We report a molecular modeling study to evaluate and select conducting polymers (CPs) as the sensing materials of carbon dioxide (CO2) sensor. The interaction between polymer and gas and the adsorption of the gas molecules in the polymer matrix are investigated. Polymers considered for this work include emeraldine base polyaniline (EB–PANI) and unprotonated sodium polyaniline salt (NaSPANI) with sulfur to nitrogen ratio (S/N) of 0.4, 0.5 and 0.6. Gases studied include CO2, humidity (H2O) and carbonic acid (H2CO3). The interaction energies of polymer-gas and the loading number of the analytes predict that the NaSPAN is more suitable for CO2 sensing than the EB–PANI.