Binary Diffusion Coefficients for Methane and Fluoromethanes in Nitrogen

Binary gas-phase diffusion coefficients, of interest in physical models of atmospheric and combustion chemistry, have been measured in ${\text{N}}_2$ for the homologous series of refrigerant-related (fluoro)methanes: methane (${\text{CH}}_4$), fluoromethane (${\text{CH}}_3\text{F}$), difluoromethane (${\text{CH}}_2{\text{F}}_2$), and trifluoromethane (${\text{CHF}}_3$). Values have been determined by reverse-flow gas chromatography, which has been previously demonstrated to provide accurate results over a wide range of temperatures. Coefficients were measured at temperatures of (300 to 550) K for all species and extending up to 650 K and 723 K for ${\text{CH}}_2{\text{F}}_2$ and ${\text{CHF}}_3$, respectively, and down to 250 K for ${\text{CH}}_4$. We also performed measurements for ${\text{CH}}_4$ in air at temperatures of (250 to 350) K, obtaining values the same as in ${\text{N}}_2$ within 0.3 %, well within our experimental uncertainty. We report the first measurements for ${\text{CH}}_3\text{F}$ and compare with the limited literature data for the other compounds. Our results agree broadly with earlier measurements in both ${\text{N}}_2$ and air. The greater temperature ranges reported in this work lead to temperature dependences that differ from most previous experiments, although they are consistent with several literature estimates and are similar to temperature exponents found for small hydrocarbons in ${\text{N}}_2$. Comparison of the present work with a recent study that found different diffusion coefficients for methane when determined in a typical arrested flow apparatus and a novel "twin tube" method unaffected by sample adsorption shows much better agreement with the the arrested flow results over all common temperatures.