Synthetic membrane

Synthetic membrane can be fabricated from a large number of different materials. It can be made from organic or inorganic materials including solids such as metals, ceramics, homogeneous films, polymers, heterogeneous solids (polymeric mixtures, mixed glasses), and liquids.[3] Ceramic membranes are produced from inorganic materials such as aluminium oxides, silicon carbide, and zirconium oxide. Ceramic membranes are very resistant to the action of aggressive media (acids, strong solvents). They are very stable chemically, thermally, and mechanically, and biologically inert. Even though ceramic membranes have a high weight and substantial production costs, they are ecologically friendly and have long working life. Ceramic membranes are generally made as monolithic shapes of tubular capillaries.[3]

Polymer electrolyte membranes

Polymer membranes may be functionalized into ion-exchange membranes by the addition of highly acidic or basic functional groups, e.g. sulfonic acid and quaternary ammonium, enabling the membrane to form water channels and selectively transport cations or anions, respectively. The most important functional materials in this category include proton exchange membranes and alkaline anion exchange membranes, that are at the heart of many technologies in water treatment, energy storage, energy generation. Applications within water treatment include reverse osmosis, electrodialysis, and reversed electrodialysis. Applications within energy storage include rechargeable metal-air electrochemical cells and various types of flow battery. Applications within energy generation include proton exchange membrane fuel cells (PEMFCs), alkaline anion exchange membrane fuel cells (AEMFCs), and both the osmotic- and electrodialysis-based osmotic power or blue energy generation.

Ceramic multicanal elements

Contact angle of a liquid droplet wetted to a rigid solid surface.Young's equation: γ_LG ∙cos θ+ γ_SL= γ_SG.

One of the critical characteristics of a synthetic membrane is its chemistry. Synthetic membrane chemistry usually refers to the chemical nature and composition of the surface in contact with a separation process stream.[4] The chemical nature of a membrane's surface can be quite different from its bulk composition. This difference can result from material partitioning at some stage of the membrane's fabrication, or from an intended surface postformation modification. Membrane surface chemistry creates very important properties such as hydrophilicity or hydrophobicity (related to surface free energy), presence of ionic charge, membrane chemical or thermal resistance, binding affinity for particles in a solution, and biocompatibility (in case of bioseparations).[4] Hydrophilicity and hydrophobicity of membrane surfaces can be expressed in terms of water (liquid) contact angle θ. Hydrophilic membrane surfaces have a contact angle in the range of 0°<θ<90° (closer to 0°), where hydrophobic materials have θ in the range of 90°<θ<180°.

Wetting of a leaf.

The contact angle is determined by solving the Young's equation for the interfacial force balance. At equilibrium three interfacial tensions corresponding to solid/gas (γ_SG), solid/liquid (γ_SL), and liquid/gas (γ_LG) interfaces are counterbalanced.[4] The consequence of the contact angle's magnitudes is known as wetting phenomena, which is important to characterize the capillary (pore) intrusion behavior. Degree of membrane surface wetting is determined by the contact angle. The surface with smaller contact angle has better wetting properties (θ=0°-perfect wetting). In some cases low surface tension liquids such as alcohols or surfactant solutions are used to enhance wetting of non-wetting membrane surfaces.[4] The membrane surface free energy (and related hydrophilicity/hydrophobicity) influences membrane particle adsorption or fouling phenomena. In most membrane separation processes (especially bioseparations), higher surface hydrophilicity corresponds to the lower fouling.[4] Synthetic membrane fouling impairs membrane performance. As a consequence, a wide variety of membrane cleaning techniques have been developed. Sometimes fouling is irreversible, and the membrane needs to be replaced. Another feature of membrane surface chemistry is surface charge. The presence of the charge changes the properties of the membrane-liquid interface. The membrane surface may develop an electrokinetic potential and induce the formation of layers of solution particles which tend to neutralize the charge.

Synthetic membranes can be also categorized based on their structure (morphology). Three such types of synthetic membranes are commonly used in separation industry: dense membranes, porous membranes, and asymmetric membranes. Dense and porous membranes are distinct from each other based on the size of separated molecules. Dense membrane is usually a thin layer of dense material utilized in the separation processes of small molecules (usually in gas or liquid phase). Dense membranes are widely used in industry for gas separations and reverse osmosis applications.

Dense membranes can be synthesized as amorphous or heterogeneous structures. Polymeric dense membranes such as polytetrafluoroethylene and cellulose esters are usually fabricated by compression molding, solvent casting, and spraying of a polymer solution. The membrane structure of a dense membrane can be in a rubbery or a glassy state at a given temperature depending on its glass transition temperature .[2] Porous membranes are intended on separation of larger molecules such as solid colloidal particles, large biomolecules (proteins, DNA, RNA) and cells from the filtering media. Porous membranes find use in the microfiltration, ultrafiltration, and dialysis applications. There is some controversy in defining a "membrane pore". The most commonly used theory assumes a cylindrical pore for simplicity. This model assumes that pores have the shape of parallel, nonintersecting cylindrical capillaries. But in reality a typical pore is a random network of the unevenly shaped structures of different sizes. The formation of a pore can be induced by the dissolution of a "better" solvent into a "poorer" solvent in a polymer solution.[2] Other types of pore structure can be produced by stretching of crystalline structure polymers. The structure of porous membrane is related to the characteristics of the interacting polymer and solvent, components concentration, molecular weight, temperature, and storing time in solution.[2] The thicker porous membranes sometimes provide support for the thin dense membrane layers, forming the asymmetric membrane structures. The latter are usually produced by a lamination of dense and porous membranes.

• Membrane technology

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• Zeman, Leos J., Zydney, Andrew L., Microfiltration and Ultrafitration, Principles and Applications., New York: Marcel Dekker, Inc,1996.
• Mulder M., Basic Principles of Membrane Technology, Kluwer Academic Publishers, Netherlands, 1996.
• Jornitz, Maik W., Sterile Filtration, Springer, Germany, 2006
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• Ho C., Zydney A. Protein fouling of asymmetric and composite microfiltration membranes. Ind Eng Chem Res. 40(2001): 1412–1421.