Nanotube Alignment and Surface Chemistry in Altering Water and Salt Permeabilities for Imogolite-Polyamide Membranes

Reducing the specific energy consumption of reverse osmosis (RO) processes motivates the development of new membrane materials that have enhanced water permeability while maintaining low salt permeability (high rejection). Nanocomposite membranes have shown great promise in achieving these goals, particularly those using nanotubes as fillers. Here, we report on the relationships between the orientations and surface functionalities of imogolite nanotubes (INTs) with water and salt permeabilities for polyamide nanocomposite membranes. An external electric field was used to manipulate the INT orientation within the polyamide active layer. The INT interior and exterior chemistries, respectively, were made hydrophobic using methyl triethoxysilane as a precursor during INT synthesis and post-synthesis modification with alkali-phosphate groups. Irrespective of nanotube orientation or surface chemistry, membrane permeance increased from 0.3 to ≥1.0 L m^-2 h^-1 bar^-1. A salt permeability comparable to the conventional polyamide membrane was maintained by making the INT pore throat hydrophobic. These findings indicated that salt rejection could be tailored by manipulating the INT interior surface chemistry without sacrificing water permeability.