Time Evolution of Density Fluctuation in the Supercritical Region. 2. Comparison of Hydrogen- and Non-hydrogen-Bonded Fluids

The time evolution of the density fluctuation of molecules is investigated by dynamic light scattering in six
neat fluids in supercritical states. This study is the first to compare the dynamics of density inhomogeneity
between hydrogen- and non-hydrogen-bonded fluids. Supercritical methanol and ethanol are used as hydrogen-bonded fluids, whereas four non-hydrogen-bonded fluids were used:  CHF3, C2H4, CO2, and Xe. We measure
the time correlation function of the density fluctuation of each fluid at the same reduced temperatures and
densities and investigate the relationship between the dynamic and static density inhomogeneities of those
supercritical fluids. In all cases, the profile of the time correlation function of the density fluctuation is
characterized by a single-exponential function, whose decay is responsible for the dynamics characterized by
hydrodynamic conditions. We obtain correlation times from the time correlation function and discuss dynamic
and static inhomogeneity using the Kawasaki theory and the Landau-Placzek theory. While the correlation
times in the six fluids show noncoincidence, those values agree well with each other except for the supercritical
alcohols when scaled to a dimensionless parameter. Although the principle of corresponding state is observed
in the non-hydrogen-bonded fluids, both the supercritical methanol and ethanol deviate from that principle.
This deviation is attributed to the presence of hydrogen bonding among alcohol molecules at high temperature
and low density. The average cluster size of each fluid is estimated under the same thermodynamic conditions,
and it is shown that the clusters of supercritical alcohols are on average 1.5−1.7 times larger than those of
the four non-hydrogen-bonded fluids. Moreover, the thermal diffusivity of each neat fluid is obtained over
wide ranges of density and temperature.