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Membrane materials could be both natural and synthetic and are basically used for separation processes where the driving forces are pressure and concentration gradients. The separation process is referred to as filtration. Separating membrane is a semipermeable barrier that separates two phases and allows a permeate to pass through selectively. A good example of a natural membrane material is the cell membrane which encloses the cell contents and separates them from the surrounding extracellular fluids. Its major role is to selectively allow the passage of materials in and out of the cell. On the other hand are the synthetic membranes which carry the bulk of the material membranes that are used in day today industrial, laboratory, biotechnological and biomedical separations (Scott &Hughes 2006: 100-102). There are quite a number membrane separation techniques and differ on the basis of separation mechanisms and the sizes of the particles or molecules being separated. The processes include microfiltration and ultrafiltration which are mainly employed in separations in food and beverage processing, biotechnological processes, production of antibiotics and protein purification in pharmaceutical industries and in water treatment and purification. It is also used in micro electronics industry. Nanofiltration and reverse osmosis are membrane separation processes mainly used for water purification. Electrolysis, vapour permeation, gas separation and dialysis also use membrane materials in their separation processes. Another separation process that uses membrane materials is electrodialysis but it is the only one of the mentioned process that does not rely on pressure for separation. Pervaporation separation process uses membranes and involves phase change. Membrane distillation which is mainly used in cases like removal of carbon dioxide from natural gas and separation of azeotopic mixtures to cut down the cost of distillation also uses membrane materials (Kumar &Maria 2008: 234-236).
Synthetic membranes are made from an extensive number of materials. That is organic and inorganic materials. Solids like metals, ceramics, homogeneous or heterogeneous polymers and mixed glasses are commonly used. Of all these, ceramic membranes are generally recommended due to their high weight, good production costs, long life span and their friendliness to the environment. In addition to this, due to the materials used in its manufacture, that is aluminium, silicon, carbide and zirconium oxides, it is strong, resistant to corrosion and thermally and mechanically inert. The ceramic membranes are basically composed of monolithic shapes of tubular capillaries. (Scott &Hughes 2006: 105).
Apart from the solid membranes, there are liquid membranes which are made of non rigid material. They include: emulsion liquid membranes, immobilised liquid membranes, molten salts and hollow fibre membranes. (Scott &Hughes 2006: 108).
When selecting a membrane material, it is important to check on some requirement based on the separation process that it is intended to be used for. The membrane should provide sufficient mass transfer area to process good amounts of feed stream. It should have high selectivity properties for certain particles and must resist fouling or retention of particles creating a concentration gradient and particle flow back. Mechanical stability and reproducibility should also be checked. The membrane also should also have low manufacturing and maintenance cost. (Scott &Hughes 2006: 112).
Surface chemistry of the membrane material is important. It touches on the chemical nature and what composes the surface in contact with the separations stream. The surface of the membrane can have a very different chemical property from its main contents. The surface chemistry constitutes vital characteristics of the membrane such as hydrophobic and hydrophilic nature. Ionic charges, thermal resistance, binding affinity and biocompatibility of a membrane purely depend on its surface chemistry and dictate the quality of separation. (Scott &Hughes 2006: 120).
Synthetic membrane can be categorized by their morphology. These classes include dense membranes, porous membranes and asymmetric membranes. Dense and porous membranes are differentiated by the sizes of the separated materials. Dense membranes are thin layers of dense materials separating small particles and are usually used in gas or liquid separations and reverse osmosis. They coud be amorperous or heterogeneous structures. Porous membranes are used to separate large particles like solid or colloidal particles, DNA, RNA, proteins and other large biomolecules (Kumar &Maria 2008: 256-258).
This is the most commonly used of all the material membranes. It is made up of polymeric structures that are intertwined to selectively allow transfer of chemical particles. They mainly find use in gas separation like the oxygen –nitrogen separation and removal of carbon dioxide from natural gas. Polymeric membranes can be porous or asymmetric and separation is based on concentration, pressure, temperature gradient, electric field or diffusion gradient. The membrane is a semipermeable barrier separating two phases and filters the constituents of different sizes selectively. The feed stream or the input stream is separated by filtration through a membrane into two output streams referred to as the permeate and the concentrate. The permeate is the filtrate or what goes through the permeable membrane while the concentrate is the residue or that which does not go through the permeable membrane. The membrane separates one or more gases from feed mixture to give a specific gas rich permeate. Knudson diffusion, molecular sieving, solution diffusion, surface diffusion and capillary condensation are the mechanisms that enable separation across the polymeric membranes. In knudson diffusion, the gas molecules passing through the membrane are small enough to avoid bulk diffusion and the separation occurs due to the difference in the mean path of the gas molecules resulting from collisions with the pore walls and this is related to their molecular weight. Molecular sieving relies on size exclusion to separate gas mixtures. The permeation process through a polymer membrane is predominantly modeled by the solution-diffusion process .According to this process, the transport of penetrant through a non-porous film involves two steps: first the dissolution of the penetrant in the film, followed by a transfer of the penetrant across the membrane due to a concentration gradient.Pores of the polymeric membrane are of small size to allow small molecules to pass through faster than the larger ones. Gas transport through polymeric membranes is modelled in terms of a solution diffusion process. The process involves dissolution of penetrants in the polymer matrix followed by transfer of the penetrants under the applied potential gradient across the membrane .The relative affinity and transfer rate of each penetrant in the polymer define the transport and separation of the gases. (Kumar &Maria, 2008: 301-316).
Surface diffusion involves migration of the adsorbed gases along the pore walls of porous membranes. The rate of surface diffusion is determined by the level of interaction between the adsorbed gases and the pore surface Sand separation is achieved by the difference in the degree of this interaction for the specific gases. Solution diffusion describes the separation process by porous polymeric membranes. Separation depends on chemical and physical interaction between the various gases and the polymers which determines the amounts of the gas that can accumulate in the membrane polymeric matrix. Apart from the porous type of the polymeric membranes, asymmetric polymeric membranes have been made by combining the dense and the porous membranes. Basically the polymeric membranes are of two types; rubbery and glassy basing on the operating temperatures. Rubbery type operates above the glass transition temperature while the glassy membrane operates below the glass transition temperature. Combining the two means that the membrane never reaches the thermodynamic equilibrium and the polymer chains are arranged such that much free volume in form of microscopic voids is formed in the matrix of the polymer. This voids help a great deal in adsorption of gases and increases their solubility hence a better separation. The glassy hard part gives the structural frame and mechanical support while the soft rubbery portion forms the continuous parts within the membrane and its flexibility allows transportation of the gas creating greater permeability. All this copolymerisation is aimed at combining the selectivity of one and the permeability of the other for an effective separation. (Colin & Sandra 2008).
The first polymeric membrane was designed for permeation of carbon dioxide from flue gases by the reverse osmossis and was made of cellulose acetate. Since then enormous changes have been made to improve its separation efficiency. To increase solubility of the gas, the composition of the membrane was changed, to increase diffusion; packing within the membrane was altered in order to increase the volume of free space in the membrane. (Kumar &Maria 2008: 320).
The gas transport and separation properties of polymers have been successfully exploited in commercial ventures. Industrial applications employing membrane processes range from production of pure gases to barrier coatings for protection against environmental elements. Membrane separations are simple, energy efficient processes, which can be economically competitive with traditional separation technologies. (Kumar &Maria 2008: 323).
Common polymers that are used in making these membranes include: polyamides, polyarylates, polypyrolles, polyaniline, poly(phenylene oxide), polypropylene, polyethylene, polyacrinitrile, polytetrafluoroethylene, polyvinylidenetetrafluorine, polyvinylchloride and others. The most widely used being polysulfones due to their chemo and thermal stability. However, polyamides beat all the rest by its best permeability and selectivity properties including thermal and mechanical strength. Conclusively, when manufacturing polymeric membranes, the following properties should be taken care of: gas solubility, membrane packing, binding affinity, chain rigidity of the basic polymer and resistance to harsh environment. (Kumar &Maria 2008: 325-329).
According to a research that was carried out by Ariya Akithakul et.al (2004), it was discovered that Membranes that deliver nanoscale size selectivity are desirable for applications ranging from water treatment to molecular separations. In their study they described polymer thin film composite membranes coated with amphiphilic graft copolymers consisting of a poly(vinylidenefluoride) (PVDF) backbone and poly(oxyethylene methacrylate) (POEM) side chains, PVDF-g-POEM. Transmission electron microscopy and thermal analysis revealed that these materials molecularly self-assemble into bicontinuous nanophase domains of semicrystalline PVDF, providing structural integrity, and poly(ethylene oxide) (PEO), providing selective transport channels of defined size. This research revealed further that PVDF ultrafiltration membranes coated with PVDF-g-POEM wet instantaneously and reject >99.9% of emulsified oil from a 1000 ppm oleic acid/triethanolamine/water microemulsion feed at 66 psi without fouling. Their molecular sieving capability was demonstrated through separation of like-charged organic dyes varying in molecular dimensions by several angstroms. They further revealed that thicker films of PVDF-g-POEM also act as a chromatograph, exhibiting time-dependent permeation of vitamins B2 and B12. This study also indicated that nonporous asymmetric membranes prepared by immersion precipitation of PVDF/PVDF-g-POEM blend solutions exhibit separation capability similar to that of the thin film composites. These new nanochannel membranes hold potential utility for both high volume and high end value liquid-based separations.
Akon Higuchi et.al (2008) employed several polymeric materials for the chiral separation of pharmaceuticals. In this study several chiral separation membranes were prepared from chiral polymers where enantioselectivity was generated from chiral carbons in the main chain. They found it hard to create excellent chiral separation membranes from chiral polymers only, because racemic penetrants mainly encounter the flexible side chains of the membrane polymers. Therefore, chiral separation membranes were also prepared using polymers with a chiral branch. Furthermore, several molecules were used for specific interactions between the molecules and specific pharmaceuticals or drugs in chiral separation membranes. Cyclodextrins, crown ether derivatives, albumin, and DNA were found to be stereoselective ligands and efficient in chiral separation membranes.Kravet and Dmitriev (2009) carried out a research in which they intended to modify properties of polymeric membrane by low temperature treatment. The results they obtained from this study showed on the use of low-temperature plasma for modification of porous polymer membranes is a quite effective tool for both improving the properties of existing polymer membranes and manufacturing new composite membranes with unique characteristics.
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