Intermolecular interaction between the pendant chain of perfluorinated ionomer and methanol

A comparison between water and methanol on the interaction with a pendant chain model for perfluorosulfonic ionomers (PFI), CF₃OCF ₂CF₂SO₃^-, was made by using molecular orbital calculation. Intermolecular interaction energy (E _int) of the most stable complex for CF₃OCF ₂CF₂SO₃^- + CH₃OH, where methanol associates with sulfonic acid group, is -10.38 kcal/mol at the MP2/aug-cc-pVDZ//B3LYP/6-31 + G* level, and it is almost the same with that of CF₃OCF₂CF₂SO₃^- + H₂O complex (-10.58 kcal/mol). Since an association of methanol to the sulfonic acid group is quite advantageous in energy, it is expected that, similarly to water, methanol would likely populate around the acidic site. On the contrary, according to a systematic E_int analysis for 500 random configurations, dissimilar distribution of E_int was observed for methanol compared with water. This is because methyl group substitution reduces oxygen surface area and causes more attractive dispersion energy with CF ₃OCF₂CF₂SO₃^- by about 1.0 kcal/mol on average compared with water. To see how the difference in the interaction energy affects the solvation structure of methanol and water to the PFI, molecular dynamics simulations of a CF₃OCF₂CF ₂SO₃^- molecule in methanol solutions have also been carried out at the methanol concentration of 10-90 mol%. Consequently, water probably associates with the sulfonic acid group-pushing methanol close to hydrophobic sites. It was also observed that methanol molecule tends to point its methyl group toward the solute at hydrophobic sites. These results demonstrated that methanol should locate in the vicinity of hydrophobic site compared with water due to methyl group substitution.