S. Urata, J. Irisawa, A. Takada, S. Tsuzuki, W. Shinoda, and M. Mikami
J. Fluorine Chem. 126, 1312-1320 (2005).
A comparison between water and methanol on the interaction with a pendant chain model for perfluorosulfonic ionomers (PFI), CF3OCF2CF2SO3-, was made by using molecular orbital calculation. Intermolecular interaction energy (Eint) of the most stable complex for CF3OCF2CF2SO3- + 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 Eint analysis for five hundred random configurations, dissimilar distribution of Eint was observed for methanol compared with water. This is because methyl group substitution reduces oxygen surface area and causes more attractive dispersion energy with CF3OCF2CF2SO3- 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 CF3OCF2CF2SO3- molecule in methanol solutions have also been carried out at the methanol concentration of 10 to 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.
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