Some unusual features in the behaviour of cyclohexane in confined geometry studied by neutron scattering

Cyclohexane, C6H12, is one of the simplest organic molecules due to its sixfold symmetry and weak van der Waals interaction. The dynamic characteristics have been extensively studied in the liquid and plastic crystal phases by spectroscopic means and have been analysed in terms of an unhindered rotator model. The transition from the liquid to the plastic crystal at 6.55oC involves a low entropy change and leads to a fcc crystal with a large defect concentration. At lower temperatures, -87oC, the rotational motion is frozen out to give a brittle crystal phase.

One of the more interesting aspects of cyclohexane is revealed when the material is confined in a meso-porous solid. Neutron and NMR studies of cyclohexane in a porous sol-gel silica [1] showed that the phase transition temperatures were substantially depressed. Further neutron diffraction studies [2] indicated that the plastic crystal phase exhibited structural changes over a wide temperature range and that there was a hysteresis effect in the temperature sequence. The large depression of the nucleation point was shown to be inversely proportional to pore size over most of the range and subsequently used for pore size characterisation by NMR cryoporometry [3]. Comparison with other organic liquids [4] has shown that cyclohexane exhibits one of the largest effects and this behaviour has lead to a new interest in the properties.

Another unexpected phenomenon has been observed when cyclohexane liquid is added to the top of a granular powder of porous sol-gel silica in a sealed container. After a few minutes, an orange coloration develops in transmitted light (blue in reflected), which persists for up to a day. This behaviour indicates that there are spatial density variations on a scale comparable with that of light. Some test experiments using small-angle neutron scattering [5] showed that these features were changing on a time-scale of minutes; the effect could be observed in C6D12 but not in C6H12, confirming that the effect was due to the liquid behaviour in the pores. As a consequence of these recent studies, it is now interesting to make a more detailed study of the phase behaviour of cyclohexane in confined geometry and the following sections outline some of the research which is currently in progress.