Critical membrane tension

Finally there follows a brief discussion concerning the work for membrane breakdown for all the lipid systems studied to date. The work of membrane breakdown Wq has been compared to the critical membrane tensions for breakdown. The parameter is calculated from... 

The work of membrane breakdown for membranes made from different lipids and lipid-cholesterol mixtures is shown in Figure 9.10. It is seen that, for membranes with small critical membrane tensions, i.e. largely single-component liquid-phase membranes, the work of membrane breakdown increases almost linearly with the critical membrane tension. For higher-strength membranes, i.e. saturated lipids with maximum cholesterol content, the work of membrane breakdown reaches a value on the order of 0.025 kT and remains almost constant regardless of the increase of the critical membrane tension. Thus, the question arises, as to whether tension creates new pores or acts on existing defects. 

Figure 9.10 Work of membrane breakdown versus the critical membrane tension for different lipids and lipid-cholesterol mixtures. (Data from references [82], [28] and [113]).

It follows from Eqs. (73) and (74) that the only stabilizing force for a-modes at long X is the membrane tension, and critical voltage vanishes as cr 0. In experiments with black lipid membranes the surface tension a arises from the contact of the bilayer with the bulk phase contained in the surrounding rim and is typically < 0.002 N/m. Then choosing... 

The ability of a liquid to "wet" the membrane material is an indication of that liquids ability to establish and maintain such an interfacial layer. Liquids of surface tension values less than the critical surface tension iy ) of the membrane material are capable of completely "wetting" the polymer. It may be possible therefore, to select membrane materials capable of accomplishing specific separations by their ability to be wet by one solution component but not by the other. For this reason Yc membrane materials is important. By employing the standard techniques of Zisman (43), the critical surface tension for PSF and CA were determined to be 43.0 and 36.5 dynes/cm, respectively. This data indicates that PSF is more readily wet by a larger number of liquids than is CA. Similar measurements for the various sulfonated polysulfones are underway. 

The viscosity or resistance to flow increases as the number of repeat units increases, but physical properties, such as surface tension and density, remain about the same after a DP of about 25. The liquid surface tension is lower than the critical surface tension of wetting, resulting in the polymer spreading over its own absorbed films. The forces of attraction between polysiloxane films are low resulting in the formation of porous films that allow oxygen and nitrogen to readily pass though, but not water. Thus, semipermeable membranes, films, have been developed that allow divers to breath air under water for short periods. 

Mohandas and co-workers (18), confirming previous findings of Weiss and Blumenson (19), have also shown that cells in an environment free of adsorbable proteins (which rapidly modify the surface properties of polymeric or inorganic substrates) will exhibit a similar direct relationship between their adhesion and the critical surface tension of the surface they contacted. DiflFerential adhesion of red blood cells was measured by determining the fraction of cells retained on a surface after the application of well-calibrated shear stresses (IS). In protein-free experiments, the red cells (themselves dominated in adhesive interactions by their protein membranes) had greatest adhesion to glass, intermediate adhesion to polyethylene and siliconized glass, and least adhesion to Teflon. 

Above some electroporation threshold, the transmembrane potential cannot be further increased, and can even decrease due to transport of ions across the membrane [91, 95]. The phenomenon of membrane electroporation can also be understood in terms of tension. If the total membrane tension exceeds the lysis tension c ys, the vesicle ruptures. This corresponds to building up a certain critical transmembrane potential, = Pc- According to Eqs. (7.3) and (7.4), this porahon potenhal Pc depends on the inihal membrane tension Co as previously reported [59, 89, 90, 96, 97]. The crihcal hansmembrane potenhal for cell membranes is about IV (e.g., [98, 99]). 

Two common liquid membrane support materials, polytetrafluoroethylene and polypropylene, have critical surface tensions of 18 mN/m and 35 mN/m, respectively. Manufacturers often supply critical surface tensions for their porous films. Liquids with a surface tension, y, less than the critical surface tension will probably wet the surface. Therefore, hydrocarbons will wet polypropylene, but water (y = 72 mN/m) will not. Shafrin and Zisman (30) have summarized critical surface tension data for many materials and correlated the data such that critical surface tensions may be estimated from knowledge of the functional groups in the chemical structure of the surface. 

Surface characteristics of the membranes Two of the key characteristics are the hy-drophobicity and the surface charge. Hydrophobic surfaces (or water-hating surfaces) are those where the critical surface tensions of the membrane material... 

The first step in biofilm formation prior to microbial adhesion is the irreversible adsorption of macromolecules, which leads to a conditioning film (humic substances, lipo-polysaccharides, microbial products). This conditioning film alters the effect of the membrane the electrostatic charge and the critical surface tension may change. 

There are a few points to be kept in mind. The liquid membrane must be nonvolatile since it is subjected to vacuum on the permeate side and can evaporate on the feed side and be lost to the feed solution (Figure 8.1.49(d)). Further, wetting of the porous membrane structure spontaneously by the membrane liquid present outside requires that the surface tension of the membrane liquid should be equal to or lower than the critical surface tension of the polymeric or ceramic substrate being employed. For example, y,. for polypropylene (PP) is -33 dyne/cm. Many organic liquids will spontaneously wet it. However, y,. values for various fluoropolymers (FPs) are usually <25... 

Dispersion of either phase into the other in gas-liquid systems is undesirable. By having the pores filled with gas, the resistance to species transfer through the membrane pores is kept at a low level (Qi and Cussler, 1985a). For nondispersive operation, the liquid phase must not spontaneously wet the membrane pores. For porous hydrophobic membrane-based contactors, aqueous solutions are, therefore, preferred liquid phases for any non wetting liquid, the value of 0 is >90°. The surface tension of the liquid should be greater than the critical surface tension of the polymer, 7,-. However, dissolved substances, especially surface-active ones, in water can reduce the value of 7 and can lead... 

In membrane contactor processes, various types of aqueous liquid have been employed such as pure water, aqueous solution of NaOH, KOH, amine solution, and amino acid salts. Each of the absorbent has its own specialties that define a selective process application. Li and Chen [63] conducted a study on the selection of liquid absorbent in a membrane contactor, in which they highlighted criteria for choosing the chemical solvent to be implemented in membrane contactor. The criteria included high reactivity with CO2, liquids with low surface tension, good chemical compatibility with membrane material, regenerability, low vapor pressure, and good thermal stability. Because any liquid that has surface tension lower than the critical surface tension of the polymers may wet the membrane spontaneously, the solvents must have a substantially higher surface tension than the critical surface tension... 

The objectives of this research are to a) to produce environmentally friendly membranes (solvent-free) which are as stable as cast membranes and b) characterize critical membrane properties such as porosity, interfacial tension, viscosity and find a correlation for these. 

It follows from the above that the mechanism for electrical potential oscillation across the octanol membrane in the presence of SDS would most likely be as follows dodecyl sulfate ions diffuse into the octanol phase (State I). Ethanol in phase w2 must be available for the transfer energy of DS ions from phase w2 to phase o to decrease and thus, facilitates the transfer of DS ions across this interface. DS ions reach interface o/wl (State II) and are adsorbed on it. When surfactant concentration at the interface reaches a critical value, a surfactant layer is formed at the interface (State III), whereupon, potential at interface o/wl suddenly shifts to more negative values, corresponding to the lower potential of oscillation. With change in interfacial tension of the interface, the transfer and adsorption of surfactant ions is facilitated, with consequent fluctuation in interface o/ wl and convection of phases o and wl (State IV). Surfactant concentration at this interface consequently decreased. Potential at interface o/wl thus takes on more positive values, corresponding to the upper potential of oscillation. Potential oscillation is induced by the repetitive formation and destruction of the DS ion layer adsorbed on interface o/wl (States III and IV). This mechanism should also be applicable to oscillation with CTAB. Potential oscillation across the octanol membrane with CTAB is induced by the repetitive formation and destruction of the cetyltrimethylammonium ion layer adsorbed on interface o/wl. Potential oscillation is induced at interface o/wl and thus drugs were previously added to phase wl so as to cause changes in oscillation mode in the present study. 

For a hydrophobic porous material with contact angle greater than 90°, the APc is >0 and depends on the liquid surface tension and the membrane pore size. As an example, considering water-air-polypropylene system, one can calculate that for a dry membrane with a pore size of 0.03 pim (30 nm) the critical entry pressure of water is more than 300 psi (>20 bar). 

The same principle of operation as described above is applicable also to liquid-liquid extraction where an aqueous liquid and an organic liquid contact each other inside the contactor for extraction of a solute selectively from one phase to another [6-8]. The critical breakthrough pressure for liquid-liquid system could be calculated by Equation 2.1, except that the term A would now be the interfacial tension between the two liquids. Further variation of membrane contacting technology is called gas membrane or gas-gap membrane where two different liquid phases flow on either side of the membrane, but the membrane pores remain gas filled [9-10]. In this situation two separate gas-hquid contact interfaces are supported on each side of a single membrane. 

---