Dynamic Nanopore Breathing Boosts Efficiency for Molecule Separation and Diffusion

Dynamic nanopores in metal-organic frameworks (MOFs) are emerging as powerful tools for separating molecules that look almost identical—down to tiny mass differences. In a new theoretical study, researchers explain how “breathing” nanopores can turn fluctuations into a mechanism for mass-selective transport.

The work builds on earlier experiments that distinguished between regular water (H₂O) and heavy water (D₂O). Although the two molecules share most chemical characteristics, their slightly different masses lead to measurable differences in how they diffuse through nanoporous channels. Until now, the physics behind that selectivity remained unclear.

Here, a team led by Professor Shinji Saito used quantum-chemical calculations to model soft porous crystals—specifically a MOF whose nanopore structure does not remain static. Instead, the nanopore energy landscape fluctuates over time and responds to interactions with the incoming molecules.

A key result is that separation performance is governed less by an average pore size and more by the fluctuations themselves. By varying the fluctuation amplitude of the nanopore energy barrier, the researchers found that transport is maximized only under the right dynamic conditions. In other words, there is an optimal fluctuation rate that enables molecules to cross the energy barrier efficiently.

Crucially, the optimal rate is not universal: it depends on molecular mass. Even the minute difference between H₂O and D₂O produces distinct diffusion behaviors under the same dynamic pore conditions.

The study suggests that nanopore dynamics can preferentially accelerate one species over another, generating transport selectivity through nonequilibrium effects rather than static confinement. This provides a mechanistic framework for predicting when and why dynamic pores separate similar molecules.

The implications are broad. Instead of designing materials solely around fixed pore dimensions, researchers can design “smart” separation media that harness internal motion to reduce energy demands and improve industrial separation efficiency.