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.