Methane interactions with polyaniline/butylmethylimidazolium camphorsulfonate ionic liquid composite

In our previous report, a polyaniline (PAn) and ionic liquid butylmethylimidazolium camphorsulfonate (BMICS) composite was used as a methane sensing material which significantly increased the sensitivity for methane detection using quartz crystal microbalance (QCM) transducers. In this paper, we focus on the interactions within the PAn/BMICS composite and between the composite and methane. UV-vis and Fourier transform infrared (FTIR) spectroscopic results indicated that the anion of BMICS, camphorsulfonate, could form hydrogen bonds with the "nitrogen" sites of protic acid doped PAn. These hydrogen bonds align the camphorsulfonate anions in a comblike manner along the PAn backbone and therefore enhance the long-range π-orbital conjugation of PAn. Methane molecules absorbed into the PAn/BMICS may sit in the "space" between the aligned anions and cations of BMICS. By measuring the methane absorption in PAn/BMICS at a temperature range of 25-65 °C, the entropy and enthalpy of dissolution were obtained following the van't Hoff equation. They are -163.2 ± 30.1 J/mol · K and -50.5 ± 8.7 kj/mol, respectively, which are relatively higher than those in pure BMICS and in PAn only. These thermodynamic parameters further support that the absorbed methane molecules might exist in PAn/BMICS in a relatively ordered manner. Molecular mechanics simulation results agree with the spectroscopic and thermodynamic results.