  A thorough analysis of the refractive index of eleven
1-alkyl-3-methylimidazolium-based ionic liquids with three different anions,
tetrafluoroborate bis(trifluoromethylsulfonyl)imide, and
trifluoromethanesulfonate, is reported. Refractive indices were estimated, in
the temperature interval from 298.15 to 323.15 K, using an Abbe refractometer
to determine the value at the sodium D line and white light spectral
interferometry to obtain dispersion in the range of wavelengths from 400 to
1000 nm. The first part of the manuscript is focused on the dependence of
refractive index with wavelength, temperature, cation alkyl chain length, and
anion nature. Once the main features are detailed, and in order to explain the
experimental trends, a model for the refractive index is considered where its
square is expressed by a single resonance Sellmeier dispersion formula. This
formula has two coefficients: the first one identifies the position of the
resonance in the spectral axis, and the second one specifies its strength. It
was found that, for a given compound, the resonances position is independent of
temperature, while the strength varies linearly with it. This model reproduces
successfully the experimental data within the refractive index uncertainty.
Furthermore, the model allows calculating the thermo-optic coefficient and its
wavelength dependence.
