Characterizing salt permeability in polyamide desalination membranes using electrochemical impedance spectroscopy
Devin L. Shaffer, Kathleen E. Feldman, Edwin P. Chan, Gery R. Stafford, Christopher M. Stafford
Improving the performance of desalination membranes requires better measurements of salt permeability in the polyamide separating layer to elucidate the thermodynamic and kinetic components of membrane permselectivity. In this work, electrochemical impedance spectroscopy (EIS) was used to measure the salt permeability and estimate the salt partition coefficient in thin polyamide films created using a molecular layer-by-layer deposition technique. The impedance of supported polyamide films ranging in thickness from 3.5 nm to 28.5 nm were measured in different electrolyte solutions. Impedance spectra were modeled with equivalent circuits containing resistive and capacitive elements associated with the EIS measurement system as well as characteristic low-frequency parallel resistive and capacitive elements that are associated with the polyamide film. The characteristic polyamide membrane resistance increases with film thickness, decreases with solution concentration, and is an order of magnitude greater for a divalent cationic solution than for a monovalent cationic solution. For each polyamide film, salt permeability is calculated from the membrane resistance, and a salt partition coefficient is estimated. Calculated salt permeabilities range from Ps = 1.3 × 10^-16 m s-1 to 3.9 × 10^-16 m s-1, and associated salt partition coefficients range from Ks = 0.01 to 0.05. EIS is demonstrated as a valuable tool for studying membrane permselectivity through the measurement of salt permeability in thin polyamide films.