THz ATR-TDS Spectroscopy of Acetone-Water Mixtures: Hydrogen Bonding to Dipole-Dipole Dynamics.

Attenuated total reflection time-domain spectroscopy (ATR-TDS) in the terahertz regime was employed to investigate the dielectric response of water-acetone mixtures over the full molar concentration range. The ATR configuration enabled stable measurements in a controlled and nearly closed environment, minimizing acetone evaporation and allowing reliable characterization of this highly volatile binary system. The complex dielectric function, retrieved in the 0.4-1.6 THz range, was analyzed by means of a double Cole-Cole model, which provided a more consistent description of the mixtures than a simple Debye-based approach. A strongly nonlinear dependence on composition was observed, with the highest sensitivity in the water-rich region, where even small amounts of acetone produced a marked change in both the real and imaginary parts of the dielectric function. The extracted parameters indicate that acetone primarily suppresses the slow, cooperative relaxation channel associated with the hydrogen-bond network of water, whereas the faster channel remains comparatively less affected, consistent with its more local intermolecular origin. The evolution of the Kirkwood-Fröhlich correlation factors and of the broadening parameters further supports a progressive transition from a highly correlated hydrogen-bonded liquid to a structurally heterogeneous and weakly cooperative dipolar environment. These results demonstrate that THz ATR-TDS is a sensitive tool for probing intermolecular reorganization in aqueous binary mixtures, providing a physically grounded framework for the detection of acetone and other volatile hydrogen-bond-active species in water-based systems.