1. INTRODUCTION
Understanding interfacial phenomena of organic liquids is essential in the mass and heat transfer of industrial applications. The distinct properties of the interface contrast to those of bulk phase generate not only from the different compositions but also from the asymmetry of the forces that the molecules experience at the interface.1 However, the experimental observations of the interfacial structural properties, especially for those varied with the chemical environment and ambient environment are still challenging.
The sum-frequency generation vibrational spectroscopy (SFG-VS) is a sensitive means of probing the molecular behavior at an asymmetric environment.2-5 It has an extensive application in the study of vapor/liquid interface of pure compounds or mixture at a room temperature.6-20 The characteristics of the vapor/liquid interface such as anisotropic molecular orientation, unique adsorption character have been revealed by these studies. But as for the effect of temperature on the organizations of interfacial liquids, the investigations have been limited to the examples of water/air interface, ice/water interface or solid/liquid interface.21-26The exploration of temperature effect on the air-liquid interface of organic-organic binary mixture is a relatively unexplored realm.
In the current work, the vapor/liquid interface of the binary mixture of 1,2-butanediol (BUT) and ethylene glycol (EG) is investigated by polarization-dependent SFG-VS and MD simulation. The investigation on this binary system is significant in both theoretical studies and practical applications. EG is the simplest diol with no methyl group and has been served as a typical sample in SFG spectroscopy study of CH stretching modes of the methylene group.27, BUT has rarely been investigated by any spectroscopy method compared with EG. It is valuable to research on the competitive C-H vibrational mode in an organic-organic binary mixture. On the other hand, they both play important roles in the chemical industries. EG is widely used in petrochemical industry, such as antifreeze, and plasticizers. It is also an important precursor in manufacturing high value chemical compounds such as glycolic acid, methyl glycolate, etc.28 A widespread approach of producing EG is via hydrogenation of dimethyl oxalate.29,30 In this process, some propanediol and butanediol are produced at the same time. During the distillation of these glycerols, the mixture of 1,2-butanediol and EG is rather difficult to separate for their approximate boiling points (196.5℃ for BUT and 197.1℃ for EG) and the azeotrope they form.31-33 Since every single molecule in the liquid phase would have to escape through the interface before entering the gas phase, it is interesting to examine whether the surface structure of the azeotrope is formed during the evaporations and boiling. Unfortunately, a recent study on the liquid/vapor interface of the binary system of dimethyl carbonate (DMC) and methanol claimed that the room-temperature surface molecular orientations are not necessary correlated to the formation of azeotropes.34 Thus it is reasonable to examine whether special surface organization would be formed under a higher temperature. And the high boiling points of BUT and EG provide an important advantage to achieve this further verification.
Temperature change usually leads to a bright variation on the formal vibrations, such as intensity change or vibrational shift in spectra.21,35 The bright variation on the spectra leads to an implication of varied molecular adsorption or molecular structure at the surface.24 The detection of the complex surface structure is limited in scope due to inherent approximations and time- and length-scale limitations. Molecular dynamics (MD) simulation is considered as a mutually reinforced method to help fulfill the limitations of experimental measurements. The coupling between the SFG-VS experiments and computation would provide one of the most effective strategies to understand the real structure of the interfacial molecules. In this work, our contribution is demonstrating the effect of the variation of concentration and temperature on the molecular structure of this binary mixture at the liquid/vapor interface by using SFG-VS measurements and MD simulation. The SFG-VS spectra of vapor/BUT-EG mixture interface were recorded in the CH vibrational modes with SSP, PPP, SPS polarizations (the indexing was defined in the order of SFG, visible and IR beams). BUT yields stronger SFG-VS signals than EG at low 1,2-butanediol mole fraction and remain stable with increasing mole fraction at 22 ℃. After the temperature rise to 64 ℃, an obvious decline was observed in the spectra of different concentration collected, but the decline is weakened around the azeotropic mole fraction. A further analysis by MD simulation was carried out to understand the SFG-VS data and the molecular structure. At 22 ℃, the increase of the bulk BUT mole fraction has no effect on the molecular orientation of BUT or EG at the surface. When the temperature rises to 64 ℃, BUT molecules would gradually twist around its C2-O2 bond and turns from a tilt orientation to a lying orientation. At the same time, EG molecules remain a lying posture during the temperature rise.