Spectroscopic Studies Cluster Formation Abilties and Anomarous Functions in Aqueous Environment

我们最近的研究表明，乙醇和水的二元溶液在相同溶剂种类下优先形成微相。这意味着乙醇-水混合物在分子水平上不能均匀混合。为了检验微相是否也存在于其他溶剂中，我们观察了乙酸在不同摩尔分数的水、醇和非质子极性溶剂中的拉曼光谱。随着醋酸在水中的稀释，醋酸的C=O拉伸带变宽，波数移位增大。通过对实测光谱和计算光谱的比较，混合光谱中的宽频带可归因于乙酸聚集体。即使在乙酸摩尔分数为0.001的稀释溶液中，峰的位置也保持不变，表明在这个低酸浓度区域存在乙酸微相。在醇中，光谱变化与在水中观察到的相同。这表明在醇溶液中形成了相同的醋酸微相。然而，在非质子极性溶剂中，光谱变化与在质子溶剂中不同：稀释区出现两个更高波数的C=O波段。从SCRF计算中，我们将观察到的波段分配给由乙酸和溶剂单体组成的环二聚体和非络合的乙酸单体。因此，醋酸分子在质子溶剂中以微相形式存在，在非质子极性溶剂中以单体形式存在。从对其他二元溶液的若干研究中，可以提出分子水平上的混合规律：质子-质子二元混合物即使在混合比大的情况下也不容易成为均匀混合状态，而在质子溶剂浓度低的情况下，质子-非质子极性二元混合物优先出现分子水平上的均匀混合状态。

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.