Correlating Viscosity Trends and Ultrafast Structural Dynamics in TMSO–Water and SFL–Water Binary Mixtures

The hydrogen bonding dynamics in tetramethylene sulfoxide (TMSO)-water and sulfolane (SFL)-water binary mixtures were investigated by using FTIR spectroscopy, ultrafast IR spectroscopy, and molecular dynamics (MD) simulations. Despite TMSO and SFL sharing a similar cyclic backbone, markedly different viscosity behaviors were observed. By employing SCN^- as a local probe, its distinct reorientational dynamics was observed that directly correlates with the contrasting viscosity trends in the two systems. In TMSO-water solutions, strong solute-water hydrogen bonding dominates water-water interactions, leading to a viscosity maximum at intermediate concentrations. Conversely, the dynamics of water molecules in SFL-water solutions is decoupled from bulk viscosity trends due to the weaker solute-water interactions. MD simulations further elucidate how the interplay between solute-water and water-water hydrogen bonding governs the viscosity trends. This work advances our understanding of hydrogen bonding in complex aqueous environments and provides a systematic approach for connecting molecular-level interactions to macroscopic fluid properties.