Partitioning of salt ions in FT30 reverse osmosis membranes

FT30 membranes consist of a polyamide active layer interfacially polymerized on a porous polysulfone support. The ion partition coefficient κ in the active layer is a key thermodynamic parameter that partially controls the ability of the membrane to desalinate water. Membranes are soaked in aqueous solutions of CsCl, KBr, or Na2 W O4 with salt concentrations of 0.002M< Cs <2M, freeze dried to remove water without disturbing ion distribution, and analyzed by Rutherford backscattering spectrometry. For Cs <0.01M, κ is surprisingly large and independent of Cs, κ=6±2. The porosity φ of the top 500 nm of the support layer is also extracted from the RBS data: φ=0.45±0.05.