Infrared absorption by molecular gases as a probe of nanoporous silica xerogel and molecule-surface collisions: Low-pressure results

Abstract

Transmission spectra of gases confined (but not adsorbed) within the pores of a 1.4-cm-thick silica xerogel sample have been recorded between 2.5 and 5 \ensuremath{\mu}m using a high-resolution Fourier transform spectrometer. This was done for pure CO, CO${}_{2}$, N${}_{2}$O, H${}_{2}$O, and CH${}_{4}$ at room temperature and pressures of a few hectopascals. Least-squares fits of measured absorption lines provide the optical-path lengths within the confined (${L}^{C}$) and free (${L}^{F}$) gas inside the absorption cell and the half width at half maximum ${\ensuremath{\Gamma}}^{C}$ of the lines of the confined gases. The values of ${L}^{C}$ and ${L}^{F}$ retrieved using numerous transitions of all studied species are very consistent. Furthermore, ${L}^{C}$ is in satisfactory agreement with values obtained from independent measurements, thus showing that reliable information on the open porosity volume can be retrieved from an optical experiment. The values of ${\ensuremath{\Gamma}}^{C}$, here resulting from collisions of the molecules with the inner surfaces of the xerogel pores, are practically independent of the line for each gas and inversely proportional to the square root of the probed-molecule molar mass. This is a strong indication that, for the studied transitions, a single collision of a molecule with a pore surface is sufficient to change its rotational state. A previously proposed simple model, used for the prediction of the line shape, leads to satisfactory agreement with the observations. It also enables a determination of the average pore size, bringing information complementary to that obtained from nitrogen adsorption porosimetry.