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

Author： Kusano Takumi Fujii Kenta Tabata Masaaki Shibayama Mitsuhiro

Publisher： Springer Publishing Company

ISSN： 0095-9782

Source： Journal of Solution Chemistry, Vol.42, Iss.10, 2013-10, pp. : 1888-1901

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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, $$ {text{CF}}_{3} {text{SO}}_{3}^{ - } $$ ${CF}_{3} {SO}_{3}^{-}$ ). In [C₄mIm⁺]-based IL systems, no noticeable SANS intensity was observed for all of the systems examined here, although aqueous [C₄mIm⁺][ $$ {text{BF}}_{4}^{ - } $$ ${BF}_{4}^{-}$ ] 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⁺][ $$ {text{CF}}_{3} {text{SO}}_{3}^{ - } $$ ${CF}_{3} {SO}_{3}^{-}$ ] homogeneously mix with water, unlike the [C₄mIm⁺][ $$ {text{BF}}_{4}^{ - } $$ ${BF}_{4}^{-}$ ] system, due to preferential hydration of the ions. In the case of the C_nmIm 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.