Extending the study of mass transport across the gas-liquid interface by reversed-flow gas chromatography

The transfer of dichlorodifluoromethane to water was utilized as model system to provide better insight on the determination of mass transport parameters across the gas-liquid interface. Weak signals by reversed-flow gas chromatography were recorded at various temperatures, from 320.7 to 344.3 K, by digitizing and smoothing the output of the flame ionization detector. A flexible uncertainty analysis distinguished the main sources of error in the determined parameters, suggesting improvements on the utilized experimental setup. Liquid diffusivities decreased with rising temperature (4.39 to 1.01 × 10^-10 m² s^-1), approaching literature values at lower temperatures. The estimated liquid film thicknesses similarly decreased (1.8 to 1.0 × 10^-4 m) and were one order of magnitude larger than previous findings, due to the larger extent of evaporation permitted by the current experimental setup. A mass transfer coefficient was estimated, corresponding to the endothermic contribution of the reverse (liquid-to-gas) process, whose activation energy (43.4 ± 2.8 kJ mol^-1) matched the vaporization enthalpy of water in the studied temperature range. Successful comparisons were made with literature distribution coefficients and Henry's law constants. The dissolution of CFC-12 in water was found to be exothermic, slightly spontaneous at lower temperatures and approaching equilibrium at higher ones (indicated by the small negative molar Gibbs free energy values), with negative entropy change values [average: -(190.7 ± 6.9) J K^-1 mol^-1], as expected for a process of increased order.