Hydrophobicity/Hydrophilicity of 1-Butyl-2,3-dimethyl and 1-Ethyl-3-methylimodazolium Ions: Toward Characterization of Room Temperature Ionic Liquids

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• 作者：Hitoshi Kato ; Kumiko Miki ; Tomohiro Mukai ; Keiko Nishikawa ; Yoshikata Koga
• 刊名：Journal of Physical Chemistry B
• 出版年：2009
• 出版时间：November 5, 2009
• 年：2009
• 卷：113
• 期：44
• 页码：14754-14760
• 全文大小：226K
• 年卷期：v.113,no.44(November 5, 2009)
• ISSN：1520-5207

文摘

We continue to experimentally characterize the constituent ions of room temperature ionic liquids in terms of their interactions with H₂O. By using the so-called 1-propanol probing methodology, we experimentally index the relative hydrophobicity/hydrophilicity of a test ion. In this paper, we examine 1-butyl-2,3 dimethylimidazolium (abbreviated as [C₄C₁mim]⁺) and 1-ethyl-3-methylimidazolium ([C₂mim]⁺). We found that [C₄C₁mim]⁺ dissociates completely in dilute aqueous solution less than 0.006 mol fraction, and hence, its hydrophobicity/hydrophilicity could be determined. The results indicate that [C₄C₁mim]⁺ is highly amphiphilic with much stronger hydrophobicity and hydrophilicity than normal ions. Our earlier similar studies indicated the same conclusion for such typical constituent ions as 1-butyl-3-methylimidazolium ([C₄mim]⁺), PF₆⁻, CF₃SO₃⁻, and N(SO₂CF₃)₂⁻. Hence, we suggest that the constituent ions of room temperature ionic liquids that we have studied so far are all amphiphiles with much stronger hydrophobicity and hydrophilicity than normal ions. We found, furthermore, that the hydrophobicity and hydrophilicity of [C₄C₁mim]⁺ are stronger than those for [C₄mim]⁺. A possible reason for higher hydrpohilicity is discussed in terms of strong acidic character of H on the C(2) of the imidazolium ring, which tends to attract the delocalized positive charge toward itself on forming a hydrogen bond to H₂O. On replacing it with CH₃ in [C₄C₁mim]⁺, the lack of acidic H enhances the positive charge in the vicinity of N−C−N in the ring that interacts with the surrounding H₂O strongly to an induced dipole of O of the H₂O. For [C₂mim]⁺, we found it does not dissociate completely, even in dilute aqueous solution, and hence, we could not characterize it within the present methodology.