Hydrophobicity/hydrophilicity of 1-butyl-2,3-dimethyl and 1-ethyl-3-methylimodazolium ions: toward characterization of room temperature ionic liquids.

We continue to experimentally characterize the constituent ions of room temperature ionic liquids in terms of their interactions with H(2)O. 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 C(4)C(1)mim) and 1-ethyl-3-methylimidazolium (C(2)mim). We found that C(4)C(1)mim dissociates completely in dilute aqueous solution less than 0.006 mol fraction, and hence, its hydrophobicity/hydrophilicity could be determined. The results indicate that C(4)C(1)mim 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 (C(4)mim), PF(6)(-), CF(3)SO(3)(-), and N(SO(2)CF(3))(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 C(4)C(1)mim are stronger than those for C(4)mim. 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 H(2)O. On replacing it with CH(3) in C(4)C(1)mim, the lack of acidic H enhances the positive charge in the vicinity of N-C-N in the ring that interacts with the surrounding H(2)O strongly to an induced dipole of O of the H(2)O. For C(2)mim, we found it does not dissociate completely, even in dilute aqueous solution, and hence, we could not characterize it within the present methodology.