Optical properties derived from the refractive index dispersion of alkyl-imidazolium based ionic liquids

Resumo

Ionic liquids are advanced materials with great potential to take part in optical applications such as optical imaging, ultrafast optics, laser technology and so on. In view of this, a precise knowledge of their optical behavior is necessary to ensure the optimal performance of a device. In this work, the chromatic dispersion of a set of twenty alkyl-imidazolium-based ionic liquids is characterized in a quasi-continuous spectral interval comprising wavelengths between 400 and 1000 nm by using a technique based on spectrally resolved white-light interferometry that we have name Refractive Index Spectral Broadband Interferometry. A model based on a single resonance Sellmeier equation is fitted to the experimental data; the analysis of the fitting parameters allows the identification of behaviors and trends related to the structure of the material (anion, cation, alkyl chain length) or environmental conditions (temperature). The model is also used to calculate other dispersive magnitudes derived from the refractive index such as the Abbe number, group index, group velocity dispersion or third-order dispersion. In this work, the linearity of the molar volume and molar refractivity with the alkyl chain length is demonstrated by combining the density and refractive index data with the Lorentz-Lorenz equation. The linearity allows developing a predictive model that accounts for the refractive index change induced by a change in the alkyl chain length with the only requirement of knowing the data corresponding to, at least, two liquids of the same family. The model also works for predicting the Abbe number.