 347 American Chemical Society Washington, D.C., 1987 86-97. 

Proton-Coupled Transport of Alkali Metal Cations Across Liquid Membranes by lonizable Crown Ethers... 

To take advantage of the attractive features of proton-coupled transport, we (9-12) and others (14-17) have utilized crown ether compounds with pendant carboxylic acid groups. The mechanism of proton-coupled transport of a monovalent metal cation across an organic liquid membrane is illustrated in Figure 1. Thus the carrier, which remains in the organic membrane, is deprotonated at the organic phase-alkaline aqueous source phase interface and complexes the metal cation (Step 1). The electroneutral complex... 

Parameters that Affect Diffusion-Limited Transport. Brown et aL varied the length of the alkyl chain of alkyl o-nitrophenyl ether membrane solvents and examined its influence on the proton-coupled transport of alkali metal cations by a crown ether derivative. By comparison of solvent characteristics, such as the dielectric constant, viscosity, and surface tension, they concluded that hexyl o-nitrophenyl ether is a better membrane solvent than NPOE (43), The effect of the solvent on the transport of NaC104 by carrier 4 has been studied by Visser et al (44), Transport parameters and were determined. A series of octyl phenyl ethers containing an electron withdrawing group (NO2, CN) of various positions on the phenyl ring were used. Data are presented in Table 5. 

Brown PR, Hallman JL, Whaley LW, Desai DH, Pugia MJ, Bartsch RA, Competitive, proton-coupled, alkah metal cation transport across polymer-supported hquid membranes containing sym-(decyl)-dibenzo-16-crown-5-oxyacetic acid Variation of the alkyl 2-mtrophenyl ether membrane solvent. J. Membr. Sci. 1991 56, 195-206. Michaels AS, Membranes, membrane processes, and their apphcations Needs, unsolved problems, and challenges of the 1990s. Desahnation, 1990 77, 5-34. 

C.Y. Zhu and R.M. Izatt, Macrocyclic-mediated separation of Eu2+ from trivalent lanthanide cations in a modified thin-sheet-supported liquid membrane system, J. Membr. Sci., 1990, 50, 319 P.R. Brown, J.L. Hallman, L.W. Whaley, D.H. Desai, M.J. Pugia and R.A. Bartsch, Competitive, proton-coupled, alkali metal cation transport across polymer-supported liquid membranes containing s>yn(decyl-dibenzo-16-crown-5-oxyacetic acid) Variation of the alkyl 2-nitrophenyl ether membrane, ibid., 1991, 56, 195. 

Figure 1. Mechanism of Proton-coupled Metal Ion Transport by an lonizable Crown Ether.

The performance of calixarenes as cation carriers through H20-organic solvent H20 liquid membranes has also been studied.137 In basic metal hydroxide solutions, the monodeprotonated phenolate anions complex and transport the cations, while [18]crown-6 does not, under the same conditions. Low water solubility, neutral complex formation and potential coupling of cation transport to reverse proton flux have been cited as desirable transport features inherent in these molecules.137... 

Quinone type carriers perform the cotransport of two protons and two electrons (2e, 2H+ symport) [6.48, 6.49] and take part in mitochondrial and photosynthetic electron transport. Cation receptor sites such as crown ethers or cryptands bearing a quinone [6.50a] or a ferrocene [6.50b] group (see also Section 8.3.1), bind and carry cations with redox coupling through switching between a low affinity state (quinone, ferricinium) and a high affinity state (reduced quinone, ferrocene). 

Proton or sometimes alkali metal cations are used for countertransport of cationic or cotransport of anionic solutes because of their good transport properties. It is not the case with the coupling anions. In fact, for K+ transport by 18-crown-6 in a BLM, the anion effect differs by more than 100 [96]. Many studies of the anion effect on transport efficiency have been conducted [97-100]. The effects of anion hydration free energy, anion lipophilicity, and anion interactions with solvents have been mentioned, although anion hydration free energy seems to be the major determinant of transport efficiency. For example, transport of K+ with dibenzo-18-crown-6 as a carrier, decreased in the order picrate > PFr, > CIO > IO >... 

Functionalized crown ethers act as carriers for the transport of potassium ions against their concentration gradient owing to the coupled counter-transport of protons. The use of macrocyclic ligands in transport processes has been reviewed. ... 

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