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

2014 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 2014 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 2014 T* is somewhat larger along an isochoric trace than along the more conventional isobaric trace. J. Physique 41 (1980) 633-638 JUILLET 1980, Classification Physics Abstracts 71.30 In a previous paper [1] hereafter, referred to as I, we reported on the P-V-T equation of state in the vicinity of the isotropic-nematic transition. It is well known that this transition displays characteristics of both first and second-order phase changes. In I we were chiefly concerned with the first-order character of the isotropic-nematic transition. In this paper we shall present measurements of one of the secondorder aspects of this transition, namely the anomalous pretransitional increase of the turbidity above the clearing temperature Tc. These data yield the effective second-order transition temperature or spinodal temperature T*. The difference Tc T * provides a measure of the first-order component of the transition. It is of special interest to examine how the difference (*) Present address : Xerox Corporation, Webster, NY 14586, U.S.A. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01980004107063300