Effect of the chain length in the structure of imidazolic ionic liquids and dimethylformamide solutions probed by Raman spectroscopy

摘要

The Raman band assigned to the v(C=O) mode in N,N-dimethylformarnide (at ca. 1660 cm~(-1)) was used as a probe to study a group of ionic liquids 1-alkyl-3-methylimidazolium bromide ([C_nMlm]Br) with different alkyl groups (n = 2,4,6,8 and 10 carbons) in binary equimolar binary mixtures with dimethylformamide. Due to the high electric dipole moment of the group C=O, there is a substantial coupling between adjacent molecules in the solution, and the corresponding Raman band involves both vibrational and reorienta-tional modes. Different chain lengths of the ILs lead to different extents of the uncoupling of adjacent molecules of dimethylformamide, resulting in different shifts for this band in the mixtures. Information about the organization of ionic liquids in solution was obtained and a model of aggregation for these systems is proposed.