Cation transport through

An important feature of such films is their low ionic conductivity that restricts cation transport through the film substance. Electronic semiconduction, however, permits other electrode processes (oxidation of H2O to O2) to take place at the surface without further significant film growth. At elevated anodic potentials adsorption and entry of anions, particularly chloride ions, may lead to instability and breakdown of these protective films (Sections 1.5 and 1.6). 

Lipophilicity. The gross lipophilicity of the ligand and of its complexes plays a very important role whenever substances soluble in organic media of low polarity are needed. This is the case in studies of anion activation and of cation transport through lipid membranes, where salts have to be dissolved in organic phases. The lipophilic character may be controlled via the nature of the hydrocarbon residues forming the ligand framework or attached to it. 

Finally, cation transport through organic membranes depends inter alia, on the mobility of the carrier-cation complex. Thus, too thick ligands may decrease the efficiency of the carrier. 

Competition between mono- and di-valent cations has an important role in biological processes. Furthermore, the lipophilicity of a ligand and its complex plays an important role in deciding whether a species is soluble in organic media of low polarity. This has important consequences in areas such as phase-transfer catalysis, the use of crown ethers as anion activators, and in cation transport through lipid membranes. Many crown ethers have now been synthesized with incorporation of long alkyl side chains and enhanced lipophilicity and used successfully in the above areas. 

Next, we consider the time-dependence of the metal atom balance at the interface Xj = 0. Since, by hypothesis, there can be no cation transport through any individual layer, the cations consumed at xt = 0 by the growth rate (dL,/dt)+ of layer i must be derived entirely from the equivalent cation current, (dL,, /df) for i = 2, 3,. . . , N, obtained... 

Izatt, R.M., McBride, D.W., Brown, P.R., Lamb, J.D., and Christensen, J.J., The influence of halocarbon solvent on macrocycle-mediated cation transport through liquid membranes. J. Membr. Sci., 1986, 28 69-76. 

Izatt RM, Bruening RL, Bruening ML, Lamb JD, Macrocycle-mediated cation transport through liquid membranes. Isr. J. Chem. 1990 30 239-245. 

Table 17. Rate constants r, k-Q, ms> ks of valinomycin and trinactin mediated cation transport through glycerol monooleate/n-decane bilayer membranes at 25 °C (cf. Fig. 18)...

The kinetics of carrier-mediated cation transport through bilayer membranes have also been investigated. Figure 18 shows a scheme of four different processes,... 

Such a preconcentration of monovalent cations at a negatively charged BLM surface has considerable impact on transmembrane ion current values. Monovalent cations transport through BLMs to a much larger degree than anions and the ion conductivity is expected to be increased. Such an increase will affect the S/N ratio associated with selective perturbation of membrane structure. 

If the electrolyte were NaCI where t+ = 0.4, the electric current would deliver only 0.4 of an equivalent of cations for each equivalent of cations transported through the cation exchange membrane. The remaining 0.6 faraday of current would be transported by anions that are moving in the opposite direction. The net result would be a depletion of anions and cations from the solution at the membrane surface. This depletion would result in a concentration gradient that would allow diffusive transport of the balance of ions needed for electroneutrality. In this case, the balance would be 0.6 of an equivalent per faraday of current flow. The diffusive flux of ion pairs down this linear concentration gradient is ... 

R.M. Izatt, D.W. McBride, Jr., P.R. Brown, J.D. Lamb and J.J. Christensen, The influence of halocarbon solvent on macrocycle-mediated cation transport through liquid membranes, J. Membr. Sci., 1986, 28, 69 H. Parham and M. Shamsipur, Highly selective and efficient transport of mercury as Hg(N02)42 ion using Ba2+-18-crown-6 as carrier, ibid., 1994, 86, 29 A. El. Bachiri, A. Hagege and M. Burgard, Recovery of silver nitrate by transport across a liquid membrane containing dicyclohexano 18 crown 6 as a carrier, ibid., 1996, 121, 159. 

The influence of lipophilic group attachment site variation was examined for competitive alkali metal cation transport through the chloroform membrane by the series of isomeric crown ether carboxylic acids 2, 4 and 5. All three carriers exhibited the transport selectivity of Na K >Li, Rb, Cs and for which showed the... 

Table 2. Cation transport through liquid membranes.

SELECTIVE SYNTHESIS OF MACROCYCLIC OLIGOESTERS FROM A BICY-CLIC OXALACTONE AND CATION TRANSPORT THROUGH THEIR ORGANIC LIQUID MEMBRANES... 

Cation Transport through Organic Liquid Membranes... 

Although the phenomenon of polarisation has been illusuated by considering cation transport through cation-selective membranes, the same description applies to anions. However, the mobility of anions with the same valence in the boundary layer is a little greater than that of cations. This implies that under similar hydrodynamic conditions (equal thickness of the boundary layer, same cell construction) for the anion and cation, the limiting current densitj will be attained faster at a cation-exchange membrane than at an anion-exchange membrane. 

Reaction-Rate Limited Transport of Monovalent Ions. From H NMR studies it is known that decomplexation rates can be very slow and, as a consequence, complexes can be kinetically stable (35,36). Until recently the role of slow rates of cation release in SLM transport was unclear. Lehn et al (24) and Fyles (37) theoretically raised the question of the influence of slow rates of alkali metal cation release on transport through a BLM. Experimentally, this phenomenon has only been observed by Yoshida et al (38). They showed that cation transport through a BLM mediated by polynactin was limited by the rate of cation release from the membrane. In 1994, Echegoyen stated that in SLM transport the rate of cation release from the membrane could never... 

Whatever the initiation mechanism, propagation causes further evolution of the local environment, which eventually becomes saturated (with concentrations estimated to be several mol/L of chloride and metal cations). At this point, a salt layer may form on the corroding surfaces. Thus, active corrosion in the crevice is rarely controlled by an activation process but rather by the ohmic drop and, for well-developed crevices, by cation transport through the salt layer, the thickness of which determines the corrosion rate. 

For a given electrolytic solute and values of kd and CL, the higher the value of i, the lower the value of C r In the limit of CLt — 0, the value of i, i/im, is called the lintUing current density its unit is usuaUy mA/cm, and practical values are around 20 vah/cvc . This condition, identified cis concentration polarization, specifies the useful limit of i. If i is increased beyond inn, the excess current is not useful for cation transport through the cation exchange membrane, and is essentially wasted. 

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