Molecular dynamics of viscous liquids. A comparison of dielectric and Kerr-effect relaxation for tritolyl phosphate, ortho-terphenyl and their mixtures

Abstract

Low frequency dielectric (10^–2 to 10⁵ Hz) and Kerr-effect (10^–5 to 10² s) relaxation behaviour of pure liquid tritolylphosphate, super-cooled o-terphenyl and three tritolylphosphate +o-terphenyl mixtures have been examined. The Kerr-effect rise and decay transients for each of these systems at a given temperature, are characterized by the same relaxation time τ_K,r and τ_K,d respectively. For both pure liquids τ_K,r and τ_K,d are essentially the same as the dielectric relaxation time τ_D, and the dielectric loss curves and Kerr-effect transients are adequately described by the William–Watts empirical relaxation function. These results suggest that reorientational motions in highly viscous liquids do not occur by rotational diffusion, but that they are entirely consistent with a “fluctuation relaxation” mechanism, and they are discussed in terms of the Ivanov and Anderson models of rotational Brownian motion. For some o-terphenyl + tritolylphosphate mixtures τ_K,r=τ_K,d > τ_D and possible explanations of this result are considered.