4. CONCLUSION
A study of the liquid/gas surface of BUT and EG mixtures has been
performed by using the SFG-VS technique and MD simulation. At 22 ℃, with
increasing bulk mole fraction of BUT, the SFG-VS signal of EG is quickly
replaced by SFG-VS signal of BUT at aroundx but=0.3 for BUT molecules would quickly occupy
the topmost surface region even at a rather low mole fraction. At the
same time, the orientation of both BUT and EG molecules is independent
of the bulk mole fractions. BUT molecules would like to lean on the
interface with the methyl group point to the vapor phase and EG
molecules are likely to be tiled on the vapor/liquid interface at the
whole concentration range.
At 64℃, the obvious decline was observed in the intensity of SFG-VS
measurement compared with the SFG results at 22℃. This decline is
weakened around the azeotropic mole fraction. The BUT molecules prefer
to tilt at the vapor/liquid interface with the hydroxyl group on the
first carbon atom head downwards to the liquid bulk. Compared with the
orientation of interfacial molecules at 22℃, the interfacial BUT
molecules rotate around the C2-O2 bond and narrow the intersection angle
formed between the carbon chain and the vapor/liquid interface at 64℃.
In contrast to BUT molecules, EG molecules are likely to lie on the
vapor/liquid interface through the whole mole fraction range and barely
change the orientation after the temperature rise.
The present work is a molecular-level investigation on the liquid/vapor
interface of azeotropic mixture by conducting the temperature-dependent
SFG-VS measurements. The further study on liquid/vapor interface of the
azeotropic mixture should focus on a larger temperature range and more
representative azeotropic mixture.