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.