The nematic-isotropic transition at high pressures II : turbidity measurements

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Abstract
Turbidity measurements at high pressures have been performed in the isotropic phases of MBBA, EBBA, and CBNA. In all cases an increase of the value of Tc — T* with pressure is observed. However, rather unusual behavior is found for the magnitude of the turbidity at the isotropic-nematic transition : in MBBA this magnitude decreases with increasing pressure, in the homologous neighbor EBBA it increases, and in CBNA it increases and then decreases. In conjunction with estimates of the volume discontinuity obtained from our previously reported PVT data, these turbidity data are used to calculate the values of the transition parameters, including in particular the order parameter Qc. Comparison of these results with previous experimental data indicates that the validity of the mean field theory, which we have used to calculate these parameters, must be seriously questioned. Finally, the data show that the value of Tc — T* is somewhat larger along an isochoric trace than along the more conventional isobaric trace