Investigation of the relationship between surface thermodynamics of the chemically synthesized polypyrrole films and their gas-sensing responses to BTEX compounds

Cl−-doped polypyrrole (PPyCl) films as sensing materials were prepared chemically using FeCl3 as an oxidant and were used to detect benzene, toluene, ethylbenzene, and xylene (BTEX) compounds in their vapor phases, which were found to be able to enhance the doping level of PPyCl and hence increased the conductivity of PPyCl upon exposure to them. Three isomers of xylene, i.e. ortho (o), meta (m), and para (p), were also investigated for comparison. The sensitivities of PPyCl sensor exposed to BTEX compounds with various concentrations were measured and found to lie in the range of 0.08 mΩ/ppm (benzene)–0.8 mΩ/ppm (o-xylene). An adsorption model based on the Langmuir isotherm correlated well with the experimental results and was used to interpret the sensing behavior of PPyCl sensor towards BTEX vapors. The adsorption equilibrium constants, Km, were therefore able to be determine according to this model. A further study of affinity between the interested BTEX compound and the PPyCl surface was carried out by a method of inverse gas chromatography (IGC) with PPyCl-coated glass beads packed in a column as the stationary phase and the interested BTEX compound as the mobile phase. The retention volumes for these compounds were measured to be in the following order: o-xylene>m-xylene>p-xylene≅ethylbenzene>toluene. This magnitude sequence was correlated well with that of the equilibrium constants Km. The free energy of adsorption ΔGa, consisting of dispersive term ΔGad and the term of acid–base interaction ΔGas, was also determined according to the results obtained from IGC and was used to interpret the sensing response. The interaction forces were analyzed in details by a method of linear solvation energy relationships (LSERs), by which the surface of the PPyCl was found to exhibit a basic nature.