Molecular dynamics of n-hexane: A quasi-elastic neutron scattering study on the bulk and spatially nanochannel-confined liquid

TL;DR

This study uses quasi-elastic neutron scattering to compare the molecular dynamics of bulk n-hexane and nanochannel-confined n-hexane, revealing distinct populations with different mobility and a pore freezing transition.

Contribution

It provides the first detailed comparison of molecular motions in bulk and nano-confined n-hexane using neutron scattering, identifying two dynamic populations and a pore freezing phenomenon.

Findings

Two molecular populations with distinct dynamics in nano-confined n-hexane.

Approximately 5% of molecules are immobile at 250K, increasing with cooling.

Pore freezing occurs at 160K, below bulk crystallization temperature.

Abstract

We present incoherent quasi-elastic neutron scattering measurements in a wavevector transfer range from 0.4 AA^{-1} to 1.6AA^{-1} on liquid n-hexane confined in cylindrical, parallel-aligned nanochannels of 6 nm mean diameter and 260 micrometer length in monolithic, mesoporous silicon. They are complemented with, and compared to, measurements on the bulk system in a temperature range from 50K to 250K. The time-of-flight spectra of the bulk liquid can be modeled by microscopic translational as well as fast localized rotational, thermally-excited, stochastic motions of the molecules. In the nano-confined state of the liquid, which was prepared by vapor condensation, we find two molecular populations with distinct dynamics, a fraction which is immobile on the time scale of 1ps to 100ps probed in our experiments and a second component with a self-diffusion dynamics slightly slower than observed for the bulk liquid. No hints of an anisotropy of the translational diffusion with regard to the orientation of the channels' long axes have been found. The immobile fraction amounts to about 5% at 250K, gradually increases upon cooling and exhibits an abrupt increase at 160K (20K below bulk crystallization), which indicates pore freezing