Raman Spectroscopic Study on Acetic Acid Clusters in Aqueous Solutions: Dominance of Acid−Acid Association Producing Microphases

Abstract

With the addition of water into liquid acetic acid, the C O stretching vibration band of acetic acid shows a high-frequency shift from 1665 to 1715 cm -1 . This means that the hydrogen bond of the C O group of acetic acid is not as strong as those seen in liquid acetic acid or in CCl 4 solution (in which the band appears at 1668 cm -1 ). A bent type hydrogen bond is accountable for this observation. On the other hand, the increase of acetic acid in water drastically decreases the intensity of the hydrogen-bonded O−H stretching Raman band of water at 3200 cm -1 . This suggests that acetic acid breaks the hydrogen-bond networks of water. Low-frequency R (ν̄) spectra of acetic acid/water binary solutions are re-examined with new experimental data and ab initio molecular orbital analysis of intermolecular vibrational modes. The R (ν̄) spectrum of the aqueous mixture at x A = 0.5 bears a very close resemblance to that of the acetic acid/methanol mixture with x A = 0.5, indicating that the molecular complexes responsible for the Raman spectra are acetic acid clusters. The calculated low-frequency Raman feature of a side-on type dimer with bent-type hydrogen bonds based on ab initio molecular orbital theory reproduces the observed Raman pattern nicely. Any evidence of the formation of stable acid−water pairs is not found in the low-frequency Raman spectra. Furthermore, an isosbestic point is seen in the region of 0.1 ≤ x A (mole fraction of acetic acid) ≤ 0.5, and another one is also observed in 0.5 ≤ x A ≤ 1.0. The observed spectra in the region of 0 < x A < 0.5 are reproduced simply by linear combinations of the pure water spectrum and the spectrum at x A = 0.5. These results strongly suggest the presence of the two microphases with homogeneously associated molecules: a water cluster phase and an acetic acid cluster phase. The spectral change in 0.5 < x A < 1.0 is attributed to the coexistence of the acetic acid cluster phase in aqueous environment and the acid associated phase characteristic of liquid acetic acid.