Characterisation of porous membranes

Characterisation data for porous membranes often give rise to misunderstandings and misinterpretations. It should be realised that even when the pore sizes and pore size distributions have been determined properly the morphological parameters have been determined. Howeven in actual separation processes the membrane performance is mainly controlled by other factors, e.g. concentration polarisation and fouling. 

One important, but often not clearly defined variable in the characterisation of porous membranes, is the shape of the pore or its geometry. In order to relate pore radii to physical equations, several assumptions have to be made about the geometry of the pore. For example, in the Poiseuille equation (see eq. IV 4) the pores are considered to be parallel cylinders, whereas in the Kozeny-Carman equation (eq. IV - 5) the pores are die voids between the close-packed spheres of equal diameter. These models and their corresponding pore geometries are extreme examples in most cases, because such pores do not exist in practice. However, in order to interpret the characterisation results it is often essential to make assumptions about the pore geometry. In addition, it is not the pore size which is the rate-determining factor, but the smallest constriction. Indeed some characterisation techniques determine the dimensions of the pore entrance rather than the pore size. Such techniques often provide better information about permeation related characteristics. 

U should be noted that this definition does not characterise the membrane nor the pores ot the membrane, but rather the size of the particles or molecules retained by it. nw separation characteristics are determined by the large pores in the membrane.. Another factor of interest is the surface porosity. This is also a very important variable in determining the flux through the membrane, in combination with the thickness of the top layer or the length of the pore. Different miciofiltration membranes exhibit a wide range of surface porosity as discussed in chapter III. from about S to 70%. In contrast, the ultrafiltration membranes normally show very low surface porosities, ranging from 0.1-1.  

Two different t3rpes of characterisation method for porous membranes can be distinguished from the above considerations  

It is often very difficult to relate the structure-related parameters directly to the permeation-related parameters because the pore size and shape is not very well defined. The configuration of the pores (cylindrical, packed-spheres) used in simple model descriptions deviate sometimes dramatically from the actual morphology, as depicted schematically in figure IV - 3. Nevenheless, a combination of well defined characterisation techniques can give information about membrane morphology which can be used as a first estimate in determining possible fields of application. In addition, it can serve as a feed-back for membrane preparation. 

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Hie characterisation of porous membranes is, for the most part, no different from the characterisation of porous solids, although in the case of membranes, the nature of the percolative network of pores is crucial for understanding transport of the desired species through the membrane and rejection of undesired species. The most common and accurate method for determining pore size and structure is by employing probe molecules to adsorb the pore walls and/or condense within the pores themselves. 

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