Glass membrane, structure

This type of membrane consists of a water-insoluble solid or glassy electrolyte. One ionic sort in this electrolyte is bound in the membrane structure, while the other, usually but not always the determinand ion, is mobile in the membrane (see Section 2.6). The theory of these ion-selective electrodes will be explained using the glass electrode as an example this is the oldest and best known sensor in the whole field of ion-selective electrodes. 

Terms such as symmetric and asymmetric, as well as microporous, meso-porous and macroporous materials will be introduced. Symmetric membranes are systems with a homogeneous structure throughout the membrane. Examples can be found in capillary glass membranes or anodized alumina membranes. Asymmetric membranes have a gradual change in structure throughout the membrane. In most cases these are composite membranes... 

A glass membrane in an electrolyte solution cannot be taken to be a homogeneous system in the direction perpendicular to the surface. When the membrane is in contact with the solution, water molecules can enter it and form a 5-100 nm thick hydrated layer [319]. The formation of this hydrated layer is actually a condition for good functioning of the glass electrode. The basic characteristics of the glass structure probably do not change in the hydrated layer, but the cation mobility increases considerably compared with the compact membrane interior... 

At the surface of a glass membrane (Fig.3.2) that is in contact with an aqueous solution, a layer, called the Haber-Haugaard layer, is established with a thickness of about 100 nm. In this layer, a fraction of the positive ions, typical for the glass structure, are exchanged by hydrogen ions (H+). Owing... 

Numerous types of materials, for instance, polymers, glasses, ceramics, and metals can be applied for membrane synthesis [180-183], The major step in the preparation of a membrane is to adapt the material through an appropriate methodology to get a membrane structure with a morphology suitable for a particular type of separation process [183], For example, in the special case of membrane reactors (see Section 10.6.2), one of the most important separations is the selective subtraction of hydrogen from the reaction zone therefore, such membranes must be hydrogen selective [184-186],... 

Template leaching is another methodology applied for the production of membranes it can be applied to produce porous glass membranes [180], The method consists in the formation of a structure with the help of a homogeneous melt of a three-component system, for example, NazO—B203 — Si02 when... 

Asymmetry potential — In case of any membrane it happens that the potential drop between the solution and either inner side of the - membrane is not completely identical so that a nonzero net potential drop arises across the entire membrane. This is best known for - glass electrodes and other - ion-selective electrodes. The reasons of asymmetry potentials are chemical or physical differences between each side of a membrane, in particular an inhomogeneous membrane structure resulting from fabrication conditions and/or curvature. Asymmetry p. can change in the course of membrane ageing. To measure asymmetry p. one should use a symmetrical cell with identical solutions and -> reference electrodes on each side of the membrane. 

In the last years increasing research activities in the fields of membrane science [1, 2], chemical sensors [3], confined matter [4] and micro-reaction engineering [5] have evoked a new interest on porous glass membranes. Furthermore, such membranes are ideal model systems for the investigation of transport processes in porous structures. This broad spectrum of applications demands variable texture properties. 

The optical transparency of the porous glass membranes is interesting for sensor applications. It depends on the structural features of the membranes [13]. The UV-VIS spectra of micro- and mesoporous glass membranes are compared in Fig. 6. The membranes with an... 

Fignre 1-6. Structure of a glass membrane as elucidated from X-ray diffraction studies. [Courtesy of the late George A. Perley and Leeds Northrop Co., Anal. Chem., 21 395 (1949).]... 

Exposures of some metal oxide membranes, both dense and porous, to extreme pH conditions (e.g., pH less than 2 or greater than 12) can cause structural degradations, particularly with extended contact time. The extent of degradation depends on the specific phase of the material, porosity, and temperature. Steam can also be deleterious to some metal oxide and Vycor glass membranes. For example, as mentioned earlier, porous glass membranes undergo slow structural changes upon exposure to water due to partial dissolution of silica. 

Figure 21-15 compares the structural features of a glass-membrane electrode and a commercially available liquid-membrane electrode for calcium ion. The sensitivity of the liquid-membrane electrode for calcium ion is reported to be 50 times... 

The modelling of gas permeation has been applied by several authors in the qualitative characterisation of porous structures of ceramic membranes [132-138]. Concerning the difficult case of gas transport analysis in microporous membranes, we have to notice the extensive works of A.B. Shelekhin et al. on glass membranes [139,14] as well as those more recent of R.S.A. de Lange et al. on sol-gel derived molecular sieve membranes [137,138]. The influence of errors in measured variables on the reliability of membrane structural parameters have been discussed in [136]. The accuracy of experimental data and the mutual relation between the resistance to gas flow of the separation layer and of the support are the limitations for the application of the permeation method. The interpretation of flux data must be further considered in heterogeneous media due to the effects of pore size distribution and pore connectivity. This can be conveniently done in terms of structure factors [5]. Furthermore the adsorption of gas is often considered as negligible in simple kinetic theories. Application of flow methods should always be critically examined with this in mind. 

The majority of todays membranes used in microfiitration, dialysis or ultrafiltration and reverse osmosis cire prepared from a homogeneous polymer solution by a technique referred to as phase inversion. Phase inversion can be achieved by solvent evaporation, non-solvent precipitation and thermcd gelation. Phase separation processes can not only be applied to a large number of polymers but also to glasses and metal alloys and the proper selection of the various process parameters leads to different membranes with defined structures and mass transport properties. In this paper the fundamentals of membrane preparation by phase inversion processes and the effect of different preparation parameters on membrane structures and transport properties are discussed, and problems utilizing phase inversion techniques for a large scale production of membranes are specified. 

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