Phase barrier membranes

Interphase transport In two-phase systems with phase barrier membranes... 

We have been introduced to various aspects of solvent extraction in the following sections Section 3.3.7.2, liquid-liquid equilibria in aqueous-organic, organic-organic, aqueous-aqueous systems Section 3.4.1.2, flux expressions in liquid-liquid systems Section 3.4.3.2, solute transport in phase barrier membranes Section 4.1.3, separation achieved in a closed vessel Section 5.2.2, role of chemical reaction in liquid extraction Section 5.3.2, rate controlled aspects of chemical reaction in liquid-liquid systems Section G.3.2.2, bulk flow parallel to force Section 6.4.1.2, mixer-settler, CSTS system. 

Fig. 23.4 Organophilic pervaporation (PV) for in situ recovery of volatile flavour compounds from bioreactors. The principle of PV can be viewed as a vacuum distillation across a polymeric barrier (membrane) dividing the liquid feed phase from the gaseous permeate phase. A highly aroma enriched permeate is recovered by freezing the target compounds out of the gas stream. As a typical silicone membrane, an asymmetric poly(octylsiloxane) (POMS) membrane is exemplarily depicted. Here, the selective barrier is a thin POMS layer on a polypropylene (PP)/poly(ether imide) (PEI) support material. Several investigations of PV for the recovery of different microbially produced flavours, e.g. 2-phenylethanol [119], benzaldehyde [264], 6-pentyl-a-pyrone [239], acetone/buta-nol/ethanol [265] and citronellol/geraniol/short-chain esters [266], have been published...

The classification of separations should reflect the patterns of component transport and equilibrium that develop in the physical space of the system. The transport equations show that we have two broad manipulative controls that can be structured variously in space to affect separative transport. First is the chemical potential which controls both relative transport and the state of equilibrium. Chemical potential, of course, can be varied as desired in space by placing different phases, membrane barriers, and applied fields in appropriate locations. A second means of transport control is flow, which can be variously oriented with respect to the phase boundaries, membranes, and applied fields—that is, with respect to the structure of the chemical potential profile. 

The membrane in a broad sense is a thin layer that separates two distinctively different phases, i.e., gas/gas, gas/liquid, or liquid/liquid. No characteristic requirement, such as polymer, solid, etc., applies to the nature of materials that function as a membrane. A liquid or a dynamically formed interface could also function as a membrane. Although the selective transport through a membrane is an important feature of membranes, it is not necessarily included in the broad definition of the membrane. The overall transport characteristics of a membrane depends on both the transport characteristics of the bulk phase of membrane and the interfacial characteristics between the bulk phase and the contacting phase or phases, including the concentration polarization at the interface. The term membrane is preferentially used for high-throughput membranes, and membranes with very low throughput are often expressed by the term barrier. ... 

Phase inversion refers to the process by which a polymer solution (In which the solvent system Is the continuous phase) inverts Into a swollen three-dimensional macromolecular network or gel (where the polymer Is the continuous phase). As a thin film designed for use as a barrier, such a gel constitutes a phase inversion membrane. 

Pervaporation is characterized by the imposition of a barrier (membrane) layer between a liquid and a vaporous phase, with a mass transfer occurring selectively across the barrier to the vapor side. Separation occurs with the efficacy of the separation effect being determined by the physio chemical structure of the membrane. 

A MEMBRANE is a special structure or phase that separates two macroscopic phases. The membrane normally forms a thin barrier between the macroscopic phases so that its total volume is small relative to the separated phases. Although the membrane is generally insoluble in... 

As a semi-permeable barrier between two fluid phases, a membrane can be used to separate chemical species if the membrane is designed to be selectively permeable to some species and not others. The selectivity of the membrane... 

The most serious obstacle to the study of models of the uptake of metal cations, in spite of the lively experimental activity, is probably the theoretical difficulties in energetics and molecular structure at the cellular barrier membranes, the chemistry of which seems to be unexpectedly complex. In principle, the uptake of metal cations is divided into the passive, physicochemical phase and an active phase that depends on metabolic sources of energy. The active uptake has been adequately demonstrated and the sites of input for different groups of cations have been revealed by competitive experiments. In spite of many attempts, however, the active carriers of cations have not been found. It is an appropriate time for new experimental and/or theoretical ideas. For example, the new, developing thermodynamics of irreversible processes has energetically outlined some very interesting possibilities of the stationary state for the coupling of the processes which are worthy of closer experimentation. 

A well-accepted definition of nanocomposite material is that one of the phases has dimensions in the order of nanometers [51]. Roy et al. [52] present in their paper on alternative perspectives on nanocomposites a summary of features of particle properties when particle size decreases beyond a critical size. As dimensions reach nanoranges, interactions improve dramatically at the interfaces of phases, as do the effect of surface area/volume on the structure-property relationship of the material [53]. There is definite increase in the modulus of the material reinforced with composites, higher dimensional stability to thermal treatment, as well as enhanced barrier, membrane (conductive properties) and flame resistance. Thus, as Paul and Robeson [54] rightly put it, the synergistic advantage of nanoscale dimensions ( nano effect ) relative to larger-scale modifications is an important consideration ... 

Conventionally, solvent extraction with chemical reaction is implemented in a dispersive system where one liquid phase is dispersed as drops in the other immiscible liquid phase. Such a separation is also implemented using a porous membrane as a phase barrier (see Figure 3.4.11) the immiscible phase interface is immobilized at the pore mouth of, for example, a solvent-resistant hydrophobic microporous/porous membrane. Solvent extraction or back extraction through such an interface is easiiy implemented without dispersing one phase in the other. Applications of solvent extraction with chemical reaction in such a nondispersive format have been studied for a variety of systems. 

Although extraction of lipids from membranes can be induced in atomic force apparatus (Leckband et al., 1994) and biomembrane force probe (Evans et al., 1991) experiments, spontaneous dissociation of a lipid from a membrane occurs very rarely because it involves an energy barrier of about 20 kcal/mol (Cevc and Marsh, 1987). However, lipids are known to be extracted from membranes by various enzymes. One such enzyme is phospholipase A2 (PLA2), which complexes with membrane surfaces, destabilizes a phospholipid, extracts it from the membrane, and catalyzes the hydrolysis reaction of the srir2-acyl chain of the lipid, producing lysophospholipids and fatty acids (Slotboom et al., 1982 Dennis, 1983 Jain et al., 1995). SMD simulations were employed to investigate the extraction of a lipid molecule from a DLPE monolayer by human synovial PLA2 (see Eig. 6b), and to compare this process to the extraction of a lipid from a lipid monolayer into the aqueous phase (Stepaniants et al., 1997). 

A membrane is defined as an intervening phase separating two phases forming an active or passive barrier to the transport of matter. Membrane processes can be operated as (1) Dead-end filtration and (2) Cross-flow filtration. Dead-end filtration refers to filtration at one end. A problem with these systems is frequent membrane clogging. Cross-flow filtration overcomes the problem of membrane clogging and is widely used in water and wastewater treatment. 

Cell membrane The cell membrane is composed of about 45% lipid and 55% protein. The lipids form a bilayer that is a continuous nonpolar hydrophobic phase in which the proteins are embedded. The cell membrane is a highly selective permeability barrier that controls the entry of most substances into the cell. Important enzymes in the generation of cellular energy are located in the membrane. 

Whereas the relationship of solute permeability with lipophilicity has been studied in a large number of in vivo systems (including intestinal absorption models [54,55], blood-brain [56 58] and blood nerve [59] barrier models, and cell culture models [60 62], to name just a few), numerous in vitro model systems have been developed to overcome the complexity of working with biological membranes [63-66]. Apart from oil-water systems that are discussed here, the distribution of a solute between a water phase and liposomes is... 

A thorough discussion of the mechanisms of absorption is provided in Chapter 4. Water-soluble vitamins (B2, B12, and C) and other nutrients (e.g., monosaccharides, amino acids) are absorbed by specialized mechanisms. With the exception of a number of antimetabolites used in cancer chemotherapy, L-dopa, and certain antibiotics (e.g., aminopenicillins, aminoceph-alosporins), virtually all drugs are absorbed in humans by a passive diffusion mechanism. Passive diffusion indicates that the transfer of a compound from an aqueous phase through a membrane may be described by physicochemical laws and by the properties of the membrane. The membrane itself is passive in that it does not partake in the transfer process but acts as a simple barrier to diffusion. The driving force for diffusion across the membrane is the concentration gradient (more correctly, the activity gradient) of the compound across that membrane. This mechanism of... 

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