What makes the polyamide membranes so unique: new insights into membrane structure and ion rejection

Noga Fridman-Bishop, Chemical engineering, Technion, Haifa, Israel
Viatcheslav Freger, Chemical Engineering, Technion, Haifa, Israel

Polymeric membranes are widely used for desalination; however, curiously, only very few materials have been found selective and permeable enough to make successful RO membranes, most notably, aromatic polyamides (PA). The present study uses electrochemical impedance spectroscopy (EIS) to directly measure polymer ions permeability and clarify features that make polymeric aromatic PA membranes SO unique.

EIS results of Nomex, a linear analogue of cross-linked PA, shows an unusual 0.5-exponent scaling of conductivity with salt concentration, inconsistent with any salt exclusion model used so far. Such scaling points to an exceptionally strong ion exclusion of salt ion by the polymer, thereby it preferentially uptakes protons rather that salt cations. EIS results for RO films show essentially the same 0.5 scaling, a signature of similarly strong salt exclusion, yet a much higher conductivity.

Collectively, EIS results for Nomex and genuine RO membranes indicate RO membranes behave essentially as an extremely thin sub-10 nm films of Nomex. Recent TEM investigations indeed confirms that RO membrane contains water-filled voids separated by thin polymer films. We believe that this unique sponge-like structure, containing barriers that are both very dense (yielding a strong salt exclusion) and just a few nanometer thick (hence a reasonable water permeability), enables exceptional performance of PA membranes in desalination. Importantly, the complex structure, formed by the interfacial polymerization process, mechanically stabilizes those thin barriers. Thereby the entire PA layer is mechanically robust despite its small effective thickness.

In addition, the high PA proton affinity, revealed by EIS, suggests a strong interplay between proton transport and salt/ion rejection in the RO process, which have many consequences. On one hand, trans-membrane potential, resulting from salt rejection, controls proton transport. On the other hand, high affinity to protons may both strongly enhance and suppress salt permeability, an effect totally ignored by membrane transport models.