Composition-mediated intermolecular interaction transition and vibrational reorganization in n-hexane-ethanol binary mixtures.

Mixing polar ethanol with nonpolar n-hexane provides a suitable liquid system for examining composition-dependent changes in local molecular organization and vibrational behavior. This study combines Raman spectroscopy and DFT calculations to clarify the molecular origin of these changes in n-hexane-ethanol binary mixtures with ethanol volume fractions from 0.1 to 0.9. As the ethanol content increased, the ethanol CCO band near 894 cm-1 exhibited a redshift-blueshift-redshift trend, whereas the n-hexane CH stretching bands at 2898 cm-1, 2923 cm-1, and 2950 cm-1 showed the opposite blueshift-redshift-blueshift trend. Both trends reversed near an ethanol volume fraction of 0.4, where pronounced spectral reshaping was also observed in the 950 cm-1-1200 cm-1 and 1240 cm-1-1340 cm-1 regions, indicating substantial local structural reorganization. DFT calculations showed that weak CH⋯O interactions dominated in n-hexane-rich environments, weakened at intermediate compositions, and were progressively replaced by OH⋯O hydrogen bonds between ethanol molecules as the ethanol content increased. Consistent with this evolution, MD trajectories and radial distribution functions revealed a nonmonotonic rearrangement of intermolecular contacts during the transition from 2Hex:1EtOH to 1Hex:1EtOH. The mean per-molecule BSSE-corrected interaction energy remained relatively weak at intermediate compositions but became markedly more negative in ethanol-rich clusters. Peak deconvolution further identified the emergence of an ethanol-related band at 2763 cm-1 near 0.4 and the evolution of the 2741 cm-1 band from an n-hexane-dominated mode toward mixed molecular contributions. These results establish a direct link between composition-driven intermolecular association and vibrational reorganization.