HPLC coupled with a spectrophotometer as a reliable setup for the study of absorption properties of imidazolium ionic liquids using bmimBF4 as an example

Abstract

The absorption spectra of the same ionic liquid published by many authors show significant differences, which are interpreted as being caused by the presence of many difficult to eliminate impurities. This paper presents an innovative use of an HPLC system coupled with a spectrophotometer, which allows separation of impurities from bmimBF₄, and to a high degree, separation of bmimBF₄-related species, determination of their relative content and measurement of their absorption spectra. The absorption spectra of bmimBF₄ aggregates and water complexes show a long-wavelength absorption band (λ_max = 278 nm) but with different molar absorption coefficients, ε. The absorption spectrum measured by a spectrophotometer is a sum of the absorption spectra of all species derived from bmimBF₄ and the impurities that it contains, while the use of the proposed setup provides reliable results on the spectral properties of different compounds showing complex and/or difficult to study properties even if they contain numerous impurities.