Self-diffusion and the isotope mass effect in single crystalline benzene

Abstract

A study has been made of the isotope mass effect in self-diffusion in single crystals of benzene as a function of temperature in the range 240 to 274 K. The tracers were ¹⁴C labelled normal benzene and ³H labelled deutero-benzene. The self-diffusion coefficients are represented by D=D₀ exp –E/RT where D₀= 1.43 × 10⁵ m² s^–1 and E= 96.7±1.5 kJ mol^–1 for the ¹⁴C species and D₀= 1.53 × 10⁵ m² s^–1 and E= 96.9±2.5 kJ mol^–1 for the ³H species.

A direct comparison of the rates of diffusion of the two labelled species yields a value for the mass factor E^a_b=fΔK= 0.78±0.03. Agreement between this factor and the probable value for the correlation factor for vacancy self-diffusion, f= 0.78, suggests that self-diffusion proceeds via a vacancy mechanism.