How Does the Ionic Liquid Organizational Landscape Change when Nonpolar Cationic Alkyl Groups Are Replaced by Polar Isoelectronic Diethers?

Journal Article · Fri Dec 21 00:00:00 EST 2012 · Journal of Physical Chemistry. B, Condensed Matter, Materials, Surfaces, Interfaces and Biophysical Chemistry

DOI:https://doi.org/10.1021/jp311032p· OSTI ID:1062431

Kashyap, Hemant K. ^[1]; Santos, Cherry S. ^[2]; Daly, Ryan P. ^[1]; Hettige, Jeevapani J. ^[1]; Murthy, N. Sanjeeva ^[2]; Shirota, Hideaki ^[3]; Margulis, Claudio J. ^[1]

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1. Univ. of Iowa, Iowa City, IA (United States)
2. State Univ. of New Jersey, Piscataway, NJ (United States)
3. Chiba Univ. (Japan)

The X-ray scattering experiments and molecular dynamics simulations have been performed to investigate the structure of four room temperature ionic liquids (ILs) comprising the bis(trifluoromethylsulfonyl)amide (NTf₂^–) anion paired with the triethyloctylammonium (N₂₂₂₈⁺) and triethyloctylphosphonium (P₂₂₂₈⁺) cations and their isoelectronic diether analogs, the (2-ethoxyethoxy)ethyltriethylammonium (N_222(2O2O2)⁺) and (2-ethoxyethoxy)ethyltriethylphosphonium (P_222(2O2O2)⁺) cations. Agreement between simulations and experiments is good and permits a clear interpretation of the important topological differences between these systems. The first sharp diffraction peak (or prepeak) in the structure function S(q) that is present in the case of the liquids containing the alkyl-substituted cations is absent in the case of the diether substituted analogs. Using different theoretical partitioning schemes for the X-ray structure function, we show that the prepeak present in the alkyl-substituted ILs arises from polarity alternations between charged groups and nonpolar alkyl tails. In the case of the diether substituted ILs, we find considerable curling of tails. Anions can be found with high probability in two different environments: close to the cationic nitrogen (phosphorus) and also close to the two ether groups. Moreover, for the two diether systems, anions are found in locations from which they are excluded in the alkyl-substituted systems. This removes the longer range (polar/nonpolar) pattern of alternation that gives rise to the prepeak in alkyl-substituted systems.

• Research Organization: Argonne National Lab. (ANL), Argonne, IL (United States). Advanced Photon Source (APS)
• Sponsoring Organization: National Science Foundation (NSF)
• OSTI ID: 1062431
• Journal Information: Journal of Physical Chemistry. B, Condensed Matter, Materials, Surfaces, Interfaces and Biophysical Chemistry, Vol. 117, Issue (4) ; 01, 2013; ISSN 1520-6106
• Publisher: American Chemical Society
• Country of Publication: United States
• Language: ENGLISH