Dielectric studies of molecular interactions in benzene solutions of 4-heptyl-4′-nitro-biphenyl

A study was made of the molecular association of 4-heptyl-4′-nitro-biphenyl (7NB4) in benzene, in the temperature range from 288 K to 323 K. With this aim both static linear and nonlinear dielectric properties were investigated. The experiments were carried out in the whole, solubility constrained, range of solubilities. The interpretation was only given for moderately diluted solutions, up to 0.3 molar fraction concentration. From the experimental values of both linear and nonlinear polarization the energy of intermolecular interactions was determined. In the adopted concentration range, a good agreement with the experiment was achieved when only binary interactions of 7NB4 molecules were assumed. Within experimental error, the obtained dipolar moment of the aggregates is zero meaning the antiparallel orientation of the molecules in the dimer. The most important and unexpected conclusions from these studies can be made when comparing the results with those of a similar study on the isomorphous compound 4-heptyl-4′-cyano-biphenyl (7CB4) which, in contrary to 7NB4, is mesogeneous. In pure 7CB4 one does not only have the nematic phase, but self-association in the dilute solution is much stronger, involving at least tetramers as well as the characteristic equilibrium constants between weakly and strongly polar multimers, which do not occur in 7NB4. Properties of the two compounds, and of the non-mesogeneous isomer 4-heptyl-3′-cyano-biphenyl (7CB3) are compared and discussed.