Discriminating layer

The location of the discriminating layer can be manipulated through control of processing conditions. The discriminating layer can lie on either side of the membrane or somewhere in the middle [24-25]. Furthermore, the pore size distribution and morphology of the underlying support can be controlled. Excellent reviews of the phase separation process and the relationship between process variables and final membrane structure are available in the literature [26-27]. 

An alternative to the asymmetric structure is the composite structure illustrated in Figure 1 which compares the two. Like the asymmetric structure, the composite structure consists of a discriminating layer supported by an underlying support. In contrast to the asymmetric structure, the support generally is not made of the same material as the discriminating layer and hence is not integrally attached to it. 

The porous support often is made separately by a phase inversion process followed by drawing to enhance porosity. The discriminating layer is added afterwards by dip or slip-casting [26] or interfacial polymerization [28]. Alternatively, the support and discriminating layers can be formed simultaneously by co-extrusion [29]. 

Transport in dense discriminating layers is most commonly described using the well developed solution-diffusion theory [36]. The theory is based on the assumptions that 1) the driving force for transport is a gradient in chemical potential, 2) at a fluid-membrane interface the chemical potential in the two phases are equal (i.e., equilibrium exists), and 3) the pressure within the membrane is constant and equal to the highest value at either interface. Baker [37] summarizes the application of the theory to a variety of membrane separation processes ranging from dialysis to gas separation. 

There are serious practical consequences for the observed thickness effect. It is common practice to reduce the thickness of the discriminating layer of a membrane to maximize the productivity of a membrane device. This is achieved either by using a thin film coating on a porous substrate or using an asymmetric membrane structure. As a consequence of the thickness effect these approaches may not be applicable to the dehydration membranes made from highly water permeable ionomers. Similar restrictions will apply to other membrane separations exhibiting similar thickness effects. 

Ramakrishnan S., McDonald C.J., Carbeck J.D., Prudhomme R.K. Latex composite membranes Structure and properties of the discriminating layer. J. Mem. Sci. 2004 231 57-70 Romero-Cano M.S., Martin-Rodriguez A., Nieves F.J.D.L. Electrosteric stabilization of polymer colloids with different functionality. Langmuir 2001 17(11) 3505-3511 Rau D.C., Parsegian V.A. Direct Measurement of the intermolecular forces between counterion-condensed DNA double helices— Evidence for long-range attractive hydration forces. Biophys. J. 1992 61(1) 246-259... 

Coercivity of Thin-Film Media. The coercivity ia a magnetic material is an important parameter for appHcations but it is difficult to understand its physical background. It can be varied from nearly zero to more than 2000 kA/m ia a variety of materials. For thin-film recording media, values of more than 250 kA / m have been reported. First of all the coercivity is an extrinsic parameter and is strongly iafluenced by the microstmctural properties of the layer such as crystal size and shape, composition, and texture. These properties are directly related to the preparation conditions. Material choice and chemical inborn ogeneties are responsible for the Af of a material and this is also an influencing parameter of the final In crystalline material, the crystalline anisotropy field plays an important role. It is difficult to discriminate between all these parameters and to understand the coercivity origin ia the different thin-film materials ia detail. 

The mechanism by which analytes are transported in a non-discriminate manner (i.e. via bulk flow) in an electrophoresis capillary is termed electroosmosis. Eigure 9.1 depicts the inside of a fused silica capillary and illustrates the source that supports electroosmotic flow. Adjacent to the negatively charged capillary wall are specifically adsorbed counterions, which make up the fairly immobile Stern layer. The excess ions just outside the Stern layer form the diffuse layer, which is mobile under the influence of an electric field. The substantial frictional forces between molecules in solution allow for the movement of the diffuse layer to pull the bulk... 

The depth resolution (i.e. the ability to discriminate between atoms in adjacent thin layers) is limited by the primary beam causing redistribution of target atoms prior to their emission as ions, and to segregation and radiation-enhanced diffusion processes. The local topography can also lead to a loss of depth resolution with sputter depth. 

CPAA may be employed to determine trace element concentrations in bulk solid material, but its importance in our present context is that it permits the characterization of a thin surface layer, i.e. the mass of the analyte element per surface unit, with a good detection limit and outstanding accuracy. For example the composition of a surface layer (or foil) of known thickness can be determined, or, conversely, the thickness of a surface layer of known concentration. Depth profiling or scanning is not possible, and a disadvantage of the method is that heating occurs during irradiation. It is also not possible to discriminate between different oxidation states of the analyte element or between different compounds. 

The silica microspheres provide some diversity but not enough for many complex discrimination tasks. To introduce more sensor variety, hollow polymeric microspheres have been fabricated8. The preparation of these hollow microspheres involves coating silica microspheres by living radical polymerization, using the surface as the initiation site. Once the polymer layer forms on the silica microbead surface, the silica core is removed by chemical etching. These hollow spheres can be derivatized with the dye of interest. The main advantage of these polymer microspheres is the variety of monomers that can be employed in their fabrication to produce sensors with many different surface functionalities and polymer compositions. 

The inclusion of radiative heat transfer effects can be accommodated by the stagnant layer model. However, this can only be done if a priori we can prescribe or calculate these effects. The complications of radiative heat transfer in flames is illustrated in Figure 9.12. This illustration is only schematic and does not represent the spectral and continuum effects fully. A more complete overview on radiative heat transfer in flame can be found in Tien, Lee and Stretton [12]. In Figure 9.12, the heat fluxes are presented as incident (to a sensor at T,, ) and absorbed (at TV) at the surface. Any attempt to discriminate further for the radiant heating would prove tedious and pedantic. It should be clear from heat transfer principles that we have effects of surface and gas phase radiative emittance, reflectance, absorptance and transmittance. These are complicated by the spectral character of the radiation, the soot and combustion product temperature and concentration distributions, and the decomposition of the surface. Reasonable approximations that serve to simplify are ... 

A quantitative discrimination between labile and nonlabile complexes is made by comparing the diffusion timescale with those of the association/dissociation reactions (or alternatively, the reaction layer, /i (equation (58)) and the diffusion layer, <5, thicknesses (e.g. equations (15), (18) and (19)). 

---