Water concentration dependence of the refractive index of various ionic liquid-water mixtures

The refractive indices of various ionic liquid (IL)-water mixtures were measured across the entire composition range from pure water to pure IL. The maximum deviation in the refractive index relative to that calculated for the corresponding ideal solution exhibited good linear correlation with the refractive index of the pure IL. On the other hand, the IL concentration corresponding to the maximum deviation was about 25 mol% regardless of the IL species. Therefore, the IL concentration dependence of the normalized deviation between ideal and experimental (pure water and pure IL are 0 and maximum deviation of the refractive index is unity) refractive index for all studied ILs were completely overlapped. These results suggest that by normalization their respective IL characters are not necessarily reflected in the refractive indexes of many IL-water mixtures, although IL-water mixtures have the unique structural properties. Therefore using refractive index of pure water and pure IL, we can estimate the refractive index of an IL-water mixture easily for an arbitrary IL concentration.