Near-Infrared Spectroscopic Investigation of Inclusion Complex Formation of Cyclodextrins in Room-Temperature Ionic Liquid

Near-infrared spectrometry has been successfully used to determine association binding constants between phenol and α-, β- and γ-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) M-1, (16 ± 2) M-1and(40 ± 6) M-1forphenol and α-, β- and γ-CD, respectively. compared to 87 M-1and 214 M-1for α- and β-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 ionic liquid including differences in the polarity and viscosity of RTIL and D2O. However, the main reason may be due to the fact that the cation of the ionic liquid (i.e., butylmethylimidazolium ion) may form inclusion complexes with the cyclodextrin, thereby preventing phenol from being included in cavity of CDs.