Self-Diffusion, Spin-Lattice Relaxation, and Density of SF6 near the Critical Point

We have measured the self‐diffusion coefficient $D$ and the ¹⁹F spin–lattice relaxation time $T_1$ of SF₆ along the liquid–vapor coexistence curve and several isochores using nuclear magnetic resonance. The density was also measured in the same samples. The samples were purified and mass spectrographically analyzed to be pure to 0.01%. The density values are interpreted in terms of the critical index $\beta$ which is found to be 0.337 ± 0.012. The results of the self‐diffusion and relaxation time measurements show no anomalous behavior in the critical region. The spin–rotation interaction appears to be the predominant relaxation mechanism in the range of our measurements. From the measured values of $D$ and $T_1$ , both the linear momentum and angular momentum correlation times can be evaluated. The ratio of these times is a measure of the effectiveness of a collision in producing a change in the angular momentum of a molecule.