Tuning refractive index of deep eutectic solvents

Abstract

Refractive index is one of the key physical properties of deep eutectic solvents (DESs). However, there is no systematical study on the refractive index of DESs to the best of our knowledge. Here, a systematical investigation on the refractive index of typical DESs is carried out. Furthermore, the effect of hydrogen-bonding donor (HBD), hydrogen-bonding acceptor (HBA), mole ratio of HBD to HBA, and temperature on the refractive index is analyzed. Results show that effect of HBD on the refractive index of DESs is CC:GLL (1:2) > CC:EG (1:2) > CC:MA (1:2) > CC:PHA (1:2) > CC:LA (1:2) > CC:FA (1:2) > CC:PA (1:2) > CC:AA (1:2), BE:GLL (1:2) > BE:LA (1:2) > BE:MA (1:2) > BE:FA (1:2) > BE:PA (1:2) > BE:AA (1:2), or LB:NMA (1:4) > LB:GLL (1:4) > LB:ACE (1:4) > LB:LA (1:4) > LB:EG (1:4) > LB:PA (1:4) > LB:AA (1:4) > LB:FA(1:4). Increasing the mole ratio could either increase or decrease the refractive index of DES, depending on the component and the intermolecular interaction. Effect of component on the refractive index on the refractive index could be affected by the original component and the added component; however, the presence of water decreases refractive index of all the DESs investigated. Refractive index of DESs decreases linearly when increasing the temperature. This work would provide theoretical guidance to tune the refractive index of DESs for their widespread applications.