Search for isotope effects in the diffusion of methane in krypton at various densities

Experimental measurements of the diffusion coefficient of C${\mathrm{H}}_4$ in Kr at $T=220\mathrm{}°$K in the density range $0<\mathrm{}\rho \le 2{\rho }_c$ are reported. The experimental data are compared with self-diffusion for Kr and C${\mathrm{H}}_4$ and with the data for diffusion of Kr in C${\mathrm{H}}_4$ at about the same reduced temperature. The behavior of the mass effect on the diffusion process is shown clearly for the first time. At $\rho \le {\rho }_c$ the deviation from Enskog's predictions is already marked, whereas at $\rho \ge 1.5{\rho }_c$ the mass effect is of the same kind as for hard spheres.