Separation of nematic and smectic-A potential effects on solute ordering in a series of binary 6OCB-8OCB mixtures

(1)H NMR and optical microscopy have been used to study four solutes dissolved in samples of a binary mixture of 4-n-hexyloxy-4'-cyanobiphenyl (6OCB) and 4-n-octyloxy-4'-cyanobiphenyl (8OCB) that are close to the region of the phase diagram where the smectic-A (SmA) and re-entrant nematic (RN) phases exist. Optical microscopy clearly indicates that one of the four studied samples shows the phase sequence of isotropic-nematic-SmA-RN. The derived solute order parameters were interpreted by means of two Maier-Saupe mechanisms in conjunction with the Kobayashi-McMillan theory in the case of the SmA phase. The novel feature of this study is that the nematic potential is extrapolated from the nematic to the SmA phase based on concentrations of the 6OCB-8OCB mixtures. The derived smectic order parameters for each of the studied solutes clearly show a maximum absolute magnitude near the centre of the SmA temperature range. The different partitioning of these solutes in the binary mixture is also discussed.