Enhancing water permeation through alumina membranes by changing from cylindrical to conical nanopores

In the quest for next-generation ceramic membranes, an embodiment of the gradient conical pore structure is foreseen as an intriguing route. In this proof-of-concept simulation study, we investigate water permeation through alumina membranes consisting of cylindrical and conical nanopores, respectively. For the cylindrical nanopores, water permeability increases with increasing the nanopore diameter (2.05, 3.10 and 7.62 nm). Due to the strong interaction of water with the hydrophilic pore surface, the effective viscosity of water in the nanopore is greater than the bulk viscosity; consequently, the simulated permeability is lower than the prediction by the Hagen-Poiseuille (HP) equation. For the conical nanopores with various apex angles (α = 0°, 9.6°, 19.2°, 28.9°, 38.9° and 49.2°), water permeability is predicted to be higher than the cylindrical counterparts and is the highest when α = 19.2°. The potentials of mean force reveal that the barrier for water permeation through the cylindrical nanopore is high and continuous over the entire nanopore, whereas the barrier through the conical nanopore is lower. Moreover, the nanofiltration of two solutes (sucralose and bisphenol A) is simulated. In the presence of solute, water permeability is reduced to a certain extent. Compared with hydrophobic bisphenol A, hydrophilic sucralose interacts more strongly with the nanopore, thus causing a greater pore blockage and a larger reduction in water permeability. This simulation study provides atomistic insights into the key factors governing water permeation through alumina nanopores, and validates the idea that water permeation can be enhanced using conical nanopores.