Pore shape, separator

Size-exclusion HPLC (SE-HPLC) separates proteins on the basis of size and shape. As most soluble proteins are globular (i.e. roughly spherical in shape), separation is essentially achieved on the basis of molecular mass in most instances. Commonly used SE-HPLC stationary phases include silica-based supports and cross-linked agarose of defined pore size. Size-exclusion systems are most often used to analyse product for the presence of dimers or higher molecular mass aggregates of itself, as well as proteolysed product variants. 

The separation efficiency (e.g. permselectivity and permeability) of inorganic membranes depends, to a large extent, on the microstructural features of the membrane/support composites such as pore size and its distribution, pore shape, porosity and tortuosity. The microstructures (as a result of the various preparation methods and the processing conditions discussed in Chapter 2) and the membrane/support geometry will be described in some detail, particularly for commercial inorganic membranes. Other material-related membrane properties will be taken into consideration for specific separation applications. For example, the issues of chemical resistance and surface interaction of the membrane material and the physical nature of the module packing materials in relation to the membranes will be addressed. 

The ratio of volume to area within a pore depends upon the pore geometry. For example, the volume to area ratios for cylinders, parallel plates and spheres are, respectively, r/2, r/2 and r/3, where r is the cylinder and sphere radii or the distance of separation between parallel plates. If the pore shapes are highly irregular or consist of a mixture of regular geometries, the volume to area ratio can be too complex to express mathematically. In these cases, or in the absence of specific knowledge of the pore geometry, the assumption of cylindrical pores is usually made, and equation (8.6) becomes... 

The exact pore shape is usually unknown and cylindrical pores are generally assumed. Mikhail, Brunauer, and Bodor show in their paper that equation (9.17) is equally valid for parallel plate or cylindrical pores and that the mean hydraulic radius in Table 9.1, is the same as the separation between plates or the cylindrical radius. 

A combination of characterization techniques for the pore structure of mesoporous membranes is presented. Equilibrium and dynamic methods have been performed for the characterisation of model membranes with well-defined structure while three-dimensional network models, combined with aspects from percolation theory can be employed to obtain structural information on the porous network topology as well as on the pore shape. Furthermore, the application of ceramic membranes in separations of condensable from noncondensable vapors is explored both theoretically and experimentally. 

The transport properties (i.e. permeation and separation efficiency) of inorganic membrane systems depend, to a Icirge extent, on the microstructural features of the membrane and the architecture of membranes and modules. The microstructural features, such as pore shape and morphology, pore size (distribution), interconnectivity/tortuosity, as well as the architecture of the membrane and membrane-support combinations will be briefly described. Here, architecture means the way the different parts of the membrane system or module are shaped and combined. 

In the SLM process, like in all membrane processes, the membrane plays a key role in the transport and separation efficiency. The permeation rate and separation efficiency depends strongly on the type of liquids and supports used for SLM construction. However, the transport properties depend on the type of liquids used as a membrane phase the hquid membrane stability and mechanical stability depend, to a large extent, on the microstructure like pore shape, size, and tortuosity of the membrane used as a support. Therefore, many types of polymeric and inorganic microporous membrane supports are studied for the liquid membrane phase immobilization. 

In recent years simultaneous progress in the understanding and engineering of block copolymer microstructures and the development of new templating strategies that make use of sol-gel and controlled crystalHzation processes have led to a quick advancement in the controlled preparation of nanoparticles and mesoporous structures. It has become possible to prepare nanoparticles of various shapes (sphere, fiber, sheet) and composition (metal, semiconductor, ceramic) with narrow size distribution. In addition mesoporous materials with different pore shapes (sphere, cyHndrical, slit) and narrow pore size distributions can be obtained. Future developments will focus on applications of these structures in the fields of catalysis and separation techniques. For this purpose either the cast materials themselves are already functional (e.g., Ti02) or the materials are further functionalized by surface modification. 

The accessibility curve given in Figure 3 is effectively a cumulative pore distribution relating pore volume to pore size. With an assumption of pore shape it supplies all the information required to calculate the surface area of the pores. In previous publications we have postulated that the most basic particle shape within the cell wall is that of lamellae or sheets of microfibrils. The pores would then be the slit like spaces between these sheets as depicted in the simple model shown in Figure 4. In the model two sheets are separated by a distance w (pore width) and contain a pore volume Ao. If A A is the area of the pore bounded by the two sheets, then obviously AA/2 is the area of one face of the pore and the three quantities, A A, At , and w are related by the equation ... 

The pore shape influences the mass transfer rate and thus the efficiency of separation. The effective diameter of pores determines the range of separated molar masses. The pore size distribution and the pore volume are decisive for selectivity of separation (section 4.6.2.3). The pore sizes of commercially available gels cover the region necessary for separation of the wide spectrum of substances — from low molecular samples to very high polymers, colloidal particles and viruses. The mean values of pore diameters range from few nanometers to about 2.5 /xm. Gels with various pore sizes, but of the same type, can be combined within the same column. 

Mesoporous MSU-X silica was synthesized with a two-step pathway, that allowed us to get a high control degree on both the final material shape and the porous size distribution. These materials were developed and tested for separating applications, including HPLC chromatography and ultrafiltration membranes. Both applications show that the specific structure of the Micellar Templated Structures exhibits a new behavior in the separation applications, compared with other materials. They are explained by the combined effect of the silica nature and the specific cylindar pore shape. 

The aim of Chapter 5 by Thornton et al. was to give systematic consideration to different types of transport in porous membranes. They developed a new model that allows one to predict the separation outcome for a variety of membranes in which the pore shape, size and composition are known, and conversely to predict pore characteristics with known permeation rates. 

Battery separators are characterized by numerous properties, including material nature, membrane stractural and functional properties. Material nature includes chemical stability, crystalline structure, hydrophilicity, thermal shrinkage, melting point, M and Mv,/M of polyolefin materials. Structural properties include thickness, porosity, pore size, pore shape, pore tortuosity, and pore distribution. Functional properties include mechanical strength, electrical resistivity, air permeability, thermal shutdown, electrolyte wettability and retention. Many of the above properties are affected with each other and may be in a trade-off relationship. For example, the mechanical strength is affected in opposite manner by the thickness, porosity and permeability, as required by the battery performance. 

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