Deep eutectic solvents (DESs) are considered as potential alternatives for ionic liquids (ILs). The evaluation of DESs as new generation of solvents for various practical application requires enough knowledge about some main physical, chemical, and thermodynamic properties. In this study, due to lack of data for DESs' refractive indices, the refractive indices of twenty four DESs based on ammonium and phosphonium salts were measured and predicted using atomic contribution method. The atomic contributions data for molar refraction proposed by Wildman and Crippen which was developed for neutral compounds were employed to calculate the molar refractions of DESs. Subsequently, the refractive indices of DESs were predicted using Lorentz-Lorenz equation through the calculated DESs' molar refractions and experimental DESs' densities values. The absolute relative percentage error (ARPE) value of 0.56% and a regression coefficient (R2) value of 0.9822 both confirmed the highly possible application of the proposed method for predicting the refractive indices. In addition, the effect of DESs' composition on refractive index of DESs was investigated and it was found that the refractive index of the DESs lies between that of the salt and HBD. Moreover, the densities of DESs were also predicted using Lorentz-Lorenz equation employing the calculated molar refraction and values of experimental DESs' refractive indices. The ARPE of 1.43% shows that this method for prediction of densities of the synthesized DESs is applicable as well.
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