A thin sheet or surface film, either natural or man-made, of microporous structure that performs as an efficient filter of particles down to the size range of chemical molecules and ions.
Such membranes are termed "semipermeable" because some substances will pass through but others will not. Usually small ions, water, solvents, gases, and other very small molecules can pass through a membrane, but other ions and macromolecules such as proteins and colloids are barred from passage.
Man-made (synthetic) membranes are highly engineered polymer films about 100 angstroms thick and with controlled distributions of pores ranging from 5 to 5,000 angstroms in diameter.
Membranes are used in reverse osmosis, electrodialysis, nanofiltration, ultrafiltration, and as pleated final filter cartridges in water treatment.
SEE ALSO cellulose acetate (CA); cellulose triacetate (CTA); charged polysulfone membrane.