Liquid-Liquid Phase Separation Produces Fast H-Bond Dynamics in DMSO-Water Mixtures.

Liquid-liquid phase separation is common in complex mixtures, but the behavior of nanoconfined liquids is poorly understood from a physical perspective. Dimethyl sulfoxide (DMSO) is an amphiphilic molecule with unique concentration-dependent bulk properties in mixtures with water. Here, we use ultrafast two-dimensional infrared (2D IR) spectroscopy to measure the H-bond dynamics of two probe molecules with different polarities: formamide (FA) and dimethylformamide (DMF). Picosecond H-bond dynamics are fastest in the intermediate concentration regime (20-50 mol % DMSO), because such confined water exhibits bulk-like dynamics. Each vibrational probe experiences a unique microscopic environment as a result of nanoscale phase separation. Molecular dynamics simulations show that the dynamics span multiple time scales, from femtoseconds to nanoseconds. Our studies suggest a previously unknown liquid environment, which we label "local bulk", in which despite the local heterogeneity, the ultrafast H-bond dynamics are similar to bulk water.