Isotope effects in liquid diffusion

Abstract

The self-diffusion coefficient of benzene has been measured using benzene-1 14C and benzene-1,2 14C tracers. These data are compared with existing data of Birkett and Lyons for the limiting diffusion coefficient of benzene-2H6 in benzene. This comparison shows that the limiting diffusion coefficient of a tracer species is dependent upon the isotopic composition of the tracer. The isotope effect study suggested that a general mass dependence for limiting diffusion data might be found. Limiting diffusion data for 24 homologous series in both polar and non-polar solvents were correlated with the empirical relation D = a +bM^(-1/2) V-bar^(-1/3) (Equation 1) For non-associating solutes in unassociated solvents the value of a was generally small compared to the second term. The value of b was found to depend upon M^(1/2) solvent, V-bar^(1/3) solvent, eta solvent. Equation 1 may be rewritten in the form D= a +b' (M_s^(1/2)V-bar_s^(1/3))/(eta_s M(1/2) V-bar^(1/3)) where the subscript s denotes a solvent property; b' assumes a value of 1 for non-cyclic solvents and a value of 1.5 for cyclic solvent. Associated solvent systems are characterized by higher values of b'. Solute association is characterized by lower values of b' while solvent-solute association leads to high values of a and low values of b'. These generalizations have been used to interpret diffusion data in aqueous systems.