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 µm 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.

Orientational order in liquids upon condensation in nanochannels: An optical birefringence study on rodlike and disclike molecules in monolithic mesoporous silica

“…The general shape of these curves ͑i.e., the finite starting value of ⌬n, the maximum at f about 0.3, and the subsequent decrease to very low values at f approaching 1͒ is similar to that observed previously for hexane. 16 It is consistent with what is expected from a model describing aligned pores filled with an isotropic medium in an optically isotropic matrix. 21 Nevertheless there are slight differences ␦ between the ⌬n͑f͒ data of the optically anisotropic molecules and their isotropic counterparts, ␦ = ⌬n aniso − ⌬n iso .…”

“…Upon desorption a sizeable light scattering typical of a coarsening of the pore condensate upon desorption ͑see a discussion of this phenomenology in Ref. 16, and references therein͒ hampered polarimetry measurements. The observed retardation R has been converted into the optical bi-refringence ⌬n by means of the formalism described in detail in Refs.…”

Thermotropic nematic order upon nanocapillary filling

“…Due to the changes in the geometrical arrangement of the liquid upon gradual filling, the latter one changes considerably in the entire region of partial fillings. This holds even in the case of the adsorption of a liquid with isotropic building blocks, like neopentane, which we experimentally and theoretically demonstrated in two previous studies [26,31]. It is, however, practically T -independent for a fixed f .…”

“…The simple relation between optical birefringence and Q the bulk system is also conserved for the liquid crystal/matrix composite material, provided the geometrical birefringence is subtracted [26]. Due to the changes in the geometrical arrangement of the liquid upon gradual filling, the latter one changes considerably in the entire region of partial fillings.…”

“…Both regimes can be inferred by light adsorption experiments. Upon onset of capillary condensation or evaporation typically a drastic decrease in optical transmission, lg(I/I 0 ) is observable [25][26][27] (Here I 0 and I are the intensities of incident and transmitted laser light, respectively). The matrix turns milky and the reduction in the transmitted light originates in scattering at the capillary bridges/void structures, which have sizes on the length scale of the wavelength of visible light [28,29].…”