Figure 6. Simulated <cos θμ> of the interfacial BUT molecules and EG molecules at 22℃ and 64℃.
It should be noted that Kataoka and Cremer observed a significant twist on the vapor/liquid interface of water-isopropyl alcohol binary mixtures at the bulk mole fraction of the azeotrope by assuming a δ-function orientational distribution.54 On the basis of the current work and our previous study on vapor/liquid interface of dimethyl carbonate and methanol binary mixture, we proved that the orientation of the interfacial molecules were not directly relevant to the mole fraction of the azeotrope at a room temperature.34 But a higher temperature compared to the ambient environment might lead to an orientation change of the interfacial molecules. Besides it has to be mentioned that the mechanism of the narrow decline in the intensity of SFG-VS spectra during the temperature rise around the azeotropic mole fraction is still not clear. It is definitely worth more systematic investigations on the molecular orientations of interfacial molecules affected by temperature.