Asymmetric membranes induced phase separation

Lipids in model systems are often found in asymmetric clusters (see Figure 9.8). Such behavior is referred to as a phase separation, which arises either spontaneously or as the result of some extraneous influence. Phase separations can be induced in model membranes by divalent cations, which interact with negatively charged moieties on the surface of the bilayer. For example, Ca induces phase separations in membranes formed from phosphatidylserine (PS)... 

Asymmetric membranes are usually produced by phase inversion techniques. In these techniques, an initially homogeneous polymer solution becomes thermodynamically unstable due to different external effects and the phase separates into polymer-lean and polymer-rich phases. The polymer-rich phase forms the matrix of the membrane, while the polymer-lean phase, rich in solvents and nonsolvents, fills the pores. Four main techniques exist to induce phase inversion and thus to prepare asymmetric porous membranes [85] (a) thermally induced phase separation (TIPS), (b) immersion precipitation (wet casting), (c) vapor-induced phase separation (VIPS), and (d) dry (air) casting. 

Matsuyama H, Berghmans S, Batarseh MT, Lloyd DR (1999) Formation of anisotropic and asymmetric membranes via thermally-induced phase separation. In Piimau I, Freeman BD (eds) Membrane formation and modification. American Chemical Society, Washington,... 

The Loeb-Sourirajan process often is referred to as diffusion induced phase separation (DIPS) to reflect the role of diffusion in forming the asymmetric structure. Liquid-liquid phase separation and the resulting asymmetric structure arise from diffusion of a solvent (acetone) out of the film and diffusion of a nonsolvent (water) into the film. This physical interpretation provided the basis for the development of asymmetric membrane manufacturing processes for other polymer - solvent - non-solvent systems. 

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. 

TCA removal was also studied using SBS asymmetric membranes prepared by non-solvent induced phase separation (NIPS), as reported in the US patent by Sikdar et al. (2008). In this work, the temperature and vacuum pressure were varied while the feed concentration ( 120 ppm) was kept constant. TCA, water and total flux increased with increasing the temperature (from 10°C to 35°C) at the constant pressure of 0.05 bar. At a permeate pressure of 0.05 bar and temperature of 34°C, TCA flux was about 0.018 kg m i. Separation factor ((Ztca/Hjo) increased from 900 to 4600 at higher temperature (from 10°C to 35°C) and from 640 to 2300 changing the permeate pressure from 0.01 to 0.05 bar. 

In the nonsolvent induced-phase separation process, a nonsolvent penetrates into a polymer solution by difrusion. Since phase separation occurs quickly in the surface where the penetration starts, and slowly in the inner part where the penetration proceeds comparatively slowly, the membrane has an asymmetrical stmeture, which has a smaller pore size in the smface and a larger pore size in the inner part. An example of UF having the asymmetrical stmeture is shown in Figure 5.3. 

The modulation of synaptosomal plasma membranes (SPMs) by adriamycin and the resultant effects on the activity of membrane-bound enzymes have been reported [58]. Again DPH was used as fluorescence probe. Adriamycin increased the lipid fluidity of the membrane labeled with DPH, as indicated by the steady-state fluorescence anisotropy. The lipid-phase separation of the membrane at 23.3 °C was perturbed by adriamycin so that the transition temperature was reduced to 16.2 °C. At the same time it was found that the Na+,K+-stimulated ATPase activity exhibits a break point at 22.8 °C in control SPMs. This was reduced to 15.8 °C in adriamydn-treated SPMs. It was proposed that adriamycin achieves this effect through asymmetric perturbation of the lipid membrane structure and that this change in the membrane fluidity may be an early key event in adriamycin-induced neurotoxicity. 

A membrane can essentially be defined as a barrier that separates two phases and selectively restricts the transport of various chemicals. It can be homogenous or heterogeneous, symmetric or asymmetric in structure, solid or liquid, and can carry a positive or negative charge, or be neutral or bipolar. Transport across a membrane can take place by convection or by diffusion of individual molecules, or it can be induced by an electric field or concentration, pressure or temperature gradient. The membrane thickness can vary from as little as 100 p.m to several millimeters. 

The membrane is essentially a barrier, that separates two phases and restricts transport of various chemicals in a selective maimer. A membrane can be homogeneous or heterogeneous, symmetric or asymmetric, sofid or liquid it can carry positive or negative charges or can be neutral. Transport through a membrane can be affected by convection or by diffusion of individual molecules, and induced by the chemical gradient or electrical gradient. 

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