Liquid n-hexane condensed in silica nanochannels: A combined optical birefringence and vapor sorption isotherm study

TL;DR

This study investigates the optical birefringence of liquid n-hexane in silica nanochannels to understand molecular orientation, finding no collective orientational order during condensation, contrasting with behavior upon freezing.

Contribution

It demonstrates that optical birefringence measurements are insensitive to the detailed arrangement of adsorbed liquids and shows that n-hexane remains disordered during condensation in nanochannels.

Findings

No optical anisotropies detected in liquid n-hexane during condensation.

Molecules remain orientationally disordered in all condensation regimes.

Freezing induces perpendicular molecular alignment, not pre-existing in the liquid phase.

Abstract

The optical birefringence of liquid n-hexane condensed in an array of parallel silica channels of 7nm diameter and 400 micrometer length is studied as a function of filling of the channels via the vapor phase. By an analysis with the generalized Bruggeman effective medium equation we demonstrate that such measurements are insensitive to the detailed geometrical (positional) arrangement of the adsorbed liquid inside the channels. However, this technique is particularly suitable to search for any optical anisotropies and thus collective orientational order as a function of channel filling. Nevertheless, no hints for such anisotropies are found in liquid n-hexane. The n-hexane molecules in the silica nanochannels are totally orientationally disordered in all condensation regimes, in particular in the film growth as well as in the the capillary condensed regime. Thus, the peculiar molecular arrangement found upon freezing of liquid n-hexane in nanochannel-confinement, where the molecules are collectively aligned perpendicularly to the channels' long axes, does not originate in any pre-alignment effects in the nanoconfined liquid due to capillary nematization.