7 research outputs found
Preferred orientation of n-hexane crystallized in silicon nanochannels: A combined x-ray diffraction and sorption isotherm study
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 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
Liquid n-hexane condensed in silica nanochannels: A combined optical birefringence and vapor sorption isotherm study
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.Comment: 7 pages, 5 figure