A paper by Hongjun Liu (Hongjun Liu and Edward J. Maginn, “Effect of Ion Structure on Conductivity in Lithium-Doped Ionic Liquid Electrolytes: A Molecular Dynamics Study”, Journal of Chemical Physics, 2013, 139, 114508 http://jcp.aip.org/resource/1/jcpsa6/v139/i11/p114508_s1) was recently published. In this work, it is shown that ionic liquid electrolytes having planar anions that weakly coordinate with Li+ ions can lead to enhanced lithium conductivities relative to that observed in ionic liquids with more conventional anions such as bistrifylimide. This is due to the way in which Li+ organizes with respect to the anions in repeating manner, as shown in the figure below on the left. Lithium (shown as a pink sphere) coordinates with two pyrrolide anions in a planar orientation, while the cations (in this case, 1-n-butyl-4-methylpyridinum) are further away. In this figure, hydrogen atoms are omitted for clarity.
In the figure on the right, it is observed that the oxygen atoms of bistrifylimide strongly coordiante with the lithium ion, with four oxygen atoms binding in a combination of bidentate and monodentate orientations. This significalty slows the mobility of Li+, which is why ionic liquids hainvg bistrifylimide anions have poor conductivities. The calculations predict that anions such as pyrrolide have much better Li+ conductivities.
A paper on new force field parameters for actinyl ions in water, written by group Vladimir Pomogaev, Surya Tiwari, and Neeraj Rai, along with collaborators George Goff and Wolfgang Runde at Los Alamos, has been published in Phys. Chem. Chem. Phys. (2013, 15, 15954-15963). The paper presents intra- and inter-molecular parameters for AnO2n+, where An = U, Np, Pu and Am and n=1 or 2.
The new potentials were used to carry out extensive molecular dynamics simulations for each hydrated ion. Computed bond lengths, bond angles and coordination numbers agree well with known values and previous simulations. Hydration free energies, computed from molecular dynamics simulations as well as from quantum simulations with a solvation model, were in reasonable agreement with estimated experimental values.
Figure showing typical arrangement of water about two actinyl ions.
Luis Fernando Mercier Franco, currently a student in the Department of Chemical Engineering at the Universidade de Sao Paulo, will be visiting the group until May, 2014. Luis will be working on simulations of protein-protein interactions in solution and trying to relate the simulations to experimental studies her has previously carried out. Welcome, Luis!