Thermodynamics of polyamide separation membrane in contact with aqueous solutions

Composite reverse osmosis (RO) membranes, which are typically comprised of a polyamide active-layer that is formed by interfacial polymerization on a porous polysulfone support, are widely used in technologies for desalination and water purification. The water permeation and the rejection of salts or other contaminants are mainly determined by the transport properties of the polyamide active layer. Both the permeabilities of water and salt are described using solution-diffusion model and the mechanism of salt distribution in polyamide is distinguished into ion-exchange and ion partitioning.;The ion partition coefficient К in the active layer is a key thermodynamic parameter that partially controls the ability of the membrane to desalinate water. FT30 membranes are soaked in aqueous solutions of CsCl, KBr, or Na 2WO4, freeze-dried to remove water without disturbing ion distribution, and analyzed by Rutherford backscattering spectrometry. К is calculated as ∼ 6 from the ion concentration in active layer measured using RBS and porosity in the polysulfone support layer is also derived as 40--50% from RBS data.;Stress change induced by salt distribution is investigated with an optical system. Stress goes up to 9 MPa for transferred polyamide active layer from commercial RO membrane FT30 and 8 MPa for lab-synthesized polyamide film. The saturation in stress change is due to the pre-occupation of ions onto all the stress-related sites.;The absorption of water in reverse osmosis membranes FT30 and LF10 is investigated by a combination of measurements of water mass uptake and biaxial stress as a function of relative humidity. Water solubility in polyamide active layer is 12 wt% at a relative humidity of 95%. The slope of a water concentration versus humidity curve can be used to calculate inter-diffusivity of water in polyamide active layers. By combining the measurements of water mass uptake and biaxial stress, we estimate the specific volume of water in the active layers to be ∼ 30%.