Refractive Index Behavior Dependent on Molecular Structure in Choline Chloride–Glycerol Deep Eutectic Solvents with Butanol Isomers

The tunable physicochemical properties and environmental compatibility of deep eutectic solvents (DESs), particularly those based on choline chloride (ChCl) and glycerol, have made them a promising green alternative to conventional solvents. The refractive index behavior of ChCl:Glycerol (1:2 molar ratio) DES was comprehensively examined in this study while being modified with three structural isomers of butanol—1-butanol, 2-butanol, and 3-butanol—over a temperature range of 298.15 K to 323.15 K. Thermal expansion and diminished molecular polarizability were identified as the causes of the linear decrease in refractive index (nᴰ) observed in all systems as temperature increased. The refractive index decreased as the mole fraction of butanol increased at fixed temperatures, following the trend of 1-butanol > 2-butanol > 3-butanol. This arrangement is indicative of the impact of molecular branching on the efficiency of hydrogen bonding and intermolecular interactions in the DES matrix. The excess refractive index (nᴱ) was calculated to assess deviations from ideality. The results were consistently negative, suggesting that the DES components and the alcohols have significant associative interactions. The highest magnitude of nᴱ was observed in systems that contained 3-butanol, which indicates that steric effects were responsible for the increased structural disruption.The Redlich–Kister polynomial model was used to fit the experimental nᴱ data, resulting in an outstanding agreement with R² values exceeding 0.99 and average absolute deviations below 0.05%. These results emphasize the sensitivity of the refractive index to both molecular structure and temperature, and they establish nᴰ as a valuable probe for evaluating the intensity of interaction and non-ideality in DES–alcohol mixtures. The findings provide valuable insights for the development of green solvent systems that are optically tunable.