Many people see membranes as filters that allow small particles to pass while rejecting large particles, thus separating the “big ones” from the “little ones”. The simplicity, and therefore the versatility, is the great strength of membranes. Today, they are widely used to convert salt water into fresh water and in kidney dialysis. But also for gas and vapour separation in petrochemical industry, to clarify beer, to concentrate fruit juices, to purify (industrial) wastewater, and for controlled administration of medicine. Membranes also are an important part of fuel cells – as electrolyte to transport charged particles between the electrodes – and of chemical reactors to selectively capture a reaction product.
Membranes usually have a porous structure; they contain pores through which liquids or gases can flow. The separating function of such a membrane is essentially very simple. You bring the mixture that you would like to separate at one side of the membrane, and you make sure that the pressure or concentration of the component that can pass through is higher at this side than at the other side. The component that can pass through the membrane will also actually do so under the pressure difference as a driving force. The other components are rejected or are transported at a lower speed through the membrane. Et voila … the separation has been accomplished.
What determines whether a compound is transported quickly or slowly through a membrane – apart from the fact that too large particles can not enter the pore? This is primarily the structure of the membrane. You can imagine that a liquid or gas flows faster through wide, straight and short pores than through long, narrow and curved pores. Simply because in the latter case the transportation path is longer, and the resistance is higher. Usually, a membrane consists of a thin, selective top layer on a coarse porous support. They merge seamlessly (as in many types of polymeric membranes) or are provided with intermediate layers to bridge the gap between a mechanically strong support and a selective top layer (as with ceramic membranes).