Cooperative motion in some simple hydrocarbons

Abstract

Dielectric-constant and loss data have been obtained for fourteen simple hydrocarbons, mostly alkylbenzenes, in the frequency range 10–10⁵ Hz between room temperature and liquid-nitrogen temperature. Most of the compounds showed asymmetric loss curves at temperatures below their melting point. The absorptions have the characteristics of cooperative relaxation, and for several compounds this α-process was observed at temperatures just above the glass-transition temperature, T_g. Unlike polar c-decalin, the non-polar t-decalin does not yield an α-process detectable by dielectric-absorption studies. Addition of chlorobenzene to t-decalin reveals an α-process where the polar solute acts as a probe to reveal the cooperative motion of the dispersion medium. This was also observed for chlorobenzene in the non-polar cyclohexane. Even small concentrations of polar impurity may lead to the detection of an α-process in glass-forming liquids. The limiting ΔH_E values for the α-process of various hydrocarbons reflect the capacity of the individual molecules to interact at room temperature.