Top skin layer

The present study shows that the asymptotic salt rejection, r, is determined by the top skin layer of a membrane. This is a result of the steady-state mass balance and the boundary conditions. Although there are no experimental data to support this, it has been shown theoretically that the asymptotic salt rejection is identical to the reflection coefficient for the homogeneous membrane, r = 0. 

Figure 7. SEM micrographs facing fracture surfaces of PEC-70HBA as-molded (a) whole surface (xlOO) (b) top skin layer (x5000) (c) mid layer (x5000) (d) core layer (x5000).

As mentioned above, the top skin layer governs the performance of a separation membrane. The surface deposition of contaminants from solutions or from gas mixtures is also affected by the surface properties of the membrane. This is particularly important when decline in the membrane flux with a prolonged operating period is observed, because it is often caused by the contaminant deposition. Hence, many attempts have been made to modify the membrane surface, aiming at prevention of contaminant deposition and maintenance of high flux. Several methods of surface modification are described below. 

An asymmetric membrane has a very thin dense top layer (or skin) with a thickness of 0.1-0.5 pm. A porous sublayer with a thickness of approximately 50-150 pm supports the dense top layer. The thin dense skin facing the feed solution acts as the selective layer, allowing water passage but rejecting dissolved solids. The resistance to mass transfer across the membrane is also mainly determined by the thin top layer. In asymmetric membranes, the selective top layer and the porous support layer are made of the same polymer material. Asymmetric membranes can be obtained by phase inversion, a technique in which a polymer in solution is transformed in a controlled manner from a liquid into a solid form. The top skin layer and the porous support layer are formed in a single-step process. 

Figure 5.4 Cross-sectional TEM images of two porous layers. There are relatively few pores on the surface compared with the bulk, with the top skin layer being most clearly visible in (a). The V-shaped branched structure of the porous network is clearly seen in (b). Reproduced from A. Sagar et al., J. Appl. Phys. 92, 4070. Copyright (2002), with permission from American Institute of Physics...

Understanding growth and maturation is important for skin regeneration, for example. In the adult mammal, an epidermis (the top skin layer) lost through injury regenerates spontaneously... 

PA composite, Dow day 1 km /day 2000 ppm NaCl solution. BBNs (miscellaneous) Removal of die top skin layer [81]... 

For laboratory research, various types of membranes mentioned earlier are used without any support in PV cells. For large-scale applications, where the membrane sizes are larger, the reinforcement of this top skin layer by an appropriate support is required to maintain dimensional stability. These types of membranes consisting of a skin layer (permselective layer) supported by a suitable support are called composite membranes. Ceramic-supported membranes already have a support, and hence, no additional support is required. 

Most membranes used in industries have an asymmetric structure. Figure 2.1 shows schematically a typical cross-sectional view of an asymmetric membrane [3]. It consists of two layers the top one is a very thin dense layer (also called the top skin layer), and the bottom one is a porous sublayer. The top dense layer governs the performance (permeation properties) of the membrane the porous sublayer only provides mechanical strength to the membrane. The membranes of symmetric structures do not possess a top dense layer. In the asymmetric membrane, when the material of the top... 

The top skin layer can also be made porous by lowering the polymer concentration in the casting solution and the solvent evaporation period. This is called, hereafter, the porous skin layer. Asymmetric membranes can also be made in a tubular form using a casting bob assembly and a hollow fiber spinneret [5]. 

The cross-sectional pictures taken by Panar et al. [3] (see Chap. 4) revealing the nodular structure near the top edge of the membranes are probably the first attempt to examine the detailed structure inside the top skin layer. Similar attempts have been recorded since then in the Uterature, but the number is limited. 

In summary, a variety of organic polymers have been used for the top skin layer of TFC membranes [22], as shown in Figure 1.6 ... 

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