Membrane instability

In our approach to membrane breakdown we have only taken preliminary steps. Among the phenomena still to be understood is the combined effect of electrical and mechanical stress. From the undulational point of view it is not clear how mechanical tension, which suppresses the undulations, can enhance the approach to membrane instability. Notice that pore formation models, where the release of mechanical and electrical energy is considered a driving force for the transition, provide a natural explanation for these effects [70]. The linear approach requires some modification to describe such phenomena. One suggestion is that membrane moduli should depend on both electrical and mechanical stress, which would cause an additional mode softening [111]. We hope that combining this effect with nonlocality will be illuminating. 

Further progress in understanding membrane instability and nonlocality requires development of microscopic theory and modeling. Analysis of membrane thickness fluctuations derived from molecular dynamics simulations can serve such a purpose. A possible difficulty with such analysis must be mentioned. In a natural environment isolated membranes assume a stressless state. However, MD modeling requires imposition of special boundary conditions corresponding to a stressed state of the membrane (see Refs. 84,87,112). This stress can interfere with the fluctuations of membrane shape and thickness, an effect that must be accounted for in analyzing data extracted from computer experiments. 

The actual cause of membrane instability, whether due to an unstable chemical bond or to a gradual, irreversible swelling of the structure, has not been determined. 

Membrane instability results in partial mixing of feed and stripping phases, which deteriorates the selectivity. In addition, raffinate and product are contaminated by the extractant, leading also to extractant losses. Economy of separation and hence industrial application of LM for separation of cephalosporins are strongly dependent on membrane stabilization. 

Significant progress has been made in alleviating the first two physical causes of membrane instability. The magnitude of the long-term chemical stability problem depends on the process. It is a major issue for carriers used to transport oxygen and olefins, but for carriers used to transport carbon dioxide, chemical stability is a lesser problem. 

Neplenbroek, A.M., Bargeman, D. and Smolders, C.A. (1992) Supported liquid membranes - instability effects. Journal of Membrane Science, 67, 121. 

Despite the unique properties of inorganic membranes vs. the rather well-established polymeric ones (see Table 1 for a comparison), issues such as membrane instability, insufficient permeability or permselectivity, or simply the unbearable costs implied still hamper the application of inorganic-membrane reactors in the process industry. 

Solvent polymeric membranes that incorporate quaternary ammonium salt anion-exchangers, such as tri-n-octylpro-pylammonium chloride decanol, are used to construct CP-selective electrodes in clinical analyzers. Although by far the most common method for measuring CP in clinical laboratories, these electrodes have been described to suffer from membrane instability and lot-to-lot inconsistency in selectivity to other anions. " Anions that tend to be problematic are other halides and organic anions, such as SCN", which can be particularly problematic because of their ability to solubilize in the polymeric organic membrane of these electrodes. 

Depletes RBC glutathione, causing cell membrane instability and hemolysis... 

Acute hemolytic anemia due to depletion of erythrocyte glutathione, causing RBC membrane instability... 

Membrane instability was believed to result from hydrolytic cleavage of sulfate and sulfonate esters in the sulfonated polyfurane resin. This hydrolysis would entail some depolymerization of the resin and concurrent formation of ionic sulfonate groups, causing the membrane to absorb water and swell in monovalent salt solutions. Oxidation by dissolved oxygen is another potential... 

During electroporation, cells or tissues are exposed to an electric field with high voltage (up to 1 kV). Short, rapid pulses cause transient membrane instability and the formation of pores with a mean lifetime of minutes [30]. Soluble DNA constructs that have been added to the culture medium or injected into the tissue may thereby enter the cell and ultimately reach the nucleus. The basic principle is also known to pharmacists as iontophoresis and is used for transdermal drug apphcation [31]. This gene transfection method has so far been well accepted by patients and is safe, especially due to the low risk of infection. To date, mainly liver, muscle, and skin cells have been transformed this way, mostly via the transdermal route [32]. 

Pd electroless baths commonly contain ethylene diamine tetra acetic acid (EDTA), a complexing agent that increases the stability of the plating bath [6]. Recently, we have determined that the use of EDTA can leave up to 6mass% carbon in the Pd membranes and this carbon reduces permeability and can react with CO2 present in the feed gas, contributing to membrane instability [13, 14]. The majority of the membranes discussed in this article were prepared without using EDTA. 

The simple rate equations for the various liquid-membrane separation processes in use prior to 1978 were reviewed previously. Since that time, much has been done to improve them, especially by considering the influence of the intemal microdroplets and the effects of leakage of the intemal reagent which was previously ignored.Cahn et al., studying copper extraction, have taken membrane instability into account by modifying the basic permeation rate equation to include a leakage term. This modified equation has the form... 

From experiments on planar bilayer membranes (BLM), it was known that lipid bilayers were not able to withstand an increase in the applied voltage above a threshold value. A conductive state followed by a rupture was observed for values of the order of 200 mV. Electropulsation induces a transmembrane potential modulation, bringing a similar membrane instability. Indeed experiments on pure lipid vesicles showed that upon the field pulse the lipid bilayer could become leaky. This was observed on line by the associated increase in conductance of a salt-filled vesicle suspension [26]. But larger molecules could leak out and be directly detected outside the vesicles as observed with radiolabelled sucrose [27] or fluorescent dyes [28]. A very fast detection of the induction of membrane leakage is obtained by electrical conductance and light scattering... 

Ginsherg, L, Xuereh, JH and Gershfeld, NL (1998) Membrane instability, plasmalogen contenL and Alzheimer s disease. J Neurochem, 70, 2533—2538. 

Membrane instability due to leaching of the carrier into the aqueous phases has been studied in our laboratory. This phenomenon is strongly related to the lipophilicity of the carrier. Enhancement of the lipophilicity has been achieved by attachment of carriers to a polymeric backbone (14) or covalent linkage of aliphatic chains to carriers (75,16). Recently, both the stability of the membrane phase and the solubility of the carrier have been improved by attachment of one or more solvent molecules to the carrier (17). Long-term membrane stability was demonstrated for a period of 5 weeks, while carrier concentrations up to 50 wt% could be used. 

Membrane stability is the primary problem associated with the use of SLMs. Solvent loss is most often the causative factor for membrane instability. Such solvent loss arises from evaporation and dissolution, as well as from an excessive pressure differential applied across the membrane which forces solvent (and carrier) out of the pores of the membrane. Carrier may also be lost due to irreversible side reactions within the membrane itself... 

Another cause of liquid membrane instability is excessive transmembrane pressure across the membrane. This may be caused by osmotic pressure or by applied pressure. The impregnated liquid phase in a SLM can be displaced when the critical applied pressure, Pc, is reached. This shortens the lifetime of the SLM and is a very important factor in hollow fiber SLMs. For cylindrical pores, the displacement pressure is given by the Laplace-Yoimg equation (44) ... 

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