Thermodynamic Effect on Viscosity and Density of a Mixture of 4-Cyano-4′-pentylbiphenyl (5CB) with Dilute Dimethyl Phthalate (DMP)

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Density ρ and viscosity η were examined for 4-cyano-4′-pentylbiphenyl (5CB) and its mixture with dilute dimethyl phthalate (DMP; w_DMP = 3.1 wt%). The mixture was in an isotropic 1-phase state at temperatures T > T_IN* (≅ 27 ºC), and exhibited the phase separation into the isotropic and nematic phases at T_IN*. Pure 5CB exhibited the (well known) first-order isotropic-to-nematic transition characterized by discontinuous changes of ρ and η at T_IN (≅ 35.0 ºC), whereas no discontinuous changes were observed for the mixture at T_IN* because only a small amount of the nematic phase was created at T_IN*. The discontinuity was expected for the local density and viscosity in the nematic phase at T_IN* and, in this sense, no essential difference was noted for that phase in the 5CB and DMP/5CB systems. Nevertheless, a characteristic difference was found in the isotropic 1-phase state. These systems commonly exhibited simple asymptotic behavior at sufficiently high T, linear thermal expansion of the specific volume 1/ρ and Eyring/Andrade-type T dependence of the kinematic viscosity, ν = η/ρ. Positive deviation from these asymptotes was noted for both systems on cooling toward respective transition temperatures, T_IN and T_IN*, but in a considerably wide range of T (up to T_IN*+10K or more) for the DMP/5CB mixture whereas only in a close vicinity of T_IN (within a few K) for pure 5CB. This difference is related to the concentration fluctuation (due to the soft, osmotic interaction of molecules) emerging only in the mixture.