8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

Fri 911:10-14:00P1.33Structure and short-time dynamics of ionic liquids: amolecular dynamics simulation and Ramanspectroscopy studyMauro Ribeiro, 1 Sérgio Urahata, 2 Leonardo Siqueira, 2 Luciano Costa, 3 BrunoNicolau, 2 and Tatiana Penna 21 Universidade de São Paulo, Instituto de Química, C. P. 26077 05513-970, São Paulo,Brazil2 Universidade Federal de São Paulo, São Paulo, Brazil3 Universidade Federal de Alfenas, Alfenas, BrazilSeveral systems with imidazolium or alkylammonium cations and different anions have been studiedby molecular dynamics (MD) simulations and low-frequency Raman spectroscopy in order todiscern the characteristic features shared by ionic liquids from more specific differences due to distinctionic species. Like (high temperature) molten salts, the charge - charge static structure factor,S CC (k), highlights charge fluctuaction in ionic liquids, and the dominant role played by charge orderingin spite of differences in molecular structures. Comparison of S CC (k) and partial S ab (k) fordifferent atoms indicate structural heterogeneity of polar and non-polar domains in ionic liquids.Raman spectra within the low-frequency range (5 lt; w lt; 100 cm −1 ) reveal short-time dynamicswith contributions of fast relaxations and intermolecular vibrations. It is shown that low-frequencyRaman spectroscopy provides similar results as optical Kerr effect (OKE) spectroscopy. A correlationlength assigned to the boson peak vibrations was estimated from the frequency of themaximum of the boson peak and experimental data of sound velocity. The correlation length relatedto the boson peak (∼19 å) does not change with the length of the alkyl chain in imidazoliumcations, in contrast to the position of the first-sharp diffraction peak observed in neutron and X-rayscattering measurements and MD simulations of ionic liquids. The temperature dependence of thequasi-elastic scattering (QES) intensity in ionic liquids illustrates relationships between short-timedynamics and long-time structural relaxation that have been proposed for glass-forming liquids.33