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Small-Angle Neutron Scattering Study on Aggregation of 1-Alkyl-3-methylimidazolium Based Ionic Liquids in Aqueous Solution

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Abstract

Aggregation structures of 1-alkyl-3-methylimidazolium based ionic liquids (ILs) in aqueous solution were investigated by small-angle neutron scattering (SANS) from the viewpoint of alkyl chain length, n, and anions (Cl−, Br− and trifluoromethanesulfonate, \( {\text{CF}}_{3} {\text{SO}}_{3}^{ - } \)). In [C4mIm+]-based IL systems, no noticeable SANS intensity was observed for all of the systems examined here, although aqueous [C4mIm+][\( {\text{BF}}_{4}^{ - } \)] solutions show a significant SANS profile originating from concentration fluctuations in the solution (Almasy et al. J Phys Chem B 112:2382–2387, 2008). This suggests that [C4mIm+][Cl−], [C4mIm+][Br−] and [C4mIm+][\( {\text{CF}}_{3} {\text{SO}}_{3}^{ - } \)] homogeneously mix with water, unlike the [C4mIm+][\( {\text{BF}}_{4}^{ - } \)] system, due to preferential hydration of the ions. In the case of the C n mIm cations with longer alkyl chain lengths (n = 8 and 12), SANS profiles were clearly observed in the aqueous solutions at IL concentrations of C IL > 230 and 20.0 mmol·dm−3, respectively. For aqueous [C8mIm+][Br−] solutions, the asymptotic behavior of the scattering function varied largely from I(q) ~ q −2 to ~q −4 with increasing C IL, indicating that the solution changes from an inhomogeneous mixing state to a nano-scale micelle state. Aqueous [C12mIm+][Br−] solutions show a typical SANS profile for micelle formation in solution. It was found from a model-fitting analysis that the structure of the [C12mIm+][Br−] micelle is ellipsoidal, not spherical, in solutions over the C IL range examined here.

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Acknowledgments

This work has been financially supported by Grant-in-Aids for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology (No. 24750066 to KF, No. 22245018 to MS). The SANS experiments were performed with the approval of the Institute for Solid State Physics, The University of Tokyo (Proposal No. 8599 and 8847K), at the research reactor, JRR-3, Japan Atomic Energy Agency, Tokai, Japan.

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  1. Institute for Solid State Physics, The University of Tokyo, Kashiwanoha, Kashiwa, Chiba, 277-8581, Japan

    Takumi Kusano, Kenta Fujii & Mitsuhiro Shibayama

  2. Department of Chemistry, Faculty of Science and Engineering, Saga University, 1 Honjo-machi, Saga, 840-8502, Japan

    Masaaki Tabata

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  1. Takumi Kusano
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  2. Kenta Fujii
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Correspondence to Kenta Fujii or Mitsuhiro Shibayama.

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Kusano, T., Fujii, K., Tabata, M. et al. Small-Angle Neutron Scattering Study on Aggregation of 1-Alkyl-3-methylimidazolium Based Ionic Liquids in Aqueous Solution. J Solution Chem 42, 1888–1901 (2013). https://doi.org/10.1007/s10953-013-0080-0

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