Amide versus amine ratio in the discrimination layer of reverse osmosis membrane by solid state 15N NMR and DNP NMR

Abstract

The state of the art membrane chemistry for reverse osmosis application is based on a classical interfacial polymerization reaction between a diamine in the aqueous phase and trifunctional acid chloride in the organic phase. Because of the very fast reaction rate of the interfacial polymerization, the extremely thin nature (typically ≤200 nm), and the insolubility of the resulting polyamide layer, the conversion rate has not been directly studied. In this work, high field (21.2 T) solid state NMR was utilized to directly measure amide to amine ratio of the polyamide layers in commercial RO membrane. Contrary to earlier indirect measurement, amines are rather abundant in these polyamide layers. Because of the dramatic reactant concentration differences on the opposing surfaces of the growing membrane, compositional heterogeneity is expected to exist across the membrane thickness. Dynamic nuclear polarization combined with solid-state NMR was utilized to probe the amide to amine ratio close to the membrane surface and was successful in differentiating membrane targeting different end user applications. The findings are important to understand both the interfacial polymerization chemistry as well as the performance of the resulting RO membrane since the amine groups present can form hydrogen bonds and ionize or deionize based on pH. The surface amine groups can be further chemically modified to acquire additional properties for membranes.