Self-Diffusion and Impurity-Controlled Proton Relaxation in Liquid Ethane

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Abstract
Self‐diffusion in liquid ethane, under its own vapor pressure, has been studied over most of the liquid range between the triple point and the critical point by means of the spin‐echo technique. The activation energy for self‐diffusion increases markedly from approximately 0.5 kcal/mole at 100°K to 3.5 kcal/mole at 300°K. The Stokes—Einstein relation is not well satisfied with either a constant molecular radius or a radius proportional to the cube root of the molar volume. The results are well correlated by a free‐volume expression in which the free volume, defined as an actual volume minus the volume of the hypothetical liquid at 0°K, replaces temperature as the independent variable. The value of the hypothetical 0°K liquid volume resulting in best fit of the experimental data is less than that determined by extrapolation of the liquid specific volume to 0°K or that based upon best fit of experimental liquidviscosity data, but it is between a value based on a hard‐sphere radius determined from gas viscosity and one derived from the second virial coefficient. Proton spin‐lattice relaxation measurements on the liquid ethane sample over a similar temperature range indicate that the relaxation is controlled by mutual diffusion of ethane and small amounts of dissolved oxygen, although the situation is somewhat complicated by temperature dependence of the distribution of oxygen between the two phases.