Evaluation of band structure and concentration of ionic liquid BMImBF4 in molecular mixtures by using second derivatives of FTIR spectra

FTIR spectra become unfeasible for detecting some narrow bands from the background of a strong broad band. We present a general method for evaluating the narrow bands in FTIR spectra by using second derivatives. As demonstrated by a synthetic Gaussian type spectrum, such information as the peak position and intensity contained in those narrow bands can be derived by using the second derivatives rather than the original FTIR spectra. The FTIR spectra of ionic liquid 1-butyl-3-methylimidazolium (BMImBF4) in the different molecular systems of H2O, ethylene glycol (EG) and tri-ethylene glycol (TEG) were then evaluated by the second derivatives to clear the influence of the broad molecular background. The peak position and even band structure of BMImBF4 were found exactly the same as that obtained from the FTIR spectrum of pure BMImBF4. In addition, this method also provides us with a new method to quantify the concentration of ionic liquid BMImBF4 in different molecular systems.