Diffusion modes of an equimolar methane-ethane mixture from dynamic light scattering

The hydrodynamic diffusion modes of an equimolar methane ethane mixture have been investigated by dynamic light scattering. Measurements were performed over a wide temperature range between the plait critical point at 263.55 K and 310 K along the critical isochore. Two relaxation modes have been observed which are commonly associated with pure mass diffusion and pun thermal diffusion, but ill near-critical binary fluid mixtures-according to recent theory may alternatively be interpreted as two effective diffusivities resulting From a coupling between mass and thermal diffusion. Diffusivity values for the slow mode were obtained with typical standard deviations of 1% over the whole temperature range, whereas the low amplitude of the fast mode only allowed values of this component with a large measurement uncertainty. The results are discussed in connection with literature data available for the thermophysical properties of this binary fluid mixture and regarding the various possibilities of theoretical interpretation.