Filtering at the Molecular Scale
Nanofiltration membranes contain pores just 0.5–5 nanometers wide — large enough to pass water molecules and small ions, but small enough to block larger solutes, divalent ions, and organic molecules. This selectivity operates at the boundary between molecular sieving and solution-diffusion transport, combining elements of both ultrafiltration's size exclusion and reverse osmosis's chemical selectivity.
Pore Transport Physics
Water flux through nanopores follows a modified Hagen-Poiseuille equation, where flux scales with the square of pore radius and linearly with applied pressure. Solute transport involves both convective drag (carried along by water flow) and diffusion through the pore. The ratio of solute to pore radius (λ) determines the steric hindrance factors that quantify how much the pore walls slow solute transport relative to free solution.
The Rejection Curve
Nanofiltration's defining feature is its sharp rejection curve: as solute size approaches pore size, rejection rises steeply from near-zero to near-complete. This enables selective separations impossible with other methods — for example, removing calcium and magnesium (water hardness) while passing sodium and chloride, or concentrating pharmaceutical intermediates while letting solvents pass through.
Engineering Better Membranes
Modern nanofiltration research focuses on next-generation membrane materials: graphene oxide laminates with atomically precise channels, metal-organic frameworks (MOFs) with tunable pore chemistry, and aquaporin-inspired biomimetic membranes. These advanced materials promise higher flux, sharper selectivity, and improved fouling resistance compared to conventional thin-film composite polyamide membranes that have dominated the field for decades.