Classification of Synthetic Membranes
A membrane can be natural or synthetic, thick or thin, its structure can be homogeneous or heterogeneous, transport across membrane can be active or passive, passive transport can be driven by various means (e.g. pressure, concentration, electrical difference), neutral or charged. As such, membranes can be classified according to different viewpoints. The first classification is by nature, i.e. biological or synthetic membranes. This is the clearest distinction possible. Synthetic membranes can be subdivided into organic (polymeric or liquid) and inorganic (e.g. ceramic, metal) membranes.
Another means of classifying membranes is by morphology or structure - for the case of solid synthetic membranes, the 2 types of membrane structures are the symmetric and asymmetric (anisotropic) membranes.
The principal types of polymeric membranes are classified as shown in the Figure below:
The Figure shows schematically the different types of membranes. This classification is, however, rather arbitrary, and many structures fit more than one of the above-mentioned classes. For example, a membrane may be microporous, asymmetric in structure, and have electrical charges.
The IUPAC classification for pores sizes are as follows:
The following membranes are briefly discussed:
Microporous Membranes
Anisotropic (Asymmetric) Membranes
Click here for examples of typical membrane structures (pictures obtained from Osmonics at www.osmonics.com)