Membrane filtration nonporous

The equipment for UF systems often looks very similar to RO systems although they operate at lower pressures. However, this similarity does not extend to the molecular level. Remeniber that RO membranes are nonporous and separate based on a solution-diffusion mechanism UF membranes are porous and separate based on size exclusion. Large molecules are excluded from pores in the thin membrane skin and thus, the large molecules are retained in the retentate. Small molecules fit into the pores and pass through to the permeate. Since there is usually a distribution of pore sizes, molecules within the range of pore sizes partially permeate and are partially retained. In a somewhat oversinplified picture, UF is cross-flow filtration at the molecular level. 

Where impurities are present as microparticulate material filtration affords a convenient technique for solvent purification. The mobile phase containing added buffers or reagents may be filtered through a 0.5 pm or smaller filter to remove particulate matter that can damage the analytical system. The equipment for filtration is simple. Usually, it consists of an Elenmayer flask connected to vacuum and a reservoir in which a porous filter disk or membrane is placed. The porous disk is usually made from nonporous spherical glass beads (1-2 pm) and/or polytetrafluoroethylene (PTEE). Membrane materials are usually made from PTEE, cellulose, or nylon. To improve the efficiency of the separation process, the surface of the filter disks or membrane surface are often modified chemically, similar to that used for chemically bonded packing materials in RP-HPLC and/or SPE. In this case, the surface properties (hydrophobic or hydrophilic) of filters and/or membranes determine the extent of purification possible. 

The concept of membrane processes is relatively simple but nevertheless often unknown. Membranes might be described as conventional filters but with much finer mesh or much smaller pores to enable the separation of tiny particles, even molecules. In general, one can divide membranes into two groups porous and nonporous. The former group is similar to classical filtration with pressure as the driving force the separation of a mixture is achieved by the rejection of at least one component by the membrane... 

The selection of a membrane for a particular type of separation is usually determined by its average pore size e.g., 10-100 yam is useful for conventional filtration, 0.1-10 yam for microfiltration, 50-1000 A for ultrafiltration, and less than 50 A for reverse osmosis, gas separation, and pervaporation, as depicted in Fig. 33.2. The latter are also described as nonporous membranes and depend on molecular interactions between the permeant and the membrane itself to affect separation. The basic terminology and theory of membrane-based separation systems are similar for both gases and liquids and are therefore treated together in this section. 

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