Impact of water addition on fluctuation dynamics in viscous solvents with varied heterogeneity: A 2D IR spectroscopic study

•FTIR and 2D IR spectroscopy was used to study the fluctuation dynamics of viscous solvents of varying degrees of heterogeneity and their aqueous solutions.•The hydrogen bond network of glycerol is disrupted upon water addition, whereas the solvent structure of the deep eutectic solution remains mostly intact.•Spectroscopic results are in very good agreement with previously reported simulation results. The addition of cosolvent, such as water, influences the overall fluctuation dynamics as sensed by the probe. It is crucial to unravel the fluctuation dynamics when water is added, particularly in viscous solvents with varying degrees of heterogeneity. In this study, we employed two-dimensional infrared spectroscopy to examine the solvation dynamics of two viscous solvents: glycerol, a molecular solvent, and malicine, an acid-based deep eutectic solvent. Additionally, we explored the relative changes in fluctuation dynamics upon water addition. Our results indicate that water disrupts the hydrogen bond network in glycerol, but no significant effect is observed in malicine. Our experimental findings align well with previously reported simulation studies. [Display omitted]