What is the chemical composition of the active layer in a typical thin-film composite (TFC) RO membrane?

The active layer in a typical thin-film composite (TFC) RO membrane is primarily composed of a polyamide material. This polyamide layer is responsible for the salt rejection properties of the membrane. TFC membranes are made using an interfacial polymerization process, where two reactive monomers are brought into contact at an interface to form a thin polymer film. Typically, an aromatic polyamine, such as m-phenylenediamine (MPD), is dissolved in water, and an acyl chloride, such as trimesoyl chloride (TMC), is dissolved in an organic solvent. These two solutions are brought into contact on the surface of a porous support layer, typically made of polysulfone. At the interface between the two solutions, the MPD and TMC monomers react to form a cross-linked polyamide polymer. The cross-linking gives the polyamide layer its strength and selectivity. The specific chemical structure of the polyamide can be modified by varying the monomers used and the reaction conditions to optimize membrane performance, such as salt rejection and water permeability. The resulting polyamide layer is typically only about 200 nanometers thick, but it provides the primary barrier to salt passage in the RO process. The chemical resistance and physical properties of the polyamide layer are critical to the membrane's overall performance and durability.