Preferred orientation of n-hexane crystallized in silicon nanochannels: A combined x-ray diffraction and sorption isotherm study

TL;DR

This study investigates how n-hexane crystallizes within silicon nanochannels, revealing a preferred orientation aligned with the channel axis that intensifies with increased filling, driven by a Bridgman-type crystallization process.

Contribution

It demonstrates the influence of spatial confinement and filling fraction on the preferred crystallite orientation of n-hexane in nanochannels, linking crystallization dynamics to channel filling.

Findings

Crystallizes in triclinic phase similar to bulk n-hexane.

Preferred orientation aligns with the channel axis, increasing with filling fraction.

Nearly perfect texture observed at full channel filling.

Abstract

We present an x-ray diffraction study on n-hexane in tubular silicon channels of approximately 10 nm diameter both as a function of the filling fraction f of the channels and as a function of temperature. Upon cooling, confined n-hexane crystallizes in a triclinic phase typical of the bulk crystalline state. However, the anisotropic spatial confinement leads to a preferred orientation of the confined crystallites, where the <001> crystallographic direction coincides with the long axis of the channels. The magnitude of this preferred orientation increases with the filling fraction, which corroborates the assumption of a Bridgman-type crystallization process being responsible for the peculiar crystalline texture. This growth process predicts for a channel-like confinement an alignment of the fastest crystallization direction parallel to the long channel axis. It is expected to be increasingly effective with the length of solidifying liquid parcels and thus with increasing f. In fact, the fastest solidification front is expected to sweep over the full silicon nanochannel for f=1, in agreement with our observation of a practically perfect texture for entirely filled nanochannels.