DIELECTRIC STUDY OF DIPOLE-DIPOLE INTERACTIONS IN ANISOTROPIC SOLUTIONS

This paper describes an experimental investigation of dipole-dipole in teractions between molecules in an orientationally ordered environment . Measurements are reported of the dielectric properties of a series o f anisotropic solutions of two structurally similar dipolar solutes an s-4-(trans-4-propylcyclohexyl)cyclohexyl]benzene (CP1) and ns-4-(trans -4-propylcyclohexyl)cyclohexyl]-benzene (CP3)) dissolved in a nonpolar nematic liquid crystal solvent thylcyclohexyl]-2-[4-ethyl-2-fluorobip henyl]ethane (I22)). The solvent provides an orienting medium for the solute molecules, and by varying the temperature the degree of orienta tional order of the solutions is changed. The measurements have been m ade on aligned thin films of the liquid crystalline solutions for diff erent concentrations and over a range of frequencies from 1 kHz to 10 MHz. All solutions exhibited a low-frequency relaxation associated wit h end-over-end reorientation of the polar solutes in the ordered envir onment. The results indicate that the dipolar interactions in CP1 solu tions are qualitatively different from those for CP3 solutions. In par ticular, the dielectric properties of CP1 solutions can be interpreted by assuming that solute molecules are locally ordered antiferroelectr ically, while for CP3 solutions the local order appears to be ferroele ctric. Analysis of both the low-frequency permittivities and the diele ctric loss supports the interpretation. The results can be fitted to a n association model which enables the proportion of parallel and antip arallel species to be determined.