Escalating global water scarcity necessitates high-performance desalination membranes, for which fundamental understanding of structure–property–performance relationships is required. In this study, we comprehensively assess the ionization behavior of nanoporous polyamide selective layers in state-of-the-art nanofiltration (NF) membranes. In these films, residual carboxylic acids and amines influence permeability and selectivity by imparting hydrophilicity and ionizable moieties that can exclude coions. We utilize layered interfacial polymerization to prepare physically and chemically similar selective layers of controlled thickness. We then demonstrate location-dependent ionization of carboxyl groups in NF polyamide films. Specifically, only surface carboxyl groups ionize under neutral pH, whereas interior carboxyl ionization requires pH >9. Conversely, amine ionization behaves invariably across the film. First-principles simulations reveal that the low permittivity of nanoconfined water drives the anomalous carboxyl ionization behavior. Furthermore, we report that interior carboxyl ionization could improve the water–salt permselectivity of NF membranes over fourfold, suggesting that interior charge density could be an important tool to enhance the selectivity of polyamide membranes. Our findings highlight the influence of nanoconfinement on membrane transport properties and provide enhanced fundamental understanding of ionization that could enable novel membrane design.
Two-dimensional nanomaterial (2-D
NM) frameworks, especially those
comprising graphene oxide, have received extensive research interest
for membrane-based separation processes and desalination. However,
the impact of horizontal defects in 2-D NM frameworks, which stem
from nonuniform deposition of 2-D NM flakes during layer build-up,
has been almost entirely overlooked. In this work, we apply Monte
Carlo simulations, under idealized conditions wherein the vertical
interlayer spacing allows for water permeation while perfectly excluding
salt, on both the formation of the laminate structure and molecular
transport through the laminate. Our simulations show that 2-D NM frameworks
are extremely tortuous (tortuosity ≈103), with water
permeability decreasing from 20 to <1 L m–2 h–1 bar–1 as thickness increased from
8 to 167 nm. Additionally, we find that framework defects allow salt
to percolate through the framework, hindering water–salt selectivity.
2-D NM frameworks with a packing density of 75%, representative of
most 2-D NM membranes, are projected to achieve <92% NaCl rejection
at a water permeability of <1 L m–2 h–1 bar–1, even with ideal interlayer spacing. A high
packing density of 90%, which to our knowledge has yet to be achieved,
could yield comparable performance to current desalination membranes.
Maximizing packing density is therefore a critical technical challenge,
in addition to the already daunting challenge of optimizing interlayer
spacing, for the development of 2-D NM membranes.
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