T. Welton, Chem. Rev. 99, 2071 (1999) url
Bulk ionic liquid
For designing ILs for commercial applications, the fundamental physical properties should be first characterized. However, many physical properties of ILs are not well known yet. For example, ILs are in liquid phase at room temperature (T < 100 oC) unlikely to the usual ionic salts such as NaCl or MgCl2. There has been much effort to understand low melting temperature of ILs in spite of the strong ionic interaction between cations and anions.
Left below figure represents the relation between the melting temperature and cation structure which was reported by Hamaguchi and coworkers at 2003 . And right below figure represents the relation between the melting temperature and alkyl chain length of the cation which was reported by Seddon and coworkers at 2007 . These results strongly propose us that the alkyl chain attached to the imidazolium ring is one of the most important key in determing the melting temperature. From the point of view, we study the cation structure of ILs.
 S. Hayashi, R. Ozawa, H. Hamaguich, Chem. Lett. 32, 498 (2003) url
 L. Lopez-Martin, E. Burello, P. N. Davey, R. Seddon and G. Rothenberg, ChemPhysChem 8, 690 (2007) url
cation structure  Second and third rows of the below figure represent the deconvoluted peaks for the [BMIM][BF4] and [BMIM][I] respectively.
As shown in the figure, the overall peak intensity of [BMIM][I] is ~2 times larger than the [BMIM][BF4]. Moreover, the CH vibration mode region of the imidazolium ring (3000 - 3200-1) is very different. The symmetric HC(4)-C(5)H stretching mode (vSS HC(4)-C(5)H) peak at ~3163 cm-1 of imidazolium ring is ~2 times larger than the antisymmetric HC(4)-C(5)H stretching mode (vAS HC(4)-C(5)H) peak at ~3126 cm-1 for the [BMIM][BF4] and [BMIM][PF6]. While The symmetric HC(4)-C(5)H stretching mode (vSS HC(4)-C(5)H) peak at ~3141 cm-1 of imidazolium ring is ~5 times smaller than the antisymmetric HC(4)-C(5)H stretching mode (vAS HC(4)-C(5)H) peak at ~3078 cm-1 for the [BMIM][I]. Moreover, the peak position of C(2)-H stretching mode (~3032 cm-1) for the [BMIM][I] is very different. These results propose us that interaction between the imidazolium ring and I- is quite different from interaction between the imidazolium ring and BF4-.
Also, the butyl chain structure of [BMIM][I] is different with [BMIM][BF4]. Below figures represent Raman data for the IL+water mixtures with changing the concentration. At the very dilute cases (~0.005 mole fraction), gauche/trans ratio of [BMIM][I] is very similar with [BMIM][BF4]. However, if the IL concentration is increased, the gauche/trans ratio becomes different for each ILs. For the case of [BMIM][I], the butyl chain have dominantly trans form.
 Y. Jeon, J. Sung, C. Seo, H. Lim, H. Cheong, M. Kang, B. Moon, Y. Ouchi, and D. Kim, J. Phys. Chem. B, 112, 4735 (2008) pdf
 Y. Jeon, J. Sung, D. Kim, C. Seo, H. Cheong, Y. Ouchi, R. Ozawa, and H. Hamaguchi, J. Phys. Chem. B, 112, 923 (2008) pdf
relative position of anion
Ionic liquid surface
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