Abstract: Room-temperature ionic liquids are under increasing scrutiny as potential electrolytes for lithium-ion batteries. Static structure factors, S(q), determined in X-ray diffraction experiments or calculated from molecular dynamics (MD) simulations of ionic liquids, show low-wavenumber peaks attributed to “charge alternation” and “charge adjacency.” For pyrrolidinium- and imidazolium-based ionic liquids, the charge alternation peak increases in height as temperature increases. The increased peak intensity might seem to indicate increasing nanoscale ordering with increasing temperature. We present MD simulations for a representative ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide, at different temperatures. Using a new interactive plug-in for visual molecular dynamics, viewSq, we calculate S(q), quantify atomic and group contributions to the peaks in S(q), and explain why peak heights increase with increasing temperature despite decreasing nanoscale ordering. This work argues for great caution when interpreting S(q) intensities and highlights the value of simulations as a complement to X-ray or neutron scattering experiments.
Bio: Ralph Wheeler is chair of the Department of Chemistry and Biochemistry at Northern Illinois University. He received a B.S. degree in chemistry from Harvey Mudd College and a Ph.D. in computational chemistry from Cornell University. His current research spans computational materials chemistry and biochemistry using molecular dynamics, quantum chemical, and QM/MM methods, with an emphasis on new tools to interpret computer simulations.