Membrane-based charge separation

Figure 3. Schematic representations of models based on charge separation for membrane potential changes induced by host-guest complexation at liquid membrane surfaces, (a) Membrane potential change induced by a cationic guest, (b) Membrane potential change induced by an anionic guest (reproduced with permission of Elsevier Science Ltd. from Comprehensive Supramolecular Chemistry, 1996, Vol. 2,p. 176).

The Donnan potential can also be regarded as a special case of a diffusion potential. We can assume that the mobile ions are initially in the same region as the immobile ones. In time, some of the mobile ions will tend to diffuse away. This tendency, based on thermal motion, causes a slight charge separation, which sets up an electrical potential difference between the Donnan phase and the bulk of the adjacent solution. For the case of a single species of mobile cations with the anions fixed in the membrane (both assumed to be monovalent), the diffusion potential across that part of the aqueous phase next to the membrane can be described by Equation 3.11 n — El = (u- — u+)/(u + w+)](i 77F)ln (c11/ 1) that we derived for diffusion toward regions of lower chemical potential in a solution. Fixed anions have zero mobility (u = 0) hence (u — u+)/(u — u+) here is —uJu+> or —1. Equation 3.11 then becomes En — El = — (RT/F) In (cll/cl)> which is the same as the Nernst potential (Eq. 3.6) for monovalent cations [—In = In (cVc11)]. 

The process of ED is, by definition, a membrane-based separation process in which ions are driven through an ion-selective membrane under the influence of an electric field [35]. Conversely, in electrofiltration a charged pollutant particle is prevented from moving through a membrane by the influence of an electric field. Electrosorption is an electrically enhanced ion-exchange process and electroremediation is a process developed for the decontamination of polluted soil. Each different process is discussed separately. 

For evaluation of the selectivity of two metal species separation we can assume that in the same solution environment (water) the diffusion coefficients of these metal ions with the same charge have similar values and the diffusion coefficients of the metal-carrier complexes have similar values. Thus, we can represent separation factor as dependent only on the distribution coefficients at forward and backward extraction, determined experimentally through distribution coefficients at membrane-based equihbrium forward, Ep/E, and backward, Ee/r, extraction [15, 58] ... 

Based on the oriented dipole and the interfacial proton transfer mechanism described above, an equivalent circuit was established which described the relaxation time course of a photoelectric current generated by a single chemical reaction step of charge separation and recombination regardless of whether the charge separation is confined within the membrane or takes place across a membrane-water interface [21]. This equivalent-circuit analysis is notable for the absence of any adjustable parameters each and every parameter used for the computation can be measured experimentally (figure 10.2). An example of the... 

Organic chemists rely on amphiphilic lipids to build up membranes in water—the only organic reaction medium of nature. Biological molecular machinery is based on lipid membrane potentials. Artificial models so far do not work like cell membranes in vectorial transport and charge separation chains, but they look good under the electron microscope. 

This biological membrane system is ideal for porphyrin-based energy conversion systems. For energy storage by photoinduced vectorial charge separation, the elecfron must be removed faster from the excited porphyrin than about IG ps in order to avoid deactivation. Thereafter, the electron should be kept at a... 

The above data demonstrate the first experimental experience in developing photoeatalytic systems based on lipid vesicle suspensions with semiconductor nanoparticles as PhCs. First, this proves the possibility of targeted synthesis of such systems with controllable topology of the arrangement of semiconductor nanoparticles with respect to the vesicle membrane. Besides, the factors were found which permit control of the size of semiconductor nanoparticles attached to the lipid membranes and, as a result, the quantum yield of the primary charge separation on electron transfer from semiconductor nanoparticles to... 

The X-ray structural studies of the photosystem II reaction center of Rhodo-pseudomonas viridis and related aspects of plant membrane structure have been reviewed/ Photoinduced charge separation in synthetic, porphyrin-based molecules which mimic aspects of the natural photosystems have also been reviwed/ ... 

Liquid membranes for ionophore-based ISEs contain not only ionophores but also ion exchangers (ionic sites). If no ionic sites are added in the membrane, coextraction or salt extraction of ionophore/primary ion complexes with counterions occurs, thereby causing no charge separation at the interface. 

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