Membranes for reverse osmosis and nanofiltration

In integral asymmetric membranes, both toplayer and the sublayer consist of the same material. These membranes are prepared by phase inversion techniques. For this reason it is essential that the polymeric material firom which the membrane it to be prepared is soluble in a solvent or a solvent mixture. Because most polymers are soluble in one or more solvents, asymmetric membranes can be prepared from almost any material. 

However, this certainly does not imply that all such membranes are suitable for every reverse osmosis application because the material constants A and B must have optimal values for a given application. Thus for aqueous applications, e.g. the desalination of seawater and brackish water, hydrophilic materials should be used (high A value) with a low solute permeability. 

Composite membranes constitute the second type of structure frequently used in reverse osmosis while most of the nanofiltration membranes are in fact composite membranes. In such membranes the toplayer and sublayer are composed of different polymeric materials so that each layer can be optimised separately. The first stage in manufacturing a composite membrane is the preparation of the porous sublayer. Important criteria for this sublayer are surface porosity and pore size distribution and asymmetric ultrafiltration membranes are often used. Different methods have been employed for placing a thin dense layer on top of this sublayer  

These various methods have been discussed in chapter III. Since reverse osmosis membranes may be considered as intermediate between porous ultrafiltration membranes and very dense nonporous pervaporation/gas separation membranes, it is not necessary that their structure to be as dense as for pervaporation/gas separatipn. Most composite reverse osmosis and nanofiltration membranes are prepared by interfacial polymerisation (see chapter in. 6) in which two very reactive bifunctional monomers (e.g. a di-acid chloride and a di-amine) or triiunctional monomers (e.g. trimesoyicbloride) are allowed to react with each other at a water/organic solvent interface and a typical rietwork structure is obtained. Another example of monomers used for interfacial polymerisation are given in table VI.6 (see also table m.1). 

Reverse osmosis can be used in principle for a wide range of applications, which may be roughly classified as solvent purificadon (where the permeate is the product) and solute concentradon (where the feed is the product). 

---