Absorbance enhancement in the infrared spectra of CH₄/CO₂ gas mixtures: quantitative analysis of binary gas diffusion

Pressure broadening is commonly observed in the infrared spectrum of gases in the presence of a foreign species. This phenomenon produces a deviation from the Bouguer-Beer-Lambert (BBL) law preventing quantitative analysis. The aim of the present study is to measure its effect over the IR spectra of CH₄/CO₂ binary gas mixtures at ambient temperature up to 10 bar in the mid and near-infrared frequency ranges and use the enhanced IR signals to quantify the gas pair interdiffusion. The composition is accurately tuned with calibrated thermal mass flow controllers. The infrared absorbance of methane in the range [3025, 3300] cm^-1 and of carbon dioxide at 3626 cm^-1 is correlated with the molar concentration of each component to elucidate the effect of pressure and composition. The data are described with polynomial functions, and the coefficients' dependence on the gas composition is unveiled. We consider that collision-induced absorption and local field effects produce the absorbance enhancement and show how this phenomenon can be exploited in the analytical technique. The binary gas diffusion of CH₄/CO₂ at ~1 bar and 298.45 or 308.15 K is successfully investigated with the same IR signals within a closed system: for methane, the absorbance is appropriately corrected accounting for equimolar counter diffusion as a constraint; for carbon dioxide, the absorbance varies linearly with the concentration so that no correction is needed. Homonuclear foreign species transparent to IR radiation can also be indirectly analyzed if the broadening effect is appropriately described, as reported in this article.