Carrier species

Because the facilitated transport process employs a specific reactive carrier species, very high membrane selectivities can be achieved. These selectivities are often far higher than those achieved by other membrane processes. This one fact has maintained interest in facilitated transport since the 1970s, but the problems of the physical instability of the liquid membrane and the chemical instability of the carrier agent are yet to be overcome. 

Lipophilic N-tetrabenzylcylam, III, at pH 4-7 was found to be a good carrier for liquid membrane transport of dicarboxylate anions, such as o-phthalate, and of amino acid anions52). It was suggested that the diprotonated species, H2L2+, is an active carrier species for anions. 

The mobile charge carrier species may either recombine or reach the semiconductor surface, where they can be trapped by the surface adsorbates or other sites. The lifetime of electron-hole (e /h+) pairs that are generated is important in determining the reaction yield. The holes are mainly trapped by water molecules or hydroxyl ions, giving rise to very reactive hydroxyl radicals ... 

Organ/Tissue Carriers Species Diseases aimed at... 

It is found that in a-Sb cSei- c alloys, electrons (the mobile carrier species) are depleted (n-type system) during dark decay, leaving behind a deeply trapped positive space charge. Note that the same situation prevails in alkali-doped a-Se [16]. 

Of special interest, because of its biological significance, is the transport of carboxylates and phosphates. In particular, the transport of nucleotides has been achieved [6.28-6.32] and may be made selective by introduction of additional base pairing interactions [6.31,6.32]. The transport of ATP has significance with respect to bioenergetic processes. Antiviral chemotherapy may take advantage of enhancing the cellular uptake of modified nucleotides by carrier species. 

The transport equations used for facilitated transport parallel those derived for coupled transport [27], The major difference is that only one species is transported across the membrane by the carrier. The carrier-species equilibrium in the membrane is... 

One promising approach to facilitated transport pioneered by Nishide and coworkers at Wasada University is to chemically bind the oxygen carrier to the polymer backbone, which is then used to form a dense polymer film containing no solvent [28], In some examples, the carrier species is covalently bonded to the polymer matrix as shown in Figure 11.29(a). In other cases, the polymer matrix contains base liquids which complex with the carrier molecule through the base group as shown in Figure 11.29(b). Because these films contain no liquid solvent, they are inherently more stable than liquid membranes and also could be formed into thin films of the selective material in composite membrane form. So far the selectivities and fluxes of these membranes have been moderate. 

Give the definition and function of an energy carrier species in metabolism. 

Carrier prodrugs are formed by combining an active drug with a carrier species to form a compound with the desired chemical and biological characteristics, for example, a lipophilic moiety to improve transport through membranes. The link between carrier and active species must be a group, such as an ester or amide, that can be easily metabolized once absorption has occurred or the drug has been delivered to the required body compartment. The overall process may be summarized by ... 

The high sensitivity of ETA—AAS for Cu has stimulated the development of methods to measure concentrations of the Cu carrier species in biological fluids. Delves [7] analysed the Cu content of the protein fractions separated from 2 pi volumes of serum by cellulose acetate membrane (CAM) electrophoresis. The separated protein bands were cut from the CAM and placed directly into the ETA via a 6 mm x 1 mm hole cut in the wall of the graphite tube. Calibration was achieved by adding 2 pi volumes of aqueous standards to 8 mm x 6 mm strips of CAM. Background correction was essential. Approximately 94% of the Cu was located in the a2 band, where carulo-plasmin would run, whereas other fractions contained less than 5% of the total serum Cu. The recovery of Cu after electrophoresis was quantitative, 99%, and the RSD was 0.086 at 1.74 ng Cu. This method was applied to studies of Cu changes in patients with Menkes Syndrome receiving intramuscular injections of copper as the EDTA complex, and in children with acute lymphoblastic leukaemia. 

To construct the pertraction network, the particular reaction networks should be added via 0 junctions to three linear networks corresponding with the processes of diffusion of C2A, C2B, and CH carrier species. The resulting reaction-diffusion network, as presented in Figure 13.6, consists of four coupled loops representing the pertraction of A " ", and H" " cations. The loops are coupled by common capacitances Aj, Bj, and Hj (i = f, s) and by the capacitances CHj and CH for a reacting or diffusing acidic form of the carrier. From the network in Figure 13.6, all the model equations used further in numerical calculations can be deduced with the help of Kirchhoff s law for a 0 junction (KCL) ... 

A water-in-oil emulsion dispersed in an aqueous phase (W/O/W ELM) is generally stabilized by the addition of a surfactant. In the case of type 2 facihtation, a carrier species is also present. The concentration of these additives together with the composition of the internal aqueous phase, external aqueous phases, and the permeation conditions influence the properties of the ELM system. These effects are summarized as follows ... 

An expression for the internal space-charge field can be obtained through the Kukhtarev model [38] that was developed to describe photorefractivity in most inorganic materials. In this model, the photorefractive material is described by a band model. As for a traditional semiconductor, the material consists of a conduction and a valence bands separated by a band gap as shown in Fig. 13. The model describes the transport of single carrier species and the band gap of the material contains localized energy levels that can be excited optically promoting either holes in the valence band (VB) or electrons in the conduction band (CB). In the model that we adopt here, we assume that the dopant is a donor with an energy level located in the band gap with concentration N. Furthermore, the crystal contains Nj acceptors with that are all ionized and that have accepted a... 

The space-charge build-up process can be divided into two steps the electron-hole generation process followed by the transport of one carrier species. The creation of a correlated electron-hole pair (or exciton) after absorption of a photon can be followed by recombination. This process limits the formation of free carriers that can participate in the transport process and is, therefore, a loss for the formation... 

In the simple transport flux of the solute that is not complexed with the carrier species is proportional to the concentration gradient of this free solute within the liquid membrane. The solutes to be transported are simply distributed over the phases by partition coefficient Kfp ... 

Different mechanisms of diffusion take place in LM diffusion of the carrier-solute complex, diffusion of the uncomplexed carrier in the opposite direction, and diffusion of the uncomplexed solutes. The last transport mechanism is not accessible to solutes that do not react with the carrier species. It is the complexation reaction that makes facilitated LM transport highly selective. 

The second step involves the surface (adsorption) equilibrium between the carrier concentration in solution, Ct = C + CA, and the concentration of sorbed carrier species CtS, on cellular surface sites (receptors) S ... 

Liquid membranes are characterised by enhanced selectivity because separation exploits specific carrier species that have a high affinity for certain compounds or compound classes [253]. 

Fig. 25. Scheme of an evaporation-intercalation solar cell. 1 layer-type electrode into which guest molecules or atoms are intercalated and from which they are evaporated. 2 solid or liquid electrolyte. 3 porous counter electrode at which the carrier species is converted into an ion... 

D21.6 The maximum flux in mediated transport is achieved at very high concentrations of the transported species. Under such conditions, the transported species A flood the carrier species C, pushing practically all of the latter into the form of the AC complex. (The mathematical condition for saturation of the flux at 7max is that [A] K, the equilibrium constant for dissociation of the AC complex this condition puts practically all C into the complex, regardless of its inherent stability.) The value of Jm a depends on the concentration of carrier species, [C]o. For a given value of [C]o,/max represents the transport capacity of the fleet of carriers. The oversupply of A keeps the carriers transporting at full capacity. 

Because of their commercial importance, we still need to do more work on thermal cracking reactions, since their scope and complexity extend considerably beyond the world of Rice-Herzfeld mechanisms. For example, consider the pyrolysis of butane (K.J. Laidler, Chemical Kinetics, McGraw-Hill, New York, 1965). This molecule affords the formation of a number of radical chain-carrier species, and the number of elementary steps increases accordingly... 

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