Roughness of the Membrane Surface

The definitions of three roughness parameters are given in Chap. 3. All roughness parameters can be calculated from the AFM image with an AFM software program. 

However, specific roughness parameters depend on the curvature on the membrane surface and also the size of the cantilever tip, as well as on the treatment of the captured images (plane fitting, flattering, etc.). Thus, the roughness parameter should not be considered as an absolute value that represents surface roughness. 

The roughness parameter is one of the best parameters for comparing different membranes. Furthermore, it can be correlated with membrane performance and other surface properties such as pore size distribution. The ranges of roughness parameters for membranes used in different processes were discussed in the summary of Chap. 4. Bowen et al. also studied a series of UF membranes with different MW-COs and reported that the surface roughness parameter increases with an increase in pore size [10]. 

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Fig. 6.17 SPM micrograph of the surface of the carbon membrane C (a) 20 pm 3-D image reveals the roughness of the membrane surface (b) pore structure is clear from this 3513 nm x 3513 nm image. (From [11])...

It can therefore be concluded that the membrane selectivity tends to decrease as the nodule size at the membrane surface becomes larger or the roughness of the membrane surface increases. 

A minimum roughness of the support surface is also required to produce defect-free membrane layers. In the present context, surface roughness is defined as the average perpendicular (to the surface) distance between peaks and dips in the support surface. As discussed in Chapter 6, several other definitions of roughness can be given and used. The roughness of the support may limit the minimum achievable layer thickness. From a fracture mechanics point of view, surface roughness determines the maximum size and sharpness of flaws which can act as crack initiators (via the stress intensity factor). 

SEM photographs of sputtered films show that the layers are fairly dense and appear to crack into platelets when subjected to MEA fabrication. The dense films do not lend themselves to high surface areas therefore, there is substantial scope for enhancement of performance if the surface area can be increased. This may be achieved by producing porous 3-D Pt-Ru layered structures. One such method for creating such 3-D structures, that seem to be extremely promising, involves the pre-treatment of the membrane surface by ion-beam etching, which is then followed by sputter-deposition of the metal. This results in substantially enhanced surface area and very rough nanostructures. Next year s effort will include characterization of such films. 

Riedl et al. [58] employed an atomic force microscopy (AFM) technique to measure membrane surface roughness and scanning electron microscopy (SEM) to assess the fouling layer. The smoothness of the membrane surface can influence the morphology of the fouling layer. It was shown that smooth membranes produced a dense surface fouling layer. 

Characterization of the membrane surface It should be emphasized that the properties of the membrane surface strongly affect membrane performance. Contact angle is often used as a measure of surface hydrophilicity or hydrophobicity. X-ray photoelectron spectroscopy (XPS) provides the data on atomic compositions at the membrane surface. Recently, attentions have been focused on the nodular structure as well as the roughness at the membrane surface that can be measured by atomic force microscopy (AFM). 

Table 4.8. Effect of shear rate on roughness of the outer surface of hollow fiber membranes ...

Fig. 4.37. Mean roughness of the inner surface of hollow fiber against air gap used for the preparation of hollow fiber membrane at a scan size of 1 pm. Reprinted from [61 ]. Copyright 2003, with kind permission from Elsevier...

The statement of Hirose et ah [1] that higher surface roughness of the polyamide TFC membrane for RO will enhance the membrane flux has been supported by some researchers through their independent experiments. On the other hand, contradictory results have been obtained by others. This may be due to the fact that surface roughness alone caimot be varied independently from other parameters that may also affect membrane flux. One such parameter is obviously the chemistry of the membrane surface. 

The fourth chapter examines the nodular structure of the membrane surface observed under AFM. It has been known for a long time that macromolecules form nodules at the membrane surface, and the size and the shape of the nodules strongly govern the membrane performance. In conjunction with an advanced technique such as plasma etching, AFM can reveal the nodular structure at the membrane surface more clearly than any other technique. In this chapter, the relationship between the nodular structures and the membrane preparation conditions is discussed for fiat sheet membranes in the first part and hollow fibers in the second part. This chapter also deals with the roughness at the membrane surface. 

Concerning the preparation of thin membranes directly on porous supports, a lower thickness limit seemingly exists for which a dense metal layer can be obtained. This thickness limit increases with increasing surfaee roughness and pore size in the support s top layer." " Clearly, this relation puts strong demands on the support quality in terms of narrow pore size distribution, and the amount of surface defects. Therefore both pore size and roughness of the support surface are often reduced by the application of meso-porous intermediate layers prior to deposition of the permselective metal layer. This procedure facilitates the preparation of thin defect-free membranes beeause it is relatively easier to cover small pores by filling them with metal. It is therefore conceivable that for a certain low Pd-alloy thickness and support pore size, the H2 flux becomes limited by the support resistance. ... 

The bulk concentration of anolyte, CA,Na. is almost unchanged under normal operations. However, 5a is a function of the flow rate and of the surface roughness of the membrane. Chlorine bubbles, generated at the anode, agitate the anolyte solution near the membrane. As bubble action becomes more intense, the diffusional boundary layers become thinner. The membrane surface itself is not flat. Industrial membranes are normally reinforced with PTFE fiber or cloth. A metal oxide coating on zero-g membranes improves their hydrophilicity and allows easier detachment of chlorine bubbles. These coatings also affect the thickness of the diffusion layer and the limiting current density. 

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