AmericanChemicalSociety.com

Molecular dynamics (MD) simulations have recently shown that ionic liquids (ILs) based on the 1-alkyl-3-methylimidazolium cation ([C_nmim]⁺) with alkyl chains C₄ and longer are nanostructurally organized with polar regions arising from charge ordering and nonpolar regions arising from alkyl chain segregation. The goal of this project is to exam the role of nanostructural organization in the intermolecular dynamics of imidazolium ILs using optical Kerr effect (OKE) spectroscopy. During the second year of this grant two papers were published.

1. Morphology and Intermolecular Dynamics of 1-Alkyl-3-methylimidazolium Bis{(trifluoromethane)sulfonyl}amide Ionic Liquids.  We studied the structural and dynamical properties of a series of ILs, namely, 1-alkyl-3-methylimidazolium bis{(trifluoromethane)sulfonyl}amide ([C_nC₁im][NTf₂]), with varying alkyl chain length (1 ≤ n ≤ 10) at ambient temperature, where the salts are stable liquids.  This study was done in collaboration with Richard A. Bartsch’s group at Texas Tech University and Kenneth R. Seddon’s group at The Queen's University, Belfast, UK, who synthesized the ILs, Lorenzo Gontrani and Ruggero Caminiti of Università di Roma “Sapienza”, Rome, Italy, who performed the energy dispersive X-ray diffraction (EDXD) experiments, and Alessandro Triolo and Olga Russina of Istituto per i Processi Chimico-Fisici-CNR, Messina, Italy, who performed the SWAXS measurements. The SWAXS measurements revealed three major diffraction peaks: two high Q (wave vector) peaks that show little dependence on the alkyl chain length (n) and a low Q peak that strongly depends both in amplitude and position on n. This low Q peak is the signature of the occurrence of nano-scale structural heterogeneities whose sizes depend on the alkyl chain length and are related to chain segregation into nano-domains. Using optical Kerr effect (OKE) spectroscopy, we accessed intermolecular dynamic features that suggest chain aggregation only occurs for n ≥ 3, in agreement with the SWAXS data.  Moreover, the increase in the frequency and width of main band of the optical Kerr effect spectra in going from n = 2 to n = 3 is consistent with stiffening of the intermolecular potential due to chain aggregation.  Multicomponent line shape analysis suggests that there are least three modes that underlie the main band in the 0-200 cm^-1 region of the OKE spectra of these ILs. Given the similarity of ILs to other complex fluid systems, we assign the low-frequency component to a fast β-relaxation mode and the intermediate- and high-frequency components to librational modes.

2. Effect of Cation Symmetry on the Low-Frequency Spectra of Imidazolium Ionic Liquids: OKE and Raman Spectroscopic Measurements and DFT Calculations. We studied the low-frequency OKE and Raman spectra of 1-propyl-3-methylimidazolium bis[(trifluoromethane)sulfonyl]amide ([C₃C₁im][NTf₂]) and 1,3-diethylimidazolium bis[(trifluoromethane)sulfonyl]amide ([(C₂)₂im][NTf₂]).  These ILs differ only in the symmetry of the alkyl substitution on the imidazolium ring of the cation.  Density functional theory (DFT) calculations on the isolated ions provided guidance in the assignment of the OKE and Raman spectra in the 0-200 cm^-1 region to intermolecular and intramolecular vibrational modes of the liquid. This work was done in collaboration with Mark W. Holtz of Texas Tech University, who did the Raman measurements, and Kihyung Sung of the Korea National University of Education, who did the DFT calculations.  We found that the intermolecular part of the OKE spectrum of [C₃C₁im][NTf₂] is higher in frequency and broader than the intermolecular part of the OKE spectrum of [(C₂)₂im][NTf₂].  We attributed this difference to the intermolecular potential being stiffer in [C₃C₁im][NTf₂] than in [(C₂)₂im][NTf₂].  Through detailed analysis of the low-frequency Raman and OKE spectra of these two ILs in concert with DFT calculations, we assigned the anomalous excess intensity in the high-frequency tail of the OKE spectrum of [(C₂)₂im][NTf₂] to low-frequency intramolecular modes of the cation.  That the intermolecular potential is stiffer in [C₃C₁im][NTf₂] than in [(C₂)₂im][NTf₂] could be due to the fact that in imidazolium ILs with long enough alkyl chains there is the possibility of tail aggregation. Tail aggregation has been invoked to explain the nanoscale spatial heterogeneity in imidazolium ILs observed experimentally and in MD simulations.   So in addition to electrostatic interactions between the anion and the positively charged imidazolium ring, van der Waals interactions between the alkyl chains can also contribute to the intermolecular forces in ILs.