Diffuse X-ray Scattering and Monte-Carlo Study of Guest-Host Interactions in Urea Inclusion Compounds

Detailed diffuse X-ray scattering patterns have been recorded at ambient temperatures (295 K) and at low temperatures (~130-150 K) for two urea inclusion compounds, n-hexadecane-urea and 1,10-dibromo-n-decane-urea. The strong diffuse scattering that occurs in the Bragg layers normal to c for these compounds has been investigated using Monte Carlo simulation of a model system. Although the model is only two dimensional, the work convincingly demonstrates that the scattering is due to the orientational disorder of the alkanes within the channels of the urea framework, together with accompanying relaxations of the framework itself. The model shows that at temperatures just above the phase transition there are quite large domains of the herringbone superstructure that had previously been postulated as the low-temperature phase. At room temperature, the alkanes are much less ordered but show a significant amount of the same kind of order. There is also evidence in the observed diffraction patterns that the anisotropic shape of the molecule causes the urea framework to flex, expanding the framework in directions parallel to the plane of the alkane backbone and contracting it in the perpendicular direction.