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 CO 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 CO group of acetic acid is not as strong as those seen in liquid acetic acid or in CCl4 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 xA = 0.5 bears a very close resemblance to that of the acetic acid/methanol mixture with xA = 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 ≤ xA (mole fraction of acetic acid) ≤ 0.5, and another one is also observed in 0.5 ≤ xA ≤ 1.0. The observed spectra in the region of 0 < xA < 0.5 are reproduced simply by linear combinations of the pure water spectrum and the spectrum at xA = 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 < xA < 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.