Definition of a membrane

Although it is difficult to give an exact definition of a membrane, a general definition could be: A selective barrier between two phases, the term "selective"  being inherent to membrane or a membrane process.

It should be noted that this is a macroscopic definition while separation should be considered at the macroscopic level. The definition says nothing about membrane structure nor membrane function.

A membrane can be thick or thin, its structure can be  homogeneous or heterogeneous, transport can be active or passive, passive transport can be driven by pressure, concentration or a temperature difference. In addition, membrane can be nature or synthetic, neutral or charged.

To obtain a more informative understanding, membranes can be classified according to different view points. The first classification is by nature, i.e. biological or synthetic membrane, this is the clearest distinction possible.

Biological membranes can be subdivided into living and non-living membranes(liposomes and vesicles from phospholipids).

Synthetic membranes can be subdivided into organic(polymer or liquid) and inorganic (ceramic, metal) membranes.

Another means of classifying membranes is by morphology or structure. This is also a very illustrative route because the membrane structure determines the separation mechanism and hence the application. If we confine ourselves to solid synthetic membranes, two types of membrane may be distinguished, i.e. symmetric(porous or nonporous) or asymmetric membranes.

Always membrane in the syringe filters is porous, it belongs to the microfiltration.