Studies on binary mixtures of systems which exhibit a maximum as well as a minimum in the A-N transition boundary

Abstract

We have studied the binary phase diagrams of (I) p'-nitrophenyl-p-n-octyloxybenzoate with (2-hydroxy)-p-ethoxybenzylidene-p'-butylaniline, and (II) p'-n-octyloxy-p-cyanobiphenyl with p-butoxyphenyl-(p'-pentyloxy)benzoate. Both of them show a strong maximum in the A-N boundary, indicative of an induced A phase, and further, a well defined minimum for a composition rich in the highly polar component. The interactions responsible for the induced phase are much stronger for the system II than for the system I. We have investigated the temperature variations of the layer spacings for various compositions of both the systems. In system I, there is a considerable thermal expansion of the spacing, the expansion coefficient attaining a maximum value for the composition corresponding to the minimum in the A-N boundary. We have given a physical interpretation of this observation. The dielectric anisotropies of various compositions of system I have also been investigated and their temperature dependences interpreted in terms of the physical model mentioned above. The conductivity anisotropy changes sign for all compositions exhibiting the A phase. The temperature of crossover in relation to T_AN is the highest for the mixture which exhibits the minimum value of T_AN, since the A-N transition is almost of a second order character for this composition.