Another method to help reduce fouling of membranes by protein includes modifying the surface of the membrane so that the proteins are less likely to bind to it. This is especially important during the ultrafiltration of proteins, as the nominal pore size of the membrane is smaller than the size of the proteins. In general, the mechanism by which proteins adsorb to the surface of membranes is the attraction of the hydrophobic proteins to the polymer surface of the membrane, which is also hydrophobic. The surface of the membrane can be modified to become hydrophillic- a process known as hydrophilization. Surface modification to increase the hydrophilicity of the membrane by either introducing oxygen-containing radical groups or hydrophilic polymer chains to the surface of the membrane reduces the attractive force between the membrane surface and protein molecules.
Hydrophilization can be conducted by using any of four major classes of methods: plasma treatment, chemical oxidation, organic chemical functionalization, and radiation induced surface grafting.
Plasma treatment involves using plasma (a low pressure gas
containing electron,
photons, ions, and other charged particles) in the presence of
oxygen to form peroxides on the
surface of the membrane. The peroxides then decompose to form
oxygen-containing radical
groups such as hydroxyls, carbonyls, or carboxyls. Plasma
treatment may also be used to
pretreat the membranes for a subsequent technique known as
surface graft polymerization.
Surface graft polymerization involves the attachment of synthetic
monomers to the peroxide
groups established by the plasma treatment. The attached
monomers then polymerize on the
surface of the membrane, forming a hydrophilic polymer layer (see
figure below):
Chemical oxidation involves the use of oxidants such as chromic acid, nitric acid, and potassium permanganate to oxidize the membrane surface introducing oxygen-containing groups onto the surface of the membrane. Redox initiators, such as ferric chloride can also oxidize the surface to create active sites where surface graft polymerization can occur.
Hydrophilization using organic chemical reactions utilizes specific functional groups which are attached by ionic or polar bonds to membrane surface groups, such as benzene rings, double bonds, and halides. The advantage to this technique is that specific functional groups are bonded to the surface of the membrane, giving the membrane surface the characteristics specified by the specific functional groups.
A fourth technique used to hydrophilize membrane surfaces is radiation induced graft polymerization. Ultraviolet and ionization radiation are used to form active sites on the surface of the membrane. Monomers can then be grafted to these active sites. Subsequent polymerization of the monomer may then occur, forming a hydrophillic layer, as shown in the figure above.