Selective filling of n-hexane in a tight nanopore

Molecular sieving may occur when two molecules compete for a nanopore. In nearly all known examples, the nanopore is larger than the molecule that selectively enters the pore. Here, we experimentally demonstrate the ability of single-wall carbon nanotubes with a van der Waals pore size of 0.42 nm to separate n-hexane from cyclohexane-despite the fact that both molecules have kinetic diameters larger than the rigid nanopore. This unexpected finding challenges our current understanding of nanopore selectivity and how molecules may enter a tight channel. Ab initio molecular dynamics simulations reveal that n-hexane molecules stretch by nearly 11.2% inside the nanotube pore. Although at a relatively low probability (28.5% overall), the stretched state of n-hexane does exist in the bulk solution, allowing the molecule to enter the tight pore even at room temperature. These insights open up opportunities to engineer nanopore selectivity based on the molecular degrees of freedom.

Automated summary

Research has shown that single-wall carbon nanotubes can separate n-hexane from cyclohexane through nanopores, despite the fact that the kinetic diameters of these molecules are larger than the pore size. Simulations and experiments have revealed that n-hexane molecules can stretch inside the nanotube pores, challenging current understanding of nanopore selectivity.