An inspection of the dispersive properties of imidazolium-based ionic liquids in the Vis-NIR

In this work, the refractive index dependence on the wavelength of 20 ionic liquids is characterised by spectrally resolved white-light interferometry in a spectral interval comprising a range of wavelengths from 400 to 1000 nm at different temperatures between 298.15 and 323.15 K. With these data and using a refractive index model based on a single resonance Sellmeier equation, we calculate the Abbe number, group index, group velocity dispersion, and dispersion of the thermo-optic coefficient for each liquid. Using the Lorentz-Lorenz approach and density data, we demonstrate (from the empirical results) the linear variation of the molar volume and molar refraction with the number of carbons in the alkyl chain. Further, we confirm that this linear relation can be used to predict the refractive index of liquids of the same family provided the addition of CH2 groups does not lead to anisotropies in the refractive index.