Normal (forward) OSMOSIS is the spontaneous movement of solvent through a semi-permeable membrane from an area of low solute concentration to an area of high solute concentration.

OSMOTIC PRESSURE is the minimum pressure that needs to be applied to the solution to prevent the flow of solvent

through a semi-permeable membrane.

In REVERSE OSMOSIS (RO) applied pressure is used to overcome osmotic pressure and force solvent through a semi-permeable membrane that retains the solute.

DESALINATION

Reverse osmosis (RO) is widely used for the desalination of seawater.

You can find data on membranes for water purification and desalination in the Open Membrane Database.

Explore the

Open Membrane Database

CELLULOSE ACETATE MEMBRANES

The first commercial RO membranes

were based on cellulose acetate.

Cellulose acetate membranes were developed in the 1950s that showed high salt rejection but low flux.

In 1963, Sidney Loeb and Srinivasa Sourirajan published a method for fabricating high flux desalination membranes from cellulose acetate.

Cellulose acetate RO membranes are integrally skinned asymmetric membranes with a thin separation layer and a highly porous sub-layer.

The thin separation layer enables high flux to be achieved and the porous sub-layer provides mechanical support.

Integrally skinned means that the skin and the sub-layer are formed from the same material, with the structure arising from the way it is fabricated.

AROMATIC POLYAMIDE MEMBRANES

Many current RO membranes are thin film composite (TFC) membranes, which have a thin separation layer of one material on a porous support made of a different material.

Often they have a separation layer of an

AROMATIC POLYAMIDE formed by

interfacial polymerization on a porous

polysulfone support with a nonwoven polyester backing.

Synthetic polyamides are prepared by

step-growth (condensation) polymerization of a monomer with amine groups and a monomer with carboxylic acid or acyl halide groups.

Interfacial polymerization is a type of step-growth polymerization where polymerization occurs at the interface between two immiscible liquids. Usually one monomer is in an aqueous phase

and the other monomer is in an organic phase. Interfacial polymerization can be used to create a thin polymer film at the surface of a porous support.

For a typical interfacial polymerization, 1,3,5-benzenetricarbonyl trichloride (triemsyl chloride) is in the organic phase and benzene-1,3-diamene (m-phenyldiamine) is in the aqueous phase.

A crosslinked polymer film is obtained.

Some unreacted acyl chloride groups

may subsequently be hydrolysed to

carboxylic acid.