Nanofiltration membranes have pores with a diameter of around one nanometer, which is why the technique is called nanofiltration. Nanofiltration (NF) is characterised by a membrane pore size between 0.5 and 2 nm and operating pressures between 5 and 40 bar. NF is used to achieve a separation between sugars, other organic molecules and multivalent salts on one hand and monovalent salts and water on the other. NF membranes have a slightly charged surface. Because the dimensions of the pores are less than one order of magnitude larger than the size of ions, charge interaction plays a dominant role. This effect can be used to separate ions with different valences.

Thin film membranes
Polyamide thin films of less than 500 nanometers are responsible for the ability of reverse osmosis and nanofiltration membranes to separate most ions and dissolved species from water. However, transport, separation and fouling mechanisms within these films are still not well understood in part because of the difficulties associated with characterizing such extremely thin and heterogeneous films.
Thin film composite (TFC) co-polyamide membranes can be prepared by interfacially polymerizing secondary amide and highly hydrophilic aromatic primary amide (3, 5-diaminobenzoic acid, BA). Incorporation of the BA can cause an increase in pure water permeability constant and solute transport parameter. Membrane with a higher content of BA produces a loose and defected skin layer.
The membranes can retain multivalent ions (e.g., calcium, magnesium, aluminum, sulfates) and non-ionized organic compounds with a molar mass exceeding about 300 g/mol. However, most monovalent ions and organic compounds with a molar mass lower than 300 g/mol pass through. Potential applications include selective demineralization (for water softening) and concentration of organic compounds with low molar mass.