Modification of polymeric membrane’s surface for water treatment by molecular and atomic layer deposition

Deposition of electrostatically charged layers on polymeric membranes generates surfaces that are selectively permeable to specific solutes. For example, deposition of positively-charged polyelectrolytes on cation exchange membrane (CEM; a non-porous, negatively charged polymer that is permeable only to cations) enhances the electrostatic repulsion of cations from the membrane surface. Since multivalent cations (e.g., Mg⁺² and Ca⁺²) are more strongly repelled from the surface than monovalent cations (Na⁺), the polyelectrolyte layer generates permselectivity between mono and multi-valent cations. Such selectivity is a highly desirable property for applications such as enriching desalinated water with di-valent cations, preventing mineral fouling in high recovery-ratio desalination, reverse-electrodialysis and industrial ion-separation processes. However, polyelectrolyte coatings on CEMs form a diffused, non-uniform layer tens of nanometers thick, and have limited charge density, resulting in limited selectivity or high resistance, depending on the coatings thickness.¹ New strategies to coat CEMs with ultra-thin (few-nanometers thick), conformal, uniform, and densely charged compounds are needed.

We demonstrate the use of atomic layer deposition (ALD) to coat CEMs with positively charged metal oxide (Al₂O₃) layers with nanometer precision in thickness. However, coating a soft flexible substrate with a rigid inorganic layer by ALD results in elastic mismatch and the peeling or breaking of the coating. We have found the this can be overcome by using molecular layer deposition (MLD) instead of ALD. Using MLD we deposited nanometrically thin (3-30 nm) hybrid organic-inorganic layers capable of adhering to the surface in solution during operation of the membrane. We explored the deposited layer’s nano-structure and morphology, and measured its surface potential. We have found improved monovalent selectivity in MLD-coated CEMS that is comparable to commercial monovalent selective CEMs based on polyelectrolyte charged surface, while the MLD-coated CEMS exhibit lower ion transport resistance.

References:

1 Luo, T. et al. J. Memb. Sci. 555,429–454(2018)