A detail study of the microstructure of methyl benzoate/methanol mixture proved by IR spectra, excess infrared wavenumber, and physicochemical properties

By a comparative study of the concentration-dependent IR spectra of methyl benzoate (MB)/methanol- d 4 and MB/ n -hexane mixtures, it is found that the infrared wavenumber ( υ ˜ ) of ester group (O C O) of MB in the former system decreased exponentially with the increase of mole fraction of MB ( x 1 ), while that of the latter system decreased linearly, suggesting that H-bonding interaction plays an important role in the microstructure of MB/methanol- d 4 mixture. Therefore, to explore the microstructure and characterize the strength of H-bonding interaction of MB/methanol mixture, the excess infrared wavenumber of a group ( υ ˜ group E ) is proposed, showing that υ ˜ O − D E of methanol- d 4 presents an S shape with a minimum value at x 1  ≈ 0.4 and a maximum value at x 1  ≈ 0.8, respectively. Furthermore, the absolute value of υ ˜ O − D E (| υ ˜ O − D E |) of the former is greater than that of the latter, suggesting that H-bonding interaction in methanol- d 4 molecules is stronger than that between methanol- d 4 and MB. While both υ ˜ C = O E and υ ˜ C − O E of MB show a negative deviation with minimum values at x 1  ≈ 0.4, and | υ ˜ C = O E | is larger than | υ ˜ C − O E |, indicating that C O is directly involved in H-bonding interaction with O D, resulting in the change of υ ˜ C − O E through the electronic conjugation effect. To confirm the microstructure obtained from IR spectra, densities, speeds of sound , and viscosities of MB/methanol mixture are measured and excess properties are calculated. The results show that both excess molar volumes and isentropic compressibility deviations have a negative deviation with a minimum value at x 1  ≈ 0.4, which is good consistent with x 1 corresponding to the minimum value of υ ˜ O − D E , υ ˜ C = O E , and υ ˜ C − O E , respectively. Moreover, viscosity deviations exhibit a negative deviation with a minimum value at x 1  ≈ 0.8, which is good consistent with x 1 corresponding to the maximum value of υ ˜ O − D E .