Membrane surface observation

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. 

The phenomenon of concentration polarization, which is observed frequently in membrane separation processes, can be described in mathematical terms, as shown in Figure 30 (71). The usual model, which is weU founded in fluid hydrodynamics, assumes the bulk solution to be turbulent, but adjacent to the membrane surface there exists a stagnant laminar boundary layer of thickness (5) typically 50—200 p.m, in which there is no turbulent mixing. The concentration of the macromolecules in the bulk solution concentration is c,. and the concentration of macromolecules at the membrane surface is c. 

Each of the membranes acts like a hard wall for dimer molecules. Consequently, in parts I and III we observe accumulation of dimer particles at the membrane. The presence of this layer can prohibit translation of particles through the membrane. Moreover, in parts II and IV of the box, at the membranes, we observe a depletion of the local density. This phenomenon can artificially enhance diffusion in the system. In order to avoid the problem, a double translation step has been applied. In one step the maximum displacement allows a particle to jump through the surface layer in the second step the maximum translation is small, to keep the total acceptance ratio as desired. 

The conformation of bovine myelin basic protein (MBP) in AOT/isooctane/water reversed micellar systems was studied by Waks et al. 67). This MBP is an extrinsic water soluble protein which attains an extended conformation in aqueous solution 68 but is more density packed at the membrane surface. The solubilization of MBP in the AOT reversed micelles depends on the water/AOT-ratio w0 68). The maximum of solubilization was observed at a w0-value as low as 5.56. The same value was obtained for another major protein component of myelin, the Folch-Pi proteolipid 69). According to fluorescence emission spectra of MBP, accessibility of the single tryptophane residue seems to be decreased in AOT reversed micelles. From CD-spectra one can conclude that there is a higher conformational rigidity in reversed micelles and a more ordered aqueous environment. 

Membrane vendors and researchers may use the feed concentration at the membrane surface or to calculate an intrinsic passage Sint, = Cp/c . The intrinsic passage characterizes the membrane while the observed passage characterizes the module. 

Recently, we [13,14] evidenced by ATR-IR spectroscopy that the membrane potential of ionophore-incorporated, PVC-based liquid membranes is governed by permselective transport of primary cations into the ATR-active layer of the membrane surface. More recently, we [14 16] observed optical second harmonic generation (SHG) for ionophore-incorporated PVC-based liquid membranes, and confirmed that the membrane potential is primarily governed by the SHG active, oriented complexed cations at the... 

Lassgaard-Jorgensen et al. [19] calculated that the rate of reactions involved in SMR is much higher than the rate of penetration of methane through the membrane. Lin et al. [20] have observed that the methane conversion strongly depends on the space velocity and the amount of methane per membrane surface area... 

In a microscope, standard polarized epi-illumination cannot distinguish order from disorder in the polar direction (defined as the optical axis) because epi-illumination is polarized transverse to the optical axis and observation is along the optical axis at 180°. However, microscope TIR illumination can be partially polarized in the optical axis direction (the z-direction of Section 7.2) and can thereby detect order in the polar angle direction. Timbs and Thompson(102) used this feature to confirm that the popular lipid probe 3,3 -dioctadecylindocarbocyanine (dil) resides in a supported lipid monolayer with its dipoles parallel to the membrane surface, but labeled antibodies bound to the membrane exhibit totally random orientations. 

It is concluded that the pore sizes of the Loeb-Sourirajan-type membrane are under the level of 10 - 15 A. The remarkable transition of the features of the membrane surface by the annealing step is observed. 

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