Temperature-dependent structure of ionic liquids: X-ray scattering and simulations

• Published in: Faraday discussions 2012-01, Vol.154 (2012), p.133-143
• Main Authors: Kashyap, Hemant K, Santos, Cherry S, Annapureddy, Harsha V. R, Murthy, N. Sanjeeva, Margulis, Claudio J, Castner, Jr, Edward W
• Format: Article
• Language: English
• Subjects: Cationic; Charge; Crystallography, X-Ray; Diffraction; Ionic liquids; Ionic Liquids - chemistry; Models, Molecular; Molecular Dynamics Simulation; Molecular Structure; Scattering; Separation; Signatures; Temperature
• ISSN: 1359-6640; 1364-5498
• DOI: 10.1039/c1fd00059d

In this article we determine the temperature-dependent structure of the tetradecyltrihexylphosphonium bis(trifluoromethylsulfonyl)amide ionic liquid using a combination of X-ray scattering and molecular dynamics simulations. As in many other room-temperature ionic liquids three characteristic intermolecular peaks can be detected in the structure function S ( q ). A prepeak or first sharp diffraction peak is observed at about q = 0.42 Å −1 . Long range anion-anion correlations are the most important contributors to this peak. In all systems we have studied to date, this prepeak is a signature of solvation asymmetry. The peak in S ( q ) near q = 0.75 Å −1 is the signature of ionic alternation and arises from the charge ordered separation of ions of the same charge. The most intense diffraction peak near q = 1.37 Å −1 arises from short-range separation between ions of opposite charge combined with a significant contribution from cationic carbon-carbon interactions, indicating that cationic hydrophobic tails have significant contacts.