Asymmetric membrane structure

The thickness of the separative membrane layer for asymmetric membrane structures represents a trade-off between the physical integrity requirement, on the one hand, and the high flux requirement, on the other Current... 

Heterogeneity of membrane constituents may also play an important role in the stabilisation of vesicles. Amphiphiles with cationic and anionic head groups can assemble into vesicles that are stable over a year [37]. This effect may be explained by assuming an asymmetric distribution of the two constituents between the two layers. Note that the two layers have curvatures of equal magnitude but opposite sign. How such an asymmetric membrane structure would be maintained through generation of protocells is not obvious, however. 

Wang, G-M., Chen, C-H., Ho, H-O., Wang, S-S., and Sheu, M-T. (2006), Novel design of osmotic chitosan capsules characterized by asymmetric membrane structure for in situ formation of delivery orifice, Int. J. Pharm., 319, 71-81. 

During the quenching process the homogeneous polymer solution separates into two phases a polymer-rich solid phase, which forms the membrane structure, and a solvent-rich liquid phase, which forms the liquid-filled membrane pores. Generally, the pores at the film surface, where precipitation occurs first and most rapidly, are much smaller than those in the interior or the bottom side of the film, which leads to the asymmetric membrane structure. There are different variations to this general preparation procedure described in the literature e.g., Loeb and Sourirajan used an evaporation step to increase the polymer concentration in the surface of the cast polymer solution and an annealing step during which the precipitated polymer film is exposed for a certain time period to hot water of 70° to 80°C.28... 

At present, hollow-fiber membranes used for gas and liquid separations are mostly prepared from amorphous polymers by means of a phase inversion technique. In this technique, a polymer is dissolved in a suitable solvent or solvent mixtures and spim into a coagulation bath, where solvent exchange occurs between the extruded fiber and coagulant, yielding the asymmetric membrane structure. 

Asymmetric membrane structures have been created from these materials using the diffusion induced phase separation process (DIPS) as well as a thermally induced phase separation process (TIPS) [23] that relies on temperature gradients to produce a gradient in phase separated domain size. Moreover, membranes formed by either process can be further modified by stretching or drawing to alter pore size and porosity. 

The main result of the above described experiments consists in the possibility of non-stationary synthesis of ATP from ADP and P in homogeneous solutions of the coupling factors of chloroplasts at acid-base shift, with the yield of about 6 mol ATP per mol protein in the absence of any asymmetric membrane structure. 

There are serious practical consequences for the observed thickness effect. It is common practice to reduce the thickness of the discriminating layer of a membrane to maximize the productivity of a membrane device. This is achieved either by using a thin film coating on a porous substrate or using an asymmetric membrane structure. As a consequence of the thickness effect these approaches may not be applicable to the dehydration membranes made from highly water permeable ionomers. Similar restrictions will apply to other membrane separations exhibiting similar thickness effects. 

Vesicle formation with block copolymers, for example with PB-poly(L-glutamate), is well documented in the literature [330]. So-called stealth liposomes could be prepared by Nuyken et al. [331] by combining phospholipids with poly(oxazoKne) based A-B-C triblock copolymers. In this context, Meier and co-workers [332 ] succeeded in preparing asymmetric membrane structures, by insertion of membrane proteins into amphiphilic A-B-C triblock copolymers with two water-soluble blocks A and C, such as PEO-PDMS-poly(2-methyloxa-zohne) of the following structure ... 

The pores at the surface of the film, where precipitation occurs first and most rapidly, are much smaller than those inside or on the base of the film, resulting in an asymmetric membrane structure. Selection of different polymer... 

Asymmetric membrane structure showing relative thicknesses of ceramic support, interlayers and top membrane layer. 

---