Membrane Surface Roughness

FIGURE 7.50. Membrane surface roughness as a function of the solution concentration at temperatures of 30, and 90 °C. After Kwa et (Reproduced by permission of The Electrochemical Society, Inc.)... 

Membrane surface roughness has a macroscopic and microscopic scale. Ghayeni et al. (1998) have reported large scratches on RO and NF membranes. While these large scale inhomogenities may cause attachment of large particles, we are interested in the microscopic roughness that determines the surface area available for adsorption and the thickness of the boundary layer. 

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. 

Fig. 5.32. Dependence of average membrane surface roughness on total solid content and CP24 wt.% in membrane casting solution. Reprinted from [49]. Copyright 2001, with kind permission from Springer-Verlag...

Hirose et al. [1] suggested an approximately linear relationship between membrane surface roughness and permeate flux for TFC RO membranes with six different cross-Hnked aromatic polyamide skin layers. On a polysulfone (PSf) substrate, cross-linked aromatic polyamides were formed at a water/solvent interface using m-phenylene diamine (MPD) and 1,3,5-benzenetricarbonyl trichloride as monomers. Isopropyl alcohol content in the aqueous amine phase was changed from 0 to 60 wt.% to control the interfacial surface tension, which eventually led to different surface roughness values. 

Table 8.9. Correlation of flux decline data and average membrane surface roughness for systematically varied physical and chemical operating conditions ...

Membrane surface roughness. The rougher the surface the more adhesion can occur. (Note that attempts to modify membranes, i.e., change the roughness, or charge, or hydrophilicity, to make adhesion less favorable are not always successful. )... 

Surface roughness is also a crucial morpholc cal parameter since it seems to be directly related to membrane fouling phenomena. Membrane surface roughness is a numerical measure of the surface morphology variation of a membrane. It corresponds to the mean value of surface relative to the center plane, this is the plane for which the volume enclosed by the image above and below this plane is equal. 

According to another similar work (Figure 4.14), Li et al. (2010) found that different salt concentrations in the polyelectrolyte solutions can actually manipulate the membrane surface roughness produced. However, the ionic strength of polyelectrolyte solutions was not in a linear relationship with membrane surface roughness. The membrane... 

FIGURE 4.14 Membrane surface roughness study of multilayered PEC membranes prepared from PE solutions with different salt concentrations. (Reprinted with permission from Li, X. et al J. Memb. ScL, 358, 150-157, 2010.)... 

With an AFM adhesion force measurement technique, Bowen et al. [42] characterized an interaction force between a colloidal silica probe and a rough membrane surface. It was found that membrane surface roughness significantly reduced electrostatic repulsion between the colloid and the surface, and the valley regions experienced a greater adhesion force. 

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