Membranes mixed crystal

Considerable work has been devoted to the development of solid membranes that are selective primarily to anions. The solid-state membrane can be made of single crystals, polycrystalline pellets, or mixed crystals. The resulting solid-state membrane electrodes have found use in a great number of analytical applications. 

In a similar manner, as described for fluoride electrode above, pofyciystalline Ag S membrane gives a good sulphide ion electrode. Mixed crystals of AgX-Ag S compose the anion selective electrodes for chloride, bromide, iodide, and thiocyanate. 

The designation membrane is used here in a very general sense, as many materials are used. Commercially available electrodes include liquid membrane units, solid-state electrodes, glass membrane electrodes, and plastic membrane electrodes. General classes of ion specific electrodes in addition to those cited are immobilized-liquid membrane electrodes, mixed-crystal membrane electrodes, enzyme electrodes, and antibiotic electrodes (Rechnitz, 1973). Certain of these membrane electrodes are now discussed in some detail. A generalized membrane electrode is illustrated in Figure 6.1a. 

The concept of a mixed-crystal membrane is not limited to sulfides and it should be possible to develop satisfactory two- and three-component crystal mixtures which can serve as suitable membrane materials (Rechnitz, 1973 Kummer and Milberg, 1969). 

A continuous lipidic cubic phase is obtained by mixing a long-chain lipid such as monoolein with a small amount of water. The result is a highly viscous state where the lipids are packed in curved continuous bilayers extending in three dimensions and which are interpenetrated by communicating aqueous channels. Crystallization of incorporated proteins starts inside the lipid phase and growth is achieved by lateral diffusion of the protein molecules to the nucleation sites. This system has recently been used to obtain three-dimensional crystals 20 x 20 x 8 pm in size of the membrane protein bacteriorhodopsin, which diffracted to 2 A resolution using a microfocus beam at the European Synchrotron Radiation Facility. 

In heterogeneous solid membranes, the active (selective) material is mixed in the form of a powder with a suitable binder (e.g., silicone rubber) and cast in the form of thin membranes. In this case, the difficulties associated with single-crystal fabrication are avoided, and better stability is obtained than in membranes compressed from polycrystalline powder. 

In the first example, procaine penicillin, an aqueous vehicle containing the soluble components (such as lecithin, sodium citrate, povidone, and polyoxyethylene sorbitan monooleate) is filtered through a 0.22 pm membrane filter, heat sterilized, and transferred into a presterilized mixing-filling tank. The sterile antibiotic powder, which has previously been produced by freeze-drying, sterile crystallization, or spray-drying, is aseptically added to the sterile solution while mixing. After all tests have been completed on the bulk formulation, it is aseptically filled. 

The mixing of nematogenic compounds with chiral solutes has been shown to lead to cholesteric phases without any chemical interactions.147 Milhaud and Michels describe the interactions of multilamellar vesicles formed from dilauryl-phosphotidylcholine (DLPC) with chiral polyene antibiotics amphotericin B (amB) and nystatin (Ny).148 Even at low concentrations of antibiotic (molar ratio of DLPC to antibiotic >130) twisted ribbons are seen to form just as the CD signals start to strengthen. The results support the concept that chiral solutes can induce chiral order in these lyotropic liquid crystalline systems and are consistent with the observations for thermotropic liquid crystal systems. Clearly the lipid membrane can be chirally influenced by the addition of appropriate solutes. 

Various zeolites have been studied as the dispersed phase in the mixed-matrix membranes. Zeolite performance in the zeolite/polymer mixed-matrix membrane is determined by several key characteristics including pore size, pore dimension, framework structure, chemical composition (e.g., Si/Al ratio and cations), crystal morphology and crystal (or particle) size. These characteristics of zeolites are summarized in Chapter 6. 

Lyotropic liquid crystals they display liquid crystalline behavior when mixed with another material in the right concentration (typically a solvent). They can also be a mixture of more than two components (e.g., cell membranes). 

The fluoride electrode is a typical example of an ion selective electrode. Its sensitive element is a crystal of lanthanum trifluoride that allows fluorine atoms to migrate into the network formed by lanthanum atoms (Fig. 18.3). Other electrodes use a mineral membrane obtained as agglomerates of crystalline powders (for measurement of Cl-, Br-, I , Pb++, Ag+ and CN ). Generally, the internal electrolyte can be eliminated (by dry contact). However, it is preferable to insert a polymer layer with a mixed-type conductivity to ensure the passage of electrons from the ionic conductivity membrane to the electronic conductivity electrode (Fig. 18.3). 

The appearance of a shelf of crystals with the earth grown lead iodide crystals may indicate that the different ions are mixing near this level of the tube. It may be possible that crystals are nucleating above this level and then falling down to this level where they attach to other crystals and to the membrane. 

Cholesterol is a major hpid component of mammahan cell plasma membranes, accounting for approximately 35% of the total lipid of the membrane. Cholesterol has a chemical structure that is very different from the major polar hpid constiments, which is a consequence of the fused ring system of cholesterol that gives this hpid less conformational flexibihty than the straight chains of the polar lipids. It is thus not surprising that cholesterol does not mix well in membranes and that cholesterol segregates as crystals at around 50-60 mol% in bilayers of several lipids and at a much lower mol fraction of cholesterol in bilayers comprising hpids with unsaturated acyl chains (5). 

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