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.