From deep eutectic solvents to deep band gap systems

The optical properties of nine choline-based Deep Eutectic Solvents (DESs) of the type choline+X- (X- = chloride or acetate)/Y (Y = ethylene glycol, glycolic or levulinic acid) were studied by measuring their band gap through the UV-VIS-based Tauc method. All the considered systems showed a direct optical transition. A correlation between band gap and eutectic composition was discovered by plotting the variation of the band gap versus the molar fraction of choline+X-/Y in the parent systems containing 10 wt% of H2O. The main parameters influencing the energy of the optical transition in these "Deep Band Gap Systems" (DeBaGS) where studied through Design of Experiments (DoE) combined with multivariate analysis. The effect on the band gap of the anion (chloride or acetate), the acid (levulinic or glycolic), the water, and the stirring time, considered as single factor or in combination was clarified. Thus a multivariate model was implemented through surface responding analysis which allows engineering such kind of systems and opening to further applications in material science.