Diffusion of benzene confined in the oriented nanochannels of chrysotile asbestos fibers

We used quasielastic neutron scattering to study the dynamics of benzene that completely fills the nanochannels of chrysotile asbestos fibers with a characteristic diameter of about $5\mathrm{nm}$. The macroscopical alignment of the nanochannels in fibers provided an interesting opportunity to study anisotropy of the dynamics of confined benzene by means of collecting the data with the scattering vector either parallel or perpendicular to the fibers axes. The translational diffusive motion of benzene molecules was found to be isotropic. While bulk benzene freezes at $278.5\mathrm{K}$, we observed the translational dynamics of the supercooled confined benzene on the time scale of hundreds of picoseconds even below $200\mathrm{K}$, until at about $160\mathrm{K}$ its dynamics becomes too slow for the $\mu \mathrm{eV}$ resolution of the neutron backscattering spectrometer. The residence time between jumps for the benzene molecules measured in the temperature range of $260\mathrm{K}\text{to}320\mathrm{K}$ demonstrated low activation energy of $2.8\mathrm{kJ}∕\mathrm{mol}$.

Figure 1

A schematic picture of a bundle of chrysotile asbestos fibers.

Figure 2

The elastic scattering intensity measured in the fixed-window mode as a function of temperature and summed up over all $Q$ values. Open circles: “$Q$-parallel” orientation. Filled circles: “$Q$-perpendicular” orientation. An arbitrary vertical offset of the baselines is applied for clarity. Inset: freezing of bulk benzene sample measured in the fixed-window mode shown for comparison.

Figure 3

The scattering intensities measured at $Q=1.37{\AA }^{-1}$. The energy range and the elastic peaks at zero energy transfer are truncated to better demonstrate the quasielastic signal. The $100\mathrm{K}$ data used as a resolution function are plotted on every graph and shown with squares. Open symbols: “$Q$-parallel” orientation. Filled symbols: “$Q$-perpendicular” orientation.

Figure 4

The fraction of the elastic scattering, $x\left(Q\right)$, obtained using Eq. (1). Open symbols: “$Q$-parallel” orientation. Filled symbols: “$Q$-perpendicular” orientation.

Figure 5

The HWHM of the Lorentzian quasielastic broadening obtained using Eq. (1) fitted with Eq. (2) for the “$Q$-parallel” orientation (open circles, solid lines) and with a $Q$-independent constant, $\hslash ∕{\tau }_{\mathrm{res}}$, for the “$Q$-perpendicular” orientation (filled circles, dashed lines).

Figure 6

The temperature dependence of the averaged residence time and its fit with Arrhenius law.