the passage of a substance, usually a solvent, through a semipermeable membrane that separates a solution from a pure solvent or two solutions of different concentrations from each other. A semipermeable membrane allows small molecules of a solvent to pass through but is impermeable to larger molecules. The concentrations on both sides of such a membrane can be equalized only by unilateral diffusion of the solvent. Hence, osmosis always proceeds from a pure solvent to a solution or from a dilute solution to a more concentrated one. Osmosis within an enclosed volume of liquid is called endosmosis, and osmosis to the outside is called exosmosis.
A solvent is transported through a membrane by osmotic pressure, which is equal to the excess external pressure that has to be exerted by the solution in order to stop osmosis, that is, to create the condition of osmotic equilibrium. Excess pressure above the osmotic pressure may lead to reversal of osmosis, or reverse diffusion, of the solvent. In cases where a membrane is permeable not only to a solvent but also to some dissolved substances, diffusion of the solutes from the solution into the solvent is called dialysis, which is used to separate polymers and colloidal systems from low-molecular admixtures.
Osmosis was first observed by J. A. Nollet in 1748 but was not thoroughly studied until a century later. A major factor in biological processes, it is widely used in laboratory work to study various biological structures and to determine the molecular characteristics of polymers and the concentration of solutions. Osmotic phenomena are sometimes utilized in industry, for example, to produce certain polymeric materials or to purify highly mineralized water by reverse osmosis.
L. A. SHITS