Self-diffusion and viscosity of methylcyclohexane in the dense liquid region

Abstract

The self-diffusion coefficients of liquid methylcyclohexane have been measured as a function of temperature and pressure over the density range 2.5⩽ρ/ρc⩽3.3 and temperature range 0.35⩽T/Tc⩽0.5 using the NMR spin echo technique with a fixed field gradient. Shear viscosities and densities have also been determined under the same experimental conditions. An approximate analysis of the experimental diffusion and viscosity data in terms of the rough sphere model of liquids yields effective hard core diameters and a parameter A that reflects the degree of coupling between the rotational and translational motion of methylcylohexane molecules. The strong temperature dependence of this coupling arises most likely from the nonspherical shape of the methylcyclohexane molecule. The behavior of the rotational–translational coupling and its possible connection to melting properties and to the experimental finding that methylcyclohexane does not form an orientationally disordered crystalline phase is discussed in a qualitative way.