Applications permeation

In the colloidal realm, given the large surface-to-volume ratio and the relatively small range of force that can sway the disposition of a colloidal particle, it is easy to appreciate the importance of controlling surface properties. Research literature abounds with the characteristics of colloid systems and model systems that mimic colloid surfaces. Applications permeate the fields of materials processing, adhesion, coatings, food science, and medicine. 

When the solution-diffusion mechanism is applicable, permeation can usually be treated as an activated process, as described by Equation 15.3. The activation energy Ep changes as the polymer goes through major transitions such as the glass transition or melting. Pq is a pre-exponential factor which has the same units as P(T). 

This chapter contains one of the more diverse assortments of topics of any chapter in the volume. In it we discuss the viscosity of polymer solutions, especially the intrinsic viscosity the diffusion and sedimentation behavior of polymers, including the equilibrium between the two and the analysis of polymers by gel permeation chromatography (GPC). At first glance these seem to be rather unrelated topics, but features they all share are a dependence on the spatial extension of the molecules in solution and applicability to molecular weight determination. 

In most cases, hoUow fibers are used as cylindrical membranes that permit selective exchange of materials across their waUs. However, they can also be used as containers to effect the controUed release of a specific material (2), or as reactors to chemically modify a permeate as it diffuses through a chemically activated hoUow-fiber waU, eg, loaded with immobilized enzyme (see Enzyme applications). 

Pervaporation is a relatively new process with elements in common with reverse osmosis and gas separation. In pervaporation, a liquid mixture contacts one side of a membrane, and the permeate is removed as a vapor from the other. Currendy, the only industrial application of pervaporation is the dehydration of organic solvents, in particular, the dehydration of 90—95% ethanol solutions, a difficult separation problem because an ethanol—water azeotrope forms at 95% ethanol. However, pervaporation processes are also being developed for the removal of dissolved organics from water and the separation of organic solvent mixtures. These applications are likely to become commercial after the year 2000. 

Aromatic polyamide (aramid) membranes are a copolymer of 1-3 diaminobenzene with 1-3 and 1-4 benzenedicarboxylic acid chlorides. They are usually made into fine hollow fibers, 93 [Lm outer diameter by 43 [Lm inner diameter. Some flat sheet is made for spirals. These membranes are widely used for seawater desalination and to some extent for other process applications. The hollow fibers are capable of veiy high-pressure operation and have considerably greater hydrolytic resistance than does CA. Their packing density in hoUow-fiber form makes them veiy susceptible to colloidal fouling (a permeator 8 inches in diameter contains 3 M fibers), and they have essentially no resistance to chlorine. 

Vapor Permeation Vapor permeation is similar to vapor perva-poration except that the feed stream for permeation is a gas. The futnre commercial viability of this process is based npon energy and capital costs savings derived from the feed already being in the vapor-phase, as in fractional distillation, so no additional heat inpnt wonld be req iired. Its foreseen application areas wonld be the organics recov-eiy from solvent-laden vapors and pollntion treatment. One commercial nnit was installed in Germany in 1989 (Ref. 26). 

Ultrafiltration membranes are commercially fabricated in sheet, capillary and tubular forms. The liquid to be filtered is forced into the assemblage and dilute permeate passes perpendicularly through the membrane while concentrate passes out the end of the media. This technology is useful for the recovery and recycle of suspended solids and macromolecules. Excellent results have been achieved in textile finishing applications and other situations where neither entrained solids that could clog the filter nor dissolved ions that would pass through are present. Membrane life can be affected by temperature, pH, and fouling. 

Permeability. The low density of plastics is an advantage in many situations but the relatively loose packing of the molecules means that gases and liquids can permeate through the plastic. This can be important in many applications such as packaging or fuel tanks. It is not possible to generalise about the performance of plastics relative to each other or in respect to the performance of a specific plastic in contact with different liquids and gases. 

The main fluids of interest with plastics are oxygen and water vapour (for packaging applications) and CO2 (for carbonated drinks applications). Fig. 1.13 and Fig. 1.14 illustrate the type of behaviour exhibited by a range of plastics. In some cases it is necessary to use multiple layers of plastics because no single plastic offers the combination of price, permeation resistance, printability, etc. required for the application. When multi-layers are used, an overall permeation constant for the composite wall may be obtained from... 

The most common membrane systems are driven by pressure. The essence of a pressure-driven membrane process is to selectively permeate one or more species through the membrane. The stream retained at the high pressure side is called the retentate while that transported to the low pressure side is denoted by the permeate (Fig. 11.1). Pressure-driven membrane systems include microfiltration, ultrafiltration, reverse osmosis, pervaporation and gas/vapor permeation. Table ll.l summarizes the main features and applications of these systems. 

The properties of flexible polymer chains moving in porous structures, that is, in structures with geometric constraints such as tubes or slits, apart from their Tclevance for various applications such as filtration, gel permeation chromatography, oil recovery, etc., pose an exciting problem of statistical... 

The evolution of media covering aqueous and nonaqueous systems on the one hand and analytical as well as microscale and macroscale preparative applications on the other hand has resulted in an arbitrarily nomenclature within the field. Thus the current practice is to refer to the separation principle based on solute size as size exclusion chromatography (SEC) whereas the application in aqueous systems is traditionally referred to as gel filtration (GF) and the application in nonaqueous systems is designated gel-permeation... 

In general, high selectivities can be obtained in liquid membrane systems. However, one disadvantage of this technique is that the enantiomer ratio in the permeate decreases rapidly when the feed stream is depleted in one enantiomer. Racemization of the feed would be an approach to tackle this problem or, alternatively, using a system containing the two opposite selectors, so that the feed stream remains virtually racemic [21]. Another potential drawback of supported enantioselective liquid membranes is the application on an industrial scale. Often a complex multistage process is required in order to achieve the desired purity of the product. This leads to a relatively complicated flow scheme and expensive process equipment for large-scale separations. 

Possible applications of MIP membranes are in the field of sensor systems and separation technology. With respect to MIP membrane-based sensors, selective ligand binding to the membrane or selective permeation through the membrane can be used for the generation of a specific signal. Practical chiral separation by MIP membranes still faces reproducibility problems in the preparation methods, as well as mass transfer limitations inside the membrane. To overcome mass transfer limitations, MIP nanoparticles embedded in liquid membranes could be an alternative approach to develop chiral membrane separation by molecular imprinting [44]. 

A number of studies have recently been devoted to membrane applications [8, 100-102], Yoshikawa and co-workers developed an imprinting technique by casting membranes from a mixture of a Merrifield resin containing a grafted tetrapeptide and of linear co-polymers of acrylonitrile and styrene in the presence of amino acid derivatives as templates [103], The membranes were cast from a tetrahydrofuran (THF) solution and the template, usually N-protected d- or 1-tryptophan, removed by washing in more polar nonsolvents for the polymer (Fig. 6-17). Membrane applications using free amino acids revealed that only the imprinted membranes showed detectable permeation. Enantioselective electrodialysis with a maximum selectivity factor of ca. 7 could be reached, although this factor depended inversely on the flux rate [7]. Also, the transport mechanism in imprinted membranes is still poorly understood. 

---