Permselectivity charged layer

For the production of table salt by concentration of sea water monovalent cation selective membranes were prepared by forming a thin positively charged layer on the surface of a cation-exchange membrane. Monovalent anion permselective membranes have a thin highly cross-linked layer on the membrane surface have also been developed [25]. By such means the selectivity of sulfate compared to the one of chloride can be reduced from about 0.5 to about 0.01 and of magnesium compared to sodium from about 1.2 to about 0.1. 

A more simplified model was presented in Ref. 10, where the membrane was assumed to be perfectly permselective toward the counter-ion, and the salt concentration in the macropores of the electrode assumed to be unvarying in time. A basic element in the modeling of the membranes in MCDI is that in the membrane the cation concentration is different from the anion concentration, with the difference compensated by the fixed membrane charge density, X. Except for this difference, the same ion transport model can be used as in free solution (Nemst-Planck equation), thus, with ions moving under the influence of a concentration gradient and because of an electrical field (electromigration). At the edges of the membranes, a Donnan potential difference develops between the outside solution and inside the membrane. For more information on MCDI, see Section 15.4.3, where a porous electrode is modeled which has an ideally permselective membrane layer in front. 

To evaluate the contribution of the SHG active oriented cation complexes to the ISE potential, the SHG responses were analyzed on the basis of a space-charge model [30,31]. This model, which was proposed to explain the permselectivity behavior of electrically neutral ionophore-based liquid membranes, assumes that a space charge region exists at the membrane boundary the primary function of lipophilic ionophores is to solubilize cations in the boundary region of the membrane, whereas hydrophilic counteranions are excluded from the membrane phase. Theoretical treatments of this model reported so far were essentially based on the assumption of a double-diffuse layer at the organic-aqueous solution interface and used a description of the diffuse double layer based on the classical Gouy-Chapman theory [31,34]. 

All spectroscopic evidence on the composition of a relatively thin layer at the membrane surface was thus found to be in agreement with the interpretation of permselectivity as being due to the exclusion of counteranions from the membrane phase. However, the depth accessible to ATR-IR is of the order of 0.1 1.0 pm and is thus too large for the observation of phenomena in the region in closest proximity to the membrane/aqueous interface in which charge separation is assumed to take place. Optical second harmonic generation (SHG), which has an even more pronounced surface sensitivity than ATR-IR, was recently shown to be very suitable for the investigation of the interface between ISE membranes and sample solutions."" ... 

It is observed from Fig. 5.3.2c that in the modified TMS model (5.3.61) (5.3.63) permselectivity (t ) remains essentially constant in a wide range of voltages. The corresponding VC curves (Fig. 5.3.2a,b) are similar in shape to those for an ideal permselectivity membrane. This is also true regarding the space charge density and profiles of the other fields in the depletion layer 0 < x < 1. 

Problems 2 and 3 are of direct relevance for an adequate understanding of concentration polarization at, respectively, composite heterogeneous and homogeneous permselective membranes. The main difference between these formulations is that in Problem 2, relevant for a composite heterogeneous membrane, the motion in a pore of the support is induced by the electro-osmotic slip due to the interaction of the applied electric field with the space charge of the electric double layer which is present already at equilibrium. 

Cyclic voltammograms of luminol at different electrodes in 0.1 mol/L phosphate buffer (PBS, pH=7.5) were obtained (Fig. 3A). Similarly, an enhancement of redox current from the analyte was observed at the PDDA-chitosan modified GCE, compared with the response at the bare GCE. This is due to the good permselectivity and highly positive charge density of the PDDA-chitosan composite layer. The negatively charged luminol could be easily absorbed on the surface of modified GCE through electrostatic interaction, which was supported by the linear increase of oxidation current vs. scan rates. 

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