Neutral Microporous Membranes

The neutral, microporous films represent a very simple form of a membrane which closely resembles the conventional fiber filter as far as the mode of separation and the mass transport are concerned. These membranes consist of a solid matrix with defined holes or pores which have diameters ranging from less than 2 nm to more than 20 //m. Separation of the various chemical components is achieved strictly by a sieving mechanism with the pore diameters and the particle sizes being the determining parameters. Microporous membranes can be made from various materials, such as ceramics, graphite, metal or metal oxides, and various polymers. Their structure may be symmetric, i.e., the pore diameters do not vary over the membrane cross section, or they can be asymmetrically structured, i.e., the pore diameters increase from one side of the membrane to the other by a factor of 10 to 1,000. The properties and areas of application of various microporous filters are summarized in Table 1.1. 

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Nanofiltration is a membrane process involving microporous membranes, with a cutoff lower than 1000 Da. This process is used for the selective retention of small neutral molecules or of small dissociated ions found in solvents with a high dielectric constant, like water. 

When Dorman contribution can be neglected (case of neutral solutes), membrane cut-off can be determined based on respective sizes of model solutes and membrane pores. Mass transport can be described using both basic concepts of ultrafiltration and specific aspects of transport in micropores. 

Microporous membrane liquid- liquid extraction MMLLE Aq/org/org lonizable and neutral e.g. triazines in oil (38). 

A membrane is a layer of material that serves as a selective barrier between two phases and remains impermeable to specific particles, molecules, or substances when exposed to the action of a driving force. Some components are allowed passage by the membrane into a permeate stream, whereas others are retained by it and accumulate in the retentate stream. Membranes can be of various thicknesses, with homogeneous or heterogeneous structure. Membranes can also be classified according to their pore diameter. According to the International Union of Pure and Applied Chemistry (lUPAC), there are three different pore diameter (dp) size classifications microporous (dp < 2 nm), mesoporous (2 nm < dp < 50 nm), andmacroporous (dp > 50 mn). Membranes can be neutral or charged, and particle transport can be active or passive. The latter can be facilitated... 

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