Cavity-Enhanced Raman Spectroscopy Revealed Competitive Hydrogen-Bond Restructuring in Ethanol-Water Solutions.

This study investigates the competitive restructuring of hydrogen-bond networks in ethanol-water solutions using cavity-enhanced Raman spectroscopy and molecular dynamics simulations. The multireflection cavity provides a 60-fold sensitivity enhancement, enabling detection of weak intermolecular vibrations as well as direct tracking of O-H stretching features, particularly the free O-H mode, which serves as a sensitive probe of hydrogen-bond disruption. Our results identify three structural regimes: hydration-trimer-stabilized water networks at low ethanol concentrations, competitive coexistence of strong and weak ethanol-water dimers with incipient cluster segregation at intermediate concentrations, and ethanol self-association leading to microphase separation at high ethanol content. MD simulations reveal a nonlinear redistribution of hydrogen bonds, where water-water interactions are progressively replaced by water-ethanol interactions, with a crossover near 70% ethanol (from water-dominated to ethanol-associated hydrogen bonds). This transition coincides with the evolution of the free OH and intermolecular O-H spectral features, providing a molecular-level origin for the experimentally observed three-regime structural transformation. Our research demonstrates the power of cavity-enhanced Raman spectroscopy for resolving hierarchical hydrogen-bond dynamics in complex molecular liquids.