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Experimental results for the temperature dependence of the density of propyl -cyanobiphenyl (3CB), butylcyanobiphenyl (4CB) and hexyl -cyanobiphenyl (6CB) are presented. The results are compared with previous results for temperature dependence of other members of the alkyl cyanobiphenyl series (nCB). The deviation of the density from linear temperature dependence is discussed in terms of the recently discovered corresponding rule for nematic liquid crystals.

Composites were prepared from an aerosil and 4-n-alkyl-4'cyanobiphenyls with five to eight carbon atoms in the alkyl chain. Their high silica density of y7 g aerosil in 1 cm 3 of liquid crystal (LC) allows the observation of the behaviour of a thin cyanobiphenyl layer (having nearly a monolayer structure) on the silica particles. The systems are investigated by dielectric spectroscopy (10 22 -10 9 Hz) in a large temperature range (220-370 K). All the composites show a (main) relaxation process at frequencies much lower than the processes observed for the bulk LC that was assigned to the dynamics of the molecules in the surface layer. The temperature dependence of its characteristic frequencies obeys the Vogel-FulcherTammann law, which is found to be typical for glass-forming liquids. The quasi twodimensional character of the glass transition in the surface layer is discussed for the first time. At the nematic-to-isotropic transition temperature of the bulk, the composites show a continuous decrease of the characteristic frequencies as a function of the alkyl chain length, while the bulk LCs show the well known odd-even behaviour. The magnitude and temperature dependence of the slow relaxation process in the composites (molecules on an outer surface) agree with those of the same molecules confined to the nanopores of molecular sieves (internal surface).

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