Mechanism of membrane formation

It is shown in this section that two types of demixing process resulting in two different types of membrane morphology can be distinguished instantaneous liquid-liquid demixing delayed onset of liquid-liquid demixing 

Instantaneous demixing means that the membrane is formed immediately after 

The occurrence of these two distinctly different mechanisms of membrane formation can be demonstrated in a number of ways by calculating the concentration profiles by light transmission measurements and visually. 

The best physical explanation is given by a calculation of the concentration profiles. To calculate the concentration profiles in the polymer film during the (delayed demixed type of) phase inversion process, some assumptions and considerations must be made[35]  

In addition, a number of parameters must be determined experimentally  

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To investigate the mechanism of membrane formation as a function of the casting techniques listed in point (4) above. 

In order to understand the mechanism of membrane formation, one must first understand the environmental condition which influences membrane biogenesis. The two most obvious effectors of mitochondria and chloroplasts are Oj and light. 

The initial layer formation step has hardly been investigated. Some qualitative guidelines can be abstracted from theoretical considerations and experimental observations. The first intensive and systematic study of the kinetics and mechanism of membrane formation was performed by Leenaars et al. [2-4] on boehmite and Y-AI2O3 membranes with pore diameters in the range of 3-5 nm. 

Information about the porous support layer rather than the skin layer. The techniques used by these authors, as well as those reviewed by Pusch and Welch (21), provide valuable Insight Into the mechanism of membrane formation and thus may assist membrane scientists In developing better membranes. However, many of these techniques do not characterize the membrane under the conditions of application for example, the ultrafiltration membranes (23,24) are dried prior to gas sorption studies and microscopy. Therefore, caution must be exercised In Interpreting the results of these characterization methods and relating them to membrane performance and transport mechanisms. 

These different structures can be correlated to the two mechanism of membrane formation, toplayer by a delayed onset of demixing, sublayer by instantaneous demixing. Moreover, a polymer concentration profile should be generated as shown schematically in figure HI -49, with a high polymer concentration at the top side and a low polymer concentration at the bottom side. Such a profile can be obtained in two ways ... 

Several articles or communications show the use of tomography for the structural characterization of membranes. Thus, studies were published relating to the mechanisms of membrane formation, the experimental acquisition of data such as porosity or tortuosity, the obtaining of realistic 3D models for the simulation of the flows in the membranes, or the detection of defects. 

The mutual interaction between P, S, and NS strongly affects the mechanism of membrane formation. For instance, when the difference between the solubility parameters of P and S is small, S has a strong dissolving capacity. As a consequence. 

Q.F. Alsalhy, K.T. Rashid, W.A. Noori, S. Simone, A. FigoU, and E. DrioU, Poly(vinyl chloride) hoUow-fiber membranes for ultrafiltration applications Effects of the internal coagulant composition. Journal of Applied Polymer Science 124 (2012) 2087—2099. M.A. Frommer, R.M. Messalem, Mechanism of membrane formation. 6. Convective flows and large void formation during membrane precipitation. Industrial and Engineering Chemistry Product Research and Development 12 (1973) 328—333. 

Like mitochondria, chloroplasts (when illuminated) pump protons across their membranes (Fig. 23-18). However, while mitochondria pump protons to the outside, the protons accumulate on the inside of the thylakoids. The ATP synthase heads of coupling factor CEj are found on the outside of the thylakoids, facing the stromal matrix, while those of F, lie on the insides of mitochondrial membranes. However, the same mechanism of ATP formation is used in both chloroplasts and mitochondria (Chapter 18). 

Fig. 21 Proposed mechanisms of lipoplex formation (a) vesicle titration (DNA initially in excess) -DNA coats the vesicle surfaces as the latter are added to the DNA solution - with increase of the vesicle concentration, clusters of DNA-coated vesicles form and consequently rupture (b) DNA titration (lipid initially in excess) - DNA encounters with bare membranes result in vesicle associations - vesicle-DNA-vesicle adhesion generates stresses, which lead to vesicle rupture, followed by continued aggregation and growth of the complex upon further addition of DNA. (reproduced with permission from [67] copyright (2000) Biophysical Society)...

How the hydrophilic a-LTX inserts into lipid membranes and makes cation-permeable pores is not fully known, but an in-depth insight into the mechanisms of channel formation has been gained by combining cryo-EM, biochemical and biophysical studies with toxin mutagenesis. a-LTX pore formation consists of at least three steps toxin tetramerisation, interaction with a specific cell-surface receptor and, finally, membrane insertion. Many experimental procedures can affect some of these steps and thereby prevent or assist channel formation. 

Figure 1.7 Interaction of nisin with phospholipid model membranes mechanism of pore formation (reprinted with permission from Ref. [34], Copyright 2007 American Chemical Society).

P NMR was also used to study of the oriented membranes and pores induced by protegrin-1 (PG-1), which represents AMPs.92,93 The line shape specifies the toroidal pores and thinned membranes that are formed in membrane bilayers by the binding of AMPs. The lateral diffusion of lipids were analysed from the motion-ally averaged 2D 31P SS NMR spectra. The mechanism of pore formation due to interaction between the peptide (fallaxidin) with lipids has been investigated.94... 

Figure 4 The modified stalk mechanism of membrane fusion and inverted phase formation, (a) planar lamellar (La) phase bilayers (b) the stalk intermediate the stalk is cylindrically-symmetrical about the dashed vertical axis (c) the TMC (trans monolayer contact) or hemifusion structure the TMC can rupture to form a fusion pore, referred to as interlamellar attachment, ILA (d) (e) If ILAs accumulate in large numbers, they can rearrange to form Qn phases, (f) For systems close to the La/H phase boundary, TMCs can also aggregate to form H precursors and assemble Into H domains. The balance between Qn and H phase formation Is dictated by the value of the Gaussian curvature elastic modulus of the bIlayer (reproduced from (25) with permission of the Biophysical Society) The stalk in (b) is structural unit of the rhombohedral phase (b ) electron density distribution for the stalk fragment of the rhombohedral phase, along with a cartoon of a stalk with two lipid monolayers merged to form a hourglass structure (reproduced from (26) with permission of the Biophysical Society).

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