Molecular dynamics simulations and vibrational spectroscopic studies of local structure in tetraglyme:sodium triflate (CH3O(CH2CH2O)4CH3:NaCF3SO3) solutions

An oligomeric model for poly(ethylene oxide), tetraglyme, with NaCF3SO3 at an ether oxygen:Na+ ratio of 10:1 was used to represent the amorphous phase of PEO:salt systems in molecular dynamics simulations at 300 K and 400 K. Na+-tetraglyme interactions were examined by calculating chain dimensions, dihedral angle population distribution, and conformational triad populations of coordinating tetraglyme chains, All results consistently show that the Na+-ether oxygen coordination induces more compact chains by enforcing a gauche conformation for C-C bonds and introducing a strong preference for tgt conformations in the C-O-C-C-O-C bond sequence. Na-O(tetraglyme) and Na-O(triflate) radial distribution functions reveal that triflate ions contribute more oxygens (4.9 at 300 K and 5.3 at 400 K) than tetraglyme (2.2 at 300 K and 2.0 at 400 K) to the first coordination shell of Na+. Populations of aggregates consisting of a triflate ion coordinated to 0-3 Na+ were available from vibrational spectra and were also calculated from equilibrated trajectories. Both results agree well, and the computational results indicate that Na-CF(3)So(3)(-) association increases with increasing temperature. The results are compared with results from a previously studied tetraglyme:LiCF3SO3 system. Unlike Li+ in the tetraglyme:LiCF3SO3 system, bidentate coordination of Na+ by CF3SO3- becomes more favorable at the higher temperature, at the expense of monodentate and tridentate coordination.