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 Na2WO4 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 500nm of the support layer is also extracted from the RBS data: ϕ=0.45±0.05.