(325f) Shorter and Harder or Longer and Easier: The Effect of Pore Length on Ion Transport

Membrane-based separations are widely regarded as the most efficient techniques for critical
applications such as metal recovery and water desalination. With growing global population and
industrial demand, there is an urgent need to enhance membrane performance by increasing
both selectivity and permeability. Yet, improving one often compromises the other. Addressing
this trade-off requires either identifying novel materials that intrinsically favor selective transport,
or optimizing existing ones through fine-tuning of design parameters. Both approaches hinge on
unraveling the structure–selectivity relationships that govern membrane behavior—a task made
difficult by the vastness and complexity of the design space.

To achieve high selectivity, membranes often rely on single-digit nanopores (SDNs), where the
pore dimensions are on the order of the target molecules. At this scale, classical models of
transport break down, as molecular interactions with the pore walls become dominant.
Experimental investigation of ion dynamics in SDNs is limited by the spatio-temporal resolution
of current techniques. Computational tools, particularly molecular simulations, offer a valuable
alternative to explore these phenomena in atomistic detail.

In this work, we employ molecular dynamics simulations combined with Forward Flux Sampling
(FFS) to probe ion dynamics through hydrophobic and hydrophilic nanopores of varying lengths.
For hydrophobic pores, we observe a surprising trend: increasing the pore length from sub-
nanometer to several nanometers results in faster counter-ion transport—contradicting the
classical expectation of linearly increasing passage time with length. Water passage time, too,
displays a non-monotonic dependence on length. At the scale of SDNs, such effects are
attributed to perturbed interactions between the ion, water, and pore structure. For hydrophilic
pores, differing hydration environments are expected to impose distinct energetic penalties and
influence transport mechanisms in nontrivial ways.

Our findings highlight the critical roles of pore length and hydrophilicity in governing ion and
water transport in SDNs. Molecular simulations provide key mechanistic insights into these
systems, enabling us to uncover non-intuitive behaviors that escape continuum models. Our
ongoing efforts aim to construct a statistical-mechanics-based framework that can capture and
predict the dynamics and energetics of transport across a wide range of nanopore designs.