| Research Abstract |
Our recent studies have shown that binary solutions of ethanol and water preferentially form microphases with the same solvent species. This means that ethanol-water mixtures do not get homogeneously mixed on the molecular level. To examine whether the microphases also exist in other solvents or not, we have observed the Raman spectra of acetic acid in water, alcohols, and aprotic polar solvents with varying mole fractions of acetic acid. As acetic acid is diluted in water, the C=O stretching band of acetic acid becomes broader and shows a higherwavenumber shift. From comparison between the observed and the calculated spectra, the broad band in the mixture spectra can be ascribable to the acetic acid aggregates. The peak position remains unchanged even in a diluted solution with an acetic acid molar fraction of0.001, indicating that the acetic acid microphases exist in this low acid concentration region. In alcohols, the spectral changes are the same as those observed in water. This indicates that the same acetic acid microphases are formed in the alcohol solutions. In aprotic polar solvents, however, the spectral changes are different from those in the protic solvents : two higher-wavenumber C=O bands appear in the diluted region. From the SCRF calculations, we assign the observed bands to the cyclic dimer consisting of acetic acid and solvent monomers and to the non-complexed acetic acid monomer. Therefore, it is concluded that acetic acid molecules exist as their microphases in the protic solvents and as the monomeric forms in the aprotic polar solvents. From several studies on other binary solutions, the mixing rules at molecular levels can be proposed as follows : protic-protic binary mixtures do not easily become homogeneously mixed states even when the mixing ratio is large, while homogeneously mixed states at molecular levels preferentially occur in protic-aprotic polar binary mixtures at low concentrations of the protic solvent.
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