Structural and Optical Characteristics of Aqueous Solutions of Acetic Acid.

A refractometric method, coupled with molecular dynamics study, attenuated total reflection Fourier transform infrared (ATR FT-IR), and Raman spectroscopy, was used to determine optical characteristics of concentration features of aqueous solutions of acetic acid. Measurements of the refractive index of aqueous solutions of acetic acid in the wide range of acetic acid concentrations (∼ 0 ÷ 1 mole fraction) in a solution at a room temperature were conducted. Maximum value of refractive index was detected at a concentration of ∼0.3 mole fraction. The deviation from the parabolic form of the dependence of the refractive index on the concentration occurs at a concentration of ∼0.8 mole fraction. As far as we know, this deviation has been observed for the first time. The maximum is attributed to the largest number of molecular interactions between water and acetic acid molecules, while the deviation is associated with the parallel orientation of acetic acid molecules. To identify the reconstructing of molecules in the system, FT-IR and Raman spectra of these solutions at a concentration of ∼0.3 and ∼0.8 mole fraction were recorded and compared with pure solutions. The data obtained by using ATR FT-IR and Raman spectroscopy support the idea that the refractometric method is sensitive to determine the structural states of aqueous solutions of acetic acid.