Molecular simulation study of Nafion membrane solvation in water and methanol

Molecular mechanics and molecular dynamics simulation studies of conformation and solvation of perfluorosulfate oligomers, representing fragments of Nafion membrane, have been performed. Two typical conformations of the oligomer, composed of 10 monomer units, have been found in a vacuum. A stretched geometry of the fluorocarbon skeleton with tortuosity of 2.4 was reached by energy optimization starting from the regular conformation with all CCCC angles in the trans position. A highly folded spiral-like configuration was obtained when the randomly bent chain was taken as the initial configuration. Molecular dynamics simulations of shorter oligomers solvated in water and methanol revealed a noticeable difference between the geometries of the fluorocarbon skeleton in different solvents. The skeleton structure in water was substantially more folded than in methanol. The side chain of the Nafion oligomers was found to be quite stiff; only a few conformational transitions in the side chain were detected. Both water and methanol formed stable hydrogen bonds with the oxygens of SO3- group with the hydrogen bond lifetimes several times longer than rotational correlation times of individual solute molecules in the bulk. The other parts of the side chain were found to be hydrophobic, including ester oxygens. On average, each SO3- group formed ca. five hydrogen bonds to water and four bonds to methanol. The spatial distribution of solvent molecules bonded to the SO3- group was characterized by a pronounced anisotropic first solvation shell.