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.