Determination of binding constants of cyclodextrins in room-temperature ionic liquids by near-infrared spectrometry

Near-infrared spectrometry has been successfully used to determine association binding constants between phenol and alpha-, beta- and gamma-cyclodextrin (CD) in [butylmethylimidazolium][chloride] room-temperature ionic liquid (RTIL). It was found that adding CD into the RTIL solution of phenol resulted in an enhancement in the absorption coefficient of the stretching overtone of the aromatic C-H groups. However, the enhancement induced by CDs in RTIL is much lower (order of magnitude) than those corresponding in D2O. The binding constants in RTIL are also much lower than those in D2O ((11 +/- 2), (16 +/- 2), and (40 +/- 6) M-1 for phenol and alpha-, beta- and gamma-CD, respectively, as compared to 87 and 214 M-1 for alpha- and beta-CD in D2O). The results obtained seem to suggest that in ionic liquid, the main interaction between phenol and CDs may not be inclusion complex formation but rather external adsorption. A variety of reasons may be responsible for relatively weaker interactions and lower binding constants in the ionic liquid, including differences in the polarity and viscosity of RTIL and D2O. However, the main reason may be due to the possibility that the 1-butyl-3-methylimidazolium cation of the ionic liquid may form inclusion complexes with CDs either through its imidazolium moiety or its butyl group. Such complex formation would prevent phenol from being included in the cavity of the CDs.