An experimental and computational study on the material dispersion of 1-alkyl-3-methylimidazolium tetrafluoroborate ionic liquids

The material dispersion of the [Ckmim][BF4] (k = 2, 3, 4, 6, 7, 8, 10) family of ionic liquids is measured at several temperatures over a broad spectral range from 300 nm to 1550 nm. The experimental curves are fitted to a modified three-resonance Sellmeier model to understand the effects of temperature and alkyl chain length on the dispersion behaviour. From the parameters of the fitting, we analyze the influence that the different constituents of these ionic liquids have on the dispersion behaviour. In addition, a semi-empirical approach combining simulated electronic polarizabilities and experimental densities for predicting the material dispersion is successfully tested by using a direct comparison with the experimental results. The limitations of this method are analyzed in terms of the molecular structure of the ionic liquids. The results of this work aim to increase our knowledge about how the molecular structure of an ionic liquid influences its material dispersion. Understanding this influence is fundamental to producing ionic liquids with tailored optical properties.This work was supported by the Ministerio de Economía y Competitividad (MINECO) and FEDER Program through the projects (MAT2017-89239-C2-1-P); Xunta de Galicia and FEDER (AGRU 2015/11 and GRC ED431C 2016/001, ED431D 2017/06, ED431E2018/08). C. D. R. F. acknowledges the support of Xunta de Galicia through the grant ED481A-2018/032. We also thank the Centro de Supercomputación de Galicia (CESGA) facility, Santiago de Compostela, Galicia, Spain, for providing the computational resources employed in this work.S