Asymmetric support

The film-coating process is applied using suspensions on macroporous substrates to produce intermediate films with macropores in order to obtain microfiltration membranes or to obtain composite, asymmetric supports suitable for production of ultra-fine, mesoporous membranes as discussed in Chapter 6. 

The use of supports in asymmetric, supported membranes introduces a number of complications in the interpretation of permeation and separation data as well as in the optimalisation of membrane systems. If the flow resistance of the support is not negligible, there is a pressure drop across the support. This implies that the pressure and so the occupancy at the interface of separation layer and support is different from the (directly accessible) pressure at the support surface, usually the permeate side. Consequently, the driving force for permeation through the separation layer is different from the total driving force across the membrane system. In cases where one wants to calculate or compare transport properties of the separation layer material, it is necessary to correct for this effect (for illustration see below). 

In asymmetric supported membranes the use of permeability data can give rise to much confusion and erroneous conclusions for several reasons. In most cases the layer thickness is not precisely known and usually it is not known whether this layer is homogeneous or has property gradients (e.g. a "skin" and a more porous part). In many cases the material of the layer penetrates the support to some extent and so it is not possible to separate properties of separation layer and support without giving account of the interface effect. Finally, even if all these complications can be avoided, a comparison based on separation layer properties expressed in terms of permeabilities can give a completely wrong impression of the practical possibilities (as done in e.g. Ref. [109]). This is illustrated by comparison of hydrogen permeabilities of ultra-thin silica layers (see Tables 9.14-9.16) with other materials such as zeolites and metals. The "intrinsic" material properties of these silica layers are not impressive ... 

Second, several types of asymmetric supported bilayer structures, in which the composition of the two monolayers is different, have been created as alternatives to symmetric PSLBs. One example is the HBM [45,46], The inner monolayer in an HBM is an alkyl SAM, typically an alkanethiol on gold, upon which an outer lipid monolayer is deposited by either LBS or vesicle fusion methods. A more sophisticated type of asymmetric supported bilayer is the tethered bilayer lipid membrane (tBLM) in which the SAM is replaced with an inner lipid monolayer. Some or all of the molecules in the inner monolayer are covalently tethered to the underlying support, usually through a hydrophilic linker that creates a water-swollen spacer layer between the tBLM and the substrate surface [47-51],... 

Fig. 1. Schematic representations of a thin top layer membrane (Al) and a composite infiltrated membrane (Bl) on/in asymmetric supports. The corresponding FE-SEM observations of MF1 membranes on/in an aAbO support (Pall-Exekia, with 200 nm top-layer pore size) are shown in A2 (seeding/secondary growth) and B2 (in-situ direct crystallisation).

Asymmetric phase-inversion membranes like the membranes employed in reverse osmosis are difficult to prepare as gas permeation is much more sensitive to micropores than RO due to the much higher diffusion coefficients of gases. For the same reason, the composite membrane differs from RO composite membranes in gas permeation, the top layer of the asymmetric support structure is responsible for the separation while it is the sole duty of the coating to plug the micropores. Consequently, the material of the coating chosen (silicone) has a high permeability but a low selectivity while the membrane material (poly-sulfone) has a high selectivity (and a much lower permeability). 

Methods of Preparation. The major obstacle to fabricating ultra-thin membranes with asymmetric supports is fmding a method to selectively immobilize the liquid phase in... 

With the increasing number of commercially available, extremely pure chiral auxiliaries, thin-layer chromatographic purity control via formation of diastereomers has gained increasing importance. In contrast to direct enantiomer separations, antipode separation via diastereomers usually is not achieved with chiral adsorbents however, enhanced diastereomer selectivity is also noted for asymmetric supports. The type of chiral reagent for formation of the diastereomer depends among other parameters on the structure—mono- or bifunctional—of the compound to be derivatized (see Table 7). 

In these cases, the polymer was used as an asymmetric support to induce the formation of optically pure product (cf. Worster et al., 1979). Few reports of the use of polymer-bound asymmetric reagents seem to exist in the literature. In this application, the reagent is used either to promote the asymmetric coupling of two groups or to add a group to a compound in an asymmetric manner. By far the largest number of applications have been those in which the polymer-bound asymmetric centers act as catalysts. Asymmetric catalysts, based on either amino acids or cinchona alkaloids, have been used to catalyze the Michael reaction in an... 

Shusen et al. [12] used one-step preparation method of asymmetric supported carbon molecitlar sieve membranes, consisting of the formation of phenol formaldehyde film followed by pyrolysis and unequal oxidation steps. Micro-pores were formed as a resirlt of small gaseous molecules charmeling their way out of the solid matrix of the polymer dming pyrolysis. The micropore stracture was further... 

Figure 2.6 Schematic representation of (a) a thin supported layer on an asymmetric support and (b) a composite membrane infiltrated in an asymmetric support. Reproduced from [17]. With permission from Elsevier.

Principle of a supported membrane the im-thick separation layer is deposited on a macroporous ceramic or metallic support. To reduce the pressure drop across the support, that is, to minimize the flow resistance, asymmetric supports are often used. 

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