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

Abstract

The optical birefringence of liquid n-hexane condensed in an array of parallel silica channels of 7 nm diameter and $400\text{ }\ensuremath{\mu}\text{m}$ 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 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 prealignment effects in the nanoconfined liquid due to capillary nematization.