Experimental study reveals self-diffusion mechanisms across liquid noble gases and methane, indicating that momentum autocorrelations dominate temperature-dependent transport.
Key Points
To measure the self-diffusion coefficients of liquid argon, krypton, xenon, and methane across varying temperatures and pressures and evaluate current theories of liquid diffusion.
Measured self-diffusion coefficients for liquid Ar, Kr, Xe, and CH4 as functions of temperature and pressure.
Evaluated the empirical data against corresponding states relations, free volume theory, a fluid continuum model incorporating momentum autocorrelation, and the dense square-well fluid model.
Ar, Kr, and Xe adhere to a single corresponding states equation, log D̃ = 0.05 + 0.07p̃ − (1/T̃)(1.04 + 0.1p̃), whereas CH4 deviates significantly due to its distinct intermolecular repulsion steepness.
The activation volume for CH4 is one-third of its molar volume, compared to one molar volume or greater for Ar, Kr, and Xe; free volume theory proved inadequate to describe these experimental results.
A fluid continuum model accounting for negative portions of the momentum autocorrelation function accurately matches experimental diffusion and temperature derivatives, whereas the dense square-well fluid model predicts temperature dependence quantitatively but exhibits an error of about 30% in absolute diffusion values.