Studies on interaction of ionic liquids with cyclodextrins in aqueous solution

Abstract

Interactions between <img src='/image/spc_char/alpha.gif'>-, <img src='/image/spc_char/beta.gif'>-, <img src='/image/spc_char/gamma.gif'>-CDs and two ionic liquids, 1-butyl-3-methylimidazolium tetrafluroborate ([bmim][BF₄]) and 1-octyl-3-methylimidazolium tetrafluroborate ([omim][BF₄]), have been studied with titration microcalorimetry and UV and NMR spectra at 298.15 K. The results are discussed in terms of the hydrophobic interaction of CD molecular cavity with alkyl chain of IL cation and the change in iceberg structure formed by water molecules existing around the hydrophobic tail of the ionic liquids. The stoichiometry of CDs with the two ionic liquids is 1:1 and 1:2 except that of <img src='/image/spc_char/alpha.gif'>-CD with [omim][BF₄], which is only 1:1. The binding process of <img src='/image/spc_char/alpha.gif'>-CD with both the ionic liquids is entropy driven, while that of <img src='/image/spc_char/beta.gif'>-CD and <img src='/image/spc_char/gamma.gif'>-CD with the ionic liquids is characterized by both enthalpy favorable and entropy favorable processes. The ¹H NMR spectra reveal that chemical shift data of all protons in <img src='/image/spc_char/alpha.gif'>-CD, <img src='/image/spc_char/beta.gif'>-CD and <img src='/image/spc_char/gamma.gif'>-CD molecules move to high field in the presence of ionic liquids. UV absorbance for the CD-IL systems has been detected, which indicates that the inclusion complex is formed through weak interaction.