Fluid liquid membranes

Liquid membranes are most useful where there is a low driving force for mass transfer. In this case, the fluid liquid membrane can serve as an extracting phase for a desired solute. The solute partitions to satisfy thermodynamic equilibrium constraints. Since the liquid membrane is usually very thin, this partitioning will be completed in a relatively short time and with minimal concentrative effect. In standard liquid-liquid extraction processes, one would employ a stripping step to replenish the extractant and concentrate the extracted solute. For liquid membranes, such a stripping step may be carried out on the opposite side of the liquid membrane (i.e. in the receiving phase). Thus, liquid membrane separations are often called liquid membrane extraction processes in view of the analogy to traditional... 

Hydrated bilayers containing one or more lipid components are commonly employed as models for biological membranes. These model systems exhibit a multiplicity of structural phases that are not observed in biological membranes. In the state that is analogous to fluid biological membranes, the liquid crystal or La bilayer phase present above the main bilayer phase transition temperature, Ta, the lipid hydrocarbon chains are conforma-tionally disordered and fluid ( melted ), and the lipids diffuse in the plane of the bilayer. At temperatures well below Ta, hydrated bilayers exist in the gel, or Lp, state in which the mostly all-trans chains are collectively tilted and pack in a regular two-dimensional... 

From the frequency measurements of the LB-film-deposited QCM plate in water, the behavior of phospholipid LB films can be classified into three types (i) phospholipids having relatively hydrophilic head groups such as DPPC and DPPG are hydrated and then easily flaked from the substrate in the fluid liquid crystalline state above Tc (ii) DPPE and DPPS having the less hydrophilic head groups are hydrated only near their Tc (iii) cholesterol LB films show relatively large hydration behavior even at low temperatures due to the water penetration into the structure defects in the membrane. 

ISEs are well suited for flow measurements because the instrumentation and signal handling are simple, the measurement is almost independent of the liquid flow-rate, the linear dynamic range is broad, the temperature dependence is not very pronounced and the measurement is selective (the selectivity is, however, a drawback in applications to chromatography). The experimental conditions are readily adjusted and often only consist of ionic strength and pH maintenance. ISEs with solid membranes usually exhibit better performance than liquid membrane electrodes and gas probes, because their response is faster and they are mechanically stronger. The most difficult problem is passivation of the electrodes in some media, for example, biological fluids or surface and waste waters. 

The membranes used are typically composed of cross-linked silicones and are suitable for on-line monitoring of volatile organic and inorganic compounds [93-94]. An alternative material is microporous PTFE, which has more rapid responses as well as lower selectivities and higher fluxes of the mobile phase compared to nonporous silicone membranes. More recently, developments in membrane introduction systems include the use of liquid membranes composed, for example, of a polyphenyl ether diffusion pump fluid [95-96]. This membrane has the advantage that it can take any desirable analyte and the selectivity can be modified using appropriate reagents. 

As the distribution ratio between phases 1 and 3 is the product of those in the two pairs of fluids, the potential effectiveness of the liquid membrane process is considerably greater than that of conventional solvent extraction. Thus the liquid membrane process is particularly suitable for the treatment of dilute feeds. In addition, if the liquid membrane is an organic phase, its small volume reduces the solvent duty considerably. 

Liquid/liquid partition constants within pharmaceutical chemistry have been of primary interest because of tlieir correlation with liquid/membrane partitioning behavior. A sufficiently fluid membrane may, in some sense, be regarded as a solvent. With such an outlook, tlie partitioning phenomenon may again be regarded as a liquid/liquid example, amenable to treatment with standard continuum techniques. Of course, accurate continuum solvation models typically rely on the availabihty of solvation free energies or bulk solvent properties in order to develop useful parameterizations, and such data may be sparse or non-existent for membranes. Some success, however, has been demonstrated for predicting such data either by intuitive or statistical analysis (see, for example. Chambers etal. 1999). 

Mechanical forces can disturb the elaborate structure of the enzyme molecules to such a degree that de-activation can occur. The forces associated with flowing fluids, liquid films and interfaces can all cause de-activation. The rate of denaturation is a function both of intensity and of exposure time to the flow regime. Some enzymes show an ability to recover from such treatment. It should be noted that other enzymes are sensitive to shear stress and not to shear rate. This characteristic mechanical fragility of enzymes may impose limits on the fluid forces which can be tolerated in enzyme reactors. This applies when stirring is used to increase mass transfer rates of substrate, or in membrane filtration systems where increasing flux through a membrane can be accompanied by increased fluid shear at the surface of the membrane and within membrane pores. Another mechanical force, surface... 

Liquid membranes of the water-in-oil emulsion type have been extensively investigated for their applications in separation and purification procedures [6.38]. They could also allow extraction of toxic species from biological fluids and regeneration of dialysates or ultrafiltrates, as required for artificial kidneys. The substrates would diffuse through the liquid membrane and be trapped in the dispersed aqueous phase of the emulsion. Thus, the selective elimination of phosphate ions in the presence of chloride was achieved using a bis-quaternary ammonium carrier dissolved in the membrane phase of an emulsion whose internal aqueous phase contained calcium chloride leading to phosphate-chloride exchange and internal precipitation of calcium phosphate [6.1]. 

The change in the two-state distribution is easily monitored by a convenient one-wavelength measurement of the neutral form fluorescence, and this can be used for probing the membrane. The fairly large differences in wavelengths of excitation (300 nm), fluorescence of the neutral form (360 nm), and fluorescence of the anion form (480 nm) makes the fluorescence free from spectral interference. The variation of the P form fluorescence intensity with temperature showed a maximum at phase-transition temperatures (Tc) for both DMPC (23°C) (Fig. 2) and DPPC (42°C) membranes (Fig. 3). Figures 2 and 3 show a very nice correspondence of this variation with DPH fluorescence polarization and self-diffusion rate [93] of 22Na+. The coexistence of solid gel and fluid liquid-crystalline phases at Tc and the consequent imperfection of the membrane [93] result in a redistribution of... 

Partitioning of components between two immiscible or partially miscible phases is the basis of classical solvent extraction widely used in numerous separations of industrial interest. Extraction is mostly realized in systems with dispergation of one phase into the second phase. Dispergation could be one origin of problems in many systems of interest, like entrainment of organic solvent into aqueous raffinate, formation of stable, difficult-to-separate emulsions, and so on. To solve these problems new ways of contacting of liquids have been developed. An idea to perform separations in three-phase systems with a liquid membrane is relatively new. The first papers on supported liquid membranes (SLM) appeared in 1967 [1, 2] and the first patent on emulsion liquid membrane was issued in 1968 [3], If two miscible fluids are separated by a liquid, which is immiscible with them, but enables a mass transport between the fluids, a liquid membrane (LM) is formed. A liquid membrane enables transport of components between two fluids at different rates and in this way to perform separation. When all three phases are liquid this process is called pertraction (PT). In most processes with liquids membrane contact of phases is realized without dispergation of phases. 

Table 5.10 summarizes the presently available electrodes categorized as glass, ion-exchange membrane, crystal membrane, and liquid membrane. These electrodes can be used either for direct potentiometric measurements of ionic activity after calibration of the Nemst expression for the particular electrode or to monitor a potentiometric titration when a selected reaction that involves the monitored ion is available. Table 5.10 also indicates the common interfering ions. Several instrument companies are endeavoring to develop potentiometric-membrane electrodes to monitor directly ions in body fluids. 

In the 1970s, the fluid mosaic concept emerged as the most plausible model to account for the known structure and properties of biological membranes [41]. The fact that membranes exist as two-dimensional fluids (liquid disordered) rather than in a gel state (solid ordered) was clearly demonstrated by Frye and Ededin [42], who showed that the lipid and protein components of two separate membranes diffuse into each other when two different cells were fused. Since that time, numerous studies have measured the diffusion coefficient of lipids and proteins in membranes, and the diffusion rates were found to correspond to those expected of a fluid with the viscosity of olive oil rather than a gel phase resembling wax. 

As water is withdrawn from a membrane during entry into anhydrobiosis, the strengths of interactions among acyl chains increase, and a shift from the fluid to the gel state is favored. This shift is noticeable as water content is reduced below about 20% (Crowe et al., 1997). Transition temperatures (Tm), the temperatures at which the fluid (liquid crystalline) to gel conversion occurs, increase significantly during dehydration. A membrane composed of palmitoyloleoylphosphatidyl choline has a Tm of —7°C when fully hydrated, but the Tm rises to approximately 60°C in the dry lipid (Crowe et al., 1997). Thus, a phospholipid membrane that would be in a fluid state at normal cell temperatures when hydrated acquires a rigid gel structure when dehydrated. 

Acetylcholineesterase Miniaturized multichannel transduc-tor with planar Au electrode which was first covered with a choline-selective liquid membrane made from 66% PVC-polyvinyl acetate (PVA), 33% 2-nitrophenyl octyl ether plasticizer and 1% ion-pair choline phosphotungstate. A second layer of 2% AChE in the PVA-polyethylene dispersion was spread on the top. The electrode was used as working electrode versus Ag/AgCl for potentiometric measurement of Ch and ACh in 0.1 M Tris buffer at 7.4. Optimum pH range for the sensor was 7-9. The calibration graph was linear from 0.02-10 mm ACh and detection limit was 5 pM. Response time was 3-5 min. Sensor was suitable for determination of ACh in biological fluids. [86]... 

Evans EA, Sackmann E. Translational and rotational drag coefficients for a disk moving in a liquid membrane associated with a rigid substrate. J. Fluid Mech. 1988 194 553-561. 

Cholesterol affects a large variety of membrane properties in animal cells (39). It is involved in modifying dynamical membrane properties by reducing passive permeation, slowing down the lateral diffusion of molecules in fluid-like membranes, and speeding up diffusion in gel-phase membranes. It also affects bilayer properties by condensing the bilayer, which changes its elastic properties and promotes the order of phospholipid acyl chains in the hydrophobic membrane core. In this manner, cholesterol develops the formation of the liquid-ordered... 

Continuous Phase Composition Emulsion liquid membrane properties can be significantly influenced by changing the composition of the external aqueous phase. Emulsion stability can be improved by an increase in the viscosity as a result of the decrease in the rate of fluid drainage between the liquid films [88]. An increase in ionic strength of the external phase has been shown to cause a decrease in entrainment phenomena during permeation. This has been attributed to an alteration of the stmcture of the interface between the emulsion and the external phase promoted by the presence of electrolytes in the external phase. A reduction in osmosis also occurs due to a reduction in the chemical potential difference between both sides of the membrane [94,98]. 

Gallego-Lizon T and Perez de Ortiz ES. Drop sizes in liquid membrane dispersions. Ind Eng Chem Res 2000 39 5020-5028. CalderbankPHandMoo-YoungMB. The continuous phase heat and mass-transfer properties of dispersions. Chem Eng Sci 196 16 39-54. Rowe PN, Claxton KT, and Lewis JB. Heat and mass transfer from a single sphere in an extensive flowing fluid. Trans Instn Chem Engrs 1965 43 T14-T31. 

These membrane systems, mainly provided in the form of low cost hollow fibres, offer a high interfacial area, significantly greater than most traditional absorbers, between two phases to achieve high overall rates of mass transfer. Furthermore, whereas the design of the conventional devices is restricted by limitations in the relative flows of the fluid streams, membrane contactors give an active area which is independent of the liquid fluid dynamics. 

These techniques involving the measurement of membrane permeability to a fluid (liquid or gas) lead to a mean pore radius (usually the effective hydraulic radius Th) whose quantitative value is often highly ambiguous. The flux of a fluid through a porous material is sensitive to all structural aspects of the material [129]. Thus, in spite of the simplicity of the method, the interpretation of flux data, even for the simplest case of steady state, is subject to uncertainties and depends on the models and approximations used. 

Criteria for immobilized liquid membrane (ILM) support selection can be divided into two categories structural properties and chemical properties. Structural properties include geometry, support thickness, porosity, pore size distribution and tortuosity. Chemical criteria consist of support surface properties and reactivity of the polymer support toward fluids in contact with it. The support thickness and tortuosity determine the diffusional path length, which should be minimized. Porosity determines the volume of the liquid membrane and therefore the quantity of carrier required. The mean pore size determines the maximum pressure difference the liquid membrane can support. The support must be chemically inert toward all components in the feed phase, membrane phase, and sweep or receiving phase. 

The pressure difference is directly proportional to the cosine of the contact angle. For a nonwetting fluid, 6 approaches 90°, and AP approaches zero. The implication of immobilizing a nonwetting or poorly wetting fluid through solvent-exchange or other methods is that the Immobilized liquid membrane would have little resistance to small transmembrane pressures. 

Aside from intracellular work, the liquid membrane micro-ISEs described above may also be used for extracellular studies involving a wide range of tissues. For example, accumulation and depletion of ions that flow across extracellular space between nerve groups in the brain can be easily monitored by these types of microelectrodes (N2). The size of the electrodes used in such studies need only be 2-4 pm. In these experiments, the ISEs are placed in the extracellular fluid and ion activities may be directly monitored without concern for the cells membrane potentials. 

---