Semipermeable Membrane
A barrier that allows solvent molecules to pass while blocking larger solute particles.
Unit
Dimensionless
In depth
Selectivity arises from pore size, from solubility differences within the membrane material, or from active molecular channels in biological systems. Membranes are characterised by their molecular weight cut-off, and their performance defines both dialysis and reverse osmosis technology.
Examples in the real world
Cellulose acetate in a desalination module; a cell membrane; parchment or pig bladder in classic osmosis demonstrations.