  We present the results of a Molecular Dynamics computer simulations of a two
component isotope mixture of Lennard-Jones particles, monodisperse in size but
different in masses, at a fixed average density and temperature. We study
changes in properties that result from mass heterogeneity, by measuring the
pair distribution function, diffusion coefficient, velocity autocorrelation
function, non-Gaussian and isotope effect parameters, as functions of the
degree of mass difference. Our results show that if static properties are not
influenced by a variation of mass variation, the dynamic properties are
significantly affected and even exhibit the presence of ``critical'' values in
the mass difference. We also demonstrate that our model gives a simple contra
example to a recently proposed a universal scaling law for atomic diffusion in
condensed matter [M. Dzugudov, Nature, v.381, 137 (1996)].
